Nonlinear index of refraction is of great importance in evaluation of laser-induced damage threshold, as it relates to parameters defining self-focusing critical power and avalanche breakdown. An accurate method to measure the nonlinear refraction can help restrict the damage caused by these mechanisms. Beam-Deflection (BD) method is a useful tool to calculate nonlinear refractive index. The critical parameters in the calculations are probe-to-excitation spot size ratio, r, and the relative displacement of the two overlapping beams. Similar to the Z-Scan method, an empirical function, solely dependent on r, is obtained which provides a relation between the BD signal and the phase shift.
Non-degenerate two-photon absorption (ND-2PA) across the indirect gap of silicon is theoretically and experimentally investigated in the current work. We present measurements of the 2PA coefficient of bulk silicon using femtosecond pump and probe pulses in the IR and mid-IR range. Enhancement of ND-2PA was observed from the results of our measurements with increasing non-degeneracy. This can be utilized when designing sensitive silicon-based mid-IR detectors. Modeling of the 2PA was performed by considering three theoretical pathways across the band structure of silicon for the two photons and a phonon, and the most dominant processes are determined by comparison to our measurements.
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