Enhanced Localized Electric Field from Surface Plasmon Coupling in a Silver Nanostructure Array with a Silver Thin Film for Bioimaging and Biosensing

Abstract

The electric field enhancement effect induced by localized surface plasmon resonance (LSPR) plays a critical role in imaging and sensing applications. In particular, nanocube structures with narrow gaps provide large hotspot areas, making them highly promising for high-sensitivity applications. This study predicts the electric field enhancement effect of structures combining silver nanocubes and a 10 nm thick silver thin film using the finite-difference time-domain (FDTD) method. We demonstrate that the interaction between the silver nanocubes and silver thin film allows control over sharp LSPR peaks in the visible wavelength range. Specifically, the structure with a spacer layer between the silver nanocubes and the silver thin film is suitable for multimodal imaging, while the direct contact structure of the silver nanocubes and the silver thin film shows potential as a highly sensitive refractive index sensor. The 10 nm thick silver thin film enables backside illumination due to its transparency in the visible wavelength region, making it compatible with inverted microscopes and allowing for versatile applications, such as living cell imaging and observations in liquid media. These structures are particularly expected to contribute to advancements in bioimaging and biosensing.

1. Introduction

Advancements in nanotechnology have increasingly highlighted metamaterials engineered to exhibit physical properties absent in nature. By designing dielectric and metallic structures at subwavelength scales, a wide range of optical functionalities have been explored, including perfect absorbers [1], metasurfaces [2], and refractive index sensors [3]. Among them, plasmonic metamaterials that leverage localized surface plasmon resonance (LSPR) on metallic nanoparticles or nanostructures have garnered significant interest. These materials exhibit strong absorption and scattering of specific wavelengths of light [4,5,6]. Furthermore, precise engineering of metal–dielectric combinations, along with shape and size optimization, enables the enhancement of localized electric fields [7,8], tuning of resonance spectra [9,10,11], and control over light propagation [12,13], surpassing conventional optical technologies. Silver, in particular, has been widely studied as an exceptional plasmonic material, owing to its low optical losses and resonance wavelengths within the visible spectrum. The size and shape of nanostructures can be precisely tailored through techniques such as electron beam lithography (EBL) [14,15], nanoimprint lithography (NIL) [16,17], chemical synthesis [18,19,20,21,22], and focused ion beam (FIB) processing [23,24]. Moreover, reducing gap distances to the nanometer scale enables the exploitation of inter-particle interactions. These characteristics of LSPR are particularly advantageous for non-invasive optical analysis at the cellular and molecular levels, and they hold great promise for bioapplications such as highly sensitive biosensors [25,26] and super-resolution imaging [27,28]. In imaging applications, conventional optical microscopes are constrained by the diffraction limit, which restricts resolution. However, by utilizing LSPR, it is possible to achieve high-resolution observations approaching this limit. Notably, nanoparticles generate localized optical fields, facilitating the visualization of nanoscale structures and movements. This advancement allows for the observation of previously challenging biological processes, such as biomolecular interactions and dynamics [29]. Additionally, LSPR-based techniques are highly suitable for multimodal imaging in complex biological environments, including live cells, owing to their high selectivity and sensitivity at specific wavelengths. In biosensing applications, the enhanced localized electric fields induced by LSPR significantly improve the sensitivity of molecular detection. When target molecules bind to the surface of a nanostructure, the resulting change in the local refractive index manifests as a shift in the LSPR peak, which can be detected through spectral measurements. Notably, gold- and silver-based nanostructures have been reported to enable real-time, highly accurate detection of trace biomolecules, including DNA, proteins, and even cancer markers [30]. This technique provides a significant advantage over conventional fluorescence-based detection methods, as it eliminates the need for labeling and preserves the physiological conditions of the sample.

We previously demonstrated that by placing metallic nanohemispheres on a metal substrate through a dielectric spacer layer, the interactions between the nanohemispheres and their mirror images enable resonance peak splitting and sharpening [31,32,33]. Similarly, numerous studies have proposed metal–insulator–metal (MIM) structures composed of metallic nanostructures, a dielectric layer, and a mirror substrate [34,35,36,37]. However, mirror-based structures face limitations in bioapplications where adjusting focus from the front side is challenging, as they cannot be excited from the backside. In our previous study [37], we systematically compared the optical properties of nanoparticles with various shapes and demonstrated that nanocubes exhibit larger closest surface areas than disk-shaped structures, which are easier to fabricate with high precision using electron beam lithography. This characteristic allows cubic nanoparticles to support stronger and more extended ‘hotspots’—regions of intense electric field localization—making them advantageous for imaging and sensing applications. However, reducing the inter-particle distance to enhance these hotspots introduces challenges such as peak broadening and redshifting [38,39,40].

To address these challenges, in this study, we designed structures that leverage the interaction between a thin silver film and nanocubes, utilizing a 10 nm thick silver film as an alternative to a mirror substrate to enable the transmission of incident light from the backside. The finite-difference time-domain (FDTD) method was employed throughout the calculations to investigate the optical responses of the structures, including the strength of localized electric fields on their surface and their response to changes in the surrounding refractive index. Furthermore, we explored the tunability of multiple resonance modes resulting from these interactions, as well as the enhancement of refractive index sensitivity, by optimizing the spacer layer thickness.

2. Methods

FDTD simulations were performed using commercially available software (Poynting for Optics, V03L10R131, Fujitsu, Tokyo, Japan). Periodic and absorption boundary conditions were applied in the X and Y directions, as well as in the Z-direction, respectively. A nonuniform mesh with grid sizes ranging from 0.1 to 1.5 nm was employed. The incident pulse, a differential Gausian function with a pulse width of 0.3 fs, electric field of 1 V/m, center wavelength of approximately 500 nm (600 THz), and polarized in the X-direction, was irradiated from the backside of the structures. The refractive index of the glass was set at 1.5 without dispersion. To reduce the computational cost, the dielectric function of silver was approximated using the adjusted Drude model in the visible light region, based on the Lorentz–Drude model values reported in [41]. The detailed fitting method and the validity of the adjusted Drude model are thoroughly discussed in our previous work [42].

3. Results and Discussions

3.1. Resonance Peak Tuning via Structural Parameters

Figure 1a,b show schematic diagrams of the nanocube structures with and without the silver thin film, respectively. In these structures, $L$ and $h$, respectively, represent the edge length and height of the nanocube, $G$ is the gap length between adjacent nanocubes, and $sp$ denotes the thickness of the spacer layer between the nanocube and the silver thin film. The thickness of the silver thin film, $d$, was fixed at 10 nm in this study. For the case without the silver thin film, $d$ was set to 0 nm. Reflectance spectra and electric field intensity distributions on detection planes for nanocube structures with $L$ = 100 nm, $h$ = 50 nm, $G$ = 20 nm, and $sp$ = 10 nm are presented in Figure 1c. The steady-state electric field distributions at the phase of maximum intensity and at the peak wavelength were evaluated on detection planes positioned 5 nm above the top surfaces of the structures. As demonstrated in our previous study [37], hotspots resulting from interactions between nanocubes were confined to the gap regions extending across the entire closest surface area. However, interactions between the nanocubes resulted in a broad resonance peak that appeared in the infrared region. In contrast, the introduction of a silver thin film induced a resonance peak at 620 nm in the visible wavelength region, which arose from interactions between the thin film and the nanocubes. This configuration also led to an increase in the electric field intensity across the hotspots at the phase of maximum intensity.

In general, the resonance wavelength of LSPR strongly depends on structural parameters. The following results demonstrate how changes in these parameters can control the resonance wavelength. Figure 2a illustrates the reflectance spectra of the nanocube structures, with $h$, $G$,$sp$, and $d$ set to 50 nm, 20 nm, 10 nm, and 10 nm, respectively, while $L$ is varied from 60 nm to 140 nm. Figure 2b also presents the reflectance spectra for structures where $L$, $G$, $sp,$ and $d$ are fixed at 100 nm, 20 nm, 10 nm, and 10 nm, respectively, while $h$ is varied from 20 nm to 50 nm. Increasing $L$ from 60 nm to 140 nm, which corresponds to the polarization direction of the incident light, resulted in a redshift in the resonance wavelength by over 200 nm in the visible wavelength region due to size effects. In contrast, varying $h$ from 20 nm to 50 nm, which is perpendicular to the polarization of the incident light, shifted the resonance wavelength by only 30 nm, enabling more precise wavelength control. Figure 2c presents the reflectance spectra of the nanocube structures where $L$, $h$, $sp,$ and $d$ were set to 100 nm, 50 nm, 10 nm, and 10 nm, respectively, while $G$ was varied from 10 nm to 30 nm. As the gap distance increased, the interaction between the nanocubes weakened, which caused a slight blueshift in the resonance wavelength. Increased gap spacing also allowed light transmission through the structure, leading to peak broadening. Conversely, as the gap size decreased, the reflectance peak diminished, and the structure’s optical response approached that of a mirror-like reflection. For sensing applications requiring a sharp resonance peak, a gap size of at least 20 nm is therefore necessary. Since variations in periodicity did not significantly affect the resonance wavelength, it is considered that the interaction with diffracted light is minimal. Although precise dimensional control at the nanometer scale remains technically challenging in practical fabrication, gap sizes as small as 10 nm have been successfully achieved using electron beam lithography (EBL) [14,15,43], while focused ion beam (FIB) milling has enabled the realization of gaps in the range of 20–30 nm [23,24]. The fact that the LSPR peak position does not vary significantly with changes in the gap length indicates that variations in the gap size have minimal influence on the LSPR characteristics. This result demonstrates that the resonance wavelength in the visible region can be effectively tuned by adjusting the structural parameters.

3.2. Mode Analysis Through Spacer Layer Thickness

In the preceding section, we demonstrated that the presence of the silver thin film resulted in resonance peak sharpening through its interaction with the nanocube structures, facilitating resonance wavelength control within the visible light range. To gain deeper insight into the interaction between the silver thin film and the nanocube structures, we varied the distance between them and examined the resulting resonance modes in the x-z cross-section at the center of the structure. In a typical MIM structure utilizing Fabry–Pérot resonance, the resonance peak blueshifts as the spacer layer thickness decreases, following the Fabry–Pérot condition in the dielectric layer. On the other hand, in the case of the coupling mode between the localized electric field of a metallic nanostructure and its mirror image mode on the metal substrate, the resonance peak splits. As the spacer layer thickness decreases, the shorter-wavelength peak undergoes a blueshift, while the longer-wavelength peak experiences a redshift, enabling resonance wavelength tuning over a broad spectral range. Figure 3a exhibits the reflectance spectra with $L$, $h$, $G$, and $d$ fixed at 100 nm, 50 nm, 20 nm, and 10 nm, respectively, while $sp$ was varied from 3 nm to 15 nm. Three peaks are observed: two within the visible wavelength range and one in the infrared region. To examine the resonance modes, reflectance spectra in the near-infrared range are also included. Although this range extends beyond the fitted wavelength region of silver’s dielectric function, the values of the dielectric function in the infrared region differ slightly from those of the Lorentz–Drude model. As a result, the peak position and sharpness are slightly different; however, the overall trend of the peaks and the types of resonance modes remain unchanged. This is because, in the near-infrared region, the dielectric function closely follows Drude-type behavior. Therefore, these minor discrepancies do not affect the validity of the mode analysis. Figure 3b,c present the reflectance spectra with an expanded view of the visible wavelength range and the corresponding peak positions, respectively. All three resonance peaks exhibited redshifts as the spacer layer thickness decreased. Therefore, it can be concluded that these modes are distinct from Fabry–Pérot resonances or nanostructure–mirror image interactions. This redshift behavior suggests that, as the coupling between the nanocubes and the silver film becomes stronger with decreasing spacer thickness, spectral splitting into bonding and anti-bonding modes occurs. All three observed modes correspond to the bonding branches [44,45] resulting from this coupling. In this configuration, no blueshifted peaks corresponding to anti-bonding modes were found in the simulated spectra, indicating that the anti-bonding modes were not observed. The distributions of the electric field component in the z-direction at three resonance wavelengths for $sp$ = 3 nm are shown in Figure 3d. In all modes, dipole oscillations of the nanocubes were identified, along with standing wave modes localized within the spacer layers. The longest wavelength mode observed in the infrared region corresponds to the fundamental mode, while the third- and fifth-order modes appear in the visible wavelength region [46,47]. These standing waves arose from the interaction between the dipole oscillations of the nanocubes and the silver film, which exhibited antisymmetric modes relative to the nanocube center. Consequently, even-order modes, which exhibited symmetry about the nanocube center, were not induced. Similarly to Fabry–Pérot resonances or nanostructure–mirror image interactions, the resonance wavelength can be controlled by adjusting the thickness of the spacer layer. The spacer thickness can be precisely controlled at the atomic scale via atomic layer deposition (ALD), enabling fine-tuning of the coupling strength. These findings indicate that even when the silver film is sufficiently thin to allow incident light transmission, optimizing the spacer layer thickness allows two modes in the visible region to be utilized for imaging applications.

Figure 4a exhibits the reflectance spectra for structures with $L$, $G$, $sp$, and $d$ set to 100 nm, 20 nm, 0 nm, and 10 nm, respectively, while $h$ is varied from 20 nm to 50 nm. Compared to the case with a spacer layer, when the silver thin film was directly in contact with the silver nanocubes, the resonance wavelength shift exhibited a stronger dependence on the nanocube height, enabling more flexible tuning in the visible wavelength region. The electric field distribution in the z-direction for structures with h = 50 nm at a resonance wavelength is presented in Figure 4b. Due to the direct contact between the silver thin film and the silver nanocubes, no mode could be supported in the spacer region, and only the dipolar oscillations of the nanocubes were observed. The large resonance wavelength shift induced by the variations in nanocube height attributed to the absence of localized modes within the spacer layer, which made the resonance wavelength more sensitive to the structural dimensions. Figure 4c shows the electric field intensity distribution at the resonance wavelength on the detection plane, which is located 5 nm above the structure, for a structure with $h$ = 50 nm. Figure 4d illustrates the line profile along the dashed region in Figure 4c, including the profile of a structure without a silver thin film for reference. When the silver thin film was in contact with the silver nanocubes, the presence of the film inhibited dipole oscillations at the bottom of the nanocubes, causing a shift in the localized electric field toward the upper surface of the structure. As a result, the field intensity increased by approximately 1.5 times compared to the case without the film. Although numerous studies have reported electric field enhancement using metamaterials—such as power-flow conformal metamirrors [48], coupled dipole nanoantennas and grating nanostructures [49], and plasmonic coupling architectures based on closely spaced nanoparticles [50]—the present structure exhibited a unique distribution of the field, leading to increased intensity above the nanostructure. This field configuration is particularly compatible with the enhancement of fluorescence signals from molecules positioned above the structure, making it promising for various applications including imaging.

3.3. Evaluation of Refractive Index Sensitivity

The resonance conditions of plasmonic metamaterials are influenced by the surrounding refractive index. Therefore, by measuring shifts in the resonance wavelength, it is possible to sensitively detect molecular adsorption onto the nanostructure or changes in the surrounding environment. In general, the sensitivity to refractive index variations, as defined by the resonance wavelength shift, depends on the wavelength, with higher sensitivity at longer wavelengths. [51,52]. As shown in Figure 5a, sensitivity ($S$= ${\displaystyle \frac{∆\lambda }{∆n}}$) was calculated by varying the surrounding refractive index above the metal structure from 1 to 1.5. Figure 5b,c show the reflectance spectra under varying surrounding refractive indices for structures with different parameters: Figure 5b corresponds to $L$ = 100 nm, $h$ = 30 nm, $G$ = 20 nm, $sp$ = 0 nm, and $d$ = 10 nm, while Figure 5c presents a reference case with $L$ = 100 nm, $h$ = 30 nm, $G$ = 100 nm, and $d$ = 0 nm. The structural parameters were optimized to allow for sensing within the visible wavelength range. To evaluate the effect of the silver thin film on refractive index sensitivity, a reference structure was designed without the silver thin film, with nanocubes of similar size and resonance wavelength adjusted to a comparable position. The sensitivity $S$ was 215 nm/RIU without the silver thin film but significantly increased to 546 nm/RIU when the nanocubes were in direct contact with the silver thin film. This corresponds to an approximately 2.5-fold enhancement in sensitivity. The electric field distributions in the z- and x-direction for each structure are shown in Figure 5d,e. In the absence of the silver thin film, the localized electric fields of the nanocubes were primarily concentrated at their four corners. The regions extending into the glass substrate were unresponsive to changes in the surrounding refractive index. When the gap length was small, the localized electric field appeared not at the corners but within the gap regions between adjacent nanocubes. Moreover, in the configuration where the silver thin film was in contact with the nanocubes, the field shifted toward the upper regions of the gaps, thereby increasing its overlap with the surrounding medium. This likely enhanced the sensitivity to variations in the surrounding refractive index [53]. Several structures with sensitivities exceeding 546 nm/RIU have been reported in previous studies. For example, a sensitivity of 596 nm/RIU at a wavelength of 700 nm was achieved using a metal–insulator–metal (MIM) waveguide coupled with double rectangular cavities [54]. Similarly, a sensitivity of 636 nm/RIU at 800 nm was reported for a silicon nanoring located inside a circular cavity [55]. However, these high sensitivities are typically realized in the longer wavelength region. In contrast, at 600 nm—an important wavelength for visible-range sensing—reported sensitivities are generally around 500 nm/RIU or lower. The structure proposed in this study demonstrates superior sensitivity at this wavelength.

Various methods for fabricating densely packed nanocube arrays have been reported [20,21,22], most of which rely on the chemical synthesis of nanocubes. In contrast, the structure proposed in this study can be fabricated using either electron beam lithography (EBL) or focused ion beam (FIB) techniques. For instance, with FIB, the structure can be realized by directly processing the silver thin film to form nanocube patterns, followed by additional silver deposition. Since the enhanced sensitivity of this structure arises from the electric field localized within the gap, it is thought to be less sensitive to fabrication imperfections such as edge rounding. Although precisely fabricated MIM structures using techniques such as electron beam lithography have also been employed in sensing applications, the localized electric field in those structures is typically confined within the dielectric spacer, limiting its interaction with the surrounding environment [35,56]. In contrast, the present structure, in which the silver thin film directly contacts the nanocubes without a spacer layer, overcomes this limitation. Therefore, nanocube structures in direct contact with the silver thin film hold significant potential for applications as refractive index sensors.

4. Conclusions

We have demonstrated that the combination of silver nanocubes with small gaps and large hotspot areas, along with a silver thin film, enables the control of sharp resonance spectra in the visible wavelength range and successfully enhances the electric field intensity on the top surfaces of the structures. Furthermore, by optimizing the distance between the silver thin film and the nanocube structure, multiple resonance modes can be utilized for imaging applications. Additionally, bringing the silver thin film into direct contact with the nanocube structure significantly enhances the refractive index sensitivity. The proposed small-gap nanocube structures could potentially be fabricated on a large scale by directly processing a silver thin film via FIB techniques. Since the silver thin film is transparent to excitation light, this structure facilitates highly sensitive analysis in bioimaging and sensing applications, particularly in liquid environments where focusing from above the structure can be challenging. While silver is advantageous in terms of optical properties, silver nanoparticles are known to exhibit cytotoxicity and are less stable compared to other metals, particularly due to silver’s susceptibility to oxidation and sulfidation in aqueous environments. However, it has been reported that coating silver nanoparticles with an oxide layer can effectively suppress their toxicity and improve their stability [57], making them suitable for biological applications. These findings suggest promising applications in the fields of medicine and drug discovery, paving the way for advancements in high-sensitivity detection and analysis.