Resonance enhancement of harmonics in the vicinity of 32 nm spectral range during propagation of femtosecond pulses through the molybdenum plasma

Abstract

Abstract We demonstrate the enhancement of harmonics in the vicinity of 32 nm while generating them in the Mo laser-produced plasma. The enhanced harmonics centered at around the 25th harmonic of 806 nm radiation were analyzed by different means (delay between heating and driving pulses, dependences of harmonic yield on the driving and heating pulse intensities and durations, different regimes of plasma formation, two-color pump of plasma, application of chirped pulses, etc). The role of strong ionic transition possessing large oscillator strength leading to the growth of the nonlinear optical response is discussed. Comparative studies using molybdenum, silver, and chromium laser-produced plasmas are presented. To validate the experimental findings, we numerically simulate harmonic generation from Mo using full-dimensional, all-electron, first-principles calculations. The calculated harmonic spectra exhibit a prominent peak around 32 nm and a suppression around 36–38 nm, successfully reproducing the experimentally observed features. Our transition-resolved analyses of the calculation results reveal both that the 4 p -4 d transitions are responsible for the observed enhancement and that the destructive interference between the 4 p -4 d transitions and the recombination to the 4 d orbitals leads to the observed suppression.