The fifth harmonic pulses of an intense femtosecond Ti:sapphire laser are experimentally shown to be negatively chirped by using a LiF plate as a positive dispersive medium. The chirp of the harmonic pulse is originated from the intensity-dependent atomic dipole phase, which is estimated to be proportional to ${25U}_p$, where $U_p$ is the ponderomotive energy. Consequently, we have succeeded in compressing the chirped pulses to 13 fs by compensating the intrinsic negative chirp. Chirp effects of the fundamental laser on the pulse width of the fifth harmonic were also investigated.

• Received 7 June 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.2564