Enhancement of high-order harmonic generation efficiency by double pulse lasers

Abstract
We have investigated the efficiency of the fifth order harmonics from solid surface plasma by varying the electron density gradient using two 2.2 ps, 1054 nm laser pulses. A prepulse irradiates the Al deposited target 0-20 ps prior to the arrival of the main Nd : Glass laser pulse in order to modify the electron density gradient. It is found that 5 ps time difference between the prepulse and main pulse results in an increase in the third harmonic signal by a factor of 2-3, which then decreases for time delays greater than 10 ps, and the fourth and fifth harmonic signal also increase. Experiments and calculations are presented that demonstrate a strong dependence of the conversion efficiency on the plasma scale length.