Ultrafast Dynamics of Chiral Phonons in Monolayer Semiconductors

The optical and valleytronic properties of monolayer transition metal dichalcogenide semiconductors strongly interact with phonons. Chiral phonons that break time-reversal symmetry are predicted to modify the excitons and spins in these materials. Here we measured the ultrafast dynamics of linearly and circularly polarized phonons at the Brillouin zone center in single-crystalline monolayer WS₂. The phonons were excited by intense, resonant, and polarization-tunable terahertz pulses, and probed by time-resolved anti-Stokes Raman spectroscopy. We separated the coherent phonons, which emit coherent sum-frequency generation, from the incoherent phonon population, which are detected by incoherent scattering. We found the incoherent population lifetime is much longer than what was expected from coherence lifetime, indicating that inhomogeneous broadening and momentum scattering play important roles in phonon decoherence at room temperature. Meanwhile, we found a faster depolarization rate in circular bases than in linear bases, suggesting that lattice anisotropy makes linearly polarized modes the phonon eigenstates. Our results provide crucial information for improving the lifetime of chiral phonons and potentially facilitate dynamic magnetic control in two-dimensional materials .