Nonlinear absorption due to shallow donors in germanium at 10.6 μm

Nonlinear absorption due to shallow donors in phosphorus-doped germanium (ND ≊1.5×1016 cm−3) has been measured at 1.8 °K, using a single-mode CO2 TEA laser operating at 10.6 μm. The absorption coefficient is found to decrease with increasing intensity up to 10 MW/cm2. A simple two-level model provides a good fit to the data and yields values for the donor photoionization, free-carrier absorption, and capture cross sections.