Femtosecond laser-induced sub-wavelength plasma inside dielectrics: I. Field enhancement

TL;DR

This study investigates femtosecond laser-induced sub-wavelength plasma inside dielectrics, analyzing field enhancement and absorption mechanisms through simulations and experiments, revealing how plasma density profiles influence energy absorption and plasma behavior.

Contribution

It provides a detailed analysis of plasma formation and absorption mechanisms using PIC simulations and experiments, highlighting the effects of different plasma density profiles.

Findings

Good agreement between simulations and experiments on absorption and field distribution.

Resonance absorption occurs in Gaussian plasma profiles.

Ambipolar fields and electron sound waves are observed at plasma surfaces.

Abstract

The creation of high energy density ($\gtrsim10^6$ joules per cm$^3$) over-critical plasmas in a large volume has essential applications in the study of warm dense matter, being present in the hot cores of stars and planets. It was recently shown that femtosecond Bessel beams enable creating over-critical plasmas inside sapphire with sub-wavelength radius and several tens of micrometers in length. Here, the dependence of field structure and absorption mechanism on the plasma density transverse profile are investigated by performing self-consistent Particle-In-Cell (PIC) simulations. Two { limiting} cases are considered: one is a homogeneous step-like profile, that can sustain plasmon formation, the second is an inhomogeneous Gaussian profile, where resonance absorption occurs. Comparing experimental absorption measures to analytical predictions allows determining the plasma parameters used in PIC simulations. The PIC simulation results are in good agreement with experimental diagnostics of total absorption, near-field fluence distribution, and far-field radiation pattern. We show that in each case an ambipolar field forms at the plasma surface due to the expansion of the hot electrons and that electron sound waves propagate into the over-critical region.