Chiral Phonons in α-HgS Nanocrystal

Chiral phonon is a quantized particle that represent collective atomic motions with broken reflection and inversion symmetries. Chiral phonons have gathered significant interest due to their unique properties observed in various types of materials such as two-dimensional (2D) materials, biomolecular complexes, and inorganic chiral crystals [1,2,3]. Here, we observe chiral phonons in mercury sulfide cinnabar (α-HgS) nanocrystals with terahertz time-domain polarimetry (THz-TDP). α-HgS has two enantiomeric crystal structure that belongs to space group P3₁21 or P3₂21 depending on the rotational direction of the helical crystal lattice. By employing phonon dispersion calculations, we identify the IR active phonon modes in α-HgS that lies in the spectral range of 1 ~ 10 meV, specifically the A₂ (1.16 THz) and E modes (1.3 THz), which can be experimentally measured using THz-TDP. Mirror-symmetrically synthesized P/M α-HgS nanocrystals show identical THz absorption (TA) peaks, while their THz circular dichroism (TCD)/THz optical rotary dispersion (TORD) signals exhibit opposite caracteristics, corresponding to the A₂ phonon mode. This clearly proves the existence of chiral phonons in α-HgS nanocrystals. Moreover, it is important to mention that strong TCD signals can be measured despite the substantial disparity in scales between the THz wavelength (~300 μm) and α-HgS nanocrystals (~20 nm). Chiral phonons of α-HgS nanocrystals within the THz range not only provide new insights for THz photonics/phononics, but also bridges the gap between THz photonics and nano-engineering, opening a new door for various applications.