Systematic studies of two-color pump-induced high-order harmonic generation in plasma plumes

High-order harmonic generation (HHG) has been studied in various laser-produced plasma plumes using a two-color orthogonally polarized beam with a 12:1 energy ratio between the fundamental and second-harmonic (SH) components. The influence of the relative phase between the fundamental and SH waves on the HHG efficiency has been investigated. Odd and even harmonic generation in plasma plumes containing nanoparticles, fullerenes, carbon nanotubes, and other samples was optimized. The effect of the variation in the SH intensity on the HHG conversion efficiency in carbon aerogel and silver plasma plumes has also been studied. It is shown that by increasing the SH intensity, one can generate only even harmonics by suppressing the odd harmonics.

Figure 1

Experimental setup: FL, focusing lenses; SHC, second-harmonic crystal; T, target; S, slit; CM, cylindrical mirror; FFG, flat field grating; ZOS, zero-order beam stop; MCP, microchannel plate; CCD, charge coupled device.

Figure 2

(a) Harmonic spectra generated in fullerene plasma. Thin and thick curves show the harmonic spectra in the cases of single-color and two-color pumps, respectively. (b) Harmonics generated in carbon nanotube plasma for negatively chirped 100 fs pulses (thick curve) and positively chirped 150 fs pulses (thin curve). (c) Harmonic spectra obtained from the Au nanoparticle-containing plasma for (1) two-color pump and (2) single-color pump. The experimental conditions are described in the text.

Figure 3

Harmonic spectra for two-color pump (thick curve) and single-color pump (thin curve) schemes of harmonic generation in the silver plasma. One can see that at certain phase and intensity relation between the pump waves, the high-order odd harmonics considerably decrease in intensity compared to the even harmonics. These experimental conditions are described in the text.

Figure 4

Variation of the two-color pump plasma HHG harmonic spectra with the growth of SH pump intensity for (a) silver plasma and (b) carbon aerogel plasma (thin and thick curves correspond to the energies of SH pulse of 52 and 220 arb. units, respectively). The curves in (a) are vertically shifted toward each other for better visibility.

Figure 5

Variation of the harmonic spectra by rotation of the BBO crystal along the non-phase-matching axis for (a) Ag plasma (thick and thin curves correspond to $0^{°}$ and $1^{°}$ rotations of the BBO crystal), and (b) ${\mathrm{B}}_4$C plasma (thick and thin curves correspond to $0^{°}$ and $7^{°}$ rotations of the BBO crystal).