High‐Q Chiral Janus Metasurfaces Based on Multipolar Resonances

Optical chiral metasurfaces provide distinct responses to circularly polarized light of different handedness, which hold great promise for practical applications in chiral lasers, biomedical sensors and drug development, etc. Recently, some fascinating Janus metasurfaces featuring asymmetric chiral transmission/reflection are demonstrated to boost the information capacity and enable efficient optical applications. However, the quality factor (Q‐factor) of the realized Janus chiral metasurfaces so far is relatively low due to substantial radiative loss under broken symmetry or the nonradiative loss inherent to the plasmonic structures. Apart from that, the realized asymmetric circular dichroism functionality is often limited. To address the issue, a multifunctional all‐dielectric chiral Janus metasurface supporting high‐Q multipolar resonances is proposed. By controlling both the in‐plane and out‐of‐plane symmetry of the metasurface, tunable high Q‐factor multipolar resonances with controllable far‐field polarization states can be achieved. This study showcases that for both top‐down and bottom‐up excitations, the high‐Q (Q > 3000) metasurface exhibits symmetric and maximum intrinsic chiroptical responses, and simultaneous high‐contrast asymmetric extrinsic chiroptical responses. Moreover, the near‐field unbalanced chiral responses of the metasurface also contribute to enhanced and unidirectional third‐harmonic generation. The study may offer a broadly applicable strategy for the design of high‐Q multifunctional spin‐selective optical devices.