Pulse compression of a high-order harmonic by compensating the atomic dipole phase

The fifth harmonic pulses of an intense femtosecond Ti:sapphire laser are e xperimentally shown to be negatively chirped by using a LiF plate as a posi tive dispersive medium. The chirp of the harmonic pulse is originated from the intensity-dependent atomic dipole phase, which is estimated to be propo rtional to 25U(p), where U-p is the ponderomotive energy. Consequently, we have succeeded in compressing the chirped pulses to 13 fs by compensating t he intrinsic negative chirp. Chirp effects of the fundamental laser on the pulse width of the fifth harmonic were also investigated.