Mario Dagenais

Low power transverse optical bistability near bound excitons in cadmium sulfide

We report the observation of cw transverse optical bistability without power hysteresis at 1‐mW power levels in uncoated cadmium sulfide platelets. Optically induced refractive index changes as large as 0.15 are deduced from measurements of the transverse spatial ring profile.

Intracavity optical bistability due to thermally induced changes in absorption and refraction

Optical bistability in the presence of an optically induced absorption is observed when a thin 13 μm CdS sample is inserted between two mirrors of 90 percent reflectivity. In our experiment, a modulated CW laser of about 15 mW peak power is tuned on each side of the A free exciton resonance in CdS, and the transmission characteristics are studied. A theoretical calculation is presented and good agreement is obtained.

Physics Of Low Energy Optical Switching

The physics of low-energy optical switching in both ideal and experimental devices is discussed. Guided-wave optical bistable devices are predicted to have the lowest switching energy per unit gain of any optical switching devices. So far, the bistable diode laser amplifier is the only optical device which can switch with the same switching efficiency as the best electronic device.

Low Power Optical Bistability near Bound Excitons in Cadmium Sulfide

Direct gap semiconductors exhibit some of the largest optical nonlinearities measured in condensed media. Using the resonance enhancement of the material nonlinearity near the band gap, very large nonlinear phase shifts can be accumulated over micron size distances and large changes of the absorption coefficient can also be observed at relatively low incident power. For instance, the saturation of free excitons and the associated generation of free carriers following near-band gap excitation in bulk GaAs1 and InSb2 or in GaAs/AlGaAs superlatticeslc,3 have been used to demonstrate optical bistability on the 10 mW power scale. The response time of these optical bistable devices is typically tens of nanoseconds and depends directly on the carrier recombination lifetime. Rather than using the extended states (free excitons and free carriers) of a semiconductor, as is usually done, we propose here a completely different approach for demonstrating optical bistability. We would like to use those bound states of a direct gap semiconductor which have a large oscillator strength. In particular, we have studied the saturation behavior of the I2 bound exciton in cadmium sulfide (CdS), which is an exciton bound to a neutral donor with a binding energy of about 8 meV.4

Bistable Diode Laser Amplifiers In Optical Computing

The role of bistable diode laser amplifiers in optical signal processing and optical computing is discussed. A lower bound (1 pJ) on the switching energy of all-optical unguided bistable devices is obtained. Optical guided wave devices, both active and passive, promise to have the lowest switching energy. The advantages and limitations of electronic devices are presented. It is concluded that optics has important advantages over electronics in highly interconnected systems. Bistable diode laser amplifiers have the lowest demonstrated switching energy per unit gain (300 fJ) of any optical devices operating at room temperature. Application of these devices in an updatable optical crossbar switch leads to unique capabilities. Optics has significant advantages over electronics even for matrices of modest sizes (32x32).

Switching Behavior of Bistable Resonators Filled with Two-Level Atoms

A comprehensive study of the switching dynamics of bistable optical devices is reported. The intracavity medium is modelled as an ensemble of two-level atoms uniformly distributed in a plane parallel Fabry-Perot. By allowing for arbitrary atomic detunings and cavity mistunings, both the absorptive and dispersive contributions to optical bistability are included. Variations in polarization and population over wavelength distances are treated by means of expansions in spatial Fourier series, having as fundamental a half optical wavelength. The Fourier series are truncated after the first harmonic. We mostly concentrate on the dynamics of the turn-off since it is believed to be the limiting factor in the operation of an optical bistable device at high repetition rates. The turn-on of an optical bistable device can be made arbitrarily fast by increasing the intensity of the incident field. We emphasize that switching an optical bistable device on or off from steady state involves going from a certain intracavity distribution of the polarization, inversion, and electric field, to three completely different distributions. All three distributions need to be changed and each of them generally evolves in time at a different rate.

On the Validity of the Judd-Ofelt Theory for Two-Photon Absorption in the Rare-Earths

The one-photon absorption theory of JUDD and OFELT[1] has been rather succesfui in describing the oscillator strength of many forced electric-dipole f-f transitions in the trivalent rare-earths.[2,3] Such a success has very often been taken as a vindication of all the assumptions behind the Judd-Ofelt theory when really it shows only that the formalism of representing the intensity of an f-f transition by the sum of phenomenological parameters multiplied by the squares of tabulated reduced matrix elements U(λ)(λ = 2, 4, 6) is valid. There are cases where the Judd-Ofelt theory of one-photon absorption does not lead to accurate prediction Some well-known examples are given by the hypersensitive transitions.[2] These transitions are characterized by large values of U(2) matrix elements and are in general rather sensitive to the environment of the lanthanide ions. Since the Judd-Ofelt theory has also its weak points, it is very important to design new experiments to pinpoint its limitations. We would like to report here on a completely new test of the Judd-Ofelt theory. It is based on a theoretical calculation of AXE[4]on two-photon absorption in the rare-earths. His work is a direct extension of the Judd-Ofelt theory for one-photon absorption to two-photon absorption. The theoretical predictions on relative two-photon absorption cross-sections are even simpler and involve only the U(2) reduced matrix elements. It should be noted here that both the Axe and the Judd-Ofelt theories ignore the band character of the 5d states. In our work, we have observed direct two-photon f-f transitions in the Gd3+ ions in a LaF3 host, using a CW linearly polarized laser beam of about 100 mW power.