Janus Bound States in the Continuum with Asymmetric Topological Charges

Abstract

We propose a novel topological defect called Janus bound states in the continuum (BICs), featuring asymmetric topological charges in upward and downward radiation channels. Our approach involves a photonic crystal slab (PCS) that initially exhibits both out-of-plane and in-plane mirror symmetry, and this PCS possesses one BIC at the Γpoint and two BICs off the Γpoint. By introducing certain perturbations that break the out-of-plane mirror symmetry, the two off-ΓBICs decompose into four circularly polarized states (C points) with identical topological charges (each with half the topological charge of the original BIC) while the at-ΓBIC is preserved. Then, we selectively manipulate the four C points associated with the downward radiation channel to converge at the at-ΓBIC, forming a Janus BIC with distinct topological charges for upward and downward radiation. By further introducing in-plane mirror symmetry perturbation, we can bring two of the C points with the same handedness and identical topological charges for upward radiation to merge into the Janus BIC. This process results in a Janus chiral BIC which exhibits large intrinsic chirality and an infinite Q factor. Janus BICs can induce distinct Pancharatnam-Berry phase singularities in momentum space for different incident channels, providing a new approach to control orbital angular momentum. Janus chiral BICs hold promise in enhancing direction-dependent and spin-dependent asymmetric light-matter interaction, opening new pathways for improving chirality-dependent operation for on-chip devices.

References in corpus (12)

• Topological nature of bound states in the radiation continuum
• Observation of intrinsic chiral bound states in the continuum
• Planar chiral metasurfaces with maximal tunable chiroptical response driven by bound states in the continuum
• Metasurfaces with maximum chirality empowered by bound states in the continuum
• Applications of Bound States in the Continuum in Photonics
• Resonant Semiconductor Metasurfaces for Generating Complex Quantum States
• Geometric phase from Aharonov-Bohm to Pancharatnam-Berry and beyond
• Generation and Annihilation of Topologically-Protected Bound States in the Continuum and Circularly-Polarized States by Symmetry Breaking
• Strong light-matter interaction with self-hybridized bound states in the continuum in monolithic van der Waals metasurfaces
• Passive Bias-Free Nonreciprocal Metasurfaces Based on Nonlinear Quasi-Bound States in the Continuum
• Strongly enhanced light-matter coupling of a monolayer WS2 from a bound state in the continuum
• Topological unidirectional guided resonances emerged from interband coupling

Cited by in corpus (2)

• Giant and Rapidly Switching Intrinsic Chirality Enabled by Toroidal Quasi-Bound States in the Continuum
• Phase fields in momentum space of photonic crystal slabs