Excitation of dark plasmonic cavity modes via nonlinearly induced dipoles: applications to near-infrared plasmonic sensing.

We demonstrate that dark plasmon modes of cavity-shaped plasmonic structures made of metallic nanowires can be excited by local dipoles induced via second-harmonic generation. The optical properties of these plasmonic cavity modes are thoroughly characterized by using a numerical method that provides a complete description of the optical field at both the fundamental frequency and the second harmonic. In particular, we show that the optical properties of these plasmonic cavity modes are strongly dependent on the geometry of the plasmonic cavity and the material parameters of its constituents. This enhanced sensitivity of dark plasmonic cavity modes to the surrounding dielectric environment can find applications in plasmonic sensing. Specifically, this novel approach to sensing reveals that detection limits of 10(-5) refractive index units can readily be achieved by using wavelength-sized plasmonic devices.