Midinfrared optical absorption in germanium measured with a free-electron laser at room temperature

We have investigated the effects of high-power laser pulses on a transparent material using a free-electron laser (FEL). We have measured optical transmittance through a germanium crystal in the midinfrared region, $\ensuremath{\lambda}=5.3--12.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ at room temperature. In spite of the fact that germanium is transparent in the midinfrared region, we have observed strong suppression of optical transmission under high-intensity FEL excitation.