Femtosecond time-resolved absorption processes in lithium niobate crystals
Femtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient beta(p) ranges from similar to 3.5 cm/GW for lambda(p) = 388 nm to similar to 0.1 cm/GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2 lambda(p) = 776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (,8, similar or equal to 0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of less than or similar to 0.1 ps. (c) 2005 Optical Society of America.