Vector Exceptional Points with Strong Superchiral Fields

Abstract

Exceptional points (EPs)，branch points of complex energy surfaces at which eigenvalues and eigenvectors coalesce, are ubiquitous in non-Hermitian systems. Many novel properties and applications have been proposed around the EPs. One of the important applications is to enhance the detection sensitivity. However, due to the lack of single-handed superchiral fields, all of the proposed EP-based sensing mechanisms are only useful for the non-chiral discrimination. Here, we propose theoretically and demonstrate experimentally a new type of EP, which is a called radiation vector EP, to fulfill the homogeneous superchiral fields for chiral sensing. This type of EP is realized by suitably tuning the coupling strength and radiation losses for a pair of orthogonal polarization modes in the photonic crystal slab. Based on the unique modal-coupling property at the vector EP, we demonstrate that the uniform superchiral fields can be generated with two beams of lights illuminating on the photonic crystal slab from opposite directions. Thus, the designed photonic crystal slab, which supports the vector EP, can be used to perform surface-enhanced chiral detection. Our findings provide a new strategy for ultrasensitive characterization and quantification of molecular chirality, a key aspect for various bioscience and biomedicine applications.

Non-Hermitian Electromagnetic Metasurfaces at Exceptional Points (Invited Review)

“…Potential solutions could be topologically protected EP and exceptional surface that are more robust to undesired perturbations and fabrication errors [114,115]. Besides, non-Hermitian metasurfaces with the abilities of generating superchiral fields [68] can be applied to sensitive chiral molecule sensing in contrast to weak chirality detection by conventional method of circular dichroism spectroscopy. Such a non-Hermitian metasurface could be an ideal platform to perform surface-enhanced Raman spectroscopy or fluorescence spectroscopy [116].…”

“…By tuning the diameter and thickness of the holes and breaking the up-down symmetry, a pair of TM-and TE-like modes can be spectrally close to each other and become coupled. The coupling strength and radiation loss of this pair of orthogonal polarization modes can be further tuned by the system geometric parameters, leading to a "vector" EP [68]. Due to direction-dependent excitation of orthogonal polarization modes, the uniform superchiral fields can be generated at the EP by two incident beams with opposite directions.…”

Phase Enabled Circular Dichroism Reversal in Twisted Bi‐Chiral Propeller Metamolecule Arrays

“…Compared with the weak chiroptical responses in natural materials, artificial chiral structures have provided an ideal platform for investigating the advanced chiroptical responses from microwave to visible light. [4][5][6][7][8][9] The strong chiroptical response of the artificial structures has brought many new physical phenomena, such as chiral negative refraction, [10][11][12] superchiral fields, [13,14] broadband spin-selective transmission, [15,16] etc.…”

Evolution and global charge conservation for polarization singularities emerging from non-Hermitian degeneracies

“…Spurred by studies into bound states in the continuum, polarization singularities have gained enormous renewed interest in open periodic photonic structures, manifested in different morphologies with both fundamental and higher-order half-integer charges . Simultaneously, the significance of Berry phase has been further reinforced in surging fields of topological and non-Hermitian photonics (1,23,26,(51)(52)(53)(54)(55)(56). In open periodic structures involving band degeneracies, Berry phase and polarization singularity would inevitably meet, which sparks the influential work on non-Hermitian degeneracy (36) and several other following studies (40,43,45) discussing both concepts simultaneously.…”