Nonlinear spectroscopy near half‐gap in bulk and quantum well GaAs/AlGaAs waveguides

We study the nonlinear properties of bulk AlGaAs and GaAs/AlGaAs multiple quantum wells(MQW) below the half‐band‐gap energy using subpicosecond pulses between 1.65 and 1.7 μm. In the bulk material we find a value for the nonlinear index n 2 = +3.6× 10−14 cm2/W and a two‐photon absorption coefficient β = 0.26 × 10−4 cm/MW. In the MQW we measure an n 2 up to 2.4 times larger, and we attribute this enhancement to a stronger 1S‐exciton intermediate state. The β value is up to 25 times larger in the MQW. This larger value may result from midgap states that resonantly enhance the virtual intermediate state in two‐photon absorption and act as a real transition in a two‐step absorption process. The resulting figure of merit (2n 2/βλ) for the bulk (MQW)material is 17 (1.6), which means that these semiconductors below half band gap are appropriate for all‐optical switching and quantum optics applications. We confirm that n 2 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 as evidenced by the signal along the probe axis following the negative derivative of the pump intensity. This result may be explained by self‐phase modulation of the pump combined with a low‐frequency Raman process that couples the modes along orthogonal axes.