# Collisionless absorption of short laser pulses in a deuterium cluster: dependence of redshift of resonance absorption peak on laser polarization, intensity and wavelength

## TL;DR

This study uses molecular dynamics simulations to analyze how laser pulse absorption in deuterium clusters depends on wavelength, intensity, and polarization, revealing a red-shifted resonance peak and the role of dynamic resonance phenomena.

## Contribution

It demonstrates that the resonance absorption peak shifts with intensity and polarization, and clarifies the role of dynamic Mie-resonance in collisionless laser-cluster interactions.

## Key findings

- Maximum absorption occurs away from static Mie-resonance wavelength.
- Resonance peak shifts to higher wavelengths with increasing intensity.
- Absorption saturates at 100% outer ionization, causing the resonance peak to disappear.

## Abstract

We study collisionless absorption of short laser pulses of various intensity, wavelength ($\lambda$) and polarization in a deuterium cluster using molecular dynamics (MD) simulation. For a given laser energy and a pulse duration $\approx$ 5-fs (fwhm), it is found that maximum laser absorption does not happen at the welknown static Mie-resonance or linear resonance (LR) wavelength of $\lambdaM\approx 263$~nm (for deuterium cluster) irrespective of linear polarization (LP) and circular polarization (CP) state of laser. As the laser intensity increases, the absorption peak is gradually red-shifted to a higher $\lambda$ in the marginally over-dense regime of $\lambda\!\! \approx \!\!(1\!\!-\!\!1.5)\lambdaM$ from the expected static~$\lambdaM$ owing to gradual outer ionization and cluster expansion; and above an intensity the resonance absorption peak disappears (sometimes followed by {\em even} a growth of absorption) when outer ionization saturates at 100\% for both LP and CP. This disappearance of the resonance absorption peak should not be misinterpreted as the negligible (or no) role of Mie-resonance. In fact, in this marginally over-dense band of $\lambda\!\! \approx \!\!(1\!\!-\!\!1.5)\lambdaM$, some electrons undergo dynamic Mie-resonance (dynamic LR) and others anharmonic resonance when they are freed. It is also found that before the absorption peak, laser absorption due to LP and CP lasers are almost equally efficient (CP case being inappreciably higher than LP) for all intensities and $\lambda$. However, after the absorption peak, at lower intensities, absorption due to LP inappreciably dominates absorption due to CP with increasing~$\lambda$ which gradually reverses at higher intensities. MD results are also supported by a naive rigid sphere model of cluster.