Phonon drag as a mechanism of delayed terahertz response of metals

TL;DR

This paper demonstrates that electron drag by nonequilibrium phonons explains the waveform and spectrum of terahertz pulses generated during femtosecond laser irradiation of metals, revealing a picosecond delay and nonlinear effects at high fluences.

Contribution

It introduces a phonon drag mechanism as a key factor in terahertz pulse generation, contrasting with previous models and linking waveform features to ultrafast lattice dynamics.

Findings

Electron drag by nonequilibrium phonons matches terahertz pulse waveform and spectrum.

A picosecond delay in drag force development due to lattice heating and phonon lifetime.

High fluence induces a deformation wave that nonlinearly enhances the terahertz response.

Abstract

We show that electron drag by nonequilibrium phonons describes the actual waveform and spectrum of terahertz pulses generated during femtosecond laser irradiation of metals. In contrast to previous models, there is a picosecond delay in the drag force development due to the relatively slow lattice heating and finite phonon lifetime. We also predict that, at high pump fluences, a macroscopic deformation wave enhances nonlinearly the drag force and terahertz response. Our results establish the terahertz pulse waveform as a direct probe of ultrafast lattice dynamics in metals.