We measure the spectral dependence of the degenerate three photon absorption coefficient, ␥, for a Si ͓100͔ wafer using 200 fs pulses in the range 2300-3300 nm, i.e., photon energy between half and one-third the indirect band gap. For pulses linearly polarized along the ͓001͔ crystal axis ␥ increases from a value of near 0 cm 3 / GW 2 at 3300 nm to a peak value of 0.035 cm 3 / GW 2 at 2700 nm before decreasing with shorter wavelength; this is consistent with the dispersion expected from allowed-allowed-allowed transitions. At 2600 nm the ␥ value is ϳ30% larger for light polarized along ͓011͔ than along ͓001͔.
Efficient second-harmonic conversion of 4 microm radiation was demonstrated in orientation-patterned GaAs (OPGaAs) waveguides (WGs). An experimentally corrected phase-matching curve for second harmonic generation (SHG) in OPGaAs WGs is presented. Influence of WG modes on the SHG process was studied. Two distinct types of SHG in the waveguides were identified and related to the TE and TM modes. Each type has its own dependence on pump polarization. The 21% W(-1) normalized conversion efficiency is within a factor of 0.75 from the predicted value for an ideal WG.
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