Improved refractive-index sensing performance in medium contrast gratings by asymmetry engineering

Abstract

Silicon nitride (Si3N4) subwavelength medium contrast gratings (MCGs) directly integrated with CMOS photodetectors are a promising option for on-chip label-free biosensing. The narrow spectral features required for sensing are often realized in Si3N4 nanostructures by weakly corrugated gratings which limit design flexibility. We numerically investigate the optical properties of asymmetry-engineered MCG gratings and predict the formation of ultra-sharp spectral features via the excitation of quasi-bound states in continuum (QBIC) resonances. Systematic investigation of the design parameter space shows that sharp spectral features are obtained for a wide range of parameters without requiring ultrathin grating profiles. Transmission-mode refractive index sensing simulations for bulk and surface sensing, considering both wavelength-shift and intensity-shift modalities, indicate performance gains using these structures.

Quasi-guided modes resulting from the band folding effect in a photonic crystal slab for enhanced interactions of matters with free-space radiations

“…We should note that a similar band-folding effect has been proposed in the literature to improve the angular tolerance in the reflection of resonant grating filters with doubly periodic structures [16,17]. Other structures, such as diatomic [18] or dimerized [19,20] gratings, have been also investigated in recent years, but mainly with emphasis on the far-field spectrum, using one-dimensional (1D) grating structures. In addition, band folding was also employed to realize terahertz radiation from difference frequency generation (DFG) by using 1D leaky modes of binary waveguide gratings [21] and to manipulate the radiation coupling in the vertical directions in some photonic crystal cavities [22,23].…”

Study of Fano Resonance Effects in Graphene-Grating Composite Structures

“…Optimization of Fano sensor parameters to improve sensitivity and detection accuracy has become a major research challenge. In nanophotonics, grating structures are often used to achieve Fano resonance, which is due to the ability of grating structures to combine with various dielectric or metallic materials to more easily excite narrower resonance peak spectral curves and thus obtain higher sensitivity [ 9 , 10 ]. Zheng et al [ 11 ] proposed a tunable resonant two-sided dielectric grating structure, which uses a vertically incident plane wave to excite.…”

Enhanced sensitivity of an all-dielectric refractive index sensor with optical bound state in the continuum

“…The Fano resonances emerge when the system's symmetry is broken, so the otherwise localized BIC mode can couple with impinging light [16,17]. The spectral position of these extremely narrow Fano resonances is affected by the refractive index of the surrounding medium allowing to design optical sensors with an excellent figure of merit (FOM) [18][19][20][21][22][23] as the narrow Fano feature can be easily resolved in the spectral measurements. Despite the unsurpassed FOM, the major drawback of the dielectric sensors in comparison against the plasmonic ones is a noticeably (approximately five times) less sensitivity [24].…”