Multi-wavelength excellent circular dichroism with tunable Q-factor in magneto-optical crystals

The strong circular dichroism (CD) of chiral structures is essential in applications such as sensing, polarization manipulation, and imaging. However, to achieve strong CD for practical applications in most chiral metamaterials and metasurfaces, the symmetry of the structure must be deliberately broken, which greatly increases the complexity and process errors of micro-nano fabrication. Here, we achieve the multi-wavelength excellent CD governed by a pair of chiral bound states in the continuum (BICs) and a pair of chiral quasi-BICs in a magneto-optical photonic crystal (MO PhC) slab without structural symmetry breaking. By applying an external magnetic field instead of breaking the structural symmetry, both the chiral BICs and the quasi-BICs exhibit high Q-factors and CD values, with the Q-factors of up to 10⁶ and 10³ and the maximum CD values of up to 0.98 and 0.99, respectively. Furthermore, the results show that the Q-factor of the chiral BICs/quasi-BICs perfectly aligns with a linear function relationship with the external magnetic field, thereby enabling the tunable quantitative Q-factor. The performance of the proposed MO PhC at four wavelengths is comparable to that of traditional single-wavelength chiral structures, giving it significant prospects for various applications, including optical security, chiral lasers, and chiral optoelectronic devices.