Inverse-Designed Metaphotonics for Hypersensitive Detection

Abstract

Controlling the flow of broadband electromagnetic energy at the nanoscale remains a critical challenge in optoelectronics. Surface plasmon polaritons (or plasmons) provide subwavelength localization of light but are affected by significant losses. On the contrary, dielectrics lack a sufficiently robust response in the visible to trap photons similar to metallic structures. Overcoming these limitations appears elusive. Here we demonstrate that addressing this problem is possible if we employ a novel approach based on suitably deformed reflective metaphotonic structures. The complex geometrical shape engineered in these reflectors emulates nondispersive index responses, which can be inverse-designed following arbitrary form factors. We discuss the realization of essential components such as resonators with an ultrahigh refractive index of n = 100 in diverse profiles. These structures support the localization of light in the form of bound states in the continuum (BIC), fully localized in air, in a platform in which all refractive index regions are physically accessible. We discuss our approach to sensing applications, designing a class of sensors where the analyte directly contacts areas of ultrahigh refractive index. Leveraging this feature, we report an optical sensor with sensitivity two times higher than the closest competitor with a similar micrometer footprint. Inversely designed reflective metaphotonics offers a flexible technology for controlling broadband light, supporting optoelectronics' integration with large bandwidths in circuitry with miniaturized footprints.

Permittivity-Asymmetric Quasi-Bound States in the Continuum

“…The Friedrich-Wintgen BICs, being realized in finite-sized systems, have an upper limit in the achievable quality factors, although a completely decoupled resonancea true BICcan be achieved in the extreme conditions of null values of either the permittivity of shell layers or the diameter of the dielectric core in composite spheres . Recently, deformed reflective structures, emulating high refractive index profiles via inverse design, were shown to support a new family of localized BICs, effectively trapping the electromagnetic energy in their accessible vacuum vicinity …”

“…33 Recently, deformed reflective structures, emulating high refractive index profiles via inverse design, were shown to support a new family of localized BICs, effectively trapping the electromagnetic energy in their accessible vacuum vicinity. 34 While true symmetry-protected BICs are theoretical entities of infinitely high quality factors that can be only realized in lossless systems, which are spatially infinite in at least one dimension, 20 quasi-BICs (qBICs), also termed supercavity modes, 35 can be excited in finite structures and are able to couple to radiation impinging from the far-field. These signatures can be probed through geometric perturbations in the symmetry of the unit cell, for instance, via changes in the length, height, relative angle, or area of the constituent resonators, 36−38 their relative displacement in superlattice metasurfaces, 39 or angular deviations from normal incidence of light in photonic crystals (PhCs).…”