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.