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

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 nanoparticles (n ∼ 2.67), which confine plane-polarized incident light (λ = 532 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 obtained with the excitation laser polarized either parallel (θ = 0°) or perpendicular (θ = 90°) to the nanotubes. The spectral intensity increased by the nanoparticles only at θ = 90°, with the degree of enhancement being greater when the nanotube was located farther from the particle-substrate contact. Finite-difference time-domain simulations explained that such an enhancement at θ = 90° was a sub-diffraction phenomenon, which occurred when the nanotubes were located within one of the two confined bands formed by the TiO₂ nanoparticles. On repeating the measurements on a two-dimensional graphene sheet, only diminished Raman scattering of the graphene with no polarization dependence was observed, confirming the advantage of the one-dimensional nanostructure for studying sub-diffraction optics.