Resonance Engineering via Harnessing Anti-Parallel Dipole Image Coupling

TL;DR

This work addresses the challenge of bidirectional, broadband resonance tuning in plasmonic nanostructures by engineering a metal-dielectric-metal nanodisk array that leverages antiparallel dipole image coupling and Rayleigh anomaly. The authors use 3D FDTD simulations to optimize geometry and reveal a record blueshift of $457.82$ nm for a small thickness change in the Au nanodisk, along with wide NIR tunability from $1100$ to $2600$ nm. The longer-wavelength mode follows plasmon-ruler-like exponential decay with disk thickness, while the shorter-wavelength mode arises from RA and is tunable via period and incidence angle. The results offer a compact, tunable plasmonic platform with potential impact on sensing, optical communications, and dynamic displays.

Abstract

Precise control of plasmonic resonances across a broad spectral range is central to the development of tunable optical devices. Yet, achieving both redshifts and blueshifts within a single nanostructure has remained elusive. Here we introduce a metal-dielectric-metal (MDM) nanodisk array that enables bidirectional tuning of resonance wavelengths throughout the near-infrared (NIR) region. The observed spectral evolution follows the plasmon ruler relationship, with unprecedented tuning properties. In particular, we report a record blueshift response of 457.82 nm for a small nanodisk thickness variation of only 5-10 nm, the highest blueshift response demonstrated in plasmonic architectures to date. This platform offers finely tunable resonances spanning an exceptionally wide NIR range, providing new insights into electromagnetic (EM) coupling mechanisms and establishing a foundation for next-generation tunable devices in sensing, optical communications, and dynamic displays.

Figures (10)

• Figure 1: (a) Schematic illustration of the metal–dielectric–metal (MDM) nanostructure for resonance engineering. Au and Al2O3 were employed as metal and spacer materials, respectively. The nanostructure consists of an Al2O3 substrate and a top cladding layer of Al2O3, enabling the nanostructure to exhibit both blue- and red-shifted resonance behaviors. Cross-sectional (b) xz- and (c) xy-views of the periodic MDM unit cell. Here, $t_d$, $t_{\ce{Al2O3}}$, and $t_{Au}$ represent the thicknesses of the Au nanodisk, Al2O3 spacer, and Au slab, respectively. The periodicity is denoted as $P_x = P_y = P$, and the diameter of the nanodisk are given by $d_d$.
• Figure 2: Spatial distributions of the reflection of an MDM nanostructure (a) with and (b) without top cladding layer on the nanostructure.
• Figure 3: (a) Shift in the plasmon wavelength due to changes in the Au nanodisk thickness with and without a top cladding layer on the nanostructure. The solid lines are theoretical fitting curves by using the plasmon ruler equation. The R$^2$ for these fitted curves was $>$0.9. The xz-view of the spatial electric field ($E_z$) distributions for different Au nanodisk thicknesses (nm) of (b) 5, (c) 30, (d) 48, (e) 55, (f) 75, and (g) 90 for the nanostructure with top cladding layer. The inset of (d) depicts the color bar of $E_z$. The dotted black boxes and rings represent the Au nanodisk and the spatial electric field distributions at different resonance of (a), respectively.
• Figure 4: The xz-view of the spatial electric field ($E$) distributions for the nanostructures (a) with and (b) without top Al2O3 cladding layer, respectively. The color bar is provided in the insets.
• Figure 5: Spatial distributions of the transmission of an MDM nanostructure with top cladding layer for varying (a) $P$ and (b) $\theta$. Here, the $t_d$ and $d_d/2$ were set to be 25 and 125 nm, respectively.