Spin-preserving chiral mirror based on a pattern of rotated elliptical holes in thin monolithic all-dielectric photonic crystal slabs

Spin-preserving chiral mirrors, which reflect only one spin state of light under normal illumination without reversing the light's handedness, are intrinsically chiral structures. These mirrors are of particular interest for applications in optical imaging, chiral quantum optics, chiral cavities, and the recognition and sensing of chiral molecules. However, most of them are limited by complicated chiral elements of their unit cells and multilayer structures, which are challenging to fabricate and integrate in the visible-near-infrared region. Here we propose a thin monolithic photonic crystal (PhC) mirror with elliptical air holes to realize a giant intrinsic chiroptical response. Beyond the past chiral mirrors, our chiral mirrors selectively reflect only one spin state of light while preserving its handedness, exhibiting a near unity of circular dichroism (CD). The working principle of the PhC chiral mirror is based on guided-mode resonance with the simultaneous excitation of transverse electric (TE) and transverse magnetic (TM) eigenmodes. Interestingly, the TE and TM bands can be dynamically adjusted via the orientation angle of the elliptical air holes, leading to their degeneracy at a certain orientation angle. Based on the modified coupled mode theory, we show that the key point to achieving giant intrinsic CD is the angular difference in the in-plane effective orientation of the TE and TM eigenmode currents. As a proof of concept, we designed such an easy-fabrication PhC to serve as a spin-preserving chiral mirror, which shows superiority to the previously reported structure. The results of this work may provide new possibilities for compact chiral sensing, imaging, and polarization-sensitive devices.