Abstract

Recently much interest has been placed in ultrafast all-optical switching in semiconductors near half the band gap¹. In this spectral range, the problem associated with two-photon absorption (2PA) can be avoided, while the relatively large nonlinear refractive index (n₂) can still be utilized. Here we report for the first time a complete characterization of a near half beat length AlGaAs/GaAs quantum well (QW) nonlinear directional coupler (NLDC) and show the effects of multi-photon absorption (MPA) on nonlinear switching and pulse breakup. The NLDC used was made of 4 AIGaAs/GaAs QWs. A NaCl color-center laser with additive pulse mode-locking was used to produce 450 to 800 fsec pulses at λ=l.55 μm. In Figure 1, we show the switching fraction of the NLDC as a function of input intensity before the input waveguide. The solid lines represent numerical simulations with n₂=1.25×10^-13 cm²/W, β₂ (2PA)=0.033 cm/GW, and β₃ (three-photon absorption) =0.16 cm³/Gw². To show the effects of MPA, the dashed lines are drawn with β₂= β₃=O. It can be seen that MPA increases the switching intensity. Effects of MPA can also be clearly seen in Figure 2 in which the throughput as a function of input intensity is plotted. At high input intensity a change of transmission of about 30% occurs owing to MPA

© 1992 Optical Society of America