Direct Observation of Multiple Photon Absorption by Free Electrons in a Wide Band-Gap Insulator under Strong Laser Irradiation

We measure the kinetic energy of photoelectrons emitted from an $\alpha$-quartz surface irradiated by short (100 fs) and intense laser pulses. In the high intensity regime (above 100 GW/${\mathrm{cm}}^2$), we observe a high-energy tail in the photoemission spectra at 620 and 560 nm. This effect disappears for the doubled frequencies. These observations give direct evidence of free-electron multiple photon absorption in the conduction band, in keeping with recent Monte Carlo simulation results.