Abstract

Multiple quantum wells (MQWs) exhibit large optical nonlinearities associated with saturation of excitonic transitions at the band edge. Here we present a study of nonlinear absorption and refraction in an InGaAs/InP MQW structure, for the design of an all-optical reflection modulator. The sample, grown by SBE, consists of sixty periods of 77 Å InGaAs wells and 70 Å InP barriers, sandwiched between two 0.5-μm InP buffers. The wells are affected by a tensile strain due to a 0.5% lattice mismatch. In the experiment, a cw Nd:YAG pump and a white light probe (tungsten lamp) are incident collinearly on the sample. The output light is wavelength scanned with a monochromator that blocks (together with a Nd:YAG laser mirror and a silicon filter) the 1064-nm radiation. Transmittivity spectra in the 1480–1600-nm range were measured for incident pump intensities up to 4.3 kW/cm². Absorption spectra in the absence of excitation exhibit an exciton peak of unusual height (13,000/cm^-1 for the wells only). Measurements at various pump powers demonstrated a 60% maximum quenching of the absorption peak with a saturation intensity of 183 W/cm². The progressive peak lowering with increasing pump power is well fitted by a simple two-level formula, and a more complex model based on a numerical solution of the Bethe-Salpeter equation provides calculated spectra in good agreement with experimental data. A maximum index change of -0.076 at 1556 nm with 3.2-kW/cm² incident pump intensity was estimated from the absorption curves through the Kramers-Kroenig relations.

© 1991 Optical Society of America