A reliable fabrication technique for obtaining a high density of regular nanogaps is critical for ultrasensitive surface-enhanced Raman scattering (SERS). However, nanogaps produced between nanostructures have suffered from the lack of controllability. Taking the sphere−film structure as a starting point for its straightforward fabrication technique and well-controlled gaps, we propose a novel nanostructure by combining Au nanospheres and inverse pyramidal holes. The proposed nanostructure is obtained by trapping Au nanospheres in inverse pyramidal hole arrays to create multiple, uniform, and reproducible geometrical gaps near the contact points between the nanospheres and the inverse pyramids. The combined nanostructure, referred to as SIP (spheres in pyramids), supports augmented plasmon hybridization due to mirroring of gap plasmons and induces much stronger electromagnetic enhancement at normal incidence when compared to the contacting sphere−film structure. In contrast to the sphere−film structure exhibiting a maximum enhancement for slant incidence, the SIP achieves a maximum enhancement at normal incidence. The effective plasmon hybridization from SIP arrays sustains SERS intensities 9 times stronger than those of conventional contacting sphere−film structures and achieves 64% of the total electromagnetic field enhancement obtained from the sphere−spacer−film structure with a 0.6−0.8 nm thick spacer.

This study aims to apply the self-assembly process of particles to the fabrication of inverse opal structures, which improve the fabrication of catalysts and sensors. The process consists of two dip-coating steps. The first one is the production of sacrificial silica particles 1 or 2 µm in diameter. The second one is the fabrication of silver or gold nano-particles. After these processes, silica particles are dissolved to create the inverse opal structure. We demonstrate how changing the diameter of the sacrificial particle varies the size of the pores. Finally, we present how the patterned Ag and Au inverse opal structure can be created using the hydrophobic/hydrophilic patterned substrate.

We studied closely spaced gold nanosphere chains on Pt mirrors exhibiting strong plasmon coupling between both horizontally and vertically oriented modes relative to the chain. Resonance modes originating from the plasmon couplings realized effective light concentration around the short nanosphere chain and showed red shifts with decreasing interparticle gap length, revealing the hybrid nature of the two plasmonic modes. Thanks to the effective light concentration, the short nanosphere chain demonstrated strong surface enhanced Raman scattering (SERS) that was not strongly affected by variations in the length of the gaps or when some of the spheres made contact with each other. Even with large gaps, the short nanosphere chain exhibited consistent SERS under a low excitation power of only 0.4 mW/μm2, owing to the geometrical robustness of the nanospheres and Pt plane supporting enhancement of the electric field in the sphere-plane gaps.

Metallic nanostructures sustain surface plasmons that strongly couple incident light to the surface of the metal. They have received a great amount of attention as a possible means of light manipulation. Among the various reported structures, cavities are promising for spectroscopic applications because the cavity mode, with its narrow band, large modulation, and tunability over a wide range of wavelengths, provides a means to improve the resonances in far-field measurements. This report describes a suspended plasmonic nanofin-cavity structure capable of producing tunable reflected resonances with high quality factors (Q) in the infrared (IR) region. The nanofin-cavity structure having plasmonic hot spots on the ridges supports standing-wave resonances with enhanced electric fields in the horizontal and vertical directions; strong optical flows are thus generated and light turns in a loop, resulting in high reflectance. When the nanofin cavity is further coupled to the propagating surface plasmon resonance (SPR), a strong and narrow-band reflectance resonance due to the stringent condition of SPR arises with a bandwidth having a full width at half-maximum (FWHM) of 92 nm and a Q as large as 60. Furthermore, as result of the coupled modes, the intensity of the resonance peak depends on the angle of the incident light and thus presents a potential means of angle-controlled optical switching in the IR region. Finally, high-order modes of a nanofin-cavity structure were observed in the near-infrared (NIR) region for which wavelengths are much shorter than the cavity scale, thus, demonstrating the possibility of easing the difficulty of fabrication.

No abstract