Ultrafast transport mediated homogenization of photoexcited electrons governs the softening of the $A_\mathrm{1g}$ phonon in bismuth

TL;DR

This study investigates how ultrafast hot carrier transport in bismuth films influences phonon behavior, revealing that carrier dynamics are critical for understanding energy dissipation and phonon softening in photoexcited materials.

Contribution

It demonstrates that ultrafast hot carrier transport causes homogeneous excitation in thin bismuth films, affecting phonon dynamics and energy dissipation mechanisms.

Findings

Ultrafast hot carrier transport leads to homogeneous excitation in films up to 50 nm.

Carrier penetration depth is approximately 60 nm, independent of laser energy.

The phonon frequency redshift correlates with carrier distribution and excitation mechanisms.

Abstract

In order to determine the role of non-thermal transport of hot carriers which is decisive for the dissipation of energy in condensed matter we performed time-resolved broadband femtosecond transient reflectivity measurements on $7-197 \mathrm{nm}$ thick Bi(111) films epitaxially grown on Si(111). We monitored the behavior of the Fourier amplitude and the central frequency of the coherent $A_\mathrm{1g}$ phonon mode as function of the incident fluence, film thickness, and probe wavelength in the range of $580 -700 \mathrm{nm}$. The frequency redshift that follows photoexcitation was used as a robust quantity to determine the effective distribution of excited carriers that governs the displacive excitation mechanism of coherent $A_\mathrm{1g}$ phonons in Bi. For Bi films up to $50 \mathrm{nm}$ thickness a homogeneous excitation due to the ultrafast transport of hot charge carriers is observed, limited by a carrier penetration depth of $60 \mathrm{nm}$ independent of the totally deposited laser energy.