Plasma Formation Dynamics in Intense Laser-Droplet Interaction

TL;DR

This paper investigates the complex ionization dynamics in intense laser-droplet interactions, revealing how localized fields penetrate overdense plasmas and create highly inhomogeneous charge distributions, using advanced 3D simulations.

Contribution

It introduces a detailed analysis of nonlinear ionization processes in laser-droplet interactions, highlighting the role of local field enhancements and deriving a self-similar ionization model.

Findings

Electric fields penetrate overdense plasma under certain angles.

Charge density becomes highly inhomogeneous inside the droplet.

A self-similar exponential fit describes ionization degree based on key parameters.

Abstract

We study the ionization dynamics in intense laser-droplet interaction using three-dimensional, relativistic particle-in-cell simulations. Of particular interest is the laser intensity and frequency regime for which initially transparent, wavelength-sized targets are not homogeneously ionized. Instead, the charge distribution changes both in space and in time on a sub-cycle scale. One may call this the extreme nonlinear Mie-optics regime. We find that - despite the fact that the plasma created at the droplet surface is overdense - oscillating electric fields may penetrate into the droplet under a certain angle, ionize, and propagate in the just generated plasma. This effect can be attributed to the local field enhancements at the droplet surface predicted by standard Mie theory. The penetration of the fields into the droplet leads to the formation of a highly inhomogeneous charge density distribution in the droplet interior, concentrated mostly in the polarization plane. We present a self-similar, exponential fit of the fractional ionization degree which depends only on a dimensionless combination of electric field amplitude, droplet radius, and plasma frequency with only a weak dependence on the laser frequency in the overdense regime.