Femtosecond pump-probe study of preformed plasma channels

Abstract

We report femtosecond pump–probe experiments in He plasma waveguides using 800-nm, 80-fs pump pulses of 0.2×1018 W/cm2 peak guided intensity and single orthogonally polarized 800-nm probe pulses ~0.1% as intense as the pump. Single-shot spectra and spatial profiles of the probe pulses exiting the channels are measured through a crossed-polarization analyzer at various pump–probe time delays Δt. At |Δt|≳100 fs, we observe frequency-domain interference between the probe and a weak component of the exiting pump created by hybridization of its polarization state through interaction with the channel. Frequency-domain interference measurements show this “depolarized” component differs substantially in mode structure from the injected pump pulse. This component is nearly undetectable by direct measurements of pump leakage without the probe. We analyze possible causes of depolarization within the channel and near its entrance and exit regions. At Δt≲0, i.e., the probe pulse propagates in the leading edge of the pump pulse, we observe spectral blueshifts in the transmitted-probe spectrum that are not evident in the transmitted pump. The evidence indicates that pump depolarization and probe blueshifts both originate primarily near the channel entrance.