Resonance-Induced Enhancement of the High-Order Harmonic Generation in Plasma

Resonance-induced enhancement of the single harmonic generating in laser-produced plasmas allows achieving the microJoule energies for the coherent pulses in the extreme ultraviolet region using the moderate-intensity lasers. We review various approaches for improving the enhancement of single harmonic, which paves the way for a growth of har- monic yield in the short-wavelength range for various applications. enabled the overlap of these harmonics with the ionic transi- tions with strong oscillator strengths. Another approach for the tuning of harmonic wavelength based on the chirp varia- tions of driving radiation has been reported during gas HHG studies (15-17). Moreover, the tuning of harmonics and their "sharpness" were demonstrated using a combination of the external control of laser chirp and the intensity-induced variation of laser chirp inside a nonlinear medium (16, 17). In this mini-review, we discuss recent studies of the en- hancement of a single harmonic intensity in different parts of plateau in the case of plasma HHG. After brief overview of the experimental details, we analyze the giant single har- monic generation in indium plasma and other plasmas where the harmonic enhancement was reported. We also discuss the conditions of single harmonic enhancement at strong excita- tion of the plasma by femtosecond radiation and using the ultraviolet laser.