Probing sub-diffraction optical confinement via the polarized Raman spectroscopy of a single-walled carbon nanotube

Abstract

Polarized Raman spectroscopy of a single-walled carbon nanotube (SWNT) was shown to serve as a simple alternative to sophisticated imaging tools for probing sub-diffraction optical phenomena. As a model system, we used TiO2 nanoparticle (n ~ 2.67), which confines plane-polarized incident light (λ = 523 nm) into two bands less than 150 nm apart. After depositing the nanoparticles onto SWNTs and measuring the nanoparticle-SWNT distance, Raman spectra of individual SWNTs were collected with the excitation laser polarized either parallel (θ = 0o) or perpendicular (θ = 90o) to the nanotubes. Only at θ = 90o did the spectral intensity increase by the nanoparticle, with the degree of enhancement greater when the nanotube was located farther from the particle-substrate contact. Finite-difference time-domain simulations explained that such enhancement at θ = 90o was a sub-diffraction phenomenon, which occurred when the nanotubes were located within one of the two confined bands formed by the TiO2 nanoparticle. Repeating the measurements on a two-dimensional graphene sheet only diminished Raman scattering of the graphene with no polarization dependence, confirming the advantage of one-dimensional nanostructure for studying sub-diffraction optics.