Femtosecond two-photon absorption phase change spectroscopy of a ZnSe single crystal.

We measured time- and frequency-resolved phase changes owing to two-photon absorption induced by ultrashort (10 fs) pulses transmitted through a transparent material (ZnSe crystal), using femtosecond two-photon phase change spectroscopy. The maximal phase change was 0.02 rad when a pulse with the energy density of 1.5 nJ was focused with the focus diameter of 100 μm. The maximal phase change occurred when a probe pulse was delayed with respect to a pump pulse by 64 fs. A calculation assuming two-photon absorption by noninteracting atomic gases was consistent with the experimental observations, which supported the observed phase change of the pulse that is due to the two-photon absorption.