THz Excitation of Coherent Chiral Phonon in a chiral material

Chiral phonons, which carry intrinsic angular momentum, have emerged as a promising new platform for controlling material properties. However, the direct and real-time observation of their coupling to electron spins, a cornerstone for phonon-driven spintronics, remains a significant challenge. Here, we employ intense circularly-polarized terahertz (THz) pulses to excite chiral phonons in the prototypical chiral semiconductor HgS coherently. Time-resolved optical probing with polarization analysis reveals two distinct signals: (1) an odd signal under THz field reversal, exhibiting coherent oscillations that unambiguously signature the chiral phonon mode, and (2) a non-oscillatory even signal which emerges simultaneously. The helicity dependence of the even signal is measured and will be discussed in the context of the magneto-optic Kerr effect as potential evidence for transient spin polarization. Our work provides compelling evidence of ultrafast angular momentum transfer from chiral phonons, suggesting a novel pathway for controlling optical spin.