We report the first application of broadband time-resolved pump-probe ellipsometry to study the ultrafast dynamics of the photoinduced insulator-to-metal transition (IMT) in vanadium dioxide (VO2) thin films driven by 35 fs laser pulses. This novel technique enables the direct measurement of the time-resolved evolution of the complex pseudodielectric function of VO2 during the IMT. We have identified distinct thermal and nonthermal dynamics in the photoinduced IMT, which critically depends on the pump wavelength and fluence, while providing a detailed temporal and spectral phase map. A comparison of the pseudodielectric function of the VO2 thin film during thermally and photoinduced phase transitions reveals that the primary differences in the IMT pathways occur within the first picosecond after the pump, driven by nonequilibrium dynamics in this ultrafast time scale. The ultrafast spectroscopic ellipsometry introduced in this work offers a complementary probe to study phase changes in condensed matter and emerging photonic device materials.

Departamento de Física Aplicada Universidad de Cantabria Avenida de los Castros s n 39005 Santander Spain

Ecole Centrale de Lyon CNRS INSA Lyon Université Claude Bernard Lyon 1 CPE Lyon CNRS INL UMR5270 Université Lyon 69130 Ecully France

ELI Beamlines Facility The Extreme Light Infrastructure ERIC Za Radnicí 835 25241 Dolní Břežany Czech Republic

ICMATE CNR Istituto di Chimica della Materia Condensata e delle Tecnologie per l'Energia C so Stati Uniti 4 35127 Padova Italy

School of Materials Engineering Purdue University West Lafayette Indiana 47907 United States

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