Free-carrier-induced third-order optical nonlinearities in semiconductors

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Abstract
We have studied a variety of dissipative, free-carrier-induced optical nonlinearities in semiconductors with CO2 lasers and observed third-order nonlinear susceptibilities as large as 2 × 10−3 esu with picosecond recovery times. These fast nonlinear processes are capable of inducing huge perturbations in the dielectric function because they do not saturate at high intensities, as slower processes do. At laser intensities above a few tens of kilowatts per square centimeter, fast nonlinear processes dominate over the slower processes, which are stronger at low intensities. The large nonlinearities that we report can operate at room temperature. They are nonresonant, requiring no exact matching of the material parameters to the light frequency, and are insensitive to ambient-temperature changes. Hence they offer considerable versatility for design and stability in operation.