Cavity-Induced NIR Tunability in Optical Response and Energy Confinement of Dumbbell-Shaped Au Nanorod

Abstract

Gold nanorod has been shown to exhibit two distinct plasmon resonances in extinction spectra, a transverse mode and a longitudinal mode, which can be tuned by changing the aspect ratio of length and width. What will happen when a nanorod with a constant aspect ratio compounds with semispheres? A theoretical model that combines gold nanorod and semispheres constructing a dumbbell-shaped nanorod is performed in this paper. We report a detailed theoretical investigation on how the nanostructures with a cavity based on gold nanorods can tune the optical properties and enhance localized field. The results show that the topography of the nanorods provides other degrees of freedom to tune the optical properties. Indeed, when the direction of incident light is along the x-axis, the resonance frequency and spectral width of longitudinal modes can be tuned in a wide wavelength range by varying the radius of semisphere-suspended heads. The resonance modes in the infrared zone can be significantly tuned up to 800 nm by varying the radius of semispheres. Also, the field distribution of the dumbbell-shaped nanorod can be significantly changed by plasmon excitement of semisphere-suspended heads. Field amplification up to four orders of magnitude can be obtained, which can provide a new path to tune the optical response of the nanorod and will have wide applications in localized surface plasmon resonance (LSPR)-based biosensing and intracellular imaging and photothermal therapy.