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Nonlinear Transmission Of Semiconductor Thin Films

We observe two-photon absorption (2PA) of picosecond, 0.6 μm, 0.25 nJ pulses in a 0.4 μm film of ZnSe. We use high-frequency modulation phase- sensitive detection to be near the shot noise limit, followed by a low frequency pulse delay modulation to discriminate against large thermal nonlinearities. This dual modulation technique, combined with the field enhancement produced by placing the film in a resonant cavity, allows observation of 2PA and other fast nonlinearities using pulses having peak powers of the order of a Watt.