Anharmonic resonance absorption of short laser pulses in clusters: A molecular dynamics simulation study

TL;DR

This study uses molecular dynamics simulations to confirm that anharmonic resonance (AHR) is a universal, dominant collisionless absorption mechanism for short laser pulses interacting with clusters, bridging previous theoretical and simulation approaches.

Contribution

It provides the first direct molecular dynamics evidence that AHR is a key collisionless absorption process in short laser pulse-cluster interactions, connecting models and simulations.

Findings

AHR occurs only when the electron's position-dependent frequency matches the laser frequency.

Electrons are outer ionized exclusively during AHR events.

The anharmonic oscillator model captures key features of MD electron behavior.

Abstract

Linear resonance (LR) absorption of an intense 800~nm laser light in a nano-cluster requires a long laser pulse > 100~fs when Mie-plasma frequency ($\omegaMie$) of electrons in the expanding cluster matches the laser frequency~($\omega$). For a short duration of the pulse the condition for LR is not satisfied. In this case, it was shown by a model and particle-in-cell (PIC) simulations [Phys. Rev. Lett. 96, 123401 (2006)] that electrons absorb laser energy by anharmonic resonance (AHR) when the position-dependent frequency $\Omega[r(t)]$ of an electron in the self-consistent anharmonic potential of the cluster satisfies $\Omega[r(t)]=\omega$. However, AHR remains to be a debate and still obscure in multi-particle plasma simulations. Here, we identify AHR mechanism in a laser driven cluster using molecular dynamics (MD) simulations. By analyzing the trajectory of each MD electron and extracting its $\Omega[r(t)]$ in the self-generated anharmonic plasma potential it is found that electron is outer ionized {\em only} when AHR is met. An anharmonic oscillator model, introduced here, brings out most of the features of MD electrons while passing the AHR. Thus, we not only bridge the gap between PIC simulations, analytical models and MD calculations for the first time but also unequivocally prove that AHR processes is a universal dominant collisionless mechanism of absorption in the short pulse regime or in the early time of longer pulses in clusters.