High-Order Harmonics Generation in Selenium-Containing Plasmas

The studies of the high-order harmonics generated in Se-containing plasmas are reported. The ablation of selenium in a vacuum allowed for the formation of a plasma demonstrating high-order harmonics generation and resonance enhancement of the harmonic at the shortest wavelength reported so far (λ ≈ 22.9 nm, E_ph ≈ 54.14 eV). This harmonic corresponds to the 35th order of the 800-nm-class lasers. The influence of the presence of selenium in the molecular state (ZnSe and HgSe) on the suppression of the resonance effect during harmonics generation in plasma is studied. The enhanced 35th harmonic was analyzed by different methods of plasma formation using nanosecond, picosecond, and femtosecond pulses. The enhancement factor of the resonance-enhanced harmonic was measured to be 32× compared with the neighboring lower-order harmonics in the case of the picosecond-pulses-induced Se plasma. The role of the strong ionic transition of Se in the region of 22.7 nm in the observation of the resonance-induced enhancement of a single harmonic is discussed.