Effective wavelength and resonance characteristics of carbon nanotube optical antenna
Authors: Wu Xiao-Fang  , Xie Shu-Guo  , He Yun-Tao  , Li Li  , Li Xiao-Lu  ,   School of Electronic and Information Engineering, Beihang University, Beijing 100191, China;,   School of Instrumentation Science and Opto-Electronics Engineering, Beihang University, Beijing 100191, China
Published: December 2020

The effective wavelength scaling theory for optical antennas indicates that an optical antenna does not respond to the wavelength of incident electromagnetic wave, but to a shorter effective wavelength which depends on the plasma wavelength and optical dielectric permittivity of the antenna material, and also on the geometric structure of the antenna. In this paper, based on the effective wavelength scaling theory for optical antennas and on the assumption that metallic carbon nanotube (CNT) can be described by a free electron gas according to the Drude model, the general relationship between effective wavelength and dielectric properties of the antenna material for a metallic carbon nanotube optical antenna is derived. According to this relationship, the investigation into the effective wavelength that a metallic CNT optical antenna responds to can be transferred to easier theoretical calculation for the dielectric properties of CNT, instead of exploring its plasma wavelength. Following first-principle calculations for dielectric properties of CNT with 4 diameter, the effective wavelength versus incident wavelength for each of two types of metallic 4 CNT antennas is investigated. In addition, the resonance characteristics of metallic 4 CNT dipole antennas are analyzed. It is shown that the effective wavelength approximately follows a linear relationship with wavelength of the incident light for the 4 metallic CNT antenna, which is consistent with the wavelength scaling theory. In addition, CNT optical antenna has good wavelength scaling performance compared with nano-antennas made of conventional metals like silver and gold; hence metallic CNTs as optical antennas are beneficial for constructing more compact devices. Moreover, according to the simulation results of resonance characteristics of metallic 4 CNT dipole antennas, there are several 4 metallic CNT dipole antennas with small difference in length meeting the resonance conditions for incident electromagnetic wave with a certain frequency, while there are one or more corresponding resonant modes in the optical and near-infrared spectral range concerned for a 4 metallic CNT dipole antenna with fixed length. Therefore, it is easier to meet the resonance conditions for CNT optical antenna than for conventional metal optical antenna, which also arises from the superior wavelength scaling ability of CNT. These advantages of CNT can help to miniaturize the optical antenna and improve the efficiency of energy conversion of the incident radiation in the optical and near-infrared spectral range. Reliability of the assumption and the theoretical process in this paper are validated by comparing the simulation results with existing investigations. Therefore, the theoretical investigations in this paper may provide a new approach to studying metallic CNT optical antennas. The simulation results also demonstrate the potential applications of CNT optical antenna, including solar energy harvesting and conversion.