Near-unity asymmetric transmission of linearly polarized light with high Q-factor based on the coherent coupling of dual resonances in bilayer dielectric chiral metasurfaces

This report shows that near-unity asymmetric transmission (AT) of linearly polarized light with high Q-factor can be realized in bilayer dielectric chiral metasurfaces, the unit cell of which consists of silicon slab with rectangular holes on its both sides which are arranged perpendicularly to each other. The preceding work has revealed that the theoretical upper limit of the AT is given as the amplitude reflection coefficient of background system without nanostructure, when a single resonance mainly contributes. If double resonances with overlapping spectral shapes concurrently contribute to the AT, however, their coherent coupling enables breaking the above upper limit and near-unity AT can be obtained. As one of the contributing resonances in the metasurface is quasi-BIC, the resultant AT has also high Q-factor in addition to the near-unity value. Thus, the presented photonic system exhibits highly efficient asymmetric third-harmonic generation behavior, which has the efficiency ratio between forward and backward pumping in the orders from several hundreds to thousands. High-Q near-unity AT shown in this work will find practical applications for direction-dependent nonlinear frequency conversion, information processing, optical sensing, and polarization multiplexing.