Brian S. Wherrett

Anomalous Optical Freedericksz Transition in an Absorbing Liquid Crystal

Abstract Laser-induced reorientation was studied in absorbing nematic films. The optical Freedericksz threshold occured at an intensity level of 50 W/cm2 in contrast to the normally observed few KW/cm2 value for transparent layers. Thermomechanical coupling is considered as a possible explanation of the observed anomaly.

Room temperature, visible wavelength optical bistability in ZnSe interference filters

Abstract The observation of milliwatt-power optical bistability at room-temperature in interference filters based on ZnSe spacers is reported. Switching times of a few milliseconds have been achieved using 514 nm wavelength radiation at an irradiance of ∼250 W cm -2 . The device has been operated in a steady-state manner and switched between the two stable states by an external beam.

Theory of degenerate four-wave mixing in picosecond excitation-probe experiments

A comparative theoretical study of the transient grating coherent effects in resonant picosecond excitation-probe experiments is presented. Signals in both the probe and conjugate directions are discussed. The effects of recombination, nonradiative scattering, and spatial and orientational diffusion are included. The analysis is applied to both a molecular and a semiconductor model. Signal contributions from concentration and orientational gratings are distinguished and their temporal natures discussed. From the solid-state point of view, we show that state-filling, and in particular, anisotropic state-filling, can be identified from the configurational and time dependence of the detected signal, and thereby establish a theoretical explanation for our recent observations in germanium.

Fabry-Perot bistable cavity optimization on reflection

An analysis of bistable Fabry-Perot action in reflection is presented. Optimum values for the cavity reflective coatings and cavity length are determined. Reasons are given why reflection-mode operation rather than transmission-mode operation should be preferred for device applications. Two prototype devices, an all-optical SR (Set-Reset) flip-flop and an all-optical clock, are described.

Nonlinear optical properties of chalcogenide glasses: Observation of multiphoton absorption

We report observation of four- and five-photon absorption in the chalcogenide glasses at the telecommunication wavelengths. The nonlinear refractive index is sufficiently large that the optical switching criterion is satisfied.

The microscopic mechanism of third-order optical nonlinearity in InSb

Abstract Microscopic models are proposed for the bandgap-resonant low intensity nonlinear refraction observed in InSb by Miller et al., predicting the self-defocusing deduced from beam patterns. The physical arguments differ from earlier treatments of third-order nonlinearity in semiconductors by explicitly including excitation and saturation effects, and can explain the exceptionally large experimental value of ∾ (3) − 10 -2 esu. The results are consistent with observations of optical bistability, two- beam differential gain and degenerate four wave mixing.

The saturation limit to picosecond, induced absorption in dyes

Abstract The increase in transmission coefficient at high fluences, following low-fluence reverse saturable absorption (induced absorption), is demonstrated for the tricarbocyanine dye, HITCI. This novel effect is explained in terms of a modified three-level model of the singlet states. The absorption cross-section at 532 nm of the first excited state is determined to be 4.8x10-16 cm2, 29 times that of the ground state. We find the lifetime of the first excited state to be 1.5 ns and that of the second excited state to be of the order of 8 ps.

Picosecond transient orientational and concentration gratings in germanium

We present and discuss the results of picosecond transient grating self-diffraction measurements in germanium that can be understood in terms of an orientational grating produced by anisotropic (in k space) state-filling. Although there have been predictions and indirect experimental evidence for isotropic state-filling in germanium, this is the first direct experimental indication of anisotropic state-filling in a semiconductor. We compare the self-diffracted signals from orientational gratings with those from band-filling induced concentration gratings and find several distinctions, all of which can be explained in terms of the theory developed in the preceding paper. In addition, we have performed three-pulse transient grating lifetime measurements which indicate that the concentration grating decay is tens of picoseconds, while the orientational grating decay is too rapid to resolve with our 8 ps pulses.

Magnitude, origin, and evolution of piezoelectric optical nonlinearities in strained [111]B InGaAs/GaAs quantum wells

The magnitudes of excitonic nonlinearities were compared at 12 K in InGaAs/GaAs multiple quantum well structures with growth directions oriented along the [100] and [111] crystal axes by measuring both the steady‐state and time‐resolved differential transmission spectra. As expected, the spectra for the [100] sample are indicative of excitonic bleaching at all times and for all excitation levels, and a carrier recombination time of 0.8 ns and a nonlinear cross section (change in absorption coefficient per carrier pair) of ∼8×10−14 cm2 are extracted for the [100] sample. By comparison, for low excitation levels, the spectra for the [111] sample are consistent with a blueshift of the exciton, indicating a screening of the strain‐induced piezoelectric field. At higher excitation levels, the spectra are dominated by excitonic bleaching. Under identical 1 ps pulsed excitation conditions, the magnitudes of the changes in the absorption coefficient caused by screening in the [111] sample are comparable to those ...

Comparison between the optical limiting behavior of chloroaluminum phthalocyanine and a cyanine dye

The picosecond optical limiting characteristics of the optical limiting dyes hexamethylindotricarbocyanine iodide (HITCI) and chloroaluminum phthalocyanine (CAP) are compared and contrasted at 532 nm. From single pulse transmittance experiments, HITCI is shown to possess a serious limitation in its optical limiting behavior, where the regime of induced absorption becomes one of induced transmission for high irradiances. CAP, on the other hand, although possessing a smaller ratio of the excited-state to ground-state absorption cross-section, continues to exhibit optical limiting over a much broader dynamic range. From cross-polarized time-resolved excitation-probe measurements, HITCI exhibits a sharp negative spike around zero delay which originates from orientational coherent grating effects, while CAP does not exhibit any spike. These nonlinearities are explained theoretically as a complex interplay between a series of excited manifolds for the dyes and the radiation properties of the interacting laser p...