Ultrafast broadband optical spectroscopy for quantifying subpicometric coherent atomic displacements in $WTe_2$

TL;DR

This paper demonstrates a method using broadband optical spectroscopy to measure atomic displacements with femtometer precision, validated on WTe2, and applicable to various materials with ultrafast lattice dynamics.

Contribution

The authors introduce a parameter-free approach to quantify coherent phonon amplitudes with femtometer accuracy using optical spectroscopy and density functional theory comparison.

Findings

Successfully measured phonon oscillation amplitudes in WTe2.

Identified spectral fingerprints of two optical phonons at 8 and 80 cm^-1.

Method can be extended to other materials with ultrafast lattice responses.

Abstract

Here we show how time-resolved broadband optical spectroscopy can be used to quantify, with femtometer resolution, the oscillation amplitudes of coherent phonons through a displacive model without free tuning parameters, except an overall scaling factor determined by comparison between experimental data and density functional theory calculations. $WTe_2$ is used to benchmark this approach. In this semimetal, the response is anisotropic and provides the spectral fingerprints of two $A_1$ optical phonons at $\sim$8 and $\sim$80 $cm^-$$^1$. In principle, this methodology can be extended to any material in which an ultrafast excitation triggers coherent lattice modes modulating the high-energy optical properties.