Influence of ablated and tunneled electrons on quasi-phase-matched high-order-harmonic generation in laser-produced plasma

Studies of the quasi-phase-matched harmonics generated in spatially modulated silver plasma are presented. We analyze the influence of the driving and heating pulse energies on the enhancement and spectral tuning of the quasi-phase-matched harmonics and show the tuning of those enhanced groups of harmonics by tilting the multislit mask placed in front of the ablated targets. Calculations performed in the laser-produced plasmas demonstrate the joint influence of the electrons that appeared during target ablation and tunnel ionization on the tuning of the quasi-phase-matching conditions in the plasmas.

Figure 1

(a) Experimental scheme for the harmonic generation in the multijet plasmas. FDP, femtosecond driving pulse; PHP, picosecond heating pulse; SL, spherical lens; CL, cylindrical lens; MSM, multislit mask; VC, vacuum chamber; T, target; MJP, multijet plasma; S, slit; XUVS, extreme ultraviolet spectrometer; CM, cylindrical mirror; FFG, flat field grating; MCP, microchannel plate detector; CCD, charge-coupled device camera. (b) Images of the extended imperforated (upper panel) and eight-jet (bottom panel) plasma formations produced on the surface of the silver target. The number of jets was increased and the sizes of jets were decreased by tilting the MSM.

Figure 2

Harmonic spectra generated in different multijet silver plasmas produced by tilting the MSM. Upper panel shows the harmonic spectrum generated in the 5-mm Ag plasma.

Figure 3

Variation of the spectral and spatial shapes of the harmonics generated in the extended and multijet silver plasmas using the (a) 1.85-mJ and (b) 5-mJ driving pulses. Insets in (a) and (b) show the far-field images of the 37th harmonic in the cases of (1) multijet and (2) extended imperforated plasmas. One can see the equal divergences of the harmonics generated in the extended and perforated plasmas in the case of the 1.85-mJ driving pulse, while in the case of the 5-mJ pulse, the harmonic beam from the extended plasma became more divergent and strongly modulated compared with the one generated from the eight-jet plasma.

Figure 4

Variations of maximally enhanced harmonic order at different energies of heating (filled squares) and driving (empty circles) pulses. One can see that the threefold growth of driving pulse energy (from 1.85 to 5 mJ) did not significantly change the $q_{\mathrm{qpm}}$. In the meantime, the $q_{\mathrm{qpm}}$ was strongly dependent on the variation of the heating pulse energy. Solid and dotted lines are inserted for better viewing of the influence of heating and driving pulse energies on the QPM conditions.

Figure 5

Time evolution of pulse envelope and fractional populations for a medium initially composed of $\mathrm{A}{\mathrm{g}}^{+}$ ions. In this case harmonics are produced mainly by $\mathrm{A}{\mathrm{g}}^{+}$ in the leading part of the driving pulse.

Figure 6

Spatial $(r,z)$ maps of harmonic orders H25 (plateau) and H45 (cutoff). Parameters: $I_{\mathrm{dp}}=5\times {10}^{14}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2},p=1\mathrm{Torr}(3.3\times {10}^{16}\mathrm{c}{\mathrm{m}}^{-3}),\mathrm{A}{\mathrm{g}}^{+}$. Left column: no ablated electrons are present; right column: all ablated electrons are included.

Figure 7

Spatial $(r,z)$ maps of harmonic orders H25 (plateau) and H45 (cutoff). Parameters: $I_{\mathrm{dp}}=5\times {10}^{14}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2},p=2\mathrm{Torr}(6.6\times {10}^{16}\mathrm{c}{\mathrm{m}}^{-3}),\mathrm{A}{\mathrm{g}}^{+}$. Left column: no ablated electrons are present; right column: all ablated electrons are included.

Figure 8

Spatial $(r,z)$ maps of harmonic H35. Parameters: $I_{\mathrm{dp}}=3\times {10}^{14}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}$ (two upper panels) and $5\times {10}^{14}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}$ (two bottom panels), $p=2\mathrm{Torr}(6.6\times {10}^{16}\mathrm{c}{\mathrm{m}}^{-3}),\mathrm{A}{\mathrm{g}}^{+}$. Left column: no ablated electrons are present; right column: all ablated electrons are included.