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Polymer dye lasers

Profile image of Søren BalslevSøren Balslev

2006

Abstract

The purpose of this project has been to develop miniaturized polymer dye lasers that can be integrated on microchips containing other polymer "laboratory on a chip" circuitry. The envisioned function of the polymer dye lasers has been to deliver light for sensing purposes, either for wavelength dependent absorption or for interference based sensing, for example with integrated Mach-Zehnder interferometers. v Preface This thesis is submitted as a partial fulfillment of the requirements for obtaining the Ph.D. degree from the Technical University of Denmark (DTU). The research reported has been conducted at the Department of Micro-and Nanotechnology (MIC) at DTU in the period from February 2003 to January 2006. The project has been supervised by Associate Professor Anders Kristensen and financed by a grant from the Danish Technical Research Council (Statens Teknisk-Videnskablige Forskningsråd (STVF) (grant No. 26-02-0064)). This project would not have been possible without the help from numerous people. First of all I would like to thank my supervisor Anders Kristensen who through enthusiasm, encouragement and insight into physics has been an outstanding µαί υσις.

Figures (142)
Figure 1.1: The concept of lab-on-a-chip systems is to replace laboratory equipment such as beakers etc. with small chips that has the same functionality as a macroscale laboratory procedure. Figure 1.1: The concept of lab-on-a-chip systems is to replace laboratory equipment such
Figure 1.1: The concept of lab-on-a-chip systems is to replace laboratory equipment such as beakers etc. with small chips that has the same functionality as a macroscale laboratory procedure. Figure 1.1: The concept of lab-on-a-chip systems is to replace laboratory equipment such
The realization of the laser has enabled entirely new kinds of light-based sen- sors based on interference, such as the Mach-Zehnder interferometer, and based on monochromaticity, such as polarization spectroscopy. These kind of sensing techniques are impossible or at best, rather cumbersome to perform using in- candescent or arc light. Using lasers, the techniques have made it possible tc determine minute changes in length, refractive index, details in molecular elec- tronic transitions and the distance to the moon. The widespread use of lasers today is a testimony to the usefulness of laser based detection systems, which is also the motivational foundation for the work dedicated to develop cheap polymet based light sources for LOC type systems.
The realization of the laser has enabled entirely new kinds of light-based sen- sors based on interference, such as the Mach-Zehnder interferometer, and based on monochromaticity, such as polarization spectroscopy. These kind of sensing techniques are impossible or at best, rather cumbersome to perform using in- candescent or arc light. Using lasers, the techniques have made it possible tc determine minute changes in length, refractive index, details in molecular elec- tronic transitions and the distance to the moon. The widespread use of lasers today is a testimony to the usefulness of laser based detection systems, which is also the motivational foundation for the work dedicated to develop cheap polymet based light sources for LOC type systems.
Figure 1.3: Sketch of the first organic dye laser using a phthalocyanine solution (reproduced from [13]). cell. The fluid cell was placed in a macroscopic optical resonator consisting of twe flat dielectric mirrors. Later, in 1967, Sorokin and Lankard also demonstrated las- ing with an organic dye solution pumped by a flash lamp [14]. The demonstration of direct flash lamp pumping of dye lasers is an important step, since dye laser systems thus can become much simpler, when they do not need to be pumped by another laser.
Figure 1.3: Sketch of the first organic dye laser using a phthalocyanine solution (reproduced from [13]). cell. The fluid cell was placed in a macroscopic optical resonator consisting of twe flat dielectric mirrors. Later, in 1967, Sorokin and Lankard also demonstrated las- ing with an organic dye solution pumped by a flash lamp [14]. The demonstration of direct flash lamp pumping of dye lasers is an important step, since dye laser systems thus can become much simpler, when they do not need to be pumped by another laser.
Figure 1.6: Left: Operational principle of square resonators in dye doped polymer. Right Electron microscope image of realized dye doped PMMA resonator array (reproducec from [19]).
Figure 1.6: Left: Operational principle of square resonators in dye doped polymer. Right Electron microscope image of realized dye doped PMMA resonator array (reproducec from [19]).
The concept of a laser oscillator based on distributed feedback from a Bragg grating formed along the gain medium was first demonstrated in 1970 (published 1971) by Kogelnik and Shank [20]. In order to demonstrate the principle, they used a 14 um thick gelatine film doped with the organic dye Rhodamine 6G (see Figure 1.7). The Bragg grating on the film had a period of about 300 nm and was formed by interference lithography, and the size of the grating was 10 mm x 0.1 mm. The laser emitted light at a single wavelength (within the observational imit) when pumped by a UV Nitrogen laser (337 nm). However, the grating was continuous and did not contain a phase-shift to accommodate for the grating stop band. It is therefore possible that the laser operated on two narrowly spaced wavelengths simultaneously. Later in 1971 Shank, Bjorkholm and Kogelnik demonstrated another type of DFB
The concept of a laser oscillator based on distributed feedback from a Bragg grating formed along the gain medium was first demonstrated in 1970 (published 1971) by Kogelnik and Shank [20]. In order to demonstrate the principle, they used a 14 um thick gelatine film doped with the organic dye Rhodamine 6G (see Figure 1.7). The Bragg grating on the film had a period of about 300 nm and was formed by interference lithography, and the size of the grating was 10 mm x 0.1 mm. The laser emitted light at a single wavelength (within the observational imit) when pumped by a UV Nitrogen laser (337 nm). However, the grating was continuous and did not contain a phase-shift to accommodate for the grating stop band. It is therefore possible that the laser operated on two narrowly spaced wavelengths simultaneously. Later in 1971 Shank, Bjorkholm and Kogelnik demonstrated another type of DFB
laser [21]. In this laser, the grating was not formed by periodic structural corru- gation of the material, instead a periodic variation of the gain was introduced in the material by using two interfering pump beams to create the grating pattern during the pumping itself (see Figure 1.8). The gain medium was a bulk volume of liquid dye solution and the grating was thus formed only during pumping in a thin layer of the liquid. This method enabled wavelength tuning of the dye laser, simply by changing the angle of the two interfering pump beams. The gain grating further has the advantage that no phase shift is needed in the grating in order to obtain a single resonance at the Bragg reflection wavelength [22].
laser [21]. In this laser, the grating was not formed by periodic structural corru- gation of the material, instead a periodic variation of the gain was introduced in the material by using two interfering pump beams to create the grating pattern during the pumping itself (see Figure 1.8). The gain medium was a bulk volume of liquid dye solution and the grating was thus formed only during pumping in a thin layer of the liquid. This method enabled wavelength tuning of the dye laser, simply by changing the angle of the two interfering pump beams. The gain grating further has the advantage that no phase shift is needed in the grating in order to obtain a single resonance at the Bragg reflection wavelength [22].
Figure 2.1: Absorption in the short-wavelength range for varying thin-film thicknesses of unexposed and uncured (but pre-exposure baked) SU-8 (reproduced from [28]). 30 nm to 2 mm can be used.
Figure 2.1: Absorption in the short-wavelength range for varying thin-film thicknesses of unexposed and uncured (but pre-exposure baked) SU-8 (reproduced from [28]). 30 nm to 2 mm can be used.
Figure 2.2: Propagation loss measured on a SU-8 waveguide 100 um thick and 30 um wide. The distribution of loss contributions between the SU-8 absorption and imperfections in the waveguide structure is unknown (reproduced from [29]).
Figure 2.2: Propagation loss measured on a SU-8 waveguide 100 um thick and 30 um wide. The distribution of loss contributions between the SU-8 absorption and imperfections in the waveguide structure is unknown (reproduced from [29]).
Figure 2.3: Propagation loss measured on a SU-8 waveguide 10 tm thick and 30 um wide. The distribution of loss contributions between the SU-8 absorption and imperfections in the waveguide structure is unknown (reproduced from [30]). ‘igure 2.3: Propagation loss measured on a SU-8 waveguide 10 um thick and 30 um wide.
Figure 2.3: Propagation loss measured on a SU-8 waveguide 10 tm thick and 30 um wide. The distribution of loss contributions between the SU-8 absorption and imperfections in the waveguide structure is unknown (reproduced from [30]). ‘igure 2.3: Propagation loss measured on a SU-8 waveguide 10 um thick and 30 um wide.
Figure 2.4: Absorbance of SU-8 as function of wavelength (reproduced from [31]) supplied from the manufacturer [27], the dispersion at 600 nm is —1-10~4 nm7!
Figure 2.4: Absorbance of SU-8 as function of wavelength (reproduced from [31]) supplied from the manufacturer [27], the dispersion at 600 nm is —1-10~4 nm7!
PMMA (poly-methyl methacrylate) is a well known highly transparent thermo- plast. It is cheap and widely used, also in everyday items where it is also known under the name of plexiglass. PMMA comes dissolved for example in anisole, and is available with well controlled chain lengths. The chain length determines the viscosity of the polymer above the glass transition temperature, where the poly- mer changes from a glassy state where the polymer is elastic to a state where it can be deformed permanently [35]. This is an important parameter for nanoimprint
PMMA (poly-methyl methacrylate) is a well known highly transparent thermo- plast. It is cheap and widely used, also in everyday items where it is also known under the name of plexiglass. PMMA comes dissolved for example in anisole, and is available with well controlled chain lengths. The chain length determines the viscosity of the polymer above the glass transition temperature, where the poly- mer changes from a glassy state where the polymer is elastic to a state where it can be deformed permanently [35]. This is an important parameter for nanoimprint
Figure 2.6: The refractive index of SU-8 as function of post-exposure bake temperature. The SU-8 film was baked for 3 minutes (the reference does not state at which wavelength, but it may have been 633 nm) (reproduced from [33]). or bonding applications. The chain length of the PMMA used in this thesis for bonding is 950 k and the chain length for imprint was 50 k. The building block for PMMA is shown in Figure 2.7.
Figure 2.6: The refractive index of SU-8 as function of post-exposure bake temperature. The SU-8 film was baked for 3 minutes (the reference does not state at which wavelength, but it may have been 633 nm) (reproduced from [33]). or bonding applications. The chain length of the PMMA used in this thesis for bonding is 950 k and the chain length for imprint was 50 k. The building block for PMMA is shown in Figure 2.7.
combination of grades with different refractive indices and glass transition tem- peratures, which can be exploited in the fabrication. It must be noticed however, that the different grades of COC have different behaviors, specifically relating to the formation of cracks in advanced fabrication steps (this has been examined by Dan Johansen in [38]). The optical loss for short wavelengths is illustrated in Figure 2.8, further measurements on loss can be found in [39].
combination of grades with different refractive indices and glass transition tem- peratures, which can be exploited in the fabrication. It must be noticed however, that the different grades of COC have different behaviors, specifically relating to the formation of cracks in advanced fabrication steps (this has been examined by Dan Johansen in [38]). The optical loss for short wavelengths is illustrated in Figure 2.8, further measurements on loss can be found in [39].
Figure 2.10: Figure showing the structure of the cyclo-olefin copolymer. The polymer consists of ethylene (left) and norbornene (right). The X and Y subscripts indicates that any of the two monomers may be followed by either of the two monomers in a random fashion.
Figure 2.10: Figure showing the structure of the cyclo-olefin copolymer. The polymer consists of ethylene (left) and norbornene (right). The X and Y subscripts indicates that any of the two monomers may be followed by either of the two monomers in a random fashion.
The organic laser dye that has been used throughout the work described in this thesis is Rhodamine 6G. Use of this dye as amplifying medium in lasers goes back to the 1970’ties, and the dye has remained a preferred laser dye due to its large quantum efficiency and relatively long lifetime before bleaching, compared to other dyes. Even today it is used extensively in tunable lasers for visible wavelengths in research laboratories for many applications such as spectroscopy. Rhodamine 6G has an organic molecular structure with the feature of conjugated double bonds. The structure is depicted in Figure 2.11.
The organic laser dye that has been used throughout the work described in this thesis is Rhodamine 6G. Use of this dye as amplifying medium in lasers goes back to the 1970’ties, and the dye has remained a preferred laser dye due to its large quantum efficiency and relatively long lifetime before bleaching, compared to other dyes. Even today it is used extensively in tunable lasers for visible wavelengths in research laboratories for many applications such as spectroscopy. Rhodamine 6G has an organic molecular structure with the feature of conjugated double bonds. The structure is depicted in Figure 2.11.
Figure 2.13: Cross sections for singlet-singlet absorption (a) and fluorescence (a~), and triple-triplet (a) absorption for Rhodamine 6G in ethanol. (from [43]). the organic dye. Consider a single molecule: at room temperature, the molecule will rest at the lowest vibrational and rotational states in the Sp band (thermally distributed). In order to excite the molecule to a state in the S, band, a mini- mum energy of Eg — E, must be used. When the molecule has been excited to the 5S, band, it will decay on a picosecond timescale to the lowest states in the band via non-radiative decay. When the molecule decays to the Sp band, however, the smallest possible energy difference will be Eg — Ec, and the emitted photon may have a wavelength that is long enough not to be reabsorbed by other dye molecules. In the Sg band, the molecule will decay non-radiatively on a picosec- ond timescale to the lowest states in the S9 band. This mechanism separates the absorption and emission spectrally, and the shift of the spectra is denoted Stokes shift [40]. The shifted absorption and fluorescence cross sections are illustrated in Figure 2.13.
Figure 2.13: Cross sections for singlet-singlet absorption (a) and fluorescence (a~), and triple-triplet (a) absorption for Rhodamine 6G in ethanol. (from [43]). the organic dye. Consider a single molecule: at room temperature, the molecule will rest at the lowest vibrational and rotational states in the Sp band (thermally distributed). In order to excite the molecule to a state in the S, band, a mini- mum energy of Eg — E, must be used. When the molecule has been excited to the 5S, band, it will decay on a picosecond timescale to the lowest states in the band via non-radiative decay. When the molecule decays to the Sp band, however, the smallest possible energy difference will be Eg — Ec, and the emitted photon may have a wavelength that is long enough not to be reabsorbed by other dye molecules. In the Sg band, the molecule will decay non-radiatively on a picosec- ond timescale to the lowest states in the S9 band. This mechanism separates the absorption and emission spectrally, and the shift of the spectra is denoted Stokes shift [40]. The shifted absorption and fluorescence cross sections are illustrated in Figure 2.13.
Figure 2.14: Principle of four level laser energy level diagram.
Figure 2.14: Principle of four level laser energy level diagram.
Figure 2.15: Quantum yield ¢ of Rhodamine 6G dissolved in ethanol as function of concen- tration. The measurements were taken at 20°C. The concentration c is in mol/L. A rapid drop of ¢ is observed at c~ 10~? mol/L (reproduced from [49]) Figure 2.15: Quantum yield @ of Rhodamine 6G dissolved in ethanol as function of concen-
Figure 2.15: Quantum yield ¢ of Rhodamine 6G dissolved in ethanol as function of concen- tration. The measurements were taken at 20°C. The concentration c is in mol/L. A rapid drop of ¢ is observed at c~ 10~? mol/L (reproduced from [49]) Figure 2.15: Quantum yield @ of Rhodamine 6G dissolved in ethanol as function of concen-
Figure 2.16: Sketch of the behavior of the fluorescence and absorption spectra during pumping (dotted arrows). As pumping is increased the gain increases and the peak of the gain moves to shorter wavelengths. to the loss and the gain will change - thus shifting the net-gain maximum (see Figure 2.16).
Figure 2.16: Sketch of the behavior of the fluorescence and absorption spectra during pumping (dotted arrows). As pumping is increased the gain increases and the peak of the gain moves to shorter wavelengths. to the loss and the gain will change - thus shifting the net-gain maximum (see Figure 2.16).
Figure 3.1: Illustration of reflection and refraction of an incident beam of light travelling in a material with refractive index n; and impinging on the dielectric interface where the refractive index changes to n;. Part of the beam is refracted with an exit angle of ©; and part of the beam is reflected with. where n; and n; are the refractive indices of the ”incident” and the ” transmitted” medium and the angles 0; and ©, are given in Figure 3.1.
Figure 3.1: Illustration of reflection and refraction of an incident beam of light travelling in a material with refractive index n; and impinging on the dielectric interface where the refractive index changes to n;. Part of the beam is refracted with an exit angle of ©; and part of the beam is reflected with. where n; and n; are the refractive indices of the ”incident” and the ” transmitted” medium and the angles 0; and ©, are given in Figure 3.1.
Figure 3.2: Principle drawing of waveguide. Light with incidence angles above the critical angle is reflected successively on the dielectric interface between the core and the cladding. This dependence on the angle leads to a restriction on how light can be coupled into the waveguide — the angle must lie within the acceptance cone (reproduced from [51]). In a ray picture, waveguiding occurs due to successive total internal reflection on dielectric surfaces (see Figure 3.2), and the phenomenon serves to confine and direct light (or other waves) within the waveguide structure. In order to obtain total internal reflection, the refractive index of the so-called core of the waveguide must be higher than the cladding (the material surrounding the core). angle is reflected successively on the dielectric interface between the core and the cladding.
Figure 3.2: Principle drawing of waveguide. Light with incidence angles above the critical angle is reflected successively on the dielectric interface between the core and the cladding. This dependence on the angle leads to a restriction on how light can be coupled into the waveguide — the angle must lie within the acceptance cone (reproduced from [51]). In a ray picture, waveguiding occurs due to successive total internal reflection on dielectric surfaces (see Figure 3.2), and the phenomenon serves to confine and direct light (or other waves) within the waveguide structure. In order to obtain total internal reflection, the refractive index of the so-called core of the waveguide must be higher than the cladding (the material surrounding the core). angle is reflected successively on the dielectric interface between the core and the cladding.
Figure 3.3: Basic three-layer planar waveguide structure. Three modes are shown, repre- senting distributions of electric field in the x-direction. The planes illustrate the dielectric interfaces between the three materials (reproduced from [52]).
Figure 3.3: Basic three-layer planar waveguide structure. Three modes are shown, repre- senting distributions of electric field in the x-direction. The planes illustrate the dielectric interfaces between the three materials (reproduced from [52]).
modes, as later chapters will refer to in this thesis) and the lowest mode is termed the fundamental mode.
modes, as later chapters will refer to in this thesis) and the lowest mode is termed the fundamental mode.
Figure 4.1: Sketch of planar waveguide consisting of three layers of dielectric material with refractive indices n1, n2 and n;. The thickness of the middle layer is tg and the surrounding layers extend to infinity. Consider a planar waveguide, as in Figure 4.1, consisting of three layers with re- fractive indices, n1, n2 and n3 respectively, where the core layer thickness is tg. Consider the field in the waveguide as described by Ey(a,z,t) = Ey(x)e-), and let this be defined as the T] F} mode, for which the non-zero components are Ey, H, and H,. This fundamental type of waveguide is the one most commonly encountered in the polymer dye asers concerned in this thesis. The planar wave- guide approach is a good approximation in most cases where, for example, a waveguide defined by a strip of sandwiched polymer is much wider than it is high. propagation constant that they good. Especially when the waveguide strip is so wide that the optical transverse modes in the horizontal direction (TE or TM) are so closely spaced with respect to their are effectively degenerate, the approximation is
Figure 4.1: Sketch of planar waveguide consisting of three layers of dielectric material with refractive indices n1, n2 and n;. The thickness of the middle layer is tg and the surrounding layers extend to infinity. Consider a planar waveguide, as in Figure 4.1, consisting of three layers with re- fractive indices, n1, n2 and n3 respectively, where the core layer thickness is tg. Consider the field in the waveguide as described by Ey(a,z,t) = Ey(x)e-), and let this be defined as the T] F} mode, for which the non-zero components are Ey, H, and H,. This fundamental type of waveguide is the one most commonly encountered in the polymer dye asers concerned in this thesis. The planar wave- guide approach is a good approximation in most cases where, for example, a waveguide defined by a strip of sandwiched polymer is much wider than it is high. propagation constant that they good. Especially when the waveguide strip is so wide that the optical transverse modes in the horizontal direction (TE or TM) are so closely spaced with respect to their are effectively degenerate, the approximation is
Figure 4.2: Segment of a grating in the transfer matrix approach. The forward and back- ward propagating waves are denoted with the letters A and B respectively. The relationship between the wave amplitudes can be expressed by a matrix. The relation between the forward and backward propagating waves for the 7’th segment of a grating can be represented by a matrix, Tetement,;, Such that
Figure 4.2: Segment of a grating in the transfer matrix approach. The forward and back- ward propagating waves are denoted with the letters A and B respectively. The relationship between the wave amplitudes can be expressed by a matrix. The relation between the forward and backward propagating waves for the 7’th segment of a grating can be represented by a matrix, Tetement,;, Such that
The system T relates the four field amplitudes to each other with a complex coupling coefficient for each pair. The system T is represented by a 2 by 2 matrix T, such that
The system T relates the four field amplitudes to each other with a complex coupling coefficient for each pair. The system T is represented by a 2 by 2 matrix T, such that
Figure 4.5: Dielectric interface between two materials with refractive indices n, and ng. The interface has a reflection and transmission denoted by rj2 and tj2. Ai, Ao, By and Bo represents the ports of the system. where rj is the reflectivity of the dielectric interface in Figure 4.5 for light im- pinging from the left side. The reflectivity is given by
Figure 4.5: Dielectric interface between two materials with refractive indices n, and ng. The interface has a reflection and transmission denoted by rj2 and tj2. Ai, Ao, By and Bo represents the ports of the system. where rj is the reflectivity of the dielectric interface in Figure 4.5 for light im- pinging from the left side. The reflectivity is given by
Figure 4.6: Transmission line of length LZ in a dielectric material of refractive index no. A, Ag, By and Bo represents the ports of the system. The transmission matrix for light travelling a distance L in a medium where the propagation constant is 3 is given by
Figure 4.6: Transmission line of length LZ in a dielectric material of refractive index no. A, Ag, By and Bo represents the ports of the system. The transmission matrix for light travelling a distance L in a medium where the propagation constant is 3 is given by
Figure 4.7: Screenshot from COMSOL Multiphysics showing the result of a simulation of an electromagnetic wave travelling in a waveguide that terminates in a triangle. The color scale depicts the magnitude of the electric field out of the plane. The scale is given in meters and the wavelength of the light is 570 nm. The model shows how light in the reflection is scattered. Figure 4.7: Screenshot from COMSOL Multiphysics showing the result of a simulation of an electromagnetic wave travelling in a waveguide that terminates in a triangle. The color
Figure 4.7: Screenshot from COMSOL Multiphysics showing the result of a simulation of an electromagnetic wave travelling in a waveguide that terminates in a triangle. The color scale depicts the magnitude of the electric field out of the plane. The scale is given in meters and the wavelength of the light is 570 nm. The model shows how light in the reflection is scattered. Figure 4.7: Screenshot from COMSOL Multiphysics showing the result of a simulation of an electromagnetic wave travelling in a waveguide that terminates in a triangle. The color
Gathering the boundary conditions in the second member yields
Gathering the boundary conditions in the second member yields
There are two sweep steps in the solution algorithm. The first step reduces the number of non-zero elements in the matrix by ”removing” the a; coefficients. The following new coefficients are introduced:
There are two sweep steps in the solution algorithm. The first step reduces the number of non-zero elements in the matrix by ”removing” the a; coefficients. The following new coefficients are introduced:
The second step is to backward sweep the new matrix in order to obtain the solution. Firstly,
The second step is to backward sweep the new matrix in order to obtain the solution. Firstly,
Figure 4.8: Sketch of optical resonator consisting of two mirrors with complex reflectivities ry and rg. The distance between the mirrors is Ly + L2. No photons begin a round trip in the resonator at the plane py, and Ny photons are left after one round trip. Figure 4.8: Sketch of optical resonator consisting of two mirrors with complex reflectivities
Figure 4.8: Sketch of optical resonator consisting of two mirrors with complex reflectivities ry and rg. The distance between the mirrors is Ly + L2. No photons begin a round trip in the resonator at the plane py, and Ny photons are left after one round trip. Figure 4.8: Sketch of optical resonator consisting of two mirrors with complex reflectivities
For each time segment, the inversion of the Rhodamine molecular electronic states and the penetration of the optical pump light is calculated via rate equations. The time is divided into Tax segments, each with a duration of At, short enough to adequately resolve the modal gain can wavelength at whic the pump pulse shape. In this manner, the spectral shape of be calculated throughout the duration of the pump pulse. The h the modal gain just reaches the assumed resonator loss, can be interpreted as the resulting lasing wavelength, using the assumption that lasing action will clamp t reality, the gain wil he gain. In this way, the lasing wavelength can be found. In not be uniformly clamped spectrally and the Rhodamine 6G containing media will also have a spatially distributed gain, giving rise to spectral and spatial hole-burning respectively. The result from the model, however, gives a pointer to the dynamics of the system, and the model will give more accurate results if more precise cross-section functions are used. Preliminary note: The model presented below is a suggestion for how to approach the problem of finding the spectral gain distribution in dye doped layers which are optically dense with respect to the pump light. It must be mentioned that the sta- bility of the method has not been confirmed. The issue concerning the distribution of absorbed pump power has also been addressed by Blit and Ganiel [67].
For each time segment, the inversion of the Rhodamine molecular electronic states and the penetration of the optical pump light is calculated via rate equations. The time is divided into Tax segments, each with a duration of At, short enough to adequately resolve the modal gain can wavelength at whic the pump pulse shape. In this manner, the spectral shape of be calculated throughout the duration of the pump pulse. The h the modal gain just reaches the assumed resonator loss, can be interpreted as the resulting lasing wavelength, using the assumption that lasing action will clamp t reality, the gain wil he gain. In this way, the lasing wavelength can be found. In not be uniformly clamped spectrally and the Rhodamine 6G containing media will also have a spatially distributed gain, giving rise to spectral and spatial hole-burning respectively. The result from the model, however, gives a pointer to the dynamics of the system, and the model will give more accurate results if more precise cross-section functions are used. Preliminary note: The model presented below is a suggestion for how to approach the problem of finding the spectral gain distribution in dye doped layers which are optically dense with respect to the pump light. It must be mentioned that the sta- bility of the method has not been confirmed. The issue concerning the distribution of absorbed pump power has also been addressed by Blit and Ganiel [67].
Figure 4.11: Sketch of energy level diagram for Rhodamine 6G with relevant life times. Sq and S; denote ground singlet state band and excited state band, while 7, denote the lowest triplet state band. The encircled numbers 1 and 2 denote the lower and the upper laser level respectively. Since the time steps are much shorter than the characteristic decay times for the dye molecules, and much shorter than the characteristic rise and fall time of the pump pulse, but longer than the time it takes for the pump light to propagate through the dye medium, it is reasonable to calculate the pump light intensity distribution throughout the layers, for each time step, based on the density of absorbing molecules present at each particular time step.
Figure 4.11: Sketch of energy level diagram for Rhodamine 6G with relevant life times. Sq and S; denote ground singlet state band and excited state band, while 7, denote the lowest triplet state band. The encircled numbers 1 and 2 denote the lower and the upper laser level respectively. Since the time steps are much shorter than the characteristic decay times for the dye molecules, and much shorter than the characteristic rise and fall time of the pump pulse, but longer than the time it takes for the pump light to propagate through the dye medium, it is reasonable to calculate the pump light intensity distribution throughout the layers, for each time step, based on the density of absorbing molecules present at each particular time step.
Figure 5.2: The lithographic steps for UV lithography on SU-8.
Figure 5.2: The lithographic steps for UV lithography on SU-8.
Figure 5.3: 26 um SU-8 strips on a glass substrate fabricated via UV lithography. The structure is seen from two different angles.
Figure 5.3: 26 um SU-8 strips on a glass substrate fabricated via UV lithography. The structure is seen from two different angles.
The minimum structure size with UV lithography is determined by the finite wavelength of the light, which leads to diffraction, and thus also by the thickness of the resist. With optimization, good resolution and aspect ratios can be achieved with UV lithography on SU-8. In Figure 5.4 a structure with an aspect ratio of 6 and a width of about 1 um is illustrated (from [33]). There are some recurring phenomena associated with SU-8. First of all, SU-8 has the tendency to form cracks to release internal stress. These cracks either appear around (dust-) particles or they may appear at corners in the lithographically defined structure. To lower the risk of cr acking, therefore particle contamination should be minimized and sharp corners in the design pattern can be rounded in order to distribute the stress along a longer edge. Cracking of the SU-8 film during development is a common problem, and it seems that the density of cracks increase with time the film is developed in PGMI] EA (propylene glycol monomethyl] ether acetate). Therefore the development time should be no longer than necessary. A simple method to remove cracks that may have appeared, is to apply a quick heat treatment, such as 160 °C for 60 seconds. This will in many cases heal cracks. The treatment can be repeated.
The minimum structure size with UV lithography is determined by the finite wavelength of the light, which leads to diffraction, and thus also by the thickness of the resist. With optimization, good resolution and aspect ratios can be achieved with UV lithography on SU-8. In Figure 5.4 a structure with an aspect ratio of 6 and a width of about 1 um is illustrated (from [33]). There are some recurring phenomena associated with SU-8. First of all, SU-8 has the tendency to form cracks to release internal stress. These cracks either appear around (dust-) particles or they may appear at corners in the lithographically defined structure. To lower the risk of cr acking, therefore particle contamination should be minimized and sharp corners in the design pattern can be rounded in order to distribute the stress along a longer edge. Cracking of the SU-8 film during development is a common problem, and it seems that the density of cracks increase with time the film is developed in PGMI] EA (propylene glycol monomethyl] ether acetate). Therefore the development time should be no longer than necessary. A simple method to remove cracks that may have appeared, is to apply a quick heat treatment, such as 160 °C for 60 seconds. This will in many cases heal cracks. The treatment can be repeated.
Figure 5.5: Exposure step of resist with electron beam. Figure 5.6 shows an electron microscope picture of a SU-8 structure defined via EBL. Compared to UV lithography, the corners are very well defined. The radius of curvature appears from the image to lie below 50 nm. The walls of the resist are well defined, and do not exhibit a ”skirt”. Nor does the top surface curve down before the wall appears, as can be the case for UV lithography (Figure 5.3). Since a 20 nm layer of gold was deposited on the wafer, this may have increased the roughness of the resist surface due to aggregation of metal in islands, rather than a uniform layer. The top surface of the SU-8 has a corrugation with a period of 180 nm, which was made by varying the dose delivered to the resist in a line pattern.
Figure 5.5: Exposure step of resist with electron beam. Figure 5.6 shows an electron microscope picture of a SU-8 structure defined via EBL. Compared to UV lithography, the corners are very well defined. The radius of curvature appears from the image to lie below 50 nm. The walls of the resist are well defined, and do not exhibit a ”skirt”. Nor does the top surface curve down before the wall appears, as can be the case for UV lithography (Figure 5.3). Since a 20 nm layer of gold was deposited on the wafer, this may have increased the roughness of the resist surface due to aggregation of metal in islands, rather than a uniform layer. The top surface of the SU-8 has a corrugation with a period of 180 nm, which was made by varying the dose delivered to the resist in a line pattern.
Even though the dose curve in Figure 5.7 appears to warrant the possibility of varying the resist height with 40 % or more, the quality of the resist deteriorates with too low doses. Figure 5.8 shows a series of electron microscope pictures of resist structures exposed with doses from 4 to 14 wC/cm?. It is apparent that the resist quality is bad for doses below ~6 wC/cm?, where a spongy network forms. The sponginess is likely to cause scattering of light, leading to losses for light propagating in the resist. The resist was exposed with a beam energy of 100 keV and a beam current of 0.2 nA.
Even though the dose curve in Figure 5.7 appears to warrant the possibility of varying the resist height with 40 % or more, the quality of the resist deteriorates with too low doses. Figure 5.8 shows a series of electron microscope pictures of resist structures exposed with doses from 4 to 14 wC/cm?. It is apparent that the resist quality is bad for doses below ~6 wC/cm?, where a spongy network forms. The sponginess is likely to cause scattering of light, leading to losses for light propagating in the resist. The resist was exposed with a beam energy of 100 keV and a beam current of 0.2 nA.
The major feature of the apparatus is the beam source. At the moment, there exist no laboratory sized machine that is able to deliver the intensity needed tc do effective X-ray lithography. The beam is delivered by a synchrotron, which is a large scale facility. Figure 5.9 shows the round building housing the synchrotror that was used as X-ray source for the work in this thesis. The synchrotron con- sists of en electron storage ring with a circumference of 260 m, where bunches o: electrons are accelerated to high energies, usually chrotron radiation itself, is produced by bending 2 GeV for ELETTRA. The syn- the electron beam with magnets and the intensity of the radiation depends on the electron beam current in the synchrotron ring, while the spectrum depends on high energy X-radiation is emitted into a so called the energy. The broad spectrum beam line, which is a dedicatec X-ray beam management setup. Figure 5.10 shows a photograph taken down the LILIT beamline that was used as X-ray source or the work in this thesis. Fo X-ray lithography, the beam line will contain wavelength selective mirrors, that picks out energy ranges of choice. —™ 1 Pow 1 - wa es oe x~rmrT a6 + aa TTTTm 1 ae » acme mrmrmemo At the end of the beam line, the beam tube opens up to free air, the X-rays pas through a beryllium window that ensures a continuous vacuum behind the windov (see Figure 5.11). The mask and substrate can be mounted on a so-called steppe in front of the beam exit. The stepper scans the mask and substrate assembly across the beam with a controlled speed. By scanning with different speeds, th X-ray dose reaching the resist can be varied, although the X-ray intensity is fixec by the beamline setup and the synchrotron electron beam current. The bean current drops slowly to half over a period of 36 hours due to loss or electrons it the storage ring. New electron bunches have to be injected to raise the curren once more.
The major feature of the apparatus is the beam source. At the moment, there exist no laboratory sized machine that is able to deliver the intensity needed tc do effective X-ray lithography. The beam is delivered by a synchrotron, which is a large scale facility. Figure 5.9 shows the round building housing the synchrotror that was used as X-ray source for the work in this thesis. The synchrotron con- sists of en electron storage ring with a circumference of 260 m, where bunches o: electrons are accelerated to high energies, usually chrotron radiation itself, is produced by bending 2 GeV for ELETTRA. The syn- the electron beam with magnets and the intensity of the radiation depends on the electron beam current in the synchrotron ring, while the spectrum depends on high energy X-radiation is emitted into a so called the energy. The broad spectrum beam line, which is a dedicatec X-ray beam management setup. Figure 5.10 shows a photograph taken down the LILIT beamline that was used as X-ray source or the work in this thesis. Fo X-ray lithography, the beam line will contain wavelength selective mirrors, that picks out energy ranges of choice. —™ 1 Pow 1 - wa es oe x~rmrT a6 + aa TTTTm 1 ae » acme mrmrmemo At the end of the beam line, the beam tube opens up to free air, the X-rays pas through a beryllium window that ensures a continuous vacuum behind the windov (see Figure 5.11). The mask and substrate can be mounted on a so-called steppe in front of the beam exit. The stepper scans the mask and substrate assembly across the beam with a controlled speed. By scanning with different speeds, th X-ray dose reaching the resist can be varied, although the X-ray intensity is fixec by the beamline setup and the synchrotron electron beam current. The bean current drops slowly to half over a period of 36 hours due to loss or electrons it the storage ring. New electron bunches have to be injected to raise the curren once more.
Figure 5.10: Photograph down the "LILIT” beamline (from [69]).
Figure 5.10: Photograph down the "LILIT” beamline (from [69]).
Figure 5.11: Beam exit and stepper at LILIT beamline at the ELETTRA synchrotron facility, Trieste, Italy. The substrate to be exposed is mounted on the stepper plate in front of the X-ray beam exit tube.
Figure 5.11: Beam exit and stepper at LILIT beamline at the ELETTRA synchrotron facility, Trieste, Italy. The substrate to be exposed is mounted on the stepper plate in front of the X-ray beam exit tube.
Figure 5.12: X-ray mask fabrication steps. In order to fabricate thick heavy metal structures with vertical walls, they are made via an electroplating process. The mask fabrication sequence consists of the following steps; 1) preparation of thin silicon nitride fi and a resist layer, 2) definition of resist free areas via m with thin gold layer EBL, where gold is to be electroplated 3) electroplating of gold and 4) removal of EBL resist. The fabrication steps for making a mask are shown in Figure 5.12. During the exposure step of the resist film, the mask is placed in contact with the resist and mounted in the X-ray beam path on the stepper. The lithographic process is much the same as for UV lithography, except for the radiation type and the mask type. It is necessary to adjust post-bake time and temperature to optimize the process.
Figure 5.12: X-ray mask fabrication steps. In order to fabricate thick heavy metal structures with vertical walls, they are made via an electroplating process. The mask fabrication sequence consists of the following steps; 1) preparation of thin silicon nitride fi and a resist layer, 2) definition of resist free areas via m with thin gold layer EBL, where gold is to be electroplated 3) electroplating of gold and 4) removal of EBL resist. The fabrication steps for making a mask are shown in Figure 5.12. During the exposure step of the resist film, the mask is placed in contact with the resist and mounted in the X-ray beam path on the stepper. The lithographic process is much the same as for UV lithography, except for the radiation type and the mask type. It is necessary to adjust post-bake time and temperature to optimize the process.
Figure 5.14: Electron microscope image of gold X-ray mask detail.
Figure 5.14: Electron microscope image of gold X-ray mask detail.
on SU-8. The surface of the resist has polymerized after the post-exposure bake plus development and forms a film on top of the nominal resist pattern. The film can be eliminated by lowering the X-ray dose, which indicates that the film is caused by the residual X-rays penetrating the shadow areas of the mask. If the mask contrast is too low, it is not possible to lower the dose enough, without underexposing the SU-8. It may be conjectured that the surface of the SU-8 resist is, for some reason, more sensitive than the bulk, which is why a surface film appears, even though the absorption of the X-rays through the resist is low due to the low density of the hydro-carbon based material. Figure 5.17: Electron microscope image of optimized X-ray fabricated SU-8 structure.
on SU-8. The surface of the resist has polymerized after the post-exposure bake plus development and forms a film on top of the nominal resist pattern. The film can be eliminated by lowering the X-ray dose, which indicates that the film is caused by the residual X-rays penetrating the shadow areas of the mask. If the mask contrast is too low, it is not possible to lower the dose enough, without underexposing the SU-8. It may be conjectured that the surface of the SU-8 resist is, for some reason, more sensitive than the bulk, which is why a surface film appears, even though the absorption of the X-rays through the resist is low due to the low density of the hydro-carbon based material. Figure 5.17: Electron microscope image of optimized X-ray fabricated SU-8 structure.
Figure 5.18: Electron microscope image of X-ray defined SU-8 structure.
Figure 5.18: Electron microscope image of X-ray defined SU-8 structure.
Figure 5.19: Electron microscope image of SU-8 structures made by angled X-ray exposure (AKOY\
Figure 5.19: Electron microscope image of SU-8 structures made by angled X-ray exposure (AKOY\
Figure 5.20: Fabrication steps during imprinting. protrusion areas, which has to be taken into account in further processing. n the NIL process, a film of thermoplastic resist on a substrate is patterned witl a stamp that is pressed down on the resist while the assembly is heated to above the glass temperature of the resist. The thermoplastic resist will then conforrr to the stamp. Subsequent cooling of the resist, brings the resist below the glas: temperature again, and the stamp pattern remains in the resist after separatior of the stamp and the resist coated substrate. The imprint process is illustrated ir Figure 5.20. There will always remain a residual layer of resist under the stamy protrusion areas, which has to be taken into account in further processing.
Figure 5.20: Fabrication steps during imprinting. protrusion areas, which has to be taken into account in further processing. n the NIL process, a film of thermoplastic resist on a substrate is patterned witl a stamp that is pressed down on the resist while the assembly is heated to above the glass temperature of the resist. The thermoplastic resist will then conforrr to the stamp. Subsequent cooling of the resist, brings the resist below the glas: temperature again, and the stamp pattern remains in the resist after separatior of the stamp and the resist coated substrate. The imprint process is illustrated ir Figure 5.20. There will always remain a residual layer of resist under the stamy protrusion areas, which has to be taken into account in further processing.
Figure 5.21: Electron microscope image of 26 um wide structures made with thermal nanoimprint lithography. The box with the arrow indicates the zoom illustrated in the Figure 5.22 below. The separation of substrate and stamp can be somewhat difficult, however there exist high performance anti-stiction coatings that can be applied to the mask, in order to ease the separation step.
Figure 5.21: Electron microscope image of 26 um wide structures made with thermal nanoimprint lithography. The box with the arrow indicates the zoom illustrated in the Figure 5.22 below. The separation of substrate and stamp can be somewhat difficult, however there exist high performance anti-stiction coatings that can be applied to the mask, in order to ease the separation step.
Figure 5.22: Electron microscope image of 26 um wide structures made with thermal nanoimprint lithography. The structure is a detail of the figure above.
Figure 5.22: Electron microscope image of 26 um wide structures made with thermal nanoimprint lithography. The structure is a detail of the figure above.
sidewalls, no ”skirting” and a flat top surface. The smoothness of the surfaces which reflect the smoothness of the stamp, is achieved by applying a ”smoothing step” (described in [71]) to the stamp. This, however, has the effect that the corners become somewhat rounded as seen in the closeup in Figure 5.22. It should be immediately obvious from the images, that the quality that can be achievec with NIL is very good.
sidewalls, no ”skirting” and a flat top surface. The smoothness of the surfaces which reflect the smoothness of the stamp, is achieved by applying a ”smoothing step” (described in [71]) to the stamp. This, however, has the effect that the corners become somewhat rounded as seen in the closeup in Figure 5.22. It should be immediately obvious from the images, that the quality that can be achievec with NIL is very good.
' No mask necessary. * The roughness depends on the mask, see text. In summary, several factors may be relevant when deciding which lithographic technique to use for SU-8. Table 5.2 aims to sum up pros and cons of the different lithographic methods. A few comments in connection with Table 5.2: The price varies a lot and UV lithography is by far the cheapest process. The actual cost of NIL is difficult to determine yet, since this lithographic method is still under development. Only UV lithography has a significantly lower resolution than the other types of lithography due to the wavelength of the light. The mask price for XRL and NIL is high since the masks must be fabricated with expensive technique are therefore more rare. The side wall smoothness of the XR] principle the roughness depends on the mask, which it may be the throughput per device on a wafer must be considered. slower than any of the other methods. s (Le. EBL). A UV ithography setup and a nanoimprint setup are fairly easy to install in a clean-room aboratory, while machines for EBL and especially XRL require more resources and |, fabricated devices have not been of superior quality in the work presented in this thesis, however, in possible to fabricate with a higher quality than what has been obtained in the present work. Finally, EBL is significantly
' No mask necessary. * The roughness depends on the mask, see text. In summary, several factors may be relevant when deciding which lithographic technique to use for SU-8. Table 5.2 aims to sum up pros and cons of the different lithographic methods. A few comments in connection with Table 5.2: The price varies a lot and UV lithography is by far the cheapest process. The actual cost of NIL is difficult to determine yet, since this lithographic method is still under development. Only UV lithography has a significantly lower resolution than the other types of lithography due to the wavelength of the light. The mask price for XRL and NIL is high since the masks must be fabricated with expensive technique are therefore more rare. The side wall smoothness of the XR] principle the roughness depends on the mask, which it may be the throughput per device on a wafer must be considered. slower than any of the other methods. s (Le. EBL). A UV ithography setup and a nanoimprint setup are fairly easy to install in a clean-room aboratory, while machines for EBL and especially XRL require more resources and |, fabricated devices have not been of superior quality in the work presented in this thesis, however, in possible to fabricate with a higher quality than what has been obtained in the present work. Finally, EBL is significantly
Figure 6.1: Photograph of the optical characterization laboratory showing the setup usec to characterize the polymer laser devices. The spectrometer that was used to measure the output spectra from the devices under test, was from Avantes and based on a fixed grating and a linear CCD array. The spectrometer was interfaced with a desktop computer. A multimode fibre with a core diameter of 200 um delivered the light to be measured to the spectrometer. In all experiments the other end of the fibre directly picked up the light emitted from the dye light sources into free space. The fibre was placed 1 mm to 5 cm from the devices under investigation, depending on the intensity of the emitted light. The spectrometer had a wavelength resolution of 0.15 nm (FWHM of the response to a very narrow line laser) and distinguished light in 4000 levels. Due to this limited dynamic range, the sid e mode suppression ratio (SMSR) for the lasers could not be measured reliably. | Even so, the lasers in this thesis that clearly operated in a single dominant mode are termed ”single mode”.
Figure 6.1: Photograph of the optical characterization laboratory showing the setup usec to characterize the polymer laser devices. The spectrometer that was used to measure the output spectra from the devices under test, was from Avantes and based on a fixed grating and a linear CCD array. The spectrometer was interfaced with a desktop computer. A multimode fibre with a core diameter of 200 um delivered the light to be measured to the spectrometer. In all experiments the other end of the fibre directly picked up the light emitted from the dye light sources into free space. The fibre was placed 1 mm to 5 cm from the devices under investigation, depending on the intensity of the emitted light. The spectrometer had a wavelength resolution of 0.15 nm (FWHM of the response to a very narrow line laser) and distinguished light in 4000 levels. Due to this limited dynamic range, the sid e mode suppression ratio (SMSR) for the lasers could not be measured reliably. | Even so, the lasers in this thesis that clearly operated in a single dominant mode are termed ”single mode”.
Figure 6.2: Photograph of the metal mirror laser mounted in a sample holder with fluidic connections. The chip contains a microfluidic mixer and the mixed dye fluid is carried to the resonator area (grey square) via a meander. For clarity, the lower part shows a drawing of a closeup of the meander and laser area. Figure 6.2: Photograph of the metal mirror laser mounted in a sample holder with fluidic
Figure 6.2: Photograph of the metal mirror laser mounted in a sample holder with fluidic connections. The chip contains a microfluidic mixer and the mixed dye fluid is carried to the resonator area (grey square) via a meander. For clarity, the lower part shows a drawing of a closeup of the meander and laser area. Figure 6.2: Photograph of the metal mirror laser mounted in a sample holder with fluidic
Figure 6.3: a) Top view and cross-sectional view of metal mirror laser device. Two inlets (1 and 2) allow fluids into the microfluidic channels, where the fluids are mixed and directed through a meander to the laser resonator which is located at the top mirror. The cross section shows the layered structure of the chip. b) Sketches of an area of the microfluidic channel with only a bottom mirror and an area with both a bottom and a top mirror. (1 and 2) allow fluids into the microfluidic channels, where the fluids are mixed and directed section shows the layered structure of the chip. b) Sketches of an area of the microfluidic
Figure 6.3: a) Top view and cross-sectional view of metal mirror laser device. Two inlets (1 and 2) allow fluids into the microfluidic channels, where the fluids are mixed and directed through a meander to the laser resonator which is located at the top mirror. The cross section shows the layered structure of the chip. b) Sketches of an area of the microfluidic channel with only a bottom mirror and an area with both a bottom and a top mirror. (1 and 2) allow fluids into the microfluidic channels, where the fluids are mixed and directed section shows the layered structure of the chip. b) Sketches of an area of the microfluidic
Figure 6.4: Transmitted power fraction through a resonator with mirrors having power reflectances 0.72 and 0.83 located 13.8 um from each other. The mirror distance corresponds to the optical path length (geometric length times refractive index) in the metal mirror resonator. The reflectances correspond to the values for the metal mirrors. Losses are not taken into account. ethanol and PMMA is negligible (R=0.3%). The expected transmission respons« of the micro-resonator, calculated via the formula below (from [78]), is illustratec in Figure 6.4. The mode spacing is 10.9 nm from the Fabry-Pérot free spectra range expression A\ = \?/(2L) ,where X is the wavelength of the light and L i: the optical path length between the mirrors. In the case of the metallic mirro: laser L = (ny Ly + noLe2), where ny = 1.33 and L, = 7.5 um are the refractive index and channel height for the fluid channel (containing ethanol), and ny = 1.4¢ and L, = 2.6 um are the refractive index and the thickness of the PMMA bonding laver.
Figure 6.4: Transmitted power fraction through a resonator with mirrors having power reflectances 0.72 and 0.83 located 13.8 um from each other. The mirror distance corresponds to the optical path length (geometric length times refractive index) in the metal mirror resonator. The reflectances correspond to the values for the metal mirrors. Losses are not taken into account. ethanol and PMMA is negligible (R=0.3%). The expected transmission respons« of the micro-resonator, calculated via the formula below (from [78]), is illustratec in Figure 6.4. The mode spacing is 10.9 nm from the Fabry-Pérot free spectra range expression A\ = \?/(2L) ,where X is the wavelength of the light and L i: the optical path length between the mirrors. In the case of the metallic mirro: laser L = (ny Ly + noLe2), where ny = 1.33 and L, = 7.5 um are the refractive index and channel height for the fluid channel (containing ethanol), and ny = 1.4¢ and L, = 2.6 um are the refractive index and the thickness of the PMMA bonding laver.
Figure 6.5: Process steps for fabrication of the metal mirror laser.
Figure 6.5: Process steps for fabrication of the metal mirror laser.
Figure 6.6: Metal mirror laser device holder. a. Schematic illustration of the device holder. b. Photograph of device holder with a device chip and inlet/outlet tubes mounted.
Figure 6.6: Metal mirror laser device holder. a. Schematic illustration of the device holder. b. Photograph of device holder with a device chip and inlet/outlet tubes mounted.
Figure 6.7: Output spectrum from the metal mirror laser for varying concentrations of Rhodamine 6G in ethanol. The pump energy density was 58 uJ mm~?. The label indicates the relative flow rate of pure ethanol and a 0.1 mol/L ethanolic solution of Rhodamine 6G controlled by syringe pumps. Rhodamine 6G in ethanol. The pump energy density was 58 uJ mm “*. The label indicates
Figure 6.7: Output spectrum from the metal mirror laser for varying concentrations of Rhodamine 6G in ethanol. The pump energy density was 58 uJ mm~?. The label indicates the relative flow rate of pure ethanol and a 0.1 mol/L ethanolic solution of Rhodamine 6G controlled by syringe pumps. Rhodamine 6G in ethanol. The pump energy density was 58 uJ mm “*. The label indicates
Figure 6.8: Absolute refractive indices versus wavelength for different temperatures: dashed line, ethanol; solid curves, 0.45 mol/L Rhodamine B in ethanol. 1, 23°C; 2, -5°C; 3, -20°C; 4, -70°C (reproduced from [80]). and the PMMA layer (2.6 um) and q is the mode number (found to be 49). The tuning range determined from the spectra in Figure 6.7 is AA = 4 nm, which corresponds to a change of refractive index of: An; = 0.013. | Expressed as An, /Ac this gives 1 L/mol (or 2.2. 10~* L/g), where Ac is the concentration change. The value is comparable to the value in [80] for Rhodamine B in ethanol at 570 nm, which can be found from the graph in Figure 6.8 to ~0.8 L/mol. This large value is connected to the anomalous dispersion curve around the molecular resonance.
Figure 6.8: Absolute refractive indices versus wavelength for different temperatures: dashed line, ethanol; solid curves, 0.45 mol/L Rhodamine B in ethanol. 1, 23°C; 2, -5°C; 3, -20°C; 4, -70°C (reproduced from [80]). and the PMMA layer (2.6 um) and q is the mode number (found to be 49). The tuning range determined from the spectra in Figure 6.7 is AA = 4 nm, which corresponds to a change of refractive index of: An; = 0.013. | Expressed as An, /Ac this gives 1 L/mol (or 2.2. 10~* L/g), where Ac is the concentration change. The value is comparable to the value in [80] for Rhodamine B in ethanol at 570 nm, which can be found from the graph in Figure 6.8 to ~0.8 L/mol. This large value is connected to the anomalous dispersion curve around the molecular resonance.
Figure 6.9: Output spectra from optical pumping with 111 uJ mm? of the microfluidic channel (away from the metal mirror resonator). The four spectra corresponds to Rhodamine 6G concentrations of A: 5.6- 1073 mol/L, B: 7.7. 10~% mol/L, C: 1.0- 107? mol/L and D: 1.3- 107? mol/L. The peak position moves from 573 nm to 583 nm. Inset: The peak wavelength as function of dye concentration.
Figure 6.9: Output spectra from optical pumping with 111 uJ mm? of the microfluidic channel (away from the metal mirror resonator). The four spectra corresponds to Rhodamine 6G concentrations of A: 5.6- 1073 mol/L, B: 7.7. 10~% mol/L, C: 1.0- 107? mol/L and D: 1.3- 107? mol/L. The peak position moves from 573 nm to 583 nm. Inset: The peak wavelength as function of dye concentration.
Figure 6.10: Output spectra from the microfluidic channel during pumping with increasing pump energy densities. The microfluidic channel contained a 0.01 mol/L Rhodamine 6G solution in ethanol. Inset: The pump curve shows the output power as function of pump power. pump energy densities. The microfluidic channel contained a 0.01 mol/L Rhodamine 6G
Figure 6.10: Output spectra from the microfluidic channel during pumping with increasing pump energy densities. The microfluidic channel contained a 0.01 mol/L Rhodamine 6G solution in ethanol. Inset: The pump curve shows the output power as function of pump power. pump energy densities. The microfluidic channel contained a 0.01 mol/L Rhodamine 6G
Figure 6.11: Graph of the center wavelength shift with time when the inlet flow rates are changed. The microcfluidic channel was optically pumped away from the metal mirror resonator. The relative flow rate is changed at t = 0.
Figure 6.11: Graph of the center wavelength shift with time when the inlet flow rates are changed. The microcfluidic channel was optically pumped away from the metal mirror resonator. The relative flow rate is changed at t = 0.
mode functionality. The mode field distribution in a waveguide with a 4 um thick doped SU-8 core having a refractive index 0.001 higher than the buffer and cladding is illustrated in Figure 6.14. Since there is an upper limit to how high a concentration of Rhodamine 6G in SU-8 can be used without and loosing the optical efficiency (see Section 2.2), a thick core layer means that a higher gain per length can be achieved since more dye molecules can contribute to the modal gain. Light travelling in the resonator experiences total internal reflection from three sides of the trapezoid and partial reflection from one side where the light is coupled out. Figure 6.15 illustrates some possible beam paths in a trapezoidal resonator. The angles of incidence are 62 = 03 = 47.3°, which is well above the critical angle for the SU-8 to air interface (38.9°). The angle of incidence on the output side was 6, = 38.1°, giving an angle of the out-coupled light of 6; = 79° which has been verified experimentally [81]. The most immediate interpretation of the
mode functionality. The mode field distribution in a waveguide with a 4 um thick doped SU-8 core having a refractive index 0.001 higher than the buffer and cladding is illustrated in Figure 6.14. Since there is an upper limit to how high a concentration of Rhodamine 6G in SU-8 can be used without and loosing the optical efficiency (see Section 2.2), a thick core layer means that a higher gain per length can be achieved since more dye molecules can contribute to the modal gain. Light travelling in the resonator experiences total internal reflection from three sides of the trapezoid and partial reflection from one side where the light is coupled out. Figure 6.15 illustrates some possible beam paths in a trapezoidal resonator. The angles of incidence are 62 = 03 = 47.3°, which is well above the critical angle for the SU-8 to air interface (38.9°). The angle of incidence on the output side was 6, = 38.1°, giving an angle of the out-coupled light of 6; = 79° which has been verified experimentally [81]. The most immediate interpretation of the
modes in the trapezoid resonator is the one of closed loops formed by a semi-plane
modes in the trapezoid resonator is the one of closed loops formed by a semi-plane
Figure 6.14: The distribution of the electric field for a waveguide with a core refractive index 0.001 higher than the cladding. The core thickness is 4 um. The field extends into the buffer and cladding, which must be taken into account when manufacturing the device.
Figure 6.14: The distribution of the electric field for a waveguide with a core refractive index 0.001 higher than the cladding. The core thickness is 4 um. The field extends into the buffer and cladding, which must be taken into account when manufacturing the device.
Figure 6.15: Three possible ray paths in the trapezoidal resonator, R, R’ and R”. All incidence angles except for 0, lies above the critical angle for total internal reflection. Light is coupled out in a sharp angle to the side A.
Figure 6.15: Three possible ray paths in the trapezoidal resonator, R, R’ and R”. All incidence angles except for 0, lies above the critical angle for total internal reflection. Light is coupled out in a sharp angle to the side A.
Figure 6.16: Fabrication steps for making the three layer UV defined trapezoid laser. In the steps from 1 to 6, the three layers of SU-8 are spun onto the substrate, and a UV lithographic step is performed. The three layer structure of resist was exposed in a exposure step, with subsequent development in PGMI standard UV lithography HA. It is noteworthy, that by performing simultaneous UV lithography on the t hree SU-8 layers, the wal where the internal laser light impinges on the sides can be as flat as the intrinsic resolution of the resist allows, since the imperfections are localized at the top edge and the bottom edge of the resist (confer for example with the electron microscop¢ images in Section 5.1.1).
Figure 6.16: Fabrication steps for making the three layer UV defined trapezoid laser. In the steps from 1 to 6, the three layers of SU-8 are spun onto the substrate, and a UV lithographic step is performed. The three layer structure of resist was exposed in a exposure step, with subsequent development in PGMI standard UV lithography HA. It is noteworthy, that by performing simultaneous UV lithography on the t hree SU-8 layers, the wal where the internal laser light impinges on the sides can be as flat as the intrinsic resolution of the resist allows, since the imperfections are localized at the top edge and the bottom edge of the resist (confer for example with the electron microscop¢ images in Section 5.1.1).
Figure 6.19: Output spectrum from a three layer trapezoid laser with a round-trip length | = 470um. The mode spacing is 0.39 nm.
Figure 6.19: Output spectrum from a three layer trapezoid laser with a round-trip length | = 470um. The mode spacing is 0.39 nm.
Figure 6.20: Spectra from three layer trapezoid lasers with varying side lengths /. Inset: Example of pump curve with threshold at 7 uJ mm~?.
Figure 6.20: Spectra from three layer trapezoid lasers with varying side lengths /. Inset: Example of pump curve with threshold at 7 uJ mm~?.
Figure 6.21: Lifetime of a dye doped thin film for three different pump energies. The pump pulse frequency is constant at 10 Hz and the pump spot diameter was 6 mm, with a flat intensity distribution.
Figure 6.21: Lifetime of a dye doped thin film for three different pump energies. The pump pulse frequency is constant at 10 Hz and the pump spot diameter was 6 mm, with a flat intensity distribution.
Figure 6.22: Lifetime of a dye doped thin film for three different pump pulse frequencies. The pump energy is constant at 10.1 mJ in the pump pulses impinging a spot with a diameter of 6 mm with a flat intensity distribution. Figure 6.22: Lifetime of a dye doped thin film for three different pump pulse frequencies.
Figure 6.22: Lifetime of a dye doped thin film for three different pump pulse frequencies. The pump energy is constant at 10.1 mJ in the pump pulses impinging a spot with a diameter of 6 mm with a flat intensity distribution. Figure 6.22: Lifetime of a dye doped thin film for three different pump pulse frequencies.
Figure 6.23: Photograph of the microfluidic high order DFB laser with lateral output. Two holes in the glass lid allows dye solution into the microfluidic channel and resontor, while waveguides carries the light to the edge of the chip. For many applications, such as interference based sensors, it is necessary to use a coherent single mode laser source with a narrow line width. The laser presented in this section has lateral output and can operate in a single mode®. The laser res- onator pattern is entirely defined in cross-linked undoped SU-8 resist. It consists
Figure 6.23: Photograph of the microfluidic high order DFB laser with lateral output. Two holes in the glass lid allows dye solution into the microfluidic channel and resontor, while waveguides carries the light to the edge of the chip. For many applications, such as interference based sensors, it is necessary to use a coherent single mode laser source with a narrow line width. The laser presented in this section has lateral output and can operate in a single mode®. The laser res- onator pattern is entirely defined in cross-linked undoped SU-8 resist. It consists
Figure 6.24: Sketch of the chip layer structure.
Figure 6.24: Sketch of the chip layer structure.
Figure 6.25: Sketch of the high order Bragg grating laser structure. The number of periods (actually 22) has been reduced in the drawing for clarity.
Figure 6.25: Sketch of the high order Bragg grating laser structure. The number of periods (actually 22) has been reduced in the drawing for clarity.
Figure 6.26: Illustration of a period in the high order Bragg grating. The structure is waveguiding where the core is SU-8 and anti-guiding where the core is ethanol. of the waveguide during pumping of the dye in the laser. It is apparent from the figures, that energy is lost in the antiguiding regions, especially as the mode number increases. Table 6.1 lists the calculated losses for traversing an anti- guiding section for the first six TE modes.
Figure 6.26: Illustration of a period in the high order Bragg grating. The structure is waveguiding where the core is SU-8 and anti-guiding where the core is ethanol. of the waveguide during pumping of the dye in the laser. It is apparent from the figures, that energy is lost in the antiguiding regions, especially as the mode number increases. Table 6.1 lists the calculated losses for traversing an anti- guiding section for the first six TE modes.
Figure 6.27: FDBPM used to model the propagation through an anti-guiding section of the six first allowed modes in the guiding section. The uppermost picture shows the refractive index structure used in the propagation. The TE mode is 0 to 5 for the sub-figures a) to f). The intensity on the pictures corresponds to the squared norm of the electric field. Figure 6.27: FDBPM used to model the propagation through an anti-guiding section of the
Figure 6.27: FDBPM used to model the propagation through an anti-guiding section of the six first allowed modes in the guiding section. The uppermost picture shows the refractive index structure used in the propagation. The TE mode is 0 to 5 for the sub-figures a) to f). The intensity on the pictures corresponds to the squared norm of the electric field. Figure 6.27: FDBPM used to model the propagation through an anti-guiding section of the
Figure 6.28: The theoretical round-trip loss as function of wavelength, for the fundamental transverse mode. Inset: The round-trip loss for the fundamental (m = 0) transverse mode and the next (m = 1) transverse mode.
Figure 6.28: The theoretical round-trip loss as function of wavelength, for the fundamental transverse mode. Inset: The round-trip loss for the fundamental (m = 0) transverse mode and the next (m = 1) transverse mode.
Figure 6.29: Fabrication steps for the UV defined microfluidic DFB laser
Figure 6.29: Fabrication steps for the UV defined microfluidic DFB laser
During characterization of the SU-8 defined chip, a solution of 20 mmol/L Rho- damine 6G in ethanol or ethylene glycol was pumped through the microfluidic channel with a flow rate of 10 uL/hr. This exchanged the dye solution in the res- onator area about every 1 second, which was by far enough to eliminate problems with photo-bleaching. The quantum efficiency of the dye seemed to be slightly better in ethylene glycol than in ethanol, however there are no conclusive data to support this.
During characterization of the SU-8 defined chip, a solution of 20 mmol/L Rho- damine 6G in ethanol or ethylene glycol was pumped through the microfluidic channel with a flow rate of 10 uL/hr. This exchanged the dye solution in the res- onator area about every 1 second, which was by far enough to eliminate problems with photo-bleaching. The quantum efficiency of the dye seemed to be slightly better in ethylene glycol than in ethanol, however there are no conclusive data to support this.
Figure 6.32: Output spectrum and pump curve from a fluid high order Bragg grating DFE laser fabricated with UV lithography in SU-8 .
Figure 6.32: Output spectrum and pump curve from a fluid high order Bragg grating DFE laser fabricated with UV lithography in SU-8 .
Figure 6.33: Output spectrum and pump curve from a fluid high order Bragg grating FB laser fabricated with imprint lithography in COC (Topas). The two arrows indicate alternative Bragg reflection modes.
Figure 6.33: Output spectrum and pump curve from a fluid high order Bragg grating FB laser fabricated with imprint lithography in COC (Topas). The two arrows indicate alternative Bragg reflection modes.
Figure 6.36: PMMA layer surface contour. The dashed curve shows the measurement data, the solid curve shows the measurement data deconvoluted with respect to the profilometer needle radius of curvature (5 tm).
Figure 6.36: PMMA layer surface contour. The dashed curve shows the measurement data, the solid curve shows the measurement data deconvoluted with respect to the profilometer needle radius of curvature (5 tm).
Figure 6.37: Output spectrum from a laser device at a pump energy density of 250 uJ mm. The line width reflects the resolution of the spectrometer. Inset: The dye laser energy output as function of pump energy. The laser threshold lies at 220 uJ mm~?.
Figure 6.37: Output spectrum from a laser device at a pump energy density of 250 uJ mm. The line width reflects the resolution of the spectrometer. Inset: The dye laser energy output as function of pump energy. The laser threshold lies at 220 uJ mm~?.
Figure 6.38: Output spectra from two laser devices cut from a common wafer. The emission from the lasers is highly similar, indicating that the design is tolerant with respect to process variations.
Figure 6.38: Output spectra from two laser devices cut from a common wafer. The emission from the lasers is highly similar, indicating that the design is tolerant with respect to process variations.
exposure dose, and the real reflection may be stronger.
exposure dose, and the real reflection may be stronger.
Figure 6.41 show measurement data for four lasers with Bragg grating pitches of 187 nm, 190 nm, 200 nm and 205 nm. The lasers operate in single mode. Table 6.2 ists the resulting lasing wavelengths and the lasing thresholds. In addition, the table lists the effective refractive index calculated from the grating pitches and the asing wavelengths via neg = A/2A, where X is the lasing wavelength and A is the grating pitch. The calculation assumes that the lasing wavelength corresponds to the Bragg reflection wavelength. The thresholds differ due to the difference in pump energy required to obtain gain at different wavelengths. Table 6.2: Parameters for the four laser devices, named a-d: The pitch of the Bragg grating, the observed lasing wavelength, A, the measured effective refractive index, neg of the slab waveguide, and the measured threshold for lasing, Prnr.
Figure 6.41 show measurement data for four lasers with Bragg grating pitches of 187 nm, 190 nm, 200 nm and 205 nm. The lasers operate in single mode. Table 6.2 ists the resulting lasing wavelengths and the lasing thresholds. In addition, the table lists the effective refractive index calculated from the grating pitches and the asing wavelengths via neg = A/2A, where X is the lasing wavelength and A is the grating pitch. The calculation assumes that the lasing wavelength corresponds to the Bragg reflection wavelength. The thresholds differ due to the difference in pump energy required to obtain gain at different wavelengths. Table 6.2: Parameters for the four laser devices, named a-d: The pitch of the Bragg grating, the observed lasing wavelength, A, the measured effective refractive index, neg of the slab waveguide, and the measured threshold for lasing, Prnr.
Figure 6.40: Microscope photographs of first order Bragg grating DFB laser. Left: White light photograph (the bright dots are caused by dust). Right: Photograph of the device during operation. The green pump light is filtered away in the microscope. Increased scattering is observed at the grating phase shift in the middle of the structure. The device in the photograph is 0.5 mm by 1 mm in size. Four lasers with different grating pitch, were each characterized with respect to their output spectrum and their output as function of pump pulse energy. Also the lifetime of the lasers was investigated.
Figure 6.40: Microscope photographs of first order Bragg grating DFB laser. Left: White light photograph (the bright dots are caused by dust). Right: Photograph of the device during operation. The green pump light is filtered away in the microscope. Increased scattering is observed at the grating phase shift in the middle of the structure. The device in the photograph is 0.5 mm by 1 mm in size. Four lasers with different grating pitch, were each characterized with respect to their output spectrum and their output as function of pump pulse energy. Also the lifetime of the lasers was investigated.
Figure 6.41: Measurement data for four different grey scale defined EBL lasers. Left: Output spectra for pump energy densities of approximately 4 uJ mm~?. The linewidths reflect the resolution of the spectrometer. Right: Pump curves for the lasers.
Figure 6.41: Measurement data for four different grey scale defined EBL lasers. Left: Output spectra for pump energy densities of approximately 4 uJ mm~?. The linewidths reflect the resolution of the spectrometer. Right: Pump curves for the lasers.
Figure 6.42: The effective index for the four lasers calculated from their lasing wavelength and the grating pitch. The effective index decreases with increasing wavelength with a slope of —2.4-107-4 nm7!.
Figure 6.42: The effective index for the four lasers calculated from their lasing wavelength and the grating pitch. The effective index decreases with increasing wavelength with a slope of —2.4-107-4 nm7!.
Figure 6.43: The output power from laser device c as function of the number of pump pulses. The pump energy density was 2.2 uJ mm? at 10 Hz. There is a fast initial decay, which steadies after a while. This pattern was seen for all devices. Figure 6.43: The output power from laser device c as function of the number of pump pulses. The pump energy density was 2.2 uJ mm~~ at 10 Hz. There is a fast initial decay,
Figure 6.43: The output power from laser device c as function of the number of pump pulses. The pump energy density was 2.2 uJ mm? at 10 Hz. There is a fast initial decay, which steadies after a while. This pattern was seen for all devices. Figure 6.43: The output power from laser device c as function of the number of pump pulses. The pump energy density was 2.2 uJ mm~~ at 10 Hz. There is a fast initial decay,
Figure 6.44: A photograph of the laser light impinging a white screen 10 mm away from the laser output. The lower bright spot is the laser itself, the horizontal line just above the laser is light scattered from the edge of the laser chip. The middle vertical stripe is the normally expected output from the laser, while the two angled stripes may be from a parasitic mode.
Figure 6.44: A photograph of the laser light impinging a white screen 10 mm away from the laser output. The lower bright spot is the laser itself, the horizontal line just above the laser is light scattered from the edge of the laser chip. The middle vertical stripe is the normally expected output from the laser, while the two angled stripes may be from a parasitic mode.
Figure 6.45: Six-fold reflection mode in slab waveguide with grating. The propagation direction of the light in the mode will form an angle of 45° to the grating groves.
Figure 6.45: Six-fold reflection mode in slab waveguide with grating. The propagation direction of the light in the mode will form an angle of 45° to the grating groves.
Figure 6.47: Sketch of problem during imprint of mm-sized structures. A: Before imprint the stamp is flat. B: The stamp bends during imprint which results in incomplete stamp filling or film thickness variation along the structure. Figure 6.47: Sketch of problem during imprint of mm-sized structures. A: Before imprint
Figure 6.47: Sketch of problem during imprint of mm-sized structures. A: Before imprint the stamp is flat. B: The stamp bends during imprint which results in incomplete stamp filling or film thickness variation along the structure. Figure 6.47: Sketch of problem during imprint of mm-sized structures. A: Before imprint
Figure 6.48: Height profile of imprinted structure. The residual film is removed from the part termed ” Substrate’ .
Figure 6.48: Height profile of imprinted structure. The residual film is removed from the part termed ” Substrate’ .
Figure 6.49: Microscope photograph showing the result of stamp bending during imprint, causing incomplete stamp filling. The 0.5 mm wide laser structure is seen above and below the unfilled area.
Figure 6.49: Microscope photograph showing the result of stamp bending during imprint, causing incomplete stamp filling. The 0.5 mm wide laser structure is seen above and below the unfilled area.
In the present example, the fluid high order Bragg grating DFB laser is defined in a SU-8 layer together with waveguides, microfluidic mixer and microfluidic cuvette!. Five photodiodes are embedded in the substrate. Their function is to measure the intensity of the light from the laser after it has passed through the cuvette which is filled with a fluid to be tested. Thereby it is possible to measure the absorption of the fluid in the cuvette at the wavelength of the laser light (~576 nm). The device consists of a microfluidic polymer dye laser delivering light to five polymer waveguides that direct the light to five measuring points along a cuvette (see Fig. 7.1). On the other side of the cuvette, five waveguides pick up the transmitted light and direct it to a photodiode array, enabling spatially or temporally resolved measurements. The electric signal from the photodiodes is used for detection and can be read out from metallic pads on the chip. There is a mixer in front of the microfluidic channel forming the cuvette, making it possible to mix two fluids and measure the absorption of the result. Fluids are supplied to the channels in the chip through holes in the glass lid. The complete micro-system with laser, waveguides, microfluidics and photodiodes has a footprint of 15 mm by 20 mm. Figure 7.1: Photograph of the lab-on-chip device with integrated microfluidic dye laser, optical waveguides, microfluidic network and photodiodes. The metallic contact pads for the photodiodes are seen on the far right. The chip footprint is 15 mm by 20 mm. The photograph was taken before a lid was bonded to the structures.
In the present example, the fluid high order Bragg grating DFB laser is defined in a SU-8 layer together with waveguides, microfluidic mixer and microfluidic cuvette!. Five photodiodes are embedded in the substrate. Their function is to measure the intensity of the light from the laser after it has passed through the cuvette which is filled with a fluid to be tested. Thereby it is possible to measure the absorption of the fluid in the cuvette at the wavelength of the laser light (~576 nm). The device consists of a microfluidic polymer dye laser delivering light to five polymer waveguides that direct the light to five measuring points along a cuvette (see Fig. 7.1). On the other side of the cuvette, five waveguides pick up the transmitted light and direct it to a photodiode array, enabling spatially or temporally resolved measurements. The electric signal from the photodiodes is used for detection and can be read out from metallic pads on the chip. There is a mixer in front of the microfluidic channel forming the cuvette, making it possible to mix two fluids and measure the absorption of the result. Fluids are supplied to the channels in the chip through holes in the glass lid. The complete micro-system with laser, waveguides, microfluidics and photodiodes has a footprint of 15 mm by 20 mm. Figure 7.1: Photograph of the lab-on-chip device with integrated microfluidic dye laser, optical waveguides, microfluidic network and photodiodes. The metallic contact pads for the photodiodes are seen on the far right. The chip footprint is 15 mm by 20 mm. The photograph was taken before a lid was bonded to the structures.
Figure 7.2: Outline of the cross section of the lab-on-chip device. Photodiodes are embed- ded in the silicon substrate (right), while optical waveguides and the microfluidic network are defined in the 10 um thick SU-8 film. The channels are sealed off by a borofloat glass lid bonded to the SU-8 by means of a thin PMMA film.
Figure 7.2: Outline of the cross section of the lab-on-chip device. Photodiodes are embed- ded in the silicon substrate (right), while optical waveguides and the microfluidic network are defined in the 10 um thick SU-8 film. The channels are sealed off by a borofloat glass lid bonded to the SU-8 by means of a thin PMMA film.
Figure 7.3: a) Electron microscope image closeup of a section of a waveguide. The light is guided in the central strip of SU-8. Inset: drawing of waveguide structure (not to scale). b) Electron microscope image of the microfluidic dye laser (center) and the beginning of the waveguides (lower left). The laser structure is situated in a 1 mm wide channel carrying the liquid gain media. c) Electron microscope image of the termination of a waveguide at the side of the cuvette, forming a dike towards the cuvette. d) Electron microscope image of the waveguide to photodiode coupling region. The waveguides picking up the light from the cuvette (upper right) directs the light to the photoactive region making up the last 2 mm under the waveguides which then terminates.
Figure 7.3: a) Electron microscope image closeup of a section of a waveguide. The light is guided in the central strip of SU-8. Inset: drawing of waveguide structure (not to scale). b) Electron microscope image of the microfluidic dye laser (center) and the beginning of the waveguides (lower left). The laser structure is situated in a 1 mm wide channel carrying the liquid gain media. c) Electron microscope image of the termination of a waveguide at the side of the cuvette, forming a dike towards the cuvette. d) Electron microscope image of the waveguide to photodiode coupling region. The waveguides picking up the light from the cuvette (upper right) directs the light to the photoactive region making up the last 2 mm under the waveguides which then terminates.
Figure 7.4: Design of the microfluidic mixer. There are two inlet holes connected to the inlet arms that meet at the beginning of the mixing meander. The meander opens up to form the cuvette (top right). The curved double line is the waveguide closest to the beginning of the cuvette.
Figure 7.4: Design of the microfluidic mixer. There are two inlet holes connected to the inlet arms that meet at the beginning of the mixing meander. The meander opens up to form the cuvette (top right). The curved double line is the waveguide closest to the beginning of the cuvette.
Figure 7.5: Photodiode response as function of optical pump power of the laser pumping the on-chip dye laser. The inset shows the open circuit response from a photodiode to a single pump pulse (vertical 10 mV/div, horizontal 10 ms/div).
Figure 7.5: Photodiode response as function of optical pump power of the laser pumping the on-chip dye laser. The inset shows the open circuit response from a photodiode to a single pump pulse (vertical 10 mV/div, horizontal 10 ms/div).
Figure 7.6: Photograph of COC chip with integrated components. consists of a microfluidic laser that emits light into two waveguides. One of the waveguides direct the light to the edge of the chip for reference, while the other carries the light to an absorption cell. A fluid can be injected into the absorption cell and the transmitted light is picked up by another waveguide. The second waveguide directs the transmitted light to the edge of the chip for measurement. The microfluidic mixer functions by diffusion and has three inputs.
Figure 7.6: Photograph of COC chip with integrated components. consists of a microfluidic laser that emits light into two waveguides. One of the waveguides direct the light to the edge of the chip for reference, while the other carries the light to an absorption cell. A fluid can be injected into the absorption cell and the transmitted light is picked up by another waveguide. The second waveguide directs the transmitted light to the edge of the chip for measurement. The microfluidic mixer functions by diffusion and has three inputs.
Figure 7.8: Microscope photograph of the two tapered waveguides sending light into the cuvette where an absorbing fluid can be injected. The waveguides are tapered and an air lens is used to maximize the transmission of light from the left waveguide to the right. was to minimize the path length the light had to travel through air, since the air section is anti-guiding. Figure 7.8: Microscope photograph of the two tapered waveguides sending light into the
Figure 7.8: Microscope photograph of the two tapered waveguides sending light into the cuvette where an absorbing fluid can be injected. The waveguides are tapered and an air lens is used to maximize the transmission of light from the left waveguide to the right. was to minimize the path length the light had to travel through air, since the air section is anti-guiding. Figure 7.8: Microscope photograph of the two tapered waveguides sending light into the
Figure 7.8 shows a microscope photograph of the absorbtion cell where mixed fluids interact with the laser light travelling in the optical waveguide. An air lens is used in the design to refocus the light leaving the cuvette on the signal output waveguide. As noted on the figure, the fluid delivered to the cuvette arrives from 9 diffusion mixer.
Figure 7.8 shows a microscope photograph of the absorbtion cell where mixed fluids interact with the laser light travelling in the optical waveguide. An air lens is used in the design to refocus the light leaving the cuvette on the signal output waveguide. As noted on the figure, the fluid delivered to the cuvette arrives from 9 diffusion mixer.
Figure 7.10: Dispenser chamber with a heater integrated in a printed circuit board on top. Electrical connections are on the left. The fluid is dispensed through a hole in the fluid cavity located underneath the cavity and not visible in the photograph. An O-ring is mounted around the hole to seal the connection to the laser chip. An important part of the operation of a microfluidic laser is the supply of dye fluid. In normal laboratory sized (CW) dye lasers, liters of dye fluid are stored in a container and pumped at high pressure through the laser system. Miniaturization of dye lasers would be incomplete if the dye delivery system remains a large piece of equipment. Therefore a newly developed miniaturized fluid dispenser was adapted for use with the microfluidic lasers [90, 91], in order to demonstrate a possible solution to the delivery of dye fluid®. In all other instances presented in this thesis, tabletop syringe pumps have been used.
Figure 7.10: Dispenser chamber with a heater integrated in a printed circuit board on top. Electrical connections are on the left. The fluid is dispensed through a hole in the fluid cavity located underneath the cavity and not visible in the photograph. An O-ring is mounted around the hole to seal the connection to the laser chip. An important part of the operation of a microfluidic laser is the supply of dye fluid. In normal laboratory sized (CW) dye lasers, liters of dye fluid are stored in a container and pumped at high pressure through the laser system. Miniaturization of dye lasers would be incomplete if the dye delivery system remains a large piece of equipment. Therefore a newly developed miniaturized fluid dispenser was adapted for use with the microfluidic lasers [90, 91], in order to demonstrate a possible solution to the delivery of dye fluid®. In all other instances presented in this thesis, tabletop syringe pumps have been used.
Figure 7.11: Outline of dispenser principle and dye solution flow through the system.
Figure 7.11: Outline of dispenser principle and dye solution flow through the system.
Figure 7.12: Realized assembly after priming of laser with dye and attaching electrical connection to the heater. The left cube underneath the laser is a waste disposal arrangement.
Figure 7.12: Realized assembly after priming of laser with dye and attaching electrical connection to the heater. The left cube underneath the laser is a waste disposal arrangement.
Figure 7.13: Dispensed volume of dye solution through laser at 301 mW heater power, as function of time. The maximum temperature of the liquid dye in the dispenser during actuation was measured to 39.5°C.
Figure 7.13: Dispensed volume of dye solution through laser at 301 mW heater power, as function of time. The maximum temperature of the liquid dye in the dispenser during actuation was measured to 39.5°C.
With X-ray lithography, a waveguide terminating in a triangle at both ends were
With X-ray lithography, a waveguide terminating in a triangle at both ends were
Figure 8.2: Waveguides fabricated with X-ray lithography on dye doped SU-8. The waveg- uides terminate in a triangle which reflects light back into the waveguide due to total internal reflection.
Figure 8.2: Waveguides fabricated with X-ray lithography on dye doped SU-8. The waveg- uides terminate in a triangle which reflects light back into the waveguide due to total internal reflection.
Figure 8.3: Output spectrum from a 300 um long waveguide terminating in reflecting triangles at both ends. The mode spacing is 0.3 nm. The lasing threshold lies around 20 uJ mm~?. from 300 um long and 8 um wide waveguides. A spectrum and pump curve is illustrated in Figure 8.3. Since there is no specific out-coupling mechanism, the spectrum was measured on scattered light from the devices. The mode spacing
Figure 8.3: Output spectrum from a 300 um long waveguide terminating in reflecting triangles at both ends. The mode spacing is 0.3 nm. The lasing threshold lies around 20 uJ mm~?. from 300 um long and 8 um wide waveguides. A spectrum and pump curve is illustrated in Figure 8.3. Since there is no specific out-coupling mechanism, the spectrum was measured on scattered light from the devices. The mode spacing
Figure 8.4: Example of integration of a trapezoid laser with a waveguide in undoped SU-8 resist. A microfluidic channel passes in between the two reflecting triangles. A Rhodamine 6G dye solution is injected into the fluid channel during laser operation of the chip.
Figure 8.4: Example of integration of a trapezoid laser with a waveguide in undoped SU-8 resist. A microfluidic channel passes in between the two reflecting triangles. A Rhodamine 6G dye solution is injected into the fluid channel during laser operation of the chip.
Figure 8.7: Microscope photograph of a ratchet laser structure in SU-8 defined by electron beam lithography. The deformation that has been investigated during this Ph.D. work consists of a spiral shape deformation with a notch (see Figure 8.7)?._ The radius of the laser ring is determined by equation (8.1). The general conception of the ratchet resonator, is that it is a chaotic resonator due to its shape. If the resonator is chaotic or not, and for which conditions, has not been determined though. a spiral shape deformation with a notch (see Figure 8.7)*. ‘lhe radius of the It is speculated by Chern et al., in [94], that the ratchet geometry has a lower loss for clockwise propagating light than for counter-clockwise propagating light (see Figure 8.8). This makes it possible to achieve unidirectional lasing in the clockwise direction, and the light is coupled out by scattering backwards on the notch. The angular distribution of the emission from a ratchet resonator has been measured by Chern et al. in [99, 94] and they observed a single dominant lobe (see Figure 8.9).
Figure 8.7: Microscope photograph of a ratchet laser structure in SU-8 defined by electron beam lithography. The deformation that has been investigated during this Ph.D. work consists of a spiral shape deformation with a notch (see Figure 8.7)?._ The radius of the laser ring is determined by equation (8.1). The general conception of the ratchet resonator, is that it is a chaotic resonator due to its shape. If the resonator is chaotic or not, and for which conditions, has not been determined though. a spiral shape deformation with a notch (see Figure 8.7)*. ‘lhe radius of the It is speculated by Chern et al., in [94], that the ratchet geometry has a lower loss for clockwise propagating light than for counter-clockwise propagating light (see Figure 8.8). This makes it possible to achieve unidirectional lasing in the clockwise direction, and the light is coupled out by scattering backwards on the notch. The angular distribution of the emission from a ratchet resonator has been measured by Chern et al. in [99, 94] and they observed a single dominant lobe (see Figure 8.9).
Figure 8.8: The two counter propagating set of modes in the ring experience different losses. Clockwise modes are denoted by a negative mode number, m, and counter-clockwise propagating modes by positive mode number m. The clockwise modes are scattered on the notch and some of the energy is coupled out (reproduced from [94]). losses. Clockwise modes are denoted by a negative mode number, m, and counter-clockwise
Figure 8.8: The two counter propagating set of modes in the ring experience different losses. Clockwise modes are denoted by a negative mode number, m, and counter-clockwise propagating modes by positive mode number m. The clockwise modes are scattered on the notch and some of the energy is coupled out (reproduced from [94]). losses. Clockwise modes are denoted by a negative mode number, m, and counter-clockwise
Figure 8.9: Emission pattern measured from a semiconductor ratchet laser device (r9=145 um and € = 0.1). The numbers denote the angle ¢ in equation (8.1) and the curve the angular distribution of the light intensity (reproduced from [99]).
Figure 8.9: Emission pattern measured from a semiconductor ratchet laser device (r9=145 um and € = 0.1). The numbers denote the angle ¢ in equation (8.1) and the curve the angular distribution of the light intensity (reproduced from [99]).
Figure 8.10: Microscope photograph of a lasing ratchet device. Light is emitted at the notch and partly scattered vertically into the microscope. The light from the notch has saturated the CCD camera. Figure 8.10: Microscope photograph of a lasing ratchet device. Light is emitted at the
Figure 8.10: Microscope photograph of a lasing ratchet device. Light is emitted at the notch and partly scattered vertically into the microscope. The light from the notch has saturated the CCD camera. Figure 8.10: Microscope photograph of a lasing ratchet device. Light is emitted at the
Figure 8.11: Output spectrum from ratchet laser with a radius of 125 um and no center hole, the notch is defined by « = 0.01. The lasing threshold for the device is 4 uJ mm7?.
Figure 8.11: Output spectrum from ratchet laser with a radius of 125 um and no center hole, the notch is defined by « = 0.01. The lasing threshold for the device is 4 uJ mm7?.
uniformity of the exposure, but it principally entails vertical sidewalls. Sloping sidewalls are possible via grey scale lithography, but this method relies on the properties of the resist which are in general not constant at varying exposure doses. In addition it is not possible to fabricate negative sidewall slopes with grey scale lithography. Sloping sidewalls can be obtained by tilting the substrate under exposure with respect to the exposure radiation source (see Section 5.1.3). X-rays from the ELETTRA synchrotron beam-line LILIT have good collimation with an angular distribution less than +1 mrad, and it is therefore easy to perform good quality exposures on tilted substrates (100). This makes it possible to fabricate structures in the resist with well-defined tilted sidewalls. In particular, it is noteworthy that with this technique sidewalls with negative slope can be fabricated with very good corner definition due to the high resolution of the lithographic method. Defining a negative slope of 45° at the end of a waveguide, results in total internal reflection of light travelling in the waveguide (for SU-8) as illustrated in Figure 8.13. Thereby the light is coupled out vertically. This is the principle exploited in the ASE device in this section?. ASE is generated in a length of waveguide under optical pumping, and the light is coupled out at the end of the waveguide with negative slope. An ASE device is an alternative to a laser device if coherence and narrow spectral line-width are not necessary.
uniformity of the exposure, but it principally entails vertical sidewalls. Sloping sidewalls are possible via grey scale lithography, but this method relies on the properties of the resist which are in general not constant at varying exposure doses. In addition it is not possible to fabricate negative sidewall slopes with grey scale lithography. Sloping sidewalls can be obtained by tilting the substrate under exposure with respect to the exposure radiation source (see Section 5.1.3). X-rays from the ELETTRA synchrotron beam-line LILIT have good collimation with an angular distribution less than +1 mrad, and it is therefore easy to perform good quality exposures on tilted substrates (100). This makes it possible to fabricate structures in the resist with well-defined tilted sidewalls. In particular, it is noteworthy that with this technique sidewalls with negative slope can be fabricated with very good corner definition due to the high resolution of the lithographic method. Defining a negative slope of 45° at the end of a waveguide, results in total internal reflection of light travelling in the waveguide (for SU-8) as illustrated in Figure 8.13. Thereby the light is coupled out vertically. This is the principle exploited in the ASE device in this section?. ASE is generated in a length of waveguide under optical pumping, and the light is coupled out at the end of the waveguide with negative slope. An ASE device is an alternative to a laser device if coherence and narrow spectral line-width are not necessary.
Figure 8.14: Microscope fluorescence photograph of waveguides with sloping end sidewalls, being pumped with 532 nm (filtered away in microscope). Left: Original photograph. Right: The contrast of the picture has been adjusted to allow seeing both fluorescence from the waveguides and the light coupled out of the ends at the same time. 1, 3 and 8) and length (in um: 18.75, 37.5, 75, 150 and 300). Figure 8.14 shows a microscope photograph of the array of waveguides pumped with the Nd:YAG laser. Only the waveguides with widths of 3 and 8 um show significant output. The contrast of the photograph has been decreased to make it possible to see both the vertically out-coupled light and the fluorescence from the waveguides. Figure 8.14: Microscope fluorescence photograph of waveguides with sloping end sidewalls,
Figure 8.14: Microscope fluorescence photograph of waveguides with sloping end sidewalls, being pumped with 532 nm (filtered away in microscope). Left: Original photograph. Right: The contrast of the picture has been adjusted to allow seeing both fluorescence from the waveguides and the light coupled out of the ends at the same time. 1, 3 and 8) and length (in um: 18.75, 37.5, 75, 150 and 300). Figure 8.14 shows a microscope photograph of the array of waveguides pumped with the Nd:YAG laser. Only the waveguides with widths of 3 and 8 um show significant output. The contrast of the photograph has been decreased to make it possible to see both the vertically out-coupled light and the fluorescence from the waveguides. Figure 8.14: Microscope fluorescence photograph of waveguides with sloping end sidewalls,
Figure 8.15: Amount of light coupled out from the ends of the ASE waveguide structures as function of waveguide length.
Figure 8.15: Amount of light coupled out from the ends of the ASE waveguide structures as function of waveguide length.

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