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 mum to 36 mum. The expected quadratic dependence of the two-photon induced conductivity on laser intensity is observed. This nonlinear effect is used to perform autocorrelation experiments vielding the length of the micropulses of the free electron laser on a femtosecond time scale.