Surface plasmon resonance (SPR) is the resonance of incident light waves at the metal-medium interface with collectively oscillating electrons. Gold nanoparticles exhibit a pronounced local surface plasmon resonance effect, where the position of the local surface plasmon peak is intricately linked to the geometry of the nanoparticles. The electromagnetic properties of non-spherical metal nanoparticles demonstrate excellent controllability as their shape and size undergo variations. In single-structure gold nanoparticle sensors, the increase of the aspect ratio of gold nanoparticles improves the sensitivities of the sensors. However, there is a lack of research on the regulation of surface plasmon resonance in the composite structure. Although recent studies have shown promising outcomes by incorporating gold nanoparticles into composite structures, they lack comprehensive investigations and summaries regarding the impacts of the transverse axis length, longitudinal axis length, aspect ratio, and spacing of gold nanoparticles with different shapes on the surface of gold nanocomposite structures. Fine-tuning the geometric parameters of gold nanoparticles in the composite structure can further elevate the practical value of the composite structure. Hence, this fine-tuning demonstrates significant application potential in areas such as biosensing and detection.

In this study, we establish a model for the dielectric functions of composite structures of non-spherical gold nanoparticles with different shapes. The structures consist of layers from bottom to top: a 700 nm thick SiO₂ layer, a 100 nm thick gold film, a 200 nm thick dielectric matching layer, and gold nanoparticles. Using the finite-difference time-domain (FDTD) method, perfect matched layer (PML) absorbing boundary conditions are applied at the upper and lower boundaries in the z-axis direction, and periodic boundary conditions are applied in the x- and y-axis directions. Plane waves ranging from 400 nm to 900 nm are incident from the positive z-axis. We investigate the changes in transverse and longitudinal dimensions, aspect ratios, and spacings of different-shaped gold nanoparticles, revealing correlations among mode field distributions, local field enhancement amplitudes, and absorption response amplitudes to incident light. Through precise simulations, comprehensive regulation of the surface plasmon resonance peaks can be achieved in composite structures containing non-spherical gold nanoparticles.

SPR typically manifests in both far-field scattering enhancement and near-field localized enhancement. Thus, absorption spectra and electric field intensity distributions can be used to characterize SPR. The peak wavelength of absorption corresponds to the resonance absorption peak of surface plasmons. When light irradiates the metal surface, free electrons on the metal surface collectively resonate under the influence of the incident light, inducing a significant accumulation of electron energy. The extent of this accumulation can be precisely quantified by the numerical values of the electric field intensity. When the shape of gold nanoparticles deviates from spherical, the characteristics of the surface plasmon oscillation change. As a result, the absorption spectra of three composite structures mainly manifest two surface plasmon resonance modes: the vibration mode of free electrons along the long axis of the gold nanoparticles (transverse mode) and vibration mode perpendicular to the long axis (longitudinal mode). The electric field intensity at the longitudinal resonance peak is significantly higher than that at the transverse resonance peak, and the electric field is primarily concentrated near the edges or tips of the structures. Altering the transverse and longitudinal dimensions as well as the aspect ratios of gold nanoparticles with three different shapes has minimal effect on the transverse resonance peak, whereas the longitudinal resonance peak is more sensitive to these variations. Changing the particle spacing results in a redshift of the transverse resonance peak in all three structures, whereas the longitudinal resonance peak either redshifts or remains unchanged. The absorption spectra and two-dimensional cloud maps of absorption properties obtained from simulation calculations for the composite structures remain consistent. By adjusting the structural parameters of the composite structures that comprise gold nanoparticles with different shapes, the precise control of both transverse and longitudinal resonance peaks can be achieved, allowing for matching with specific laser wavelengths required for the desired composite structure.

This study primarily investigates the controllable arrangement of gold nanoparticles and their surface plasmons. By establishing dielectric function models for composite structures consisting of non-spherical gold nanoparticles with different shapes, the study quantitatively explores the transverse and longitudinal dimensions, aspect ratios, and spacings of gold nanoparticles within the wavelength range of 400 nm to 900 nm. The model is used to analyze changes in the structural parameters, revealing correlations among mode field distributions, local field enhancement amplitudes, and absorption response amplitudes to incident light. The precise control of both transverse and longitudinal resonance peaks can be achieved by varying the structural parameters of the composite structures comprising gold nanoparticles with different shapes. This capability enables the matching of laser wavelengths required for the fabrication of specific composite structures and holds significant promise for widespread applications in biochemical sensing, biological imaging, and medical diagnostics and therapeutics.