Wavelength-dependent reflectivity changes on gold at elevated electronic temperatures

TL;DR

This study models how ultrashort laser pulses alter gold's optical reflectivity across a broad spectrum by analyzing electronic temperature effects on electronic structure and transitions, aiding understanding of laser-material interactions.

Contribution

It combines first-principles calculations with a two-temperature model to simulate temperature-dependent reflectivity changes in gold during laser excitation.

Findings

Reflectivity changes over 0.9-6.4 eV are explained by electronic temperature effects.

Broadening and shifting of electronic states influence reflectivity.

The model captures the out-of-equilibrium optical response during laser excitation.

Abstract

Upon the excitation by an ultrashort laser pulse the conditions in a material can drastically change, altering its optical properties and therefore the relative amount of absorbed energy, a quan- tity relevant for determining the damage threshold and for developing a detailed simulation of a structuring process. The subject of interest in this work is the d-band metal gold which has an absorption edge marking the transition of free valence electrons and an absorbing deep d-band with bound electrons. Reflectivity changes are observed in experiment over a broad spectral range at ablation conditions. To understand the involved processes the laser excitation is modeled by a com- bination of first principle calculations with a two-temperature model. The description is kept most general and applied to realistically simulate the transfer of the absorbed energy of a Gaussian laser pulse into the electronic system at every point in space at every instance of time. An electronic temperature-dependent reflectivity map is calculated, describing the out of equilibrium reflectivity during laser excitation for photon energies from 0.9 - 6.4 eV, including inter- and intra-band transi- tions and a temperature-dependent damping factor. The main mechanisms are identified explaining the electronic temperature-dependent change in reflectivity: broadening of the edge of the occu- pied/unoccupied states around the chemical potential $\mu$, also leading to a shift of the $\mu$ and an increase of the collision rate of free s/p-band electrons with bound d-band holes.