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1T-VSe₂ undergoes a charge density wave (CDW) transition at 110 K which results in gaps opening around the Fermi surface together with a 4a x 4a lattice distortion, and the appearance of exotic collective modes [1]. In equilibrium, the CDW state has been associated with both electronic correlations (i.e. nesting) [2] and electron-phonon interactions [3]. Here, we use ultrafast optical spectroscopy to simultaneously track the out-of-equilibrium electron and phonon dynamics across the CDW transition. At 4.3 K, we observe well-resolved coherent oscillations of the CDW amplitude mode (~1.4 THz) triggered by the optical excitation. Upon increasing laser fluence, a photoinduced phase transition occurs from the CDW to normal phase structure, evidenced by a switching of the dominant oscillation frequency from ~1.4 to ~6.2 THz, and a dramatic change in the carrier dynamics. A temperature-dependent study (4.3 – 295 K) allows us to compare the characteristics of the photoinduced transition with the thermal transition. Our results are supported by theoretical calculations of the phonon dispersion and the stability of the CDW structure under photoexcitation.

[1] X. Feng et al. PRB 104, 165134 (2021).

[2] V. N. Strocov et al. PRL 109, 086401 (2012).

[3] J. Diego et al. Nat. Comms. 12, 598 (2021).