Infrared Fel Measurements of Power Limiting by 2-Photon Absorption in Insb and Optical Pulse Length Measurements

We have performed the first optical experiment using the laser output of the CLIO free electron laser. In a transmission experiment we have observed strong power limiting at wavelengths longer than the absorption edge at 7.5 mum associated with induced free carrier absorption produced by direct interband two-photon transitions in InSb. We have estimated the two-photon absorption (TPA) coefficient (b) of InSb at 8.9 mum to be 2 cm/MW, by fitting the power-limiting effect with a simple theoretical model. An important feature of this result is that we are utilising the broad tunability of the FEL to explore the TPA theory to shorter wavelengths than hitherto available with the CO2 laser. The result is in broad agreement with the longer wavelength measurements of Sheik-Bahae et al. [J. Opt. Soc. Am. B4 (1987) 19641, and distinguishes between different theoretical models for the process. Applications that make use of the intrinsic nonlinearity of the interaction include the measurement of picosecond optical pulse lengths using an autocorrelation method, direct measurement of the optical pulse intensity and intensity stabilisation of the FEL source using the power limiting capabilities of the semiconductor.