银 长 方 体 -金 膜 结 构 等 离 激 元 纳 米 腔 的 模 式 分 析

Objective The nanogaps between adjacent metallic structures can confine light within an extremely small volume, far exceeding the optical diffraction limitation. Among various nanocavity configurations, the nanoparticle-on-mirror (NPoM) system has demonstrated significant advantages in constructing high-quality nanocavities due to its bottom-up fabrication approach. Widely applied in strong coupling, surface-enhanced Raman spectroscopy, and surface-enhanced fluorescence, this system provides an efficient platform for studying light-matter interactions. However, current research on NPoM structures primarily focuses on high-symmetry geometries (e.g. nanospheres and nanocubes) that exhibit azimuthal and polarization isotropy in their optical responses. In contrast, low-symmetry nanostructures (such as nanocuboids and nanorods) demonstrate differentiated spectral behaviors along different axes due to their anisotropic optical responses. Notably, low-symmetry nanostructures like nanocuboids combine edge-enhanced field localization, uniform gap dimensions, and anisotropic characteristics, potentially enabling richer plasmonic mode excitation. Yet their optical behaviors in NPoM systems remain systematically unexplored, presenting crucial opportunities to investigate the physical mechanisms and application potential of low-symmetry nanocavities. This study aims to reveal the excitation and modulation principles of anisotropic plasmonic modes through constructing low-symmetry nanocuboid-on-mirror (NCboM) structures. By combining experimental characterization with numerical simulations, we elucidate the origins of multiple resonance peaks in the visible spectrum and their dependence on excitation polarization states and azimuthal angles. Methods In sample preparation and characterization, we synthesize gold-core/silver-shell nanocuboids with dimensions of (104±4)nm (length) and (52±2)nm (width) using a liquid-phase seed-mediated growth method [Fig. 1(a)]. A gold film substrate with a 4 nm Al₂O₃ spacer layer is constructed via magnetron sputtering to form the NCboM configuration [Fig. 1(b)]. Single-particle scattering spectra of NCboM are acquired using conventional dark-field microscope (DFM), revealing dual resonance peaks (P1@ 638 nm and P2@760 nm) [Fig. 1(d)]. Experimentally, we develop a polarization-resolved aperture-blocking dark-field microscopy (PRAB-DFM) system [Fig. 2(a)], enabling independent control of incident light polarization states [predominantly p-polarization (PP polarization) and predominantly s-polarization (PS polarization)] and azimuthal angles (φ=0°, 45°, 90°) through a linear polarizer, half-wave plate, and quadrant-blocking aperture [Fig. 2(b)]. This system facilitates systematic investigation of the polarization-dependent dual-peak behavior of NCboM (Fig. 3). For numerical simulations, we establish a model matching experimental parameters, successfully replicating the dual-peak feature of NCboM and generating the corresponding surface charge distribution maps (Fig. 4) to decode the plasmonic modes and their evolution mechanisms associated with each resonance. Further parametric scanning explores the influence of nanocuboid dimensions, including the length (L=90‒140 nm) and width (W=44‒64 nm), on mode characteristics (Fig. 5). Results and Discussions The experiments reveal that the dual peaks (P1 and P2) of NCboM under the traditional DFM originate from the superposition of multiple modes excited by omnidirectional illumination [Fig. 1(d)]. Using the PRAB-DFM system, we discover that different combinations of polarization and azimuth angles can selectively excite orthogonal axial modes. Under PP polarization, as φ increases from 0° to 90°, the P1 intensity gradually decreases while the P2 simultaneously enhances [Figs. 3(b)‒(d)]. Conversely, under PS polarization, changes in φ cause P1 enhancement and P2 weakening [Figs. 3(f)‒(h)]. These results demonstrate that P1 and P2 correspond to two orthogonal axial modes of NCboM. Combined with numerical simulations (Fig. 4), we define the mode order by the number of antinodes in the charge distribution standing wave along specific directions, adopting a dual-axis labeling system (M_mⁿ) where the subscript m denotes the long-axis order and the superscript n represents the short-axis order. We identify four characteristic modes in NCboM: three long-axis modes (M₄¹ , M₃¹ and -M₃¹) and one short-axis mode (M₁²). Notably, the resonance peaks at identical spectral positions under different excitation conditions may correspond to entirely distinct plasmonic nanocavity modes. Further parameter scanning reveals the evolution of modes with cuboid dimensions. When the width is fixed at 52 nm and the length increases [L=90 ‒ 140 nm, Fig. 5(a)], long-axis modes (M₄¹, M₃¹ and -M₃¹) red-shift, with a fifth-order long-axis mode emerging at L=130 nm. When the length is fixed at 104 nm and the width increases [W=44‒64 nm, Fig. 5(b)], the short-axis mode (M₁²) exhibits rapid red-shifting while long-axis modes show only minor shifts. These findings demonstrate that long- and short-axis modes can be independently regulated through dimensional control, effectively avoiding mode coupling interference. This provides a theoretical foundation for nanocavity design. Conclusions We construct a low-symmetry nanostructure of NCboM using silver nanocuboids, which exhibits dual resonance peaks (P1 and P2) under the traditional dark-field microscope. To elucidate the origin of these modes, we innovatively develop a polarization-resolved azimuthal dark-field microscopy system. By systematically investigating the scattering spectral responses under PP and PS polarizations with varying incident azimuth angles, we conclusively resolve that the dual peaks arise from the excitation of intrinsic modes along the orthogonal axes of the nanocuboid. Rigorous numerical simulations replicating experimental conditions systematically reveal the rich plasmonic resonance properties of NCboM along its long and short axes. Furthermore, numerical simulations explore the evolution of NCboM modes with nanocuboid dimensions. The results show that all modes redshift with increasing size, while modes along different axes exhibit no coupling. This work establishes a foundation for studying low-symmetry plasmonic nanocavities, expands research on low-symmetry structures within the nanoparticle-on-mirror framework, and complements existing studies on high-symmetry configurations. The multimode and anisotropic response characteristics of NCboM open avenues for exploring higher-order multipole modes and their interactions. For instance, gradient dimensions or asymmetric gap designs can be introduced to excite magnetic dipole or toroidal dipole resonances. Additionally, tunability can be achieved through dynamic reconfiguration of the gap thickness or dielectric environment using active media (e. g. phase-change materials and liquid crystal layers) or mechanical stress modulation. These approaches will enable the development of tunable plasmonic nanocavities, advancing dynamic photonic devices.