Enhancement of high-order harmonic generation efficiency from solid-surface plasma by controlling the electron density gradient of picosecond laser-produced plasmas

We have investigated the efficiency of generating the fifth-order harmonic from plasma at the surface of a solid by varying the electron density gradient of the picosecond laser-produced plasmas. In order to modify the plasma scale length, we carried out experiments in which a 2.2-ps prepulse irradiates the solid target 0-20 ps prior to the arrival of the main Nd:glass laser pulse with the same pulse duration. The chirped pulse amplification Nd:glass laser system delivered pulses up to 100-mJ energy at 1054-nm wavelength. Although third-harmonic radiation can also be observed when a single beam is used, a similar to5-ps time delay results in an increase in the harmonic signal by a factor of 2-3, which then decreases after 10 ps. We also have carried out more detailed model-calculations using the simulations. As a result, it is found that it is possible to generate high harmonics efficiency by controlling the density gradient using two laser pulses.