Observation of Ion-wave Satellites to Laser Harmonics in Intense Picosecond Laser-Solid Interaction

TL;DR

Observes regular red- and blue-shifted satellites accompanying high-order harmonics from ultra-intense laser–solid interaction with near-solid density targets at the critical density $n_c$. The satellites appear at a threshold intensity and shift by a nearly constant amount across harmonics, suggesting inelastic scattering from ion waves near the critical surface. The authors attribute the effect to strong electron quiver motion partially suppressing Debye shielding, which anisotropically increases the Debye length and modifies ion-wave dispersion toward the ion-plasma regime, yielding frequencies near $0.6\,\\omega_{p,i}$. This work links Debye shielding dynamics under intense fields to observable spectral satellites and has broad implications for plasma-wave physics near $n_c$.

Abstract

Detailed spectra of harmonics produced from ultra-intense, sub-picosecond, high-contrast laser pulses incident on solid targets have shown the first observation of regular red- and blue-shifted satellites. Their frequency shift is slightly less than the frequency of a nominal, pure ion-plasma wave associated with electron critical density, where an ion-acoustic wave would be expected. We explain this as the result of a substantial reduction of Debye shielding as the intense optical fields compete to drive electrons in large-amplitude oscillations. This general effect leads to a larger, dynamical and anisotropic Debye length, which should have a broad impact on plasma physics in this regime.

Figures (4)

• Figure 1: Experimental setup: The $f=23$cm off-axis parabola focusses the 527-nm laser beam at 60$^{\circ}$ incidence, producing a 9 $\mu$m (FWHM) focal spot. Specular harmonic spectra are directed into a 1-m VUV spectrometer, using a slit-less configuration, and recorded using a microchannel plate intensifier and 16-bit CCD camera.
• Figure 2: A typical time-integrated harmonic spectrum (composite) from Si targets, I = $3.2\times10^{17}~\text{W/cm}^{2}$.
• Figure 3: Detailed harmonic spectra at intensities between $4.5\times10^{16}~\text{W/cm}^{2}$ and $6.8\times10^{17}~\text{W/cm}^{2}$, showing the appearance of regular and reproducible Stokes and anti-Stokes satellites to the harmonics. Satellites appear on all observed harmonics, and have the same frequency-shift for all harmonics; the measured shifs suggest a slight dependence on target atomic-number.
• Figure 4: Detailed spectra of the specular-scattered laser fundamental, at intensities between $3\times10^{16}~\text{W/cm}^{2}$ and $1\times10^{18}~\text{W/cm}^{2}$, showing the sudden depletion of pump-laser reflected light coinciding precisely with the appearance of satellites.