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Optical Switching And Self-Limiting With Near. Bandgap Guided Light In Silicon Waveguides

We have observed self-limiting and optical switching of near bandgap 1.06 micrometer light using MBE grown silicon waveguides on an n-type doped silicon substrate. The guided light intensity lowers the local index of refraction of the waveguide coupling region due to the photogeneration of electron-hole pairs. This brings the guide closer to its cutoff condition thus limiting the peak intensity that can be guided. Short 20 ns pulses gave rise to optical limiting with less than 160 nJ total integrated energy, while longer 150 ns pulses yielded envelope self-switching. Fiber and lens edge coupling to the silicon was used in all cases in contrast with the usual prism coupling geometry. Four guides, each with a different cutoff condition, were used to prove that the limiting is not caused simply by free carrier absorption. A theoretical self-consistent model was developed taking into account the guide's integrating Kerr-like self-defocussing nonlinearities. Since the initiation times are subpicosecond, such a nonlinear waveguide can be used as a fast optical limiter and logic gate.