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Ultrafast non-thermal control of quantum materials has gained growing interest over decades. Contrary to the conventional knowledge that the photoexcitation causes heating of materials and destroys the low temperature ordered phases, recent developments of ultrafast light sources have shown the possibility of creating symmetry-broken ordered phases before the system reaches thermal equilibrium state. As a new route for such a light-induced phase transition, we have investigated the effect of strong excitation of amplitude mode in a charge density wave (CDW) phase in a layered transition-metal dichalcogenide compound, 3R-Ta1+xSe21. A soft phonon mode associated with the CDW phase transition, namely the amplitude mode, is identified at 2.3 THz at the lowest temperature through the optical pump and optical probe experiments. When this amplitude mode is coherently driven by an intense THz pulse through the two-photon excitation process, a dynamical suppression of the CDW order is manifested by the mode softening of the CDW amplitude mode with intense THz excitation. Furthermore, a gap opening is observed in the THz-frequency optical conductivity spectrum, indicating that an insulating-like metastable state is induced by the amplitude mode excitation. The formation dynamics of the gap synchronizes with the oscillation of CDW amplitude mode, which indicates the intimate interplay between the order parameters of the equilibrium CDW and the induced metastable hidden state. In this presentation, we overview the above results which have been recently published in Ref.1.
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