High‑Performance Refractive‑Index Sensing with Au/SiO₂ Triangle Arrays on Reflective Gold Substrates

Highlights

1. The uniform metal–insulator–metal (MIM) triangle structure with extended, sharp tips generates a markedly enhanced local electromagnetic field and an extremely narrow absorption band.

2. The dense array arrangement of MIM triangles guarantees high absorption efficiency.

3. The ultranarrow FWHM of the absorption peak underpins the structure’s superior refractive‑index sensing performance.

Background

Localized surface plasmon resonances (LSPRs) in metallic nanoparticles and nanostructure arrays efficiently confine light, particularly when the features are small or possess sharp edges, leading to extreme local field enhancements that have attracted extensive research interest. Various patterned monolayer metal films and metal/dielectric/metal (MIM) multilayers have been proposed for applications ranging from plasmon sensors and broadband absorbers to surface‑enhanced Raman scattering, transparent conductors, and polarization converters. Conventional lithography techniques—electron‑beam, focused‑ion‑beam, and double‑beam interference—are limited by cost, low throughput, and resolution constraints, especially for large‑area arrays with sub‑10‑nm sharp tips. Microsphere‑assisted lithography overcomes these limitations, enabling large‑area triangular, crescent, and star‑shaped arrays with exceptionally sharp corners at low cost. Sharp corners are critical for high‑performance sensing because they amplify local fields and narrow resonance linewidths, thereby enhancing refractive‑index sensitivity (RIS) and figure of merit (FOM = RIS/FWHM). Recent breakthroughs, such as a hyperbolic metamaterial biosensor with RIS of 30,000 nm/RIU and an ultranarrow FWHM of 3 nm in engineered nanostructures, demonstrate the potential of this approach, although practical implementation remains challenging due to low absorption and fabrication complexity. Triangle arrays, owing to their inherent sharp tips, typically outperform other morphologies, yet conventional fabrication using 500‑nm or smaller spheres yields significant size deviation and broadens the absorption spectrum, limiting RIS to <500 nm/RIU and FOM <50. Larger spheres promise improved size uniformity and longer triangle arms, potentially elevating sensing performance. In this work, we design a three‑layer Au/SiO₂/Au (MIM) structure with triangle arrays fabricated by microsphere lithography, systematically study the role of tip‑to‑tip gap, spacer and top‑layer thicknesses, and optimize the structure for maximum absorption and sensing performance.

Methods

Electromagnetic simulations were performed using CST Microwave Studio. The unit cell model (Fig. 1) includes a reflective Au substrate (100 nm thick), a SiO₂ spacer, and a top Au triangle array. Periodic boundary conditions were applied in the xy‑plane and open boundaries along z. A plane wave with 1 V/m amplitude, incident along +z and polarized along +x, illuminated the structure. Material optical constants were taken from literature. The center‑to‑center spacing of adjacent triangles was fixed at 900 nm while the tip‑to‑tip gap, SiO₂ thickness, and top Au thickness were varied systematically. Absorption spectra (A = 1 − R) and field distributions were extracted. Refractive‑index sensitivity was obtained by varying the surrounding RI from 1.33 to 1.36.

Schematic of the MIM structure sensor. a Perspective view. b Cross‑sectional view. c & d Top view.

Results and Discussion

Optical Properties

We first set the top Au and spacer thicknesses to 30 nm each and kept the Au substrate at 100 nm to ensure full reflection (T≈0). With the surrounding RI fixed at 1.34, we examined how the tip‑to‑tip gap influences the absorption spectrum. Figure 2a shows that gaps from 10 nm to 50 nm do not shift the main peak (~900 nm) nor its amplitude, indicating that the peak originates from isolated MIM units rather than inter‑particle coupling. A sparse array with >500 nm gaps confirms this, producing a similar peak position but significantly reduced absorption. Replacing the bottom Au film with SiO₂ (MII configuration) yields peaks at ~1000 nm associated with surface‑lattice resonances, but the amplitude is markedly lower than in the MIM case, underscoring the importance of the reflective substrate.

Absorption spectrum versus tip‑to‑tip gap for (a) MIM and (b) MII arrays. The inset in (a) shows the isolated MIM unit. (c)–(e) Electric‑field |E| in the x‑z plane for gaps of 20 nm, 30 nm, and 50 nm. (f) |E| for the MII array with a 30 nm gap. (g) Magnetic field |H| in the x‑z plane for the MIM array. (h) |E| in the x‑y plane at the top surface (z = −30 nm).

Field maps reveal that the maximum |E| occurs at the triangle tips (≈54–61× the incident field) when the gap is ≤20 nm, and at the gap center for larger gaps, but the overall distribution remains localized. The SiO₂ spacer hosts a strong magnetic resonance, while the Au triangles sustain a bright electric dipole. Their coupling yields a high‑absorption peak with an ultranarrow FWHM (~5 nm). This performance surpasses conventional monolayer triangle arrays due to the combined lighting‑rod effect and magnetic resonance in the spacer. Adjusting the SiO₂ thickness from 10 nm to 50 nm shifts the peak to longer wavelengths and increases absorption: thin spacers (≤20 nm) exhibit weak LSPR peaks (~900 nm), whereas 25–40 nm spacers achieve >90 % absorption with a FWHM <6 nm. For the top Au layer, thicknesses of 10 nm to 50 nm were tested. Thin Au (10 nm) yields low absorption; increasing thickness red‑shifts the peak and boosts amplitude, reaching >90 % at 30 nm. Further thickening to 50 nm preserves the high absorption while maintaining a narrow FWHM (~5 nm). The red‑shift is attributed to increased free‑electron participation and reduced resonant energy requirements.

Sensing Performance

With optimized parameters—30 nm tip‑to‑tip gap, 25 nm SiO₂ spacer, and 50 nm top Au—we evaluated refractive‑index sensitivity. Fig. 4 shows a rapid red‑shift of the high‑absorption peak as the surrounding RI increases from 1.33 to 1.36. The peak remains at ~900 nm with a FWHM of ~5 nm. Calculated RIS is 660 nm/RIU and the FOM reaches 132, both markedly superior to previously reported triangle arrays. The combination of dense packing, sharp tips, and MIM coupling yields an ultranarrow resonance that is highly responsive to environmental changes, making the structure ideal for liquid‑phase RI monitoring and gas‑liquid phase discrimination.

Absorption peak shift with varying surrounding refractive index (1.33–1.36).

Conclusions

Numerical studies demonstrate that the Au/SiO₂ triangle MIM array achieves simultaneous high local electric‑field enhancement and >90 % absorption due to the combined lighting‑rod effect, plasmonic coupling, and dense packing. The tip‑to‑tip gap has negligible influence on the main peak, while SiO₂ and top‑Au thicknesses tune the resonance position and amplitude by matching electric and magnetic dipoles. The resulting ultranarrow FWHM (~5 nm) and long triangular arms provide a refractive‑index sensitivity of 660 nm/RIU and a figure of merit of 132 across RI 1.33–1.36, outperforming earlier reports. These findings suggest that microsphere‑lithography‑fabricated MIM triangle arrays are a practical and powerful platform for high‑precision RI sensing.

Abbreviations

Al₂O₃:

Aluminum oxide

FOM:

Figure of merit

FWHM:

Full width at half maximum

LSPR:

Localized surface plasmon resonance

MII:

Metal/dielectric/dielectric

MIM:

Metal/dielectric/metal

RIS:

Refractive index sensitivities

RIU:

Refractive index unit

SiO₂:

Silicon dioxide