The excitation of the higher-order multipole localized surface plasmon resonance (LSPR) modes in sub-10 nm-gap plasmonic structure is desired in many plasmon enhancement applications. However, unfortunately, the higher-order multipole modes are either hard to be excited by light or make a very small contribution to electromagnetic enhancement, generally. In this paper, we demonstrate that tunable longitudinal higher-order multipole LSPR modes can be excited in large-area metal sub-10 nm-gap structures based on mature PS@Au core-halfshell ordered array plasmonic nanostructure by experiment and numerical simulation. Our results show that the longitudinal higher-order multipole SPR modes can be regarded as the like-cavity modes and can be excited effectively in strong coupling sub-10 nm-gap array structures and, furthermore, make a major contribution to the huge electromagnetic enhancement. Additionally, the longitudinal higher-order multipole SPR modes can be tuned easily by changing the longitudinal size of the plasmonic structure. As a benefit from this, we fabricated the optimized ultrasensitive surface-enhanced Raman scattering (SERS) substrate successfully. Our study provides a new view for design of surface plasmon resonance enhanced devices.

The plasmon hybridization theory is widely used to study the plasmon response of metallic nanostructures. In this work, we study the plasmon hybridization picture of the gold–silica–gold multilayer nanoshells from the viewpoint of the optical extinction spectrum and the charge density distribution. We find that reducing the distance between the Au core and the Au shell causes the conversion from |ω–+⟩ to |ω+–⟩ modes of the high energy extinction peak. According to our opinion, it is because the increased plasmon interaction between the Au core and the Au shell induces the energy reversion of the |ω–+⟩ and |ω+–⟩ plasmon modes. The interesting contrary shift effect of the high energy extinction peaks with different dielectric constants of the middle silica shell and outer surrounding is well-explained by the |ω+–⟩ modes. The energy reversion of hybrid plasmon modes we reported would give new insight into the plasmon hybridization theory. Moreover, our study could offer a modified way based on the charge interaction analysis, which is a necessary supplement to the plasmon hybridization theory, for studying plasmon responses in the optical spectrum of metal nanostructures.

In this study, the optical properties induced by dual symmetry breaking including both shell cutting and core offsetting in the gold-silica-gold multilayer nanoshells have been studied by the discrete dipole approximation simulations and the plasmon hybridization theory. The influences of the incident polarization and geometrical parameters on the plasmon resonances of these dual-symmetry-breaking Au-silica-Au multilayer nanoshells (DSMNS) are investigated. Under the combined effect of the two types of symmetry breaking, it is found that the polarization-dependent multiple plasmon resonances can be induced in the DSMNS. By changing the polarization of 90o, the switching of the two transparency windows can be flexibly adjusted in the DSMNS with different types of Au core offsetting. This polarization-controlled transparency is likely to generate a wide range of photonic applications such as filters and color displays. Furthermore, the local refractive index sensitivity of the DSMNS is also investigated, and the triple extinction peaks simultaneous shift is found as the surrounding medium changed, which suggests the potential applications for biological sensors.

Symmetry breaking in gold nanoshell (or multilayer nanoshells) can supply many interesting optical properties, which has been studied in gold nanostrucutres such as nanocup, nanoegg, and core offset gold-silica-gold multilayer nanoshells. In this work, the optical extinction properties of the perforated gold-silica-gold multilayer nanoshells are studied by the discrete dipole approximation method simulations and plasmon hybridization theory. The extinction spectra of these particles are sensitive to the orientation of the particle with respect to polarization of the light due to the symmetry breaking. Because of the coupling of the plasmon resonance modes between the inner gold sphere and the outer nanocup structure, the perforated gold-silica-gold nanoshell provides the additional plasmon resonance peak and an even greater spectral tunability comparing with the nanocup of similar dimensions. By changing the geometry of the particles, the extinction peaks of the particles can be easily tuned into the near-infrared region, which is favorable for biological applications. The local refractive index sensitivity of the particles is also investigated, and the multiple extinction peaks simultaneous shift is found as surrounding medium is altered. The perforated gold-silica-gold multilayer nanoshells may provide various applications ranging from angularly selective filters to biological sensors.

When two coherent beams interfere in bacteriorhodopsin (bR) film with a very small crossing angle, degenerate multi-wave mixing appears. In this paper, the properties of degenerate multi-wave mixing in bR film were studied. By use of degenerate multi-wave mixing in bR film, we realized optical Boolean logic operations.