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.