Hybrid Transverse–Longitudinal Modes for High Figure‐of‐Merit Localized Plasmonic Refractometric Sensing in the Visible Spectrum

Abstract

integration of mid-IR resonators with 2D materials has been shown to enhance sensing performance. [14] For the practicality of these applications, it is often desirable to implement the platform with a simple and cost-effective optical configuration, using inexpensive light sources and photodetectors operating in the visible spectrum. The practical aspect of localized surface plasmon (LSP)-based sensing lies in the mode excitation that can be carried out in a microscope setting, in contrast to its integrated optics and surface plasmon polariton counterparts that require complex optical configurations to meet their phase-matching conditions. [15,16] The performance of a plasmon-based sensor is gauged by the sensitivity (S) and the figure of merit (FOM), with the former describing the susceptibility of the plasmonic confinement to the refractive index changes and the latter, dependent on the resonance Q factor, describing the sensing resolution. The sensitivity has been shown to increase linearly with the resonance wavelength (λ R ), that is, S∝λ R [17,18] indicating that sensitivity progressively decreases toward the visible frequency range. This presents a dilemma in the realization of practical and cost-effective sensing platform, where high sensitivity and high FOM are the desirable parameters for visible frequency operation.Various types of planar resonators, including split ring resonators (SRR), [19] nanorods, [20,21] crescents, [22] and pyramids [23] have been reported to have sensitivities of 100-300 nm per RIU and FOM of ≈2 in the visible frequency range. Higher sensitivity is achievable in self-assembled metal nanoparticles due to their lower damping loss and negligible interaction with the substrate. [24,25] However, such an approach is not preferable in the context of a practical sensing platform requiring precise control of nanoantenna dimensions and placements. Another approach for improving sensing parameters is through planar multi-cavity configurations that allow bright and dark modes to interfere and produce a narrow Fano resonance, [26][27][28][29] where the cancellation of radiative loss gives rise to linewidth reduction and consequently higher Q factors. Sensing based on Fano resonance has been reported with structures such as The nanoscale confinement of plasmonic fields, coupled with their high susceptibility to refractive index changes, makes localized surface plasmons (LSPs) an excellent platform for rapid and label-free sensing. However, the small spatial overlap of the LSP fields with the adsorbed analyte dictates the sensitivity and figure of merit (FOM) of LSP-based sensing. The linear dependence of sensitivity on resonance wavelength, coupled with the dependence of FOM on the achievable resonance Q factor, leads to a sensitivity of ≈100-300 nm per RIU and FOM < 5 for gold-based LSP resonance in the visible range. This presents a dilemma for the realization of a practical sensing platform that requires high sensitivity and high FOM in the visible spectrum. Higher sensitivity and reso...

Interplays of Dipole and Charge‐Transfer‐Plasmon Modes in Capacitively and Conductively Coupled Dimer with High Aspect Ratio Nanogaps

“…The nominal size of the dimer is s = 80 nm, while the height of h = 38 nm is chosen to represent planar region ( h < 85 nm). [ 48 ] The periodicity ( p ) is chosen to be p = 320 nm, which is sufficiently far to prevent excessive dipolar coupling between the resonators, yet is still in subwavelength range in that radiative coupling and surface lattice resonance do not dominate the resonance behavior. The exposure dose is incrementally adjusted from 65 to 80 µC cm −2 to decrease the dimer gap spacing from sub‐10 nm to sub‐5 nm.…”

“…This is illustrated by the spectral splitting in high aspect ratio plasmonic nanostructures, which is known to result from phase retardation of transverse plasmons between the top and bottom surfaces of the nanostructure. [48,49] Furthermore, at increasing height, the oscillation of these transverse modes gives rise to longitudinal modes, causing the resonance characteristics to be determined by the interplay of transverse and longitudinal modes.…”

Surface Plasmonic Sensors: Sensing Mechanism and Recent Applications

“…The trend in the design of LSPR sensors is to use hybrid modes, for example, LSPR -SPR, to improve the sensitivity and FOM simultaneously. In Figure 2 f, the “V” shape particle lattices are explored to support a hybrid mode by the transverse LSP ( Figure 2 g (left)) and longitudinal Fabry–Perot modes ( Figure 2 g (right)) [ 65 ]. The strong polarization-dependent sensing performance of the hybrid mode is shown in Figure 2 h, in which a higher sensitivity of 402 nm/RIU is achieved at longer wavelength resonance.…”

“…Reprinted ( c – e ) with permission from Reference [ 64 ]. Reprinted ( f – h ) with permission from Reference [ 65 ].…”

Hybridized surface lattice modes in intercalated 3-disk plasmonic crystals for high figure-of-merit plasmonic sensing

“…5d. The bulk sensitivity (𝑆 𝐵 ) of LSP-based sensing has been shown to be proportional to the resonance wavelength, 𝑆 𝐵 ∝ 𝜆 𝑅 , while the figure of merit, defined as 𝐹𝑜𝑀 = 𝑆 𝐵 /𝑓𝑤ℎ𝑚, is related to the Q-factor by 𝐹𝑜𝑀 ∝ 𝑄 46,52,53 . This is verified by the linear correlation between the measured 𝐹𝑜𝑀 and Q-factor values for both x and y incident polarizations, with a range of 𝐹𝑜𝑀 from ~5 to ~24 obtained from bulk sensing measurement in Fig.…”