Boundary-Induced Embedded Eigenstate in a Single Resonator for Advanced Sensing

Abstract

Embedded eigenstates, also known as bound states in the continuum (BICs), hold great potential for applications in sensing, lasing, enhanced nonlinearities, and energy harvesting. However, their demonstrations so far have been limited to largearea periodic arrays of suitably tailored elements, with fundamental restrictions on the overall footprint and performance in the presence of inevitable disorder. In this work, we demonstrate a BIC localized in a single subwavelength resonator obtained by suitably tailoring the boundaries around it, enabling a new degree of control for on-demand symmetry breaking. We experimentally demonstrate how boundary-induced BICs open exciting opportunities for sensing by tracing highly subwavelength perturbations in a tiny drop of water using a simple tabletop experimental setup. We show that the single-resonator structure can be used to trace the dissolution of NaCl in water as well as to determine the evaporation rates of distilled water and salt water with a resolution of less than 1 μL.

Topological Metamaterials

“…Topological concepts are essential in explaining and predicting unique phenomena like bound states in the continuum (BIC) also known as an embedded eigenstate. BIC is an eigenmode of an optically open system that exhibits an unbounded radiative Q-factor, even though it lies within the continuum of unbounded states. ,− BIC can be realized in symmetry-protected scenarios, such as a hedgehog-like arrangement of dipole polarization along a sphere , or over a plane, or by exploiting the destructive interference between at least two strongly coupled resonant modes linked to the same radiation channel. ,− More recently, the topological scattering features of BIC were discussed in nonperiodic, planar reflective structures using epsilon-near-zero (ENZ) and epsilon-near-pole (ENP) materials, , which was also leveraged in acoustic and nonreciprocal electromagnetic systems . The ENZ resonances are commonly found in isotropic, anisotropic, and 2D materials, such as InAs, ITO, SiC, α-MoO 3 , and hBN.…”

“…However, they can also be tailored at the frequency of interest using MMs and metasurfaces. Owing to their high Q-factor and topological properties, BIC offer promising prospects for applications in sensing, ,, lasing, ,− high-harmonic generation, , enforced nonreciprocity, , thermal emission, , energy transfer and harvesting, , polarization control, ,, and vortex beam generation. , …”

Low-Symmetry Nanophotonics

“…Although localized BICs in single resonators have also been predicted, ,, they exploit extreme materials suffering from losses and challenging their practical implementation. Recently, a novel approach to a BIC localized in a single subwavelength resonator based on suitable tailoring the boundaries around it has been reported . The symmetry breaking of the structure has been realized by introducing a tiny water droplet atop the metallic resonator, exploiting the high permittivity of water to control the BIC state, Figure k.…”

Bound states in the continuum in photonic structures

“…For individual subwavelength resonators, genuine nonradiative states require extreme values of permittivity, tending toward infinity or zero 62,198,199 or imitating periodic boundary conditions with metallic waveguides. 65,200,201 In realistic individual resonators, there is always an infinite number of radiation channels, which limits the Q factor substantially. However, the concept of quasi-BICs allows reaching almost nonradiative states for individual dielectric resonators.…”