Multidimensional Symmetry Engineering of Metasurfaces for Circular Dichroism and Advanced Photonics.

Chiral metasurfaces provide a powerful artificial platform for manipulating the spin, phase, and amplitude of light at the subwavelength scale. The ability to generate strong circular dichroism (CD), optical activity, and spin-selective light-matter interaction creates new opportunities in chiral sensing, emission, and nonlinear photonics within flat optics. However, navigating the vast and complex geometric parameter space to maximize chiroptical responses remains a formidable challenge. To address this problem, we categorize the core design principles from the perspective of symmetry breaking including in-plane symmetry breaking, out-of-plane symmetry breaking, and low-symmetry lattice engineering. Furthermore, this review highlights the transformative role of machine learning and inverse design in overcoming the computational bottlenecks of traditional heuristic optimizations. We then review the practical applications of these chiral platforms across both linear and nonlinear regimes, focusing on imaging and holography, chiral sensing and polarization detection, circularly polarized light (CPL) emission, and advanced nonlinear chiral functionalities. Finally, major challenges and future research directions are outlined to guide the rational design and scalable implementation of chiral metasurfaces in next-generation photonic systems.