Abstract

We have investigated, experimentally and theoretically, the nonlinear propagation of femtosecond collinear pump and probe pulses in lithium niobate crystals. The propagation of pump pulses is strongly affected by two-photon absorption (TPA) for the intensity- crystal-thickness product ≳ 1 GW/cm. Furthermore, a strong pump pulse at 388 nm induces a considerable absorption for a time-delayed probe pulse at 776 nm. The dependence of the probe transmission coefficient on the time delay Δt shows a narrow dip (at Δt = 0) and a long lasting plateau. The dip is due to direct TPA transitions involving pump and probe photons (the TPA coefficient is ≈ 0.7 cm / GW). The plateau absorption is caused by pump- excited carriers; their absorption cross-section at 776 nm is ≈ 10^-17 cm². No distinct manifestations of hot carrier relaxation are found on the time scale (10^-1 — 10²) ps; the hot carrier relaxation time is expected to be ≲ 0.1 ps.

© 2005 Optical Society of America