Subpicosecond all-optical modulation using the optical stark effect in a nonlinear directional coupler

All-optical modulation in a GaAs/AlGaAs multiple quantum well nonlinear directional coupler is observed at room temperature using femtosecond pulses. The ultrafast (< 1 ps) response and recovery of the device is attributed to the optical Stark effect. All-optical switching in various nonlinear optical devices is of interest because of its possible application in high-speed photonic switches. In semiconductor devices, which exploit the relatively large optical nonlinearities near a semiconductor bandedge, the source of the nonlinearity is typically the presence of free carriers photoexcited by a temporally short pump (i.e. control) beam. Previously we have demonstrated all-optical switching both in a GaAs/AlGaAs multiple quantum well (MQW) nonlinear etalon, as well as in a MQW nonlinear directional coupler (NLDC) using real carrier excitations to generate the nonlinearities. In both cases the device has a response which follows the pulse, and a recovery time which was limited to ≉10ns by the recombination and the diffusion of the photo-generated carriers. Recently, the operation of a nonlinear etalon with a subpicosecond recovery time was demonstrated. This was accomplished by using femtosecond pulses tuned in the semiconductor’s transparency region to generate instantaneous nonlinearities which were present only while the pump travelled through the etalon. In this case the source of the optical nonlinearity was the now well known optical Stark effect. Here we report the first observation of ultrafast switching, including < 1ps recovery time, in a GaAs/AlGaAs NLDC. We believe the optically-induced temporally-short index change responsible for this fast switching is generated by the optical Stark effect.