Slow light and chromatic temporal dispersion in photonic crystal waveguides using femtosecond time of flight.

We report time-of-flight experiments on photonic-crystal waveguide structures using optical Kerr gating of a femtosecond white-light supercontinuum. These photonic-crystal structures, based on engineered silicon-nitride slab waveguides, possess broadband low-loss guiding properties, allowing the group velocity dispersion of optical pulses to be directly tracked as a function of wavelength. This dispersion is shown to be radically disrupted by the spectral band gaps associated with the photonic-crystal periodicity. Increased time-of-flight effects, or "slowed light," are clearly observed at the edges of band gaps in agreement with two-dimensional plane-wave theoretical models of group velocity dispersion. A universal model for slow light in such photonic crystals is proposed, which shows that slow light is controlled predominantly by the detuning from, and the size of, the photonic band gaps. Slowed light observed up to time delays of approximately 1 ps, corresponds to anomalous dispersion of approximately 3.5 ps/nm per mm of the photonic crystal structure. From the decreasing intensity of time-gated slow light as a function of time delay, we estimate the characteristic losses of modes which are guided in the spectral proximity of the photonic band gaps.

Highly engineered mesoporous structures for optical processing.

Arranging periodic, or quasi-periodic, regions of differing refractive index in one, two, or three dimensions can form a unique class of mesoporous structures. These structures are generally known as photonic crystals, or photonic quasicrystals, and they are the optical analogue of semiconducting materials. Whereas a semiconductor's band structure arises from the interaction of electron or hole waves with an arrangement of ion cores, the photonic crystal band structure results from the interaction of light waves with an arrangement of regions of differing refractive index. What makes photonic crystals highly attractive to the optical engineer is that we can actually place the regions of differing refractive index in a pattern specifically tailored to produce a given optical function, such as an extremely high dispersion, for example. That is, we can define the geometrical arrangement of the dielectric foam to provide us with the form of band structure we require for our optical functionality. In this paper, the optical properties and applications of these highly engineered mesoporous dielectrics will be discussed.

Determination of nonlinear refractive index in a Ta2O5 rib waveguide using self-phase modulation.

Self-phase modulation has been observed for ultrashort pulses of wavelength 800nm propagating through a 1 cm-long Ta2O5 rib waveguide. The associated nonlinear refractive index n2 was estimated to be 7.23x10-19 m2/W, which is higher than silica glass by more than one order of magnitude. Femtosecond time of flight measurements based on a Kerr shutter configuration show that the group velocity dispersion is small at a wavelength of 800 nm, confirming that dispersion may be neglected in the estimation of n2 so that a simplified theory can be used with good accuracy.

Confined plasmons in metallic nanocavities.

We investigate the properties of gold surfaces patterned using a nanoscale "lost wax" technique by electrochemical deposition through a self-assembled latex template. Near-spherical gold nanocavities within the resulting porous films support localized surface plasmons which couple strongly to incident light, appearing as sharp spectral features in reflectivity measurements. The energy of the resonances is easily tunable from ultraviolet to near infrared by controlling the diameter and height of the nanocavities. The energies of these features agree well with the Mie resonances of a perfect spherical void.

Optical trirefringence in photonic crystal waveguides.

We demonstrate that 2D photonic crystals can possess optical trirefringence in which there are six field orientations for which linear incident light is not perturbed on reflection or transmission. Such a property is rigorously forbidden in homogeneous nonmagnetic dielectrics which can possess only optical birefringence. We experimentally demonstrate this phenomena in silicon-based mesostructures formed from photonic crystal waveguides embedded in a Fabry-Perot cavity. Multirefringence is controlled by the presence of submicron dielectric patterning and is well explained by an exact scattering matrix theory.

Nephelometry in the clinical assessment of glomerular proteinuria and tubular function in diabetic nephropathy.

Urinary excretion rate and total clearances of albumin, IgG, IgA and alpha 1-microglobulin, together with selectivity index and proteinuria, were determined by computerized nephelometry in 187 IDDM and NIDDM diabetic out-patients and in 39 healthy subjects in order to perform a prompt clinical assessment of diabetic nephropathy. Significant correlations between nephelometric and RIA procedures were demonstrated for the urinary excretion of albumin (p < 0.001) and total IgG (p < 0.001) in diabetic patients and healthy subjects. Nephelometry allowed us to classify diabetic patients in different stages of nephropathy: non nephropathic, normoalbuminuric with hyperfiltration, with incipient (microalbuminuric) and overt nephropathy (macroalbuminuric). Thirty consecutive subjects were analyzed within 1 h from the beginning of the procedure. A normal tubular function was demonstrated in non nephropathic, hyperfiltering and in 34% of microalbuminuric diabetic patients. On the contrary, in 66% of microalbuminuric and in 93% of macroalbuminuric patients alpha 1-microglobulin urinary levels were found above the upper normal limit. Urinary excretion of IgA was significantly increased only in macroalbuminuric diabetic patients (p < 0.001); this marker might therefore characterise the stage of overt nephropathy. Computerized nephelometry can be considered as a prompt, reproducible and high sensitive approach in the clinical evaluation of proteinuria and tubular function in diabetic renal disease.