Helical dichroism (HD) utilizing unbounded orbital angular momentum degree of freedom, has provided an important means of exploring chiral effects in diverse wave systems, surpassing the two-state constraint in circular dichroism that relies on intrinsic spin. However, the naturally feeble chiral signals that arise during wave-matter interactions pose significant challenges to the effective enlargement of HD. Here, we introduce a new paradigm for realizing maximum HD through non-Hermitian gradient metasurfaces by engineering a chiral exceptional point (EP) in intrinsic topological charge. The non-Hermitian gradient metasurfaces are empowered by the asymmetric coupling feature at the EP, enabling flexible construction to realize versatile chirality control in extreme circumstances where one chiral vortex is totally reflected and the opposite counterpart is completely absorbed or transmitted into the customized vortex modes. As the manifestation of the maximum HD, we present the first experimental demonstration of perfect chirality-selected vortex transmission in acoustics. Our findings open new venues to achieve maximum chirality and explore chiral physics of wave-matter interactions, which can boost many vortical applications in asymmetric chirality manipulation, one-way propagation, and information multiplexing.
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- Electronic resonances are unstable states that lose electrons and are important in various high-energy environments in chemistry, physics, and biology, as well as in ambient conditions.
- The article discusses complex-variable techniques like complex scaling and complex absorbing potentials, which help analyze these resonances through non-Hermitian Hamiltonians.
- It also reviews recent advancements in the field, including analytic gradient theory, rank-reduction techniques, quantum embedding, and methods to assess partial decay widths.
Unlike the ideal circular whispering gallery cavities, those without mirror symmetry intrinsically support resonant modes exhibiting chirality which indicates an imbalance between clockwise and counterclockwise wave components. In extreme cases, nearly degenerate pairs of copropagating modes can be found around the chiral exceptional points (EPs) in parameter spaces. The chiral EPs have been studied in various schemes; however, most attention has been focused on the cases with piecewise constant or periodic refractive index profiles.
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