Department of Physics and Astronomy

We put forward schemes to prepare photons in multi-dimensional vector states of orbital angular momentum. We show realizable light distributions that yield prescribed states with finite or infinite normal modes. In particular, we show that suitable light vortex-pancakes allow the add-drop of specific vector projections. We suggest that such photons might allow the generation of engineered quNits in multi-dimensional quantum information systems.

We demonstrate experimentally the tunable control of the joint spectrum, i.e. waveform and degree of frequency correlations, of paired photons generated in spontaneous parametric downconversion. This control is mediated by the spatial shape of the pump beam in a type-I noncollinear configuration. We discuss the applicability of this technique to other sources of frequency entangled photons, such as electromagnetically induced Raman transitions.

The superposition of noncoaxial light beams containing screw wave-front dislocations is shown to create light patterns with a richer vortex content than that given by the arithmetic of the topological charges of the individual beams. We report the experimental observation of this phenomenon.

We predict new features for the evolution of non-canonical screw wave front dislocations nested in di racting light beams. The evolution equations for the location of single-charge dislocations are derived and their consequences elaborated using illustrative examples. It is shown that the dynamical trajectories of the vortices can be controlled by adjusting the non-canonical strength of the input dislocations.

The orbital angular momentum of light represents a fundamentally new optical degree of freedom. Unlike linear momentum, or spin angular momentum, which is associated with the polarization of light, orbital angular momentum arises as a subtler and more complex consequence of the spatial distribution of the intensity and phase of an optical fi eld -even down to the single photon limit.

We observe and analyze formation, decay, and subsequent regeneration of ring-shaped clusters of (2+1)dimensional spatial solitons (filaments) in a medium with cubic-quintic (focusing-defocusing) self-interaction and strong dissipative nonlinearity. The cluster of filaments, which remains stable over ≈17.5 Rayleigh lengths, is produced by the azimuthal modulational instability from a parent ring-shaped beam with embedded vorticity l = 1. In the course of still longer propagation, the stability of the soliton cluster is lost under the action of nonlinear losses. The annular cluster is then spontaneously regenerated due to power transfer from the reservoir provided by the unsplit part of the parent vortex ring. Thus, a secondary interval of the robust propagation of the regenerated cluster is identified. The experiments use a laser beam (at wavelength 800 nm), built of pulses with temporal duration 150 fs, at the repetition rate of 1 kHz, propagating in a cell filled by liquid carbon disulfide. Numerical calculations, based on a modified nonlinear Schrödinger equation which includes the cubic-quintic refractive terms and nonlinear losses, provide results in close agreement with the experimental findings.

Surface-enhanced spectroscopy is a branch of techniques that enhance light emission from molecules at nanoscale proximity to plasmonic structures. Examples of SES include surface-enhanced fluorescence (SEF), surface-enhanced phosphorescence, surface-enhanced Raman spectroscopy (SERS), surface-enhanced infrared absorption and surface-enhanced chemiluminescence. We are interested in applying plasmonic structures for enhancing fluorescence signals, what may be very important for detection of low amounts of analytes in diagnosis of infectious diseases and cancer cells; and in monitoring of healthy, food and environment. Different geometries were already applied to SEF. The one that produced the highest enhancement in average (not in hot spots) was an annealed thin silver layer over a silver mirror, with an enhancement factor of 208 [1]. We studied bi-dimensional periodic arrays of silver single and double nanocylinders with 30-80 nm of diameter, and the effects of an underlying 45-nm si...

We observe and analyze formation, decay, and subsequent regeneration of ring-shaped clusters of (2+1)-dimensional spatial solitons (filaments) in a medium with the cubic-quintic (focusing-defocusing) self-interaction and strong dissipative nonlinearity. The cluster of filaments, that remains stable over ~17.5 Rayleigh lengths, is produced by the azimuthal modulational instability from a parent ring-shaped beam with embedded vorticity l = 1. In the course of still longer propagation, the stability of the soliton cluster is lost under the action of nonlinear losses. The annular cluster is then spontaneously regenerated due to power transfer from the reservoir provided by the unsplit part of the parent vortex ring. A (secondary) interval of the robust propagation of the regenerated cluster is identified. The experiments use a laser beam (at wavelength 800 nm), built of pulses with temporal duration 150 fs, at the repetition rate of 1 kHz, propagating in a cell filled by liquid carbon d...

The effect of temperature on the optical and thermal properties of pure and indocyanine green-doped albumin protein solders as a function of wavelength has been studied between 25°C and 100°C. An increase in the group refractive index by up to 4% and a decrease in absorption coefficient ͑ϳ800 nm͒ by up to 8%, after denaturing the solder specimens in a constant-temperature water bath at temperatures of 60 -100°C, were not significant. The reduced scattering coefficient, however, increased rapidly with temperature as the solder changed from being a highly nonscattering medium at room temperature to a highly scattering medium at temperatures close to 70°C. The thermal conductivity, thermal diffusivity, and heat capacity increased by up to 30%, 15%, and 10%, respectively. Finally, the frequency factor and activation energy were measured to be 3.17 ϫ 10 56 s Ϫ1 and 3.79 ϫ 10 5 J mol Ϫ1 , respectively, for liquid protein solders ͑25% bovine serum albumin͒ and 3.50 ϫ 10 57 s Ϫ1 and 3.85 ϫ 10 5 J mol Ϫ1 , respectively, for solid protein solders ͑60% bovine serum albumin͒. Incorporation of dynamic optical and thermal properties into modeling studies of laser tissue interactions could have a significant influence on the determination of the expected zone of damage.

We report the point-by-point inscription of gratings in lithographically fabricated polymer ridge waveguides using direct-write laser processing. This device exhibited narrow-band-pass spectral features of ∼ 0.8 nm full-widthhalf-maximum, suitable for wavelength division multiplexing applications.

Defects obtained during crystal growth of many laser and nonlinear crystals limit their commercial potential, as is the case of the yttrium aluminium borate family. Typically, in Yb:YAB, birefringent phase-matching is used to obtain self frequency doubled output from a fundamental laser operating near 1 µm. Many crystals however show signs of twin inversion defects during crystal growth resulting in the reversal of the sign of the nonlinear coefficient and hence can result in quasi-phasematched-like structures. In this report we examine the second harmonic conversion characteristics of a 3.8 mm thick birefringently phase matched Yb:YAB crystal which is then examined in more detail by slicing into four thinner sections each approximately 400 µm thick. Etching of crystal surfaces reveals clear boundaries between reversed domains, which under illumination with a laser at 1.064 µm shows a cancellation of the nonlinear conversion along the interface of the two regions.