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In this study, the optical response to a terahertz pulse was investigated inthe transition metal chalcogenide Ta$_2$NiSe$_5$, a candidate excitonicinsulator. First, by irradiating a terahertz pulse with a relatively weakelectric field (0.3 MV/cm), the spectral changes in reflectivity near theabsorption edge due to third-order optical nonlinearity were measured and theabsorption peak characteristic of the excitonic phase just below the interbandtransition was identified. Next, by irradiating a strong terahertz pulse with astrong electric field of 1.65 MV/cm, the absorption of the excitonic phase wasfound to be reduced, and a Drude-like response appeared in the mid-infraredregion. These responses can be interpreted as carrier generation by excitondissociation induced by the electric field, resulting in the partial melting ofthe excitonic phase and metallization. The presence of a distinct thresholdelectric field for carrier generation indicates exciton dissociation viaquantum-tunnelling processes. The spectral change due to metallization by theelectric field is significantly different from that due to the strong opticalexcitation across the gap, which can be explained by the different meltingmechanisms of the excitonic phase in the two types of excitations.

Melting of excitonic insulator phase by an intense terahertz pulse in Ta$_2$NiSe$_5$