# Evidence of non collisional femtosecond laser electron heating in dielectric materials

## TL;DR

This study reveals non-collisional femtosecond laser electron heating in dielectric crystals, showing electrons can reach high energies through multiphoton transitions without collisions, challenging traditional models.

## Contribution

It introduces a theoretical model incorporating non-collisional multiphoton processes to explain high-energy electron generation in dielectrics under femtosecond laser pulses.

## Key findings

- Electrons exceed 40-50 eV energy levels at intensities below ablation threshold.
- Non-collisional multiphoton transitions significantly contribute to electron heating.
- Bulk electron heating and laser field acceleration both influence electron energy gain.

## Abstract

Electron dynamics in the bulk of large band gap dielectric crystals induced by intense femtosecond laser pulses at 800 nm is studied. With laser intensities under the ablation threshold (a few 10 TW/cm\textsuperscript{2}), electrons with unexpected energies in excess of 40-50 eV are observed by using the photoemission spectroscopy. A theoretical approach based on the Boltzmann kinetic equation including state-of-the-art modeling for various particles interactions is developed to interpret these experimental observations. A direct comparison shows that both electron heating in the bulk and a further laser field acceleration after ejection from the material contribute equivalently to the final electron energy gain. The electron heating in the bulk is shown to be significantly driven by a non-collisional process, i.e. direct multiphoton transitions between sub-bands of the conduction band. This work also sheds light on the contribution of the standard electron excitation/relaxation collisional processes, providing a new baseline to study the electron dynamics in dielectric materials and associated applications as laser material structuring.