Photonic Intrinsic Chiral Flatband With Tailorable Quality Factor and Circular Dichroism

Engineering spin-selective light-matter interactions in photonic systems requires precise control of the radiative loss, optical chirality, and dispersion of resonant modes; however, these properties are intricately dependent on the structural parameters, making their independent tuning challenging. Herein, a partially etched dielectric metasurface supporting an intrinsic chiral quasi-guided mode (QGM) with flatband dispersion is proposed, for which the quality (Q) factor and circular dichroism (CD) can be independently modulated. The QGM is generated via Brillouin zone folding, with its flatband dispersion remaining robust against structural variations within a certain range. The radiative loss and spin-dependent coupling strength of the QGM can be independently engineered by adjusting the scaling ratio and rotation angle of individual nanostructures, enabling decoupled control of Q factor and CD while maintaining the flatband dispersion. Experimental measurements of the chiral QGM confirm the independent tunability of Q factor and CD, demonstrating angle-insensitive linear and nonlinear CD. Our work establishes a general physical mechanism for realizing flatband QGMs with independently tunable Q factor and CD, providing a versatile platform for next-generation functional chiral metasurfaces with potential applications in chiral emission, harmonic generation, and ultrasensitive optical sensing.