  In this study, the optical response to a terahertz pulse was investigated in
the transition metal chalcogenide Ta$_2$NiSe$_5$, a candidate excitonic
insulator. First, by irradiating a terahertz pulse with a relatively weak
electric field (0.3 MV/cm), the spectral changes in reflectivity near the
absorption edge due to third-order optical nonlinearity were measured and the
absorption peak characteristic of the excitonic phase just below the interband
transition was identified. Next, by irradiating a strong terahertz pulse with a
strong electric field of 1.65 MV/cm, the absorption of the excitonic phase was
found to be reduced, and a Drude-like response appeared in the mid-infrared
region. These responses can be interpreted as carrier generation by exciton
dissociation induced by the electric field, resulting in the partial melting of
the excitonic phase and metallization. The presence of a distinct threshold
electric field for carrier generation indicates exciton dissociation via
quantum-tunnelling processes. The spectral change due to metallization by the
electric field is significantly different from that due to the strong optical
excitation across the gap, which can be explained by the different melting
mechanisms of the excitonic phase in the two types of excitations.
