Plasmon-enhanced absorption spectra of carbon nanotube in an inhomogeneous near field.

The absorption spectra of hybrid systems composed of a metal nanosphere and an open-ended, finite-sized armchair single-walled carbon nanotube (SWCNT) are systematically investigated using the hybrid real-time time-dependent Hartree-Fock (RT-TDHF)/finite difference time domain (FDTD) approach. Multiscale methods combine the real-time TDHF approach based on the semi-empirical intermediate neglected differential overlap Hamiltonian for molecular electronic dynamics with the classical computational electrodynamics approach, the FDTD, for solving Maxwell's equations. The linear and nonlinear effects of scattered field gradient and incident field strength on the absorption spectra of SWCNT were investigated. It is found that the non-uniform distribution of the near field causes further enhancement of the CNTs' absorption spectra, especially for the hybrid excitation band to which the molecule excitation strongly couples with the plasmon mode. As the intensity of the incident field I increases from weak to strong, the interaction between metallic NPs and CNTs results in an observable Fano asymmetric line shape in the absorption spectra. However, the non-uniformity of the fields introduces additional nonlinear effects. This causes high-frequency signals to dominate the absorption spectra rapidly and weakens the coupling between molecular excitation and plasmons. Competition between the coupling and the Fano parameter q first causes the Fano interference to decrease and then to increase. These studies are expected to encourage further research into plasmon-molecular interactions and the regulation and applications of Fano resonance.