Free-Electron Laser-Induced 2-Photon Absorption in Hg1-Xcdxte

The powerful output of FELIX, the recently built Rijnhuizen free-electron laser, is used for the first frequency dependent study of nonlinear optical excitation of Hg1-xCdxTe in the far-infrared spectral region. Two-photon interband absorption has been investigated as a function of power and wavelength from 20 to 40 mum. This nonlinear optical absorption is used to perform autocorrelation experiments yielding the length of the micropulses of the free-electron laser on a femtosecond time scale. The predicted dependence of the micropulse length on the synchronization between optical and electron pulses in the laser cavity is observed.