Generation of low-order harmonics in laser ablation plasmas

The third and fifth harmonics of a pulsed infrared laser (1064 nm) delivering pulses of nanosecond duration have been generated in the laser ablation plasmas of various materials including the metals Al and Fe and the highly ionic insulators MgF2 and NaCl. The harmonics were generated in a process triggered by laser ablation followed by frequency up-conversion of the fundamental laser beam that propagates parallel to the target surface. The spatial and temporal behaviour of the harmonics reveals the distinctive composition and dynamics of each plasma, gives insight into the species that act as nonlinear media and allows a comparison of different processes that control the generation efficiency. Low-order harmonic generation thus emerges as a powerful and universal technique for monitoring and diagnostics of ablation plasmas, and the results presented serve to guide the choice of solid materials for efficient high harmonic generation using ultrashort pulses in the femtosecond regime.