Surface plasmon resonance "hot spots" and near-field enhanced spectroscopy at interfaces

Optical excitations and mutual couplings of surface plasmons with specific modes in metal nanostructures are the physical basis for developing the high spatial resolution, high sensitivity, and high precision spectroscopy. Here, we systematically review latest advances in optical excitations, classifications and identifications of surface plasmon resonance modes and their typical applications in several typical interfaces. We discuss several aspects below. First, the intrinsic mechanism of creating "hot spots" in metal particle-film systems is elucidated by the finite-difference time-domain numerical method. Spatial transfers and influence factors of the "hot spots" under plasmon-induced electric-resonance and plasmon-induced magnetic-resonance conditions are discussed. Second, the plasmon-induced magnetic-resonance in the visible-light region is successfully realized in a gold nanoparticle-film system. Meanwhile, experimental results of surface-enhanced Raman spectroscopy show that the "hot spots" in the magnetic-resonance mode can output Raman scattering with a much higher enhancement factor than that in the conventional electric-resonance mode. Third, we design nonlinear nanorulers that can reach approximately 1-nm resolution by utilizing the mechanism of plasmon-enhanced second-harmonic generation (PESHG). Through introducing Au@SiO2 (core@ shell) shell isolated nanoparticles, we strive to maneuver electric-field-related gap modes such that a reliable relationship between PESHG responses and gap sizes, represented by "PESHG nanoruler equation", can be obtained. Fourth, a critical and general solution is proposed to quantitatively describe the spatial resolution and directional emission in tip-enhanced Raman spectroscopy and tip-enhanced fluorescence. These results may help enhance our understanding of the intrinsic physical mechanism of the surface plasmon resonance, and offer opportunities for potential applications in surface-enhanced Raman spectroscopy, tip-enhanced Raman spectroscopy, second harmonic generation, and other plasmon-enhanced spectroscopy.