Oblique-Incidence-Excited Localized Hot Spots in Plasmonic Particle-on-Film Nanocavities

Abstract

Regarding plasmonic nanocavities formed by particle-on-film, the nanogap hot spots play an essential role in wide applications including plasmon-enhanced spectroscopy, ultrasensitive sensing, photovoltaics, and photocatalysis. However, hot spots at the nanogap are symmetrically distributed on both sides of the junction under normal incidence. Herein, under the condition of oblique incidence, properties of plasmonic hot spots in the Au@SiO2 particle-on-film have been investigated theoretically. We find that when the plane wave is incident obliquely, the hot spots in the gap are not only symmetrically distributed on both sides of the contact point but also circularly distributed around the junction at specific excitation wavelengths. With the adjustment of the incidence angle, the maximum surface-enhanced Raman spectroscopy enhancement factor from this circularly distributed hot spot reaches 11.3 orders in magnitude. Moreover, the resonant mode at 785 nm has been readily matched by optimizing the size of the Au core, and the optimal thickness of the silica shell with 1 nm for achieving the maximum electromagnetic (EM) field enhancement was further demonstrated. These findings offer a new strategy for manipulating locations of localized hot spots with strong near-field enhancement in plasmonic nanocavities.