Optical drive of amplitude and phase modes in excitonic insulators

Abstract: Motivated by recent interests in exploring excitonic condensate as the ground state of some narrow-bandgap semiconductors such as transition metal dichalcogenides and layered chalcogenide material Ta$_2$NiSe$_5$, in this work we theoretically study the dynamics of condensate in response to periodically driven laser fields with different polarizations and intensities. In particular, we consider laser light beams with bicircular and circular polarizations breaking the time-revesal symmetry, and linear polarization. We show that the amplitude of the condensate oscillates in time during irradiating by light with a magnitude depending on the light intensity. The dynamics survives even after the light is switched off. The phase mode however changes linearly with time for a condensate originating from purely electronic correlations. We further show that in the presence of electron-phonon coupling the linear-in-time behavior is replaced by a harmonically oscillating behavior, a manifestation of gapped phase modes due to relative band charge symmetry breaking. Furthermore, we show that the primarily electronic and primarily lattice cases corresponding to strong and weak electron-phonon coupling, respectively, reveal distinct dynamics of the condensate, an observation which can modify the optical response of an excitonic insulator by stimulating amplitude and phase modes in the former case.