Symmetry-breaking-induced plasmonic exceptional points and nanoscale sensing

Abstract

Singularities of open systems, known as exceptional points (EPs), have been shown to exhibit increased sensitivities, but the observation of EPs has so far been limited to wavelength-scaled systems subject to the diffraction limit. Plasmons, the collective oscillations of free electrons coupled to photons, shrink the wavelength of light to electronic and molecular length scales. We propose a novel approach to EPs based on spatial symmetry breaking and report their observation in plasmonics at room temperature. The plasmonic EPs are based on the hybridization of detuned resonances in multilayered plasmonic structures to reach a critical complex coupling rate between nanoantenna arrays, resulting in the simultaneous coalescence of the resonances and loss rates. Their utility as sensors of anti-immunoglobulin G, the most abundant immunoglobulin isotype in human serum, is evaluated. Our work opens the way to a new class of nanoscale devices, sensors and imagers based on topological polaritonic effects. The hybridized modes of an asymmetric plasmonic dimer show avoided crossing of both the real and imaginary parts. This can lead to plasmonic exceptional points, which are used for biosensing with very high sensitivity.