Modification of chirped laser pulses via delayed rotational nonlinearity.

To interpret single-shot measurements of rotational revival patterns in molecular gases excited by an ultrashort laser pulse, an analytical description of the probe pulse modulation by the impulsively excited medium is developed. A femtosecond pump laser pulse prepares a rotational wavepacket in a gas-phase sample, and the resulting periodic revivals are mapped into the frequency domain by using a substantially chirped continuum probe pulse. Since the standard approximate descriptions of probe pulse propagation are inapplicable (such as the slowly varying envelope approximation and the slowly evolving wave approximation), we propose an approach capable of incorporating both the substantial chirp of the pulse and the temporal dispersion of the medium response. Theory is presented for the case where the frequency change of the probe during the probe pulse duration is comparable with the carrier frequency. Analytical expressions are obtained for the probe signal modulation over the pump-probe interaction region and for the resulting heterodyned transient birefringence spectra. The approach is illustrated using the case of nitrogen gas.

Ionization-grating-induced nonlinear phase accumulation in spectrally resolved transient birefringence measurements at 400 nm.

We report experimental confirmation of the ionization-grating-induced transient birefringence predicted by Wahlstrand and Milchberg [Opt. Lett. 36, 3822 (2011)] and discuss its impact on the higher-order Kerr effect interpretation by Loriot et al. of pump-probe transient birefringence measurements made at 800 nm [Opt. Express 17, 13429 (2009)]. Measurement of the transient birefringence in air at 400 nm shows a negative contribution to the index of refraction at zero delay for frequencies within the pump bandwidth, the degenerate case, and no negative contribution for frequencies exceeding the pump bandwidth, the nondegenerate case. Our findings suggest that a reevaluation of the higher-order Kerr effect hypothesis of Loriot et al. is necessary.

Self-shortening dynamics measured along a femtosecond laser filament in air.

The filamentation-induced temporal shortening of a 40 femtosecond pulse propagating in air is traced using impulsive vibrational Raman scattering and measurement of the power spectrum as a function of position along the propagation axis. The N2, O2, and H2O vibrational Raman responses reveal self-shortening of pulse features to 14 fs during the first filamentation cycle and to at least 9 fs in the second cycle. Spectral measurements further demonstrate that the coherent bandwidth generated in the region from 470 to 330 nm during the self-shortening process forms the ∼9 fs pulse.

Rovibrational wave-packet dispersion during femtosecond laser filamentation in air.

An impulsive, femtosecond filament-based Raman technique producing high quality Raman spectra over a broad spectral range (1554.7-4155 cm(-1)) is presented. The temperature of gas phase molecules can be measured by temporally resolving the dispersion of impulsively excited vibrational wave packets. Application to laser-induced filamentation in air reveals that the initial rovibrational temperature is 300 K for both N2 and O2. The temperature-dependent wave-packet dynamics are interpreted using an analytic anharmonic oscillator model. The wave packets reveal a 1/e dispersion time of 3.9 ps for N2 and 2.8 ps for O2. Pulse self-compression of temporal features to 8 fs within the filament is directly measured by impulsive vibrational excitation of H2.