Tunable quasi-bound states in the continuum in magneto-optical metasurfaces

The enhancement of electromagnetic field with high Q factor in metasurfaces has attracted extensive attention of researchers. Magneto-optical metasurfaces (MOMS) provide approaches with controllable magnetic fields to modulate the optical response, which contributes towards the magneto-optical Kerr effect in nanophotonic devices. However, it is challenging for MOMS to obtain narrow spectra peak with high Q factors. Here, we propose MOMS-supported quasi-bound states in the continuum (quasi-BIC) and theoretically investigate the evolution process from BIC to quasi-BIC. By applying an external magnetic field, the BIC in our proposed metasurface can be transformed into a quasi-BIC with finite Q factor up to 33 620. Meanwhile, the quasi-BIC shows magnetization-related circular dichroism with a vortex point in the polarization graph, based on time-reversal symmetry breaking in the magneto-optical material under the condition of external magnetic field. The quasi-BIC also maintains nearly unity reflection and almost zero absorption, showing a promising future in sensing. Our results enrich the light control mechanisms of MOMS and provide a unique opportunity for applications requiring flexible tunability and high Q factors, such as sensors, laser sources, filters and chiral-related elements.