Abstract
We report on the observation of two-photon absorption for the direct gap in intrinsic germanium at room temperature using two independently tunable picosecond, mid-infrared light pulses. The two-photon absorption coefficient is determined for different wavelengths, and carrier densities down to about 1013 cm−3 could be detected at a sample length of 700 μm. Two-photon absorption in germanium is demonstrated to be a versatile tool for zero-point determination in time resolved pump-probe investigations and permits a precise measure of the time resolution of the laser system.

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