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Study of Four-Wave Mixing in Bulk Semiconductors Excited by GW/cm� 10 μm Laser Fields


Semiconductors such as GaAs, Ge, and ZnSe have long been important materials for optics and photonics applications in the middle infrared range (2-20 μm), finding use as windows or in optoelectronic devices. The nonlinear refractive index (n2) of these three semiconductors is well documented near resonance in the near infrared, but as technology progresses to longer wavelengths and higher intensities, characterization of these materials in the long-wavelength infrared (LWIR, 8-14 μm) at GW/cm� intensities becomes necessary. In this thesis, we report on measurements of the effective nonlinear refractive index in GaAs, Ge, and ZnSe using four-wave mixing of a 10 μm CO2 laser beat-wave at both high 1-10 GW/cm� and low 1-10 MW/cm� intensities, with 200 ps and 300 ns long pulses respectively. Intensity dependent nonlinear absorption is also observed. In addition, by decreasing the beat-frequency of the CO2 laser beat-wave, the nonlinear optical response is found to increase by a factor of 10 in GaAs. Simulations attribute this beat-wave enhancement to nonlinear currents of photoexcited free carriers and predict that by further decreasing the beat frequency to a few GHz, the nonlinear optical response can be increased by a factor of almost 100. Optical control of the nonlinearity in GaAs could lead to the production of high power broadband frequency combs with narrow frequency separations ideal for high resolution spectroscopy in the LWIR.

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