International Journal of Antennas and Propagation

Volume 2017 (2017), Article ID 4903747, 13 pages

https://doi.org/10.1155/2017/4903747

## Broadband Dipole-Loop Combined Nanoantenna Fed by Two-Wire Optical Transmission Line

^{1}Department of Electrical Engineering, Federal University of Para, Belém, PA, Brazil^{2}Department of Electrotechnology, Federal Institute of Education, Science and Technology of Para, Tucuruí, PA, Brazil

Correspondence should be addressed to Janilson L. de Souza

Received 21 September 2016; Revised 31 December 2016; Accepted 9 January 2017; Published 28 February 2017

Academic Editor: Yuan Yao

Copyright © 2017 Janilson L. de Souza et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper presents a broadband nanoantenna fed by a two-wire optical transmission line (OTL). The antenna is defined by a combination of a dipole and a loop, where only the dipole element is connected to the OTL. The analysis is fulfilled by the linear method of moments with equivalent surface impedance to model the conductors. Firstly, the nanoantenna alone is investigated, where the input impedance, current distribution, reflection coefficient, fractional bandwidth, radiation efficiency, and radiation pattern are analyzed. Then, the input impedance matching of this antenna with the OTL is considered. In this case the current, near field distribution, radiation pattern, and reflection coefficient are calculated for different geometrical parameters. The results show that the loop inserted in the circuit can increase the bandwidth up to 42% and decreases the reflection coefficient in the OTL to −25 dB.

#### 1. Introduction

Recently, accompanying the development of plasmonic technology, the study of antennas exceeded the microwave barrier, reaching the infrared and optical regions. In these regions, there is a profusion of possibilities and proposals of applications. These antennas are devices designed to transmit, receive, and manipulate optical fields in nanometric scale going beyond the diffraction limit [1, 2]. For example, using optical antennas with broadband spectral response one can develop more efficient photovoltaic devices [3, 4] and amplify fluorescent emission spectroscopy [5] and Raman scattering (SERS) [6]. For these and other applications, several models of optical broadband antennas can be used as, for example, in [7] where a broadband monopole nanoantenna is proposed. The increase in bandwidth for the antenna is achieved by varying the antenna dimensions. In [8], a trapezoidal plasmonic nanoantenna is presented, where the enlarged band is achieved by overlapping different dipole resonances. In [9], a large bandwidth is obtained in the plasmonic nanoantenna with six and eight particles with a common gap. In [10], a nanoantenna formed by an array of equally spaced nanorods of variable length is presented. In this case, the increase in bandwidth is achieved by the nanoantenna arrangement.

Optical antennas can also be used in conjunction with plasmonic waveguides for designing highly integrated photonic signal processing systems, because plasmonic waveguides can efficiently handle optical fields in nanometric scale beyond the diffraction limit as well [11]. For example, a plasmonic waveguide in the form of a two-wire optical transmission line (OTL) is used in papers [12–14] to make the interconnection with optical antennas forming an optical nanocircuit. Here, the optical antennas work as terminal elements transforming the far field radiation into guided waves and vice versa. In the above works, the impedance matching between the plasmonic waveguide and the optical antenna was analyzed. The optical circuit comprises one nanoantenna for reception and another one for emission connected to a two-wire OTL. In [12], the impedance matching was attained varying the geometric dimensions of the nanodipole and the gap of the OTL for a fixed frequency. In [13] some conclusions are presented about the conditions where a better impedance matching between the emitter dipole and the OTL for a fixed frequency can be obtained. Besides, an analysis of the excitation is fulfilled as well. In [14], the nanocircuit is fed by an aperture probe, where the coupling between the aperture probe and the receiving antenna is modeled by an equivalent voltage source. In this article, the authors analyzed the impedance matching varying the geometrical dimensions of the nanodipole for a given frequency range. In the aforementioned works, the optical circuits and optical antennas with broadband spectral response were not analyzed.

In this work, a theoretical analysis of a broadband nanoantenna in an optical nanocircuit is presented. The broadband nanoantenna is formed by a combination of a loop and a dipole antenna. It is connected to a two-wire optical transmission line. The nanoantenna is obtained by placing the electric nanodipole in the center of the rectangular loop with a power source connected to the nanodipole. This geometry was used due to its simplicity in manufacturing and calculus as compared, for example, with the geometries of [7, 8]. Notice that this type of antennas for microwave region was analyzed in [15].

The numerical analysis is performed via a simple and efficient computational method based on linear method of moments (MoM) [16]. The presented results concern the nanoantenna bandwidth and quantitative analysis of the impedance matching of the OTL with the nanoantenna. Some results are compared with the simulations by the commercial software Comsol [17]. The results show that the loop used in the circuit increases the bandwidth of the nanoantenna to 42% and decreases the voltage reflection coefficient of the optical nanocircuit up to −25 dB.

#### 2. Theoretical Development

This section presents the geometry of the problem and the model of the plasmonic nanocircuit by the MoM. In this model, the Lorentz-Drude model is used to represent the complex permittivity of the metal, which is used in the calculation of the surface impedance of the cylindrical conductors of the circuit. The linear MoM is used to solve the 1D integral equation of the electric field with linear approximation of the longitudinal current, sinusoidal basis functions, and test functions of rectangular pulse [16].

##### 2.1. Description of the Problem

The nanocircuit structure is shown in Figure 1, where cylindrical gold conductors located in free space form the structure. In this figure, a voltage source applied in the gap feeds the nanocircuit. The nanocircuit is composed of a two-wire OTL connected to a broadband nanoantenna formed by the combination of a dipole antenna (straight dipole) and a loop antenna (rectangular loop). The voltage source is centered at the origin of the reference system, the OTL and the dipole are on the plane , and the loop is located in the plane ; that is, is the distance between the dipole and the loop.