Ultrafast all-optical switching using near-infrared intersubband transitions in quantum wells

In this paper, we report on all-optical switching using ultrafast optical nonlinearities originating from intersubband transitions (ISB-Ts) in InGaAs/AlAsSb quantum wells. The use of coupled double quantum wells (C-DQWs) for ISB-T-based all-optical switching is proposed, and its advantages are discussed. Sub-picosecond responses of C-DQW are simulated by rate equation analysis and measured using pump-probe experiments. We describe demonstrations of all-optical demultiplexing from 1 Tb/s signal pulse trains and reshaping/retiming functions at the optical-communication wavelength of 1.55 μm, using an InGaAs/AlAsSb C-DQW waveguide. We also report on our recent attempts to improve ISB-T characteristics using a novel InGaAs/AlAs/AlAsSb quantum well structure, which led to the observation of an ultra-low absorption saturation intensity of 3 fJ/μm².