Dynamical behavior of higher harmonic generation from a picosecond-laser-produced solid surface plasma

We have studied experimentally the efficiency of the harmonics from solid s urface plasma by varying the electron density scale length produced during the interaction of a 2.2 ps, 100 mJ, 1054 nm, p-polarized laser pulse with deposited Al targets. Although third harmonic radiation can also be observe d when a single beam is used, an approximately 5 ps time difference between the prepulse and the main pulse results in an increase in the harmonic sig nal by a factor of 2-3, which then decreases after 10 ps. The results for t he 4th- and 5th-order harmonic generation are also similar. We have also ca lculated the electron density scale length by using the one-dimensional(1D) hydrodynamic code HYADES. The calculation results are in good agreement wi th our experimental results. It is found that it is possible to generate hi gh efficiency harmonics by controlling the density gradient using a double laser pulse.