Realization of Intrinsic Planar Chirality by Bound States in the Continuum With Time‐Reversal Symmetry Breaking

ABSTRACT Intrinsic chirality has garnered considerable interest due to its inherent spin–chiral interactions and exceptional capacity for optical field localization. Approaches to achieving optical chirality via bound states in the continuum (BICs) generally require breaking both in‐plane and out‐of‐plane symmetries simultaneously, which adds fabrication complexity and impedes the realization of concurrent chirality on both sides of a structure. Here, we experimentally demonstrate intrinsic planar chirality arising from intrinsic chiral BICs by concurrently breaking in‐plane and time‐reversal symmetries in a two‐dimensional self‐biased magneto‐optical photonic crystal, thereby obviating the need for any external magnetic field. Since the out‐of‐plane symmetry is preserved, the structure can support intrinsic chiral BICs on both sides at the Γ point. The resulting intrinsic chiral BIC achieves a theoretical radiation quality factor exceeding 10 7 and a circular dichroism value of up to 0.85. Moreover, we experimentally show that the chirality of these intrinsic chiral BICs can be reversibly switched by simply reversing the magnetization direction, with all structural parameters held constant. Our work establishes a general and efficient design paradigm for chiral photonic devices, with promising applications in biosensing and optical detection.