Theoretical analysis of thin-wire elliptic antennas

In this communication we extend the state-of-the art by providing closed-form equations for thin-wire elliptical antennas with arbitrary current distributions, valid from low frequencies to the infrared regime. To this end, we derive an electric-field integral equation (EFIE) for imperfectly conducting wires and elliptical geometries. Using this formulation, we obtain unknown arbitrary current distributions through a modal expansion, enabling thus the calculation of far-fields and other radiation parameters. Results shown not only achieve remarkable but also to show the superior design possibilities of elliptical geometries in comparison to the classical circular loops, which may be considered just a particular case of the methodology here presented. Special attention is paid to mathematical details of electric farfield equations, thus providing guidelines to produce efficient codes

Inverse Design of Three-Dimensional Nanoantennas for Metasurface Applications

Recent advances in manufacturing techniques have been made to match the demand for high performance optical devices. To this end, tremendous research activity has been focused on optical metasurfaces as they offer a unique potential to achieve disruptive designs when paired with innovative fabrication techniques and inverse design tools. However, most metasurface designs have revolved around canonical geometries. While these elements are relatively easy to fabricate, they represent only a small portion of the design space, and rarely offer peak performance in transmission, phase range or field of view. In this work, a Lazy Ant Colony Optimization (LACO) technique is applied in conjunction with a full-wave solver using the Periodic Finite Element Boundary Integral (PFEBI) method to reveal high performing three-dimensional nanoantenna designs with potential applications for a variety of optical devices

A Computationally Efficient Method for Simulating Metal-Nanowire Dipole Antennas at Infrared and Longer Visible Wavelengths

This paper presents a numerically efficient approach for simulating nanowires at infrared and long optical wavelengths. A computationally efficient circuit-equivalent modeling approach based on the electric-field integral-equation (EFIE) formulation is employed to simulate the highly dispersive behavior of nanowires at short wavelengths. The proposed approach can be used both for frequency-domain and for time-domain EFIE formulations. In comparison with widely used full-wave solutions achieved through the finite-difference time-domain method, the circuit-based EFIE formulation results in a sharp reduction of the computational resources while retaining high accuracy.This work was supported in part by the Spanish Ministry of Education under Project PR2009-0443, in part by the Penn State MRSEC under NSF Grant 0213623, in part by the EU FP7/2007-2013 under Grant GA 205294 (HIRF SE project), in part by the Spanish National Projects TEC2010-20841- C04-04, CSD200800068, and DEX-5300002008105, and in part by the Junta de Andalucia Project P09-TIC5327

Analytical Expressions for the Mutual Coupling of Loop Antennas Valid from the RF to Optical Regimes

Arrays of circular loop antennas are commonly employed at radio frequencies for communications and geo- physical sensing, while also holding enormous potential in the optical regime for applications such as solar energy harvesting. Exact analytical expressions exist for predicting the mutual coupling between a variety of antennas, including dipoles and slots. However, due to the complexity of the integrals involved, analytical expressions for evaluating the coupling between loop antennas have not been previously available. This paper presents straightforward analytical expressions for efficient calculation of the coupling between two circular loops at arbitrary locations. The theory is extended to the optical regime by taking into account the dispersion and loss of the material comprising the loop antenna. These analytical expressions provide insight into the physics underlying the mutual coupling phenomenon. Along with the approximate analytical expressions, a useful pseudo-analytical representation is developed which is more exact, especially in the near-field regime, and can be easily and efficiently evaluated in MATLAB via numerical integration. It is shown that full-wave simulations for a two-element array of nanoloops can take up to six hours, while the corresponding analytical and pseudo-analytical implementations derived here take less than a minute.This work was supported in part by the Spanish Ministry of Education- Commission Fulbright Program “Salvador de Madariaga” for sponsoring the join t research collaboration under Grant PRX14/00320, in part by the Spanish and A ndalusian research programs Grant TEC2013-48414-C3-01 and Grant P12-TIC-1442, in part by the Center for Nanoscale Science, and in part by an NSF Materials Research Science and Engineering Center under Grant DMR-142062

Closed-Form Expressions for the Radiation Properties of Nanoloops in the Terahertz, Infrared and Optical Regimes

This work was supported in part by the Spanish Ministry of Education through the Commission Fulbright Program “Salvador de Madariaga” under Grant PR X14/00320, in part by the Spanish and Andalusian Research Programs under Grant TEC2013-48414-C3-01 and Grant P12-TIC-1442, and in part by the Center for Nanoscale Science, NSF Materials Research Science a nd Engineering Center, under Award DMR-1420620Since the pioneering work of Heinrich Hertz, perfect-electric conductor (PEC) loop antennas for RF appli- cations have been studied extensively. Meanwhile, nanoloops are promising in the optical regime for their applications in a wide range of emerging technologies. Unfortunately, analytical expressions for the radiation properties of conducting loops have not been extended to the optical regime. This paper presents closed-form expressions for the electric fields, total radiated power, directivity, and gain for thin-wire nanoloops operating in the terahertz, infrared and optical regimes. This is accomplished by extending the formulation for PEC loops to include the effects of dispersion and loss. The expressions derived for a gold nanoloop are implemented and the results agree well with full-wave computational simulations, but with a speed increase of more than 300 × . This allows the scientist or engineer to quickly prototype designs and gain a deeper understanding of the underlying physics. Moreover, through rapid numerical experimentation, these closed-form expressions made possible the discovery that broadband superdirectivity occurs naturally for nanoloops of a specific size and material composition. This is an unexpected and potentially transformative result that does not occur for PEC loops. Additionally, the Appendices give useful guidelines on how to efficiently compute the required integrals.Spanish Ministry of Education through the Commission Fulbright Program “Salvador de Madariaga” under Grant PR X14/00320Spanish and Andalusian Research Programs under Grant TEC2013-48414-C3-01 and Grant P12-TIC-1442Center for Nanoscale Science, NSF Materials Research Science a nd Engineering Center, under Award DMR-142062

A Knotted Meta-molecule with 2-D Isotropic Optical Activity Rotating the Incident Polarization by 90{\deg}

Optical activity is the ability of chiral materials to rotate linearly-polarized (LP) electromagnetic waves. Because of their intrinsic asymmetry, traditional chiral molecules usually lack isotropic performance, or at best only possess a weak form of chirality. Here we introduce a knotted chiral meta-molecule that exhibits optical activity corresponding to a 90{\deg} polarization rotation of the incident waves. More importantly, arising from the continuous multi-fold rotational symmetry of the chiral torus knot structure, the observed polarization rotation behavior is found to be independent of how the incident wave is polarized. In other words, the proposed chiral knot structure possesses two-dimensional (2-D) isotropic optical activity as illustrated in Fig. 1, which has been experimentally validated in the microwave spectrum. The proposed chiral torus knot represents the most optically active meta-molecule reported to date that is intrinsically isotropic to the incident polarization