For many photonic applications, it is important to confine light of a specific wavelength at a certain volume of interest at low losses. So far, it is only possible to use the polarized light perpendicular to the solid grid lines to excite waveguide–plasmon polaritons in a waveguide-supported hybrid structure. In our work, we use a plasmonic grating fabricated by colloidal self-assembly and an ultrathin injection layer to guide the resonant modes selectively. We use gold nanoparticles self-assembled in a linear template on a titanium dioxide (TiO2) layer to study the dispersion relation with conventional ultraviolet–visible–near-infrared spectroscopic methods. Supported with finite-difference in time-domain simulations, we identify the optical band gaps as hybridized modes: plasmonic and photonic resonances. Compared to metallic grids, the observation range of hybridized guided modes can now be extended to modes along the nanoparticle chain lines. With future applications in energy conversion and optical filters employing these cost-efficient and upscalable directed self-assembly methods, we discuss also the application in refractive index sensing of the particle-based hybridized guided modes.

From theoretical model to experimental realization, the bound state in the continuum (BIC) is an emerging area of research interest in the last decade. In the initial years, well-established theoretical frameworks explained the underlying physics for optical BIC modes excited in various symmetrical configurations. Eventually, in the last couple of years, optical-BICs were exploited as a promising tool for experimental realization with advanced nanofabrication techniques for numerous breakthrough applications. Here, we present a review of the evolution of BIC modes in various symmetry and functioning mediums along with their application. More specifically, depending upon the nature of the interacting medium, the excitations of BIC modes are classified into the pure dielectric and lossy plasmonic BICs. The dielectric constituents are again classified as photonic crystal functioning in the subwavelength regime, influenced by the diffraction modes and metasurfaces for interactions far from the diffraction regime. More importantly, engineered functional materials evolved with the pure dielectric medium are explored for hybrid-quasi-BIC modes with huge-quality factors, exhibiting a promising approach to trigger the nanoscale phenomena more efficiently. Similarly, hybrid modes instigated by the photonic and plasmonic constituents can replace the high dissipative losses of metallic components, sustaining the high localization of field and high figure of merit. Further, the discussions are based on the applications of the localized BIC modes and high-quality quasi-BIC resonance traits in the nonlinear harmonic generation, refractometric sensing, imaging, lasing, nanocavities, low loss on-chip communication, and as a photodetector. The topology-controlled beam steering and, chiral sensing has also been briefly discussed.

Surface plasmon resonance-based sensors have emerged as commercially fostering portable biodetectors. The scientific community is engaged in extensive research to improve their performance in terms of sensitivity, selectivity, and reproducibility for the recognition of specific biomolecules. Essentially, there is a need for miniaturizing the size of existing sensors with innovative designs without compromising their bioaffinity and sensitivity performance. In this work, we propose and demonstrate a gratingcoupled surface plasmon polariton (SPP) sensor on a thin flat gold layer using a hybrid configuration. The proof of concept of the grating architecture has been realized through an innovative fabrication procedure, with experimental verification of its bulk sensitivity. The geometry is identical to the prismcoupling configuration, yet with miniaturization and compactness. Characteristics of the excited modes in the spectral regime of interest are investigated using the finite-difference time-domain simulations. The effective index calculation of the resonance conditions and the accompanying field distribution can identify the excited SPP and metal-assisted guided-mode resonance modes. Detailed probing of the electric field distribution of the desired SPP mode reveals an extended evanescent decay length of 1284 nm, close to the theoretical limit, and an extended propagation length of 270 μm. The experimental demonstration of the reflectance dip with two different analyte media perceived an increased bulk sensitivity of 1133 nm/RIU. Remarkably, this resonant mode exhibits sensing capabilities for a wide range of analyte refractive indexes. We believe that the fabricated configuration with observed high sensitivity and calculated ultradeep evanescent field penetration depth along with extended propagation length can lead to the designing of a hands-on biochip for detecting large biomolecules.