Abstract

We present measurements of the nonlinear index n₂ and two-photon absorption coefficient β as well as time resolved pump-probe data for both bulk AlGaAs and GaAs/AlGaAs multiple quantum well waveguides near half-gap at 1.68 μm. The figure of merit (2n₂/βλ) [1] for the bulk (MQW) material is 17 (1.6), which means that these semiconductors below half bandgap are appropriate for all-optical switching and quantum optics applications. The β value is up to 25 times larger in the MQW. This larger value may result from mid-gap states that act as a stepping stone in two-photon absorption (TPA). The mid-gap absorption should not be fundamental to MQW material provided that caution is exercised during the growth to avoid impurities and interface states. We confirm that n₂ is instantaneous on the 300 fs time scale of our pulses from self-phase-modulation spectra as well as time-resolved pump-probe measurements. However, we find an intriguing exchange of energy between the two orthogonal axes with the signal along the probe axis following the negative derivative of the pump. This result may be explained by self-phase-modulation of the pump combined with a low frequency Raman gain between orthogonal axes.

© 1991 Optical Society of America