Harmonic generation in plasmas of different density gradients

The effect of plasma scalelength on second and third harmonic generation in the interaction of a 700 fs, 248 nm laser pulse with solid targets was stu died at nonrelativistic intensities. The density gradient was modified by a 700 fs prepulse 0-40 ps prior to the main pulse. The appropriately chosen prepulse leads to enhanced conversion to harmonics which is interpreted as a consequence of resonant absorption. There are differences in the effect o f the prepulse for different materials and for different harmonic orders, w hich might open the possibility of selective harmonic generation. (C) 1999 Elsevier Science B.V. All rights reserved.