Mirror Symmetry Triggered Chiral Phonon Behavior and Phonon Mode‐Dependent Magneto‐Optical Modulation in 2D Space‐ and Time‐Symmetric AgCrP 2 S 6

Abstract Phonons in 2D materials offer significant potential for magneto‐optical applications. However, chiral phonon behavior in both space‐ and time‐symmetric systems, along with its magnetic control and distinction from achiral modes, remains poorly explored. Using space‐ and time‐symmetric AgCrP 2 S 6 as a model system, distinct anisotropies in A 𝑔 and B 𝑔 phonon modes are observed, governed by lattice symmetry. Importantly, B 𝑔 modes possess chiral phonon behavior at zero magnetic field, attributed to a half‐wave plate‐analogous effect under mirror symmetry ( m = −1), as predicted theoretically. Moreover, magneto‐Raman spectroscopy reveals that under a magnetic field from 0 to 5T, the degree of linear polarization (DOLP) of the mode decreases drastically from ≈90% to 0%, with a magnetic field‐dependent rate reaching ≈40%/T; The degree of circular polarization (DOCP) of the mode changes by ≈40%, while the mode varies by only ≈6%. These gigantic mode‐dependent magneto‐optical modulations arise from the different couplings/interactions between the magnetic field and phonons with chiral/achiral characters. These findings not only underscore the significance of mirror symmetry in inducing chiral phonon behavior, but also demonstrate the high efficiency and sensitivity of Raman scattering modulation by magnetic fields, establishing AgCrP 2 S 6 as a promising platform for future magneto‐optical device applications.