Coherent optical response driven by non-equilibrium electron-phonon dynamics in a layered transition-metal dichalcogenide

Layered transition-metal dichalcogenides (TMDs) are model systems to explore ultrafast many-body interactions and various nonlinear optical phenomena. For the application of TMD-based optoelectronic devices capable of ultrafast response, it is essential to understand how characteristic electron-hole and electron-phonon couplings modify ultrafast electronic and optical properties under photoexcitation. Here, we investigate the sub-picosecond optical responses of layered semiconductor 2H-MoTe₂ in the presence of an electron-hole (e-h) plasma and a long-lived coherent phonon. Transient reflectivity measurements depending on photon energy reveal that the optical response for short-time delays ( < 1 p s ) was significantly modified by band-gap renormalization and state filling due to the presence of the e-h plasma. Furthermore, octave, sum, and difference phonon frequencies transiently appeared for the early time delays ( < 2 p s ) . The emergent multiple phonon frequencies can be described as higher-order optical modulations due to deformation-potential electron-phonon coupling under resonant photoexcitation conditions. This work provides comprehensive insights into fundamental physics and the application of non-equilibrium quasiparticle generations on TMDs under time-periodic phonon driving forces.