Ultrafast Laser-Induced Electronic Dynamics and Piezo-Phototronic Response in Janus XMMX′ Monolayers (X/X′ = S, Se, and M = Ga, In)

The electronic and lattice dynamics in response to ultrafast laser excitation in Janus monolayers is one of the vital factors for designing highly efficient electronic and optoelectronic devices. Here, femtosecond excitation dynamics in Janus XMMX′ monolayers (X/X′ = S, Se, and M = Ga, In) is unveiled by using real-time time-dependent density-functional theory with Ehrenfest dynamics, which offers an optimal trade-off between accuracy and computational costs. The results demonstrate that the ultrafast laser excitation with the polarization perpendicular to monolayer planes triggers the extremely intense out-of-plane oscillation of geometric distortion in Ga2STe and Ga2SeTe. The origin of these oscillations is attributed to the motions of vibrational modes by strong electron–phonon interactions on excited states rather than by heating on the ground states, and the corresponding reorganization energies and frequencies are obtained by fitting the time evolution of lattice kinetic energies. Most significantly, the ultrafast lattice deformation generates an ultrafast-responded out-of-plane piezoelectric field, and this field can be manipulated by using laser excitation energies. The findings presented here illustrate novel avenues for ultrafast optomechanical control and strain engineering within Janus monolayers.