We investigate the growth of InAs quantum dots on patterned GaAs substrates. The GaAs substrate h... more We investigate the growth of InAs quantum dots on patterned GaAs substrates. The GaAs substrate has been structured using holographic lithography. Quantum dot formation along the patterns has been observed as well as an increase in homogeneity of the quantum dots. Furthermore, the use of ion beams focused to nanometer diameters for substrate patterning has been studied and showed promising results. For the investigation of vertically aligned InAs quantum dots, cross-sectional atomic force microscopy has been successfully employed.
CLEO:2011 - Laser Applications to Photonic Applications, 2011
THz time-domain studies on metal-metal THz quantum cascade lasers are presented. A coupled cavity... more THz time-domain studies on metal-metal THz quantum cascade lasers are presented. A coupled cavity method allows to probe the laser via modulation of coherent broadband THz pulses. Observed laser gain and loss processes are discussed. OCIS codes: (140.5965) Semiconductor lasers, quantum cascade; (300.6495) Spectroscopy, terahertz; (300.6500) Spectroscopy, time-resolved.
We present lasing spectra of mesoscopic rings processed from a GaAs-based quantum cascade materia... more We present lasing spectra of mesoscopic rings processed from a GaAs-based quantum cascade material. The emission properties are investigated for various inner ring radii. Small inner radii leave the spectra unaffected, whereas large inner radii shift the spectra to smaller wavelengths. To obtain insight into the spatial distribution of the optical modes, we performed simulations with a finite-difference time-domain method.
We fabricated photonic crystal intersubband detectors and measured the spectral photocurrent upon... more We fabricated photonic crystal intersubband detectors and measured the spectral photocurrent upon angle and polarization resolved illumination. Light is coupled into the cavity whenever phase matching occurs between a cavity mode and the incident wave. The method allows us to map out the photonic band structure including its polarization dependence, which is shown to be highly correlated with the symmetry
ABSTRACT We present recent work towards the realization of a nanowire-based terahertz quantum cas... more ABSTRACT We present recent work towards the realization of a nanowire-based terahertz quantum cascade laser. Nanowires offer an additional quantum mechanical confinement of electrons in the plane of a two-dimensional quantum cascade structure. The additional quantization can greatly increase the lifetimes of intersubband transitions and therefore increase the optical gain and also the maximum operating temperature of terahertz quantum cascade lasers. We outline a fabrication process that is fully scalable from nanowire to micropillar devices and present measurements of micropillar arrays in a double metal waveguide. The results are very promising and also show the main technological challenges for realizing nanowire-based devices.
ABSTRACT Recently, our group presented the surface emitting ring cavity quantum cascade laser (QC... more ABSTRACT Recently, our group presented the surface emitting ring cavity quantum cascade laser (QCL). This kind of light source shows lower thresholds, higher slope-efficiencies and narrower emission beams as compared to equivalent Fabry-Perot devices. Typically, such lasers show a doughnut-shaped, rotational symmetric far field with azimuthal polarization. When the excited modes in the cavity are off-resonance to the DFB grating, the light is emitted under a broader angle. This leads to a widening of this 'doughnut' and the polarization of the far field switches to radial polarization. Additionally, we present ring-QCLs with linear polarized far fields. For this task, devices with PI-shifts in the grating period were used. The opposing fields in the ring resonator interfere constructively in the centre of the far field. We observed a central lobed far field, where pure linear polarization in this central lobe was measured. To extend this polarization property to the entire far field, a wire grid polarizer was fabricated onto the substrate with the wires perpendicular to the favoured polarization direction. The devices were optimized for substrate emission.
We demonstrate the monolithic integration of a mid-infrared laser and detector utilizing a bi-fun... more We demonstrate the monolithic integration of a mid-infrared laser and detector utilizing a bi-functional quantum cascade active region. When biased, this active region provides optical gain, while it can be used as a detector at zero bias. With our novel approach we can measure the light intensity of the laser on the same chip without the need of external lenses or detectors. Based on a bound-to-continuum design, the bi-functional active region has an inherent broad electro-luminescence spectrum of 200 cm −1 , which indicates its use for single mode laser arrays. We have measured a peak signal of 191.5 mV at the on-chip detector, without any amplification. The room-temperature pulsed emission with an averaged power consumption of 4 mW and the high-speed detection makes these devices ideal for low-power sensors. The combination of the on-chip detection functionality, the broad emission spectrum and the low average power consumption indicates the potential of our bi-functional quantum cascade structures to build a mid-infrared lab-on-a-chip based on quantum cascade laser technology.
We present the design, fabrication and characterisation of an intersubband detector employing a r... more We present the design, fabrication and characterisation of an intersubband detector employing a resonant metamaterial coupling structure. The semiconductor heterostructure relies on a conventional THz quantum-cascade laser design and is operated at zero bias for the detector operation. The same active region can be used to generate or detect light depending on the bias conditions and the vertical confinement. The metamaterial is processed directly into the top metal contact and is used to couple normal incidence radiation resonantly to the intersubband transitions. The device is capable of detecting light below and above the reststrahlenband of gallium-arsenide corresponding to the mid-infrared and THz spectral region.
The increasing demand of rapid sensing and diagnosis in remote areas requires the development of ... more The increasing demand of rapid sensing and diagnosis in remote areas requires the development of compact and cost-effective mid-infrared sensing devices. So far, all miniaturization concepts have been demonstrated with discrete optical components. Here we present a monolithically integrated sensor based on mid-infrared absorption spectroscopy. A bi-functional quantum cascade laser/detector is used, where, by changing the applied bias, the device switches between laser and detector operation. The interaction with chemicals in a liquid is resolved via a dielectric-loaded surface plasmon polariton waveguide. The thin dielectric layer enhances the confinement and enables efficient end-fire coupling from and to the laser and detector. The unamplified detector signal shows a slope of 1.8-7 mV per p.p.m., which demonstrates the capability to reach p.p.m. accuracy over a wide range of concentrations (0-60%). Without any hybrid integration or subwavelength patterning, our approach allows a straightforward and cost-saving fabrication.
A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploi... more A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploit this feature and realize nominally symmetric active regions for terahertz quantum cascade lasers, which should yield equal performance with either bias polarity. However, symmetric devices exhibit a strongly bias polarity dependent performance due to growth direction asymmetries, making them an ideal tool to study the related scattering mechanisms. In the case of an InGaAs/GaAsSb heterostructure, the pronounced interface asymmetry leads to a significantly better performance with negative bias polarity and can even lead to unidirectionally working devices, although the nominal band structure is symmetric. The results are a direct experimental proof that interface roughness scattering has a major impact on transport/lasing performance.
We characterize the performance of a quantum well infrared photodetector (QWIP), which is fabrica... more We characterize the performance of a quantum well infrared photodetector (QWIP), which is fabricated as a photonic crystal slab (PCS) resonator. The strongest resonance of the PCS is designed to coincide with the absorption peak frequency at 7.6 µm of the QWIP. To accurately characterize the detector performance, it is illuminated by using single mode mid-infrared lasers. The strong resonant absorption enhancement yields a detectivity increase of up to 20 times. This enhancement is a combined effect of increased responsivity and noise current reduction. With increasing temperature, we observe a red shift of the PCS-QWIP resonance peak of −0.055 cm −1 /K. We attribute this effect to a refractive index change and present a model based on the revised plane wave method.
We present a detailed investigation of higher order modes in photonic crystal slabs. In such stru... more We present a detailed investigation of higher order modes in photonic crystal slabs. In such structures the resonances exhibit a blue-shift compared to an ideal two-dimensional photonic crystal, which depends on the order of the slab mode and the polarization. By fabricating a series of photonic crystal slab photo detecting devices, with varying ratios of slab thickness to photonic crystal lattice constant, we are able to distinguish between 0th and 1st order slab modes as well as the polarization from the shift of resonances in the photocurrent spectra. This method complements the photonic band structure mapping technique for characterization of photonic crystal slabs.
Increased coupling is observed in distributed-feedback quantum cascade lasers when placing a shal... more Increased coupling is observed in distributed-feedback quantum cascade lasers when placing a shallow second order grating between a continuous surface-plasmon layer and the active region. The combined effect of an air cladding and a metallic layer on the opposite sides of the waveguide increases the overlap with the grating region resulting in calculated coupling coefficients up to 100 cm(-1). The waveguide design was implemented by Au thermo-compression bonding after grating formation and subsequent backside processing of ridges with air claddings. Lasers as short as 176 microm show single-mode behavior with a side-mode-suppression-ratio of 20 dB and thresholds (10 kA/cm(2)) as well as output powers (> 150 mW) close to Fabry-Pérot device performances are reached for 360 microm long devices.
InAs quantum dots were grown on AlxGa1−xAs surfaces with varying Al concentrations. Atomic force ... more InAs quantum dots were grown on AlxGa1−xAs surfaces with varying Al concentrations. Atomic force microscopy measurements conducted on surface quantum dots showed that surfaces with higher Al concentrations produce smaller dots compared to GaAs surfaces. Photoluminescence measurements performed on buried quantum dots showed a blue shift and spectral broadening of the luminescence signal for increasing Al concentrations. For Al concentrations
Using microprobe photoluminescence we measured the electronic and lattice temperatures in operati... more Using microprobe photoluminescence we measured the electronic and lattice temperatures in operating quantum cascade lasers having similar chirped-superlattice active regions but different (GaAs/AlAs or GaAs/Al 0.45 Ga 0.55 As) conduction band discontinuities. Our results demonstrate the establishment of a thermalized hot-electron distribution. Coupling between the electronic ensemble and the lattice increases with the band-offset and influences the optical characteristics of the devices.
We describe ring-cavity surface-emitting lasers (ring-CSELs) based on quantum cascade structures ... more We describe ring-cavity surface-emitting lasers (ring-CSELs) based on quantum cascade structures as an elementary building block for two-dimensional quantum cascade laser arrays. The light emitters operate at high temperatures as high as 380 K and above. The devices facilitate a reduction in threshold current density as well as enhanced radiation efficiency in comparison to Fabry-Pérot lasers. Single-mode emission is observed
We investigated the shift in emission wavelength of GaAs-based distributed feedback quantum casca... more We investigated the shift in emission wavelength of GaAs-based distributed feedback quantum cascade lasers during pulsed operation. This shift is due to current-induced self-heating. Together with a fast detector this shift in the emission spectra can be used to investigate absorption lines in ammonium isotopes. The linewidth of these single mode devices was determined to be below 0.1 cm-1. These measurements reveal the potential of these devices for chemical sensing.
We investigate the growth of InAs quantum dots on patterned GaAs substrates. The GaAs substrate h... more We investigate the growth of InAs quantum dots on patterned GaAs substrates. The GaAs substrate has been structured using holographic lithography. Quantum dot formation along the patterns has been observed as well as an increase in homogeneity of the quantum dots. Furthermore, the use of ion beams focused to nanometer diameters for substrate patterning has been studied and showed promising results. For the investigation of vertically aligned InAs quantum dots, cross-sectional atomic force microscopy has been successfully employed.
CLEO:2011 - Laser Applications to Photonic Applications, 2011
THz time-domain studies on metal-metal THz quantum cascade lasers are presented. A coupled cavity... more THz time-domain studies on metal-metal THz quantum cascade lasers are presented. A coupled cavity method allows to probe the laser via modulation of coherent broadband THz pulses. Observed laser gain and loss processes are discussed. OCIS codes: (140.5965) Semiconductor lasers, quantum cascade; (300.6495) Spectroscopy, terahertz; (300.6500) Spectroscopy, time-resolved.
We present lasing spectra of mesoscopic rings processed from a GaAs-based quantum cascade materia... more We present lasing spectra of mesoscopic rings processed from a GaAs-based quantum cascade material. The emission properties are investigated for various inner ring radii. Small inner radii leave the spectra unaffected, whereas large inner radii shift the spectra to smaller wavelengths. To obtain insight into the spatial distribution of the optical modes, we performed simulations with a finite-difference time-domain method.
We fabricated photonic crystal intersubband detectors and measured the spectral photocurrent upon... more We fabricated photonic crystal intersubband detectors and measured the spectral photocurrent upon angle and polarization resolved illumination. Light is coupled into the cavity whenever phase matching occurs between a cavity mode and the incident wave. The method allows us to map out the photonic band structure including its polarization dependence, which is shown to be highly correlated with the symmetry
ABSTRACT We present recent work towards the realization of a nanowire-based terahertz quantum cas... more ABSTRACT We present recent work towards the realization of a nanowire-based terahertz quantum cascade laser. Nanowires offer an additional quantum mechanical confinement of electrons in the plane of a two-dimensional quantum cascade structure. The additional quantization can greatly increase the lifetimes of intersubband transitions and therefore increase the optical gain and also the maximum operating temperature of terahertz quantum cascade lasers. We outline a fabrication process that is fully scalable from nanowire to micropillar devices and present measurements of micropillar arrays in a double metal waveguide. The results are very promising and also show the main technological challenges for realizing nanowire-based devices.
ABSTRACT Recently, our group presented the surface emitting ring cavity quantum cascade laser (QC... more ABSTRACT Recently, our group presented the surface emitting ring cavity quantum cascade laser (QCL). This kind of light source shows lower thresholds, higher slope-efficiencies and narrower emission beams as compared to equivalent Fabry-Perot devices. Typically, such lasers show a doughnut-shaped, rotational symmetric far field with azimuthal polarization. When the excited modes in the cavity are off-resonance to the DFB grating, the light is emitted under a broader angle. This leads to a widening of this 'doughnut' and the polarization of the far field switches to radial polarization. Additionally, we present ring-QCLs with linear polarized far fields. For this task, devices with PI-shifts in the grating period were used. The opposing fields in the ring resonator interfere constructively in the centre of the far field. We observed a central lobed far field, where pure linear polarization in this central lobe was measured. To extend this polarization property to the entire far field, a wire grid polarizer was fabricated onto the substrate with the wires perpendicular to the favoured polarization direction. The devices were optimized for substrate emission.
We demonstrate the monolithic integration of a mid-infrared laser and detector utilizing a bi-fun... more We demonstrate the monolithic integration of a mid-infrared laser and detector utilizing a bi-functional quantum cascade active region. When biased, this active region provides optical gain, while it can be used as a detector at zero bias. With our novel approach we can measure the light intensity of the laser on the same chip without the need of external lenses or detectors. Based on a bound-to-continuum design, the bi-functional active region has an inherent broad electro-luminescence spectrum of 200 cm −1 , which indicates its use for single mode laser arrays. We have measured a peak signal of 191.5 mV at the on-chip detector, without any amplification. The room-temperature pulsed emission with an averaged power consumption of 4 mW and the high-speed detection makes these devices ideal for low-power sensors. The combination of the on-chip detection functionality, the broad emission spectrum and the low average power consumption indicates the potential of our bi-functional quantum cascade structures to build a mid-infrared lab-on-a-chip based on quantum cascade laser technology.
We present the design, fabrication and characterisation of an intersubband detector employing a r... more We present the design, fabrication and characterisation of an intersubband detector employing a resonant metamaterial coupling structure. The semiconductor heterostructure relies on a conventional THz quantum-cascade laser design and is operated at zero bias for the detector operation. The same active region can be used to generate or detect light depending on the bias conditions and the vertical confinement. The metamaterial is processed directly into the top metal contact and is used to couple normal incidence radiation resonantly to the intersubband transitions. The device is capable of detecting light below and above the reststrahlenband of gallium-arsenide corresponding to the mid-infrared and THz spectral region.
The increasing demand of rapid sensing and diagnosis in remote areas requires the development of ... more The increasing demand of rapid sensing and diagnosis in remote areas requires the development of compact and cost-effective mid-infrared sensing devices. So far, all miniaturization concepts have been demonstrated with discrete optical components. Here we present a monolithically integrated sensor based on mid-infrared absorption spectroscopy. A bi-functional quantum cascade laser/detector is used, where, by changing the applied bias, the device switches between laser and detector operation. The interaction with chemicals in a liquid is resolved via a dielectric-loaded surface plasmon polariton waveguide. The thin dielectric layer enhances the confinement and enables efficient end-fire coupling from and to the laser and detector. The unamplified detector signal shows a slope of 1.8-7 mV per p.p.m., which demonstrates the capability to reach p.p.m. accuracy over a wide range of concentrations (0-60%). Without any hybrid integration or subwavelength patterning, our approach allows a straightforward and cost-saving fabrication.
A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploi... more A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploit this feature and realize nominally symmetric active regions for terahertz quantum cascade lasers, which should yield equal performance with either bias polarity. However, symmetric devices exhibit a strongly bias polarity dependent performance due to growth direction asymmetries, making them an ideal tool to study the related scattering mechanisms. In the case of an InGaAs/GaAsSb heterostructure, the pronounced interface asymmetry leads to a significantly better performance with negative bias polarity and can even lead to unidirectionally working devices, although the nominal band structure is symmetric. The results are a direct experimental proof that interface roughness scattering has a major impact on transport/lasing performance.
We characterize the performance of a quantum well infrared photodetector (QWIP), which is fabrica... more We characterize the performance of a quantum well infrared photodetector (QWIP), which is fabricated as a photonic crystal slab (PCS) resonator. The strongest resonance of the PCS is designed to coincide with the absorption peak frequency at 7.6 µm of the QWIP. To accurately characterize the detector performance, it is illuminated by using single mode mid-infrared lasers. The strong resonant absorption enhancement yields a detectivity increase of up to 20 times. This enhancement is a combined effect of increased responsivity and noise current reduction. With increasing temperature, we observe a red shift of the PCS-QWIP resonance peak of −0.055 cm −1 /K. We attribute this effect to a refractive index change and present a model based on the revised plane wave method.
We present a detailed investigation of higher order modes in photonic crystal slabs. In such stru... more We present a detailed investigation of higher order modes in photonic crystal slabs. In such structures the resonances exhibit a blue-shift compared to an ideal two-dimensional photonic crystal, which depends on the order of the slab mode and the polarization. By fabricating a series of photonic crystal slab photo detecting devices, with varying ratios of slab thickness to photonic crystal lattice constant, we are able to distinguish between 0th and 1st order slab modes as well as the polarization from the shift of resonances in the photocurrent spectra. This method complements the photonic band structure mapping technique for characterization of photonic crystal slabs.
Increased coupling is observed in distributed-feedback quantum cascade lasers when placing a shal... more Increased coupling is observed in distributed-feedback quantum cascade lasers when placing a shallow second order grating between a continuous surface-plasmon layer and the active region. The combined effect of an air cladding and a metallic layer on the opposite sides of the waveguide increases the overlap with the grating region resulting in calculated coupling coefficients up to 100 cm(-1). The waveguide design was implemented by Au thermo-compression bonding after grating formation and subsequent backside processing of ridges with air claddings. Lasers as short as 176 microm show single-mode behavior with a side-mode-suppression-ratio of 20 dB and thresholds (10 kA/cm(2)) as well as output powers (> 150 mW) close to Fabry-Pérot device performances are reached for 360 microm long devices.
InAs quantum dots were grown on AlxGa1−xAs surfaces with varying Al concentrations. Atomic force ... more InAs quantum dots were grown on AlxGa1−xAs surfaces with varying Al concentrations. Atomic force microscopy measurements conducted on surface quantum dots showed that surfaces with higher Al concentrations produce smaller dots compared to GaAs surfaces. Photoluminescence measurements performed on buried quantum dots showed a blue shift and spectral broadening of the luminescence signal for increasing Al concentrations. For Al concentrations
Using microprobe photoluminescence we measured the electronic and lattice temperatures in operati... more Using microprobe photoluminescence we measured the electronic and lattice temperatures in operating quantum cascade lasers having similar chirped-superlattice active regions but different (GaAs/AlAs or GaAs/Al 0.45 Ga 0.55 As) conduction band discontinuities. Our results demonstrate the establishment of a thermalized hot-electron distribution. Coupling between the electronic ensemble and the lattice increases with the band-offset and influences the optical characteristics of the devices.
We describe ring-cavity surface-emitting lasers (ring-CSELs) based on quantum cascade structures ... more We describe ring-cavity surface-emitting lasers (ring-CSELs) based on quantum cascade structures as an elementary building block for two-dimensional quantum cascade laser arrays. The light emitters operate at high temperatures as high as 380 K and above. The devices facilitate a reduction in threshold current density as well as enhanced radiation efficiency in comparison to Fabry-Pérot lasers. Single-mode emission is observed
We investigated the shift in emission wavelength of GaAs-based distributed feedback quantum casca... more We investigated the shift in emission wavelength of GaAs-based distributed feedback quantum cascade lasers during pulsed operation. This shift is due to current-induced self-heating. Together with a fast detector this shift in the emission spectra can be used to investigate absorption lines in ammonium isotopes. The linewidth of these single mode devices was determined to be below 0.1 cm-1. These measurements reveal the potential of these devices for chemical sensing.
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Papers by Werner Schrenk