It has been dicovered recently that the dispersive nonlinearity of MQW exhibits a strong saturating behavior[1], which constiutes a serious limitation in the design of optical bistable etalons.

Quantum well vertical cavity structures are very attractive for the large scale implementation of parallelism in photonic systems, as they allow to fabricate 2-dimensional arrays of active functional devices such as surface emitting lasers, electro-optical modulators, and bistable optical switches.

We report on a novel monolithic all-optical bistable device operating at 980 nm, based on the dispersive optical nonlinearity of strained InGaAs/GaAs quantum wells located at the antinodes of the microcavity optical field. This design maximizes the interaction with the intracavity field and allowed to use only twelve quantum wells of 10 nm thickness. The first observation of all-optical bistability with strained InGaAs/GaAs quantum wells is reported, with a contrast ratio of 7:1 and a threshold intensity of 1 kW/cm2. The operating wavelength offers key advantages such as the substrate transparency and compatibility with vertical cavity surface emitting lasers.

The saturation behavior of the nonlinear index δnNL in the band tail of GaAs/AlGaAs multiple quantum wells is experimentally studied in quasi-steady state and in a low absorption region (below 300 cm−1), where dispersive optical bistability is observed. Measurements at different detunings from the exciton resonance allow us to study the absorption dependence of the saturation parameters. The saturation intensity IS is found inversely proportional to the linear absorption coefficient, while the saturating nonlinear index δnS is approximately constant. This evidences that the nonlinear index saturation is due to a saturation of the carrier density versus light intensity, due to the saturation of the band tail absorption itself.

A pump and probe measurement of the nonlinear transmission that the origin of the saturating behaviour of the nonlinear index deltan(NL) observed in the hand tail of GaAs/GaAlAs multiple quantum wells is due to the saturation of the photoexcited carrier density.

We report on recent progress in reducing the power threshold of all-optical bistable quantum well vertical microcavities. Significant improvements are achieved through an increase of the cavity finesse, together with a reduction of the device active layer thickness. A critical intensity of 5 μW/μm 2 has been observed on a microcavity of finesse 250, with a nonlinear medium of only 18 GaAs quantum wells of 10 nm thickness. Further improvements of the Bragg mirror quality resulted in a finesse of 700 and a power-lifetime product of 15 fJ/μm 2 . Microresonator pixellation allows to obtain 2-dimensional arrays. A thermally-induced alloy-mixing technique is described, which produced a 110 meV carrier confinement energy, together with a refractive index change of −.012, averaged over the 2.6 μm nonlinear medium thickness. The resulting electrical and optical confinement is shown to improve the nonlinear characteristics, by limiting lateral carrier diffusion and light diffraction.