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Optical switching and transient carrier transport phenomena in MQW structures

We have investigated vertical transport in two different multiple quantum well (MQW) semiconductor optical switching elements; a self electro-optic device and a waveguide directional coupler, using picosecond resolution pump-probe measurements. Cross-well charge transfer mechanisms in semiconductor MQW structures are basic to the operation of a number of optical switching devices currently being researched for applications in signal routing, optical processing, and computing. The cross-well motion of charge carriers in multiple quantum wells, and specifically how rapidly photogenerated carriers can be ejected from quantum well, sets the fundamental limit on the speed of operation of some devices and can be employed to speed the recovery time of others. A self-consistent, time and spatially resolved model of cross-well transport gives good agreement with experimental results. The use of cross-well carrier sweep-out is shown to give significantly improved recovery times for all- optical waveguide directional coupler switches.