Abstract

The nonlinear spatio-temporal dynamics of a laser pulse undergoing filamentation have not been directly probed until now, given the high laser intensities, >10¹³W cm⁻², found in the filament core. Theoretical investigations¹ suggest that pulses as short as a single cycle should be obtained in a filament. Here we present a new method for probing the pulse duration within the filament using impulsive Raman spectroscopy where the filament is the impulsive pump and a 600 fs duration laser forms the probe. Impulsive excitation creates a coherent wave packet composed of the ground state of the molecule and one or more ro-vibrational excited states. This wavepacket evolves with a characteristic period, τ, defined by the energy-level spacing between the ground and the excited states. Such impulsive excitation results in a time-dependent polarizability in the sample that will imprint sidebands on a time-delayed probe pulse at ω_probe ± ω_R, where ω_probe is the carrier frequency of the probe and ω_R represents the characteristic Raman frequencies of the modes excited in the medium. We recently reported that filaments produce features short enough to impulsively excite Raman modes from rotational modes up to the 4155 cm⁻¹ stretch in H₂.² Here we demonstrate that filaments created in air can be probed using the O₂, N₂ and H₂O Raman active modes at 1554.7 cm⁻¹. 2330.7 cm⁻¹, and 3657.1 cm⁻¹ and that such measurements form a new method to probe the temporal dynamics as a function of propagation distance.

© 2010 Optical Society of America