Saturation of transmitted intensity of CO2 laser pulses in germanium

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Abstract
The transmission behavior of 90‐ns CO2 laser pulses through germanium has been studied at high intensities up to 20 MW/cm2. With increasing incident intensity, the transmitted intensity was observed to saturate quite abruptly at a level which decreased with increase in crystal length. The saturated level of transmitted intensity was found to be higher at lower temperatures. The observed transmission behavior is interpreted in terms of multiphoton absorption across the direct band gap, followed by absorption by the excess carriers thus generated. By fitting experimental data to a phenomenological model, a room‐temperature seven‐photon absorption coefficient of 1.5×10−9 (cm2/MW)6 cm−1 is obtained for germanium in the 10‐μm region.