Obtaining narrow-linewidth (high-Q-factor) plasmon resonances in metal nanostructures is of crucial importance for improving device performance, which is limited by the intrinsic Ohmic and radiative losses of metal nanostructures. Fano resonance based on the coupling between the subradiant "dark" and superradiant "bright" plasmon modes in metal nanostructures has generally been recognized as an efficient strategy to narrow the linewidths of plasmon resonances. For this reason, it has been widely utilized for improving the performance of nanodevices, such as nanolasers and nanosensors. Recently, the design and generation of multiple Fano resonances have also attracted extensive attention for the further improvement and expansion of the functionalities of metal nanostructures, which is still a challenge. Up to now, the "symmetry breaking" mechanism, the plasmon-waveguide coupling mechanism, and the complex plasmon-graphene nanostructures are three main schemes for exciting multiple high-order narrow-linewidth modes and further inducing the generation of multiple Fano resonances by coupling the high-order modes with the broad-linewidth plasmon modes. More recently, multiple Fano resonances have also been successfully and experimentally observed in simple spherical dielectric-metal core-shell resonators as a result of the near-field coupling among the multipolar sharp cavity plasmon modes and the broad sphere plasmon mode. In addition, the excitation efficiency (resonance intensity) of the multiple high-Q-factor cavity plasmon modes is also of great importance for applications in certain cases, but it has not been studied so far as well. Therefore, this paper systematically (theoretically and experimentally) studies the influences of the shape (spherical or ellipsoidal) and the integrity (with or without small openings at the sidewall equator) of the metal shell array on the excitation efficiency of the multiple high-Q-factor cavity plasmon modes.

The optical spectra (absorption, reflection, and transmission) and the near-field electromagnetic field distributions are calculated by the three-dimensional finite element simulation software COMSOL Multiphysics 5.4. To simplify the calculation, this paper sets the simulation domain to a cuboid composed of two separate quarters of the PS/Ag core-shell structures. The incident light is set to a plane wave with perpendicular incidence with respect to the array, and the perfect electric conductor boundary conditions and the perfect magnetic conductor boundary conditions are applied to the incident electric field direction and the incident magnetic field direction along the four sides of the simulation domain, respectively. Perfectly matched layers are applied to the upper and lower surfaces of the simulation domain to absorb reflected and transmitted light. The PS/Ag core-shell array is experimentally fabricated by employing the recently developed self-supporting technology. In brief, a monolayer of monodisperse PS spheres (the coefficient of variation is smaller than 2%) with a diameter of 994 nm is first self-assembled on the water/air interface by a modified Langmuir-Blodgett method. Subsequently, they are transferred onto a substrate with tens of micrometer-sized through-holes to form a self-supporting PS microsphere monolayer by exploiting the strong interparticle van der Waals interactions. Then, thin silver films with an identical thickness are successively deposited on the upper and lower half-surfaces of the self-supporting PS monolayer in the fashion of plasma sputtering. Remarkably, the existence of the small connections between adjacent PS microspheres results in the formation of six trumpet-shaped openings at the sidewall equator of the silver shell.

Specifically, the absorption, reflection, and transmission spectra of the perfect spherical silver shell array are theoretically calculated. The results demonstrate that the electric-based cavity plasmon modes (TM₂ and TM₃) can be efficiently excited while the magnetic-based cavity plasmon mode (TE₁) has low excitation efficiency (Fig. 1). In addition, the excitation efficiency of the TE₁ cavity plasmon mode can be greatly promoted by either changing the shape of the silver shell from spherical to ellipsoidal or constructing six small openings at the sidewall equator of the spherical silver shell. This is further revealed by the changes in the shape and the enhancement of the electric field (Fig. 2 and Fig. 3). Especially, an optimal opening angle (about 20?) can be obtained to maximize the excitation efficiency of the TE1 cavity plasmon mode even for different silver thickness in the range of 30-70 nm (Fig. 3). Furthermore, the TM₂, TE₁, and TM₃ cavity plasmon modes can be simultaneously efficiently excited in the non-perfect ellipsoidal silver shell array with six small openings (the opening angle is about 20?) at the sidewall equator (Fig. 4). Last but not least, the non-perfect ellipsoidal silver shell array with six small openings (the opening angle is about 20?) is successfully fabricated by applying the recently developed self-supporting technology. The measured transmission spectrum is in good agreement with the theoretical one, confirming the simultaneous high efficient excitation of the multiple high-Q-factor cavity plasmon modes (Fig. 5).

The shape and the integrity of the silver shells have a substantial influence on the excitation efficiency of the multiple high-Q-factor cavity plasmon modes. The theoretical results show that the electric-based cavity plasmon modes (TM₂ and TM₃) can be efficiently excited while the magnetic-based cavity plasmon mode (TE₁) has low excitation efficiency in the perfect spherical silver shell array. The excitation efficiency of the TE₁ mode can be significantly improved by engineering the silver shell from spherical to ellipsoidal or constructing six small openings at the sidewall equator of the spherical silver shell. In particular, an optimal opening angle (about 20?) is theoretically available to maximize the excitation efficiency of the TE₁ mode. Further theoretical investigation reveals that the TM₃, TM₂, and TE₁ modes can be efficiently excited in the non-perfect ellipsoidal silver shell array with an opening angle of about 20? simultaneously. In experiments, the non-perfect ellipsoidal silver shell array is successfully fabricated by employing self-supporting technology. The actual opening angle of the silver shells is precisely estimated to be about 20?, representing excellent agreement with the theoretical optimal value. As a result, the measured transmission spectrum is also in good agreement with its theoretical counterpart, directly confirming the simultaneous efficient excitation of the multiple cavity plasmon modes.