Nonlinear optical properties of indium arsenide with ultrafast femtosecond radiation at mid-infrared wavelengths
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Nonlinear optical properties of the narrow band-gap semiconductor InAs have been studied using ultrashort femtosecond infrared pulses from a regeneratively amplified Ti:Sapphire pumped optical parametric amplifier (OPA) and difference frequency mixing (DFM) system. Characterisation of the laser showed good stability with 1.5 µJ pulse energy in a Gaussian spatial distribution. Wavelength spectra were broad Gaussian distributions consistent with dispersion of ultrashort femtosecond duration pulses. Sub-200 fs pulse lengths were measured with a two-photon detector based autocorrelation. The measured two-photon absorption coefficient agreed well with perturbation theory from 4 to 5.5 µm. At longer wavelengths the coefficient was smaller than expected. This was attributed to the broad, polychromatic wavelength and uncertainty in the pulse length. No additional free carrier absorption was observed, even at very high irradiance. It is hypothesized that this was due to the carriers having insufficient time to undergo intra- band relaxation on the timescale of the pulses. Beam spreading and saturation effects may also have been occurring. Nonlinear refraction was measured using the z-scan technique. A strong defocusing component dominated which was consistent with free carrier plasma effects. A weaker component was also observed that was defocusing at 4 µm and self-focusing at 5.5 µm. This was in agreement with predictions of an ultrafast, n₂ contribution, corresponding to the real part of the third order susceptibility χ⁽³⁾. Pump-probe measurements showed that the nonlinear refraction was instantaneous but the absorption had a build up time of ~100 ps. This was attributed to hot-phonon screened intra-band relaxation. Fitting of the pump-probe data produced Shockley-Read-Hall and Auger recombination lifetimes consistent with prior literature data.
Thesis, PhD Doctor of Philosophy
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