Spectral probing of an extremely compressed femtosecond wave packet in calcium fluoride

We use time-resolved spectral probing to study the dynamics of transformation of the optical properties of a nonlinear medium during formation and propagation of an extremely compressed mid-IR femtosecond wave packet in calcium fluoride. Broadening, attenuation and distortion of the probe pulse spectrum have been experimentally observed varying the delay time of the probe pulse relative to the pump. By solving the unidirectional pulse propagation equation that describes propagation and filamentation of a pump pulse in a calcium fluoride crystal we determine temporal changes of the refractive index and absorption coefficient induced by an extremely compressed wave packet. A mechanism of the observed broadening and breakup of the probe pulse spectrum is proposed.