Measurement of Group Velocity Distortion due to Ultrafast Index of Refraction Transients

Abstract

The interaction of a linearly polarized laser pulse with an anisotropic molecular gas exerts a torque on molecules not initially aligned with the polarization direction [1]. Laser pulses shorter than the molecular rotation time will impulsively excite a rotational wave packet composed of a superposition of rotational eigenstates. The rotational wave packet periodically collapses and revives, producing ultrafast transients in the index of refraction [2],[3]. A time-delayed ultrafast probe pulse propagating in the ultrafast index transient accumulates a phase modulation that depends on the particular shape of the rotational revival. Strong temporal phase modulation may be applied to the probe pulse by extending the interaction length with a hollow core fiber. This large phase modulation leads to substantial spectral changes in the probe pulse and has been applied to produce transform-limited pulses both temporally compressed [4] and stretched [5].