The spatial distribution of high-order harmonics from solid surface plasmas

Investigations have been performed on harmonic generation from solid-vacuum interfaces using a 2.2-ps 1054-nm laser pulse at maximum intensities of /spl sim/2/spl times/10/sup 17/ W/spl middot/cm/sup -2/. It is found that the divergence of the harmonics gradually increases at intensities between I/spl lambda//sup 2/=2/spl times/10/sup 15/ and 2/spl times/10/sup 16/ W/spl middot/cm/sup -2//spl middot//spl mu/m/sup 2/. We experimentally observe that the divergence of the harmonics decreases as the order of the harmonic increases. We also found that the divergence of the third harmonics emission is smaller for 2.2-ps pulsewidth for the driving lasers as compared with those for 100-ps pulsewidth lasers.