Ultrafast all-optical switching in multiple-quantum-well Y-junction waveguides at the band gap resonance

An integrated Y-junction switch is used to demonstrate all-optical switching due to two mechanisms in GaAs-AlGaAs multiple quantum well structures at room temperature. Carrier-induced nonlinearities are compared to nonlinearities due to the optical Stark effect for ultrafast operation. In the first case, device geometry is exploited by a two-pulse switching process, whereby one control pulse turns the device on and a second control pulse turns the device off. Time gating of signal pulses within an 8 ps window was realized in our experiments. In the second case, the nonlinearity is ultrafast, and hence switching and recovery take place during the control pulse evolution. Consecutive switching events spaced 1.7 ps apart have been achieved. In our measurements, two-photon absorption played a significant role in the switching characteristics of the device. In order to access the carrier-induced nonlinearity, the pulse energy that is needed for switching (9 pJ in this case) results in a very high peak intensity that leads to two-photon absorption. This is observed as a strongly induced absorption exactly at the precise zero time delay between the control and signal pulses. In the second case, two-photon absorption competes directly with the optical Stark effect since both are instantaneous intensity dependent effects. Furthermore, the optical Stark effect appears to saturate at a low intensity level (0.9 pJ or 200 MW/cm² ); consequently only incomplete switching with a 2:1 contrast ratio was observed, whereas a 9:1 switching contrast was achieved with the carrier-induced nonlinearity