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.
the hybridized modes arising from the coupling of photonic and plasmonic resonances have been investigated widely. [7] The hybridized modes realized in diverse configurations using defect modes, [8] plasmonic nano-cavities, [9] Fabry-Perot metallic trench resonator, [10] etc., successfully demonstrate improved outcomes in terms of sensitivity, figure of merit, confinement, and lightmatter interaction. [11] Very recently, it has been noticed that the interaction of two different optical channels leads to the formation of a bound state in the continuum (BIC) and a resultant quasi-BIC hybrid mode. [12] The BIC is considered as a confined state in an open system with an infinite quality factor (Q-factor), which can extensively interact with the radiation channel. [13] Although the concept of BIC was proposed by von Neumann and Wigner in 1921, [14] the experimental observation of its existence is manifested only in the last decades. [15,16] Nevertheless, a true BIC is a mathematical perception that one can never achieve with either quantum or classical systems using physical parameters. [16] Fundamentally, the BICs can be of two kinds; the symmetry protected BIC (SP-BIC) [4,17,18] and the accidental BICs. [19] The SP-BICs are the inherent nature of a wave system; they are the discrete modes in the Brillouin zone near the gamma (Γ) point and are not allowed to interact with the free space radiation due to its symmetry incompatibilities. [17] Conversely, the accidental BICs are the consequence of the interference phenomena. For a system with two different resonance phenomena continuously changing with the parametric variation, at one particular set of parameters, there may occur an avoided crossing through which both the modes can interact and interfere destructively, resulting in the vanishing of one of the modes with an infinite quality factor. [19] Such an accidental vanishing of modes is explained by the Friedrich-Wintgen scenario. [20] The SP-BIC can be accessed by symmetry breaking structure [21] or by oblique illumination, resulting in the generation of resonant modes with a huge Q-factor called the quasi-BIC. [22] Although a true BIC mode is a dark mode with infinite lifetime and zero line width, in reality, the Q-factor of the BIC is finite because of restricted dimension, losses, and imperfection in the fabricated system. Thus, only the quasi-BICs with finite spectral width and Q-factor can be realized experimentally. The quasi-BIC has been explored for numerous applications including compact photonic chip for communication, [23] extreme narrowband transmission spectrum, [4] lasing, [24] The bound state in the continuum (BIC) explores the extraordinary optical properties of a system possessing wave characteristics that gained recent importance for practical applications, including lasing, sensing, optical tweezing, nonlinear interactions, and many more. Unlike a pure optical or plasmonic system, a hybrid architecture with coupled photonicplasmonic characteristics can offer the intermittent resonant modes for s...
We have investigated the antireflection and light trapping properties of two-dimensional grating arrays in the hexagonal symmetry with various texture morphologies. Optical simulation based on finite-difference time-domain (FDTD) analysis is carried out to understand the role of the structure profile for different periodicities and heights to achieve enhanced light trapping. The considered active medium of interest is 200-nm-thick hydrogenated amorphous silicon. Although the considered texture profiles possess an incremental change of refractive index from incident medium to active medium, a parabolic-shaped front side texture provides better antireflection effects owing to its high diffraction efficiencies in the higher-order modes as compared to other pattern morphologies. In the back side texture, the parabolic-shaped pattern also dominates with better light trapping efficiencies due to its ability to distribute a major amount of diffracted energy in the higher-order modes. The average reflection calculations in the wavelength range of 300-800 nm confirm that in both side textures, a periodicity of 500 nm with a height of 200 nm can be preferentially recommended for less reflection loss and improved scattering in oblique angles. The quantum efficiency calculation verifies that a device designed with these optimized parameters can offer improved efficiency for ultra-thin solar cells.
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