Ultrafast broadband optical spectroscopy for quantifying subpicometric coherent atomic displacements in WTe_{2}

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 ≈8 and ≈80cm^{−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.