Effects of pump laser chirp in high-order harmonics generated from various solid surfaces using femtosecond lasers

We investigate the effects of the pump laser chirp on high-order harmonic generation from solid surfaces of aluminium, indium and C60?film targets. Chirped pulses were produced by varying the distance between the two gratings of the laser compressor. We optimized the nanosecond contrast of our femtosecond laser pump (? = 35 fs, ? = 793 nm, I ? 1018?W cm?2) for surface harmonic generation by adjusting the trigger timing of the Pockels cell. Harmonics up to the 18th order (? = 44 nm) were observed. We show experimentally that for chirped pulses with the same pulse duration, those with negative chirp produced stronger harmonics, compared to those with positive chirp. This effect is attributed to the different rise time of the leading edge of the pulse due to the presence of high-order dispersion terms. We also show experimentally that the peak wavelength of the harmonics presents blueshift for pump lasers with negative chirp and redshift for those with positive chirp. This phenomenon appears to be due to the shift in the instantaneous wavelength of the pump laser at the maximum intensity for the different chirped pulses.