Pulse compression of a high-order harmonic

The fifth harmonic pulses of an intense femtosecond Ti:sapphire laser were experimentally shown to be negatively chirped by using an LiF plate as a po sitive dispersive medium. The chirp of the harmonic pulse originates from t he intensity-dependent atomic dipole phase, which is estimated to be propor tional to 25 U-p, where U-p is the ponderomotive energy. Consequently, we h ave succeeded in compressing the chirped pulses to 13fs by compensating the intrinsic negative chirp. Chirp effects of the fundamental laser on the pu lse width of the fifth harmonic were consistent with the negative chirp of the fifth harmonic.