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. WTe2 is used to benchmark this approach. In this semimetal, the response is anisotropic and provides the spectral fingerprints of two A1 optical phonons at ≈8 and ≈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.
