Abstract

Alfvén eigenmodes excitation by external loop antennas in the Compact Helical System heliotron/torsatron Rev. Sci. Instrum. 72, 405 (2001);

Modern antenna theory forms the bulwark of our knowledge of how radiation and metallic structures interact in the radio frequency (RF) and microwave (MW) regions. The theory has not yet penetrated the terahertz, infrared, and optical regions to the same degree. In this paper, we provide a rigorous analysis of closed circular loop antennas from first principles. Using antenna theory, we tie together their long wavelength behavior with their behavior at short wavelengths through the visible region. We provide analytic forms for the input impedance, current, quality factor, radiation resistance, ohmic loss, and radiation efficiency. We provide an exact circuit model for the closed loop in the RF and MW regions, and extend it through the optical region. We also provide an implicit analytic form for the determination of all modal resonances, allowing prediction of the resonance saturation wavelength for loops. Through simulations, we find that this behavior extends to hexagonal and square loops. All results are applicable to loop circumferences as short as 350 nm. Finally, we provide a precise analytic model of the index of refraction, as a tool in these computations, which works equally well for metals and semi-conductors. V C 2013 AIP Publishing LLC. [http://dx.

The analytical theory of perfectly conducting thinwire closed-loop antennas with multiple loads in the periphery was formally derived in the 1950s and 1960s. In this paper, it is extended to loop antennas and nanorings for use in communications, in the "Internet of things," and as metamaterials. The new derivation relies on recent work from 2013 that incorporates the surface impedance of metal wires into the standard theory, thus pushing its applicability into the gigahertz, terahertz, and optical regimes. Surface impedance effects cause losses and phase shifts in the current within the loop, which in turn cause wavelength scaling and degradation of signal strength. These effects are modeled using a critical point transition model of permittivity and of the index of refraction. The new results therefore extend standard loop antenna theory so that it now includes characteristics of multiply loaded loops over a very broad spectrum from radio frequencies to the optical region. The new model is verified using modern simulation tools. The examples given here include resistive and capacitive loading.

Many types of wavelength-scale optical structures have been investigated for light trapping in solar cells. Nanoloops have not yet been studied on solar cells, even though they play a central role in arrays for meta-materials in the microwave (MW) region. In this paper, we use standard antenna theory to provide a rigorous analysis of closed circular metallic loops as antennas in the infrared (IR) and optical region (OR), the regions of solar activity. We provide an exact impedance model for closed loops and an approximate RLC model from which we determine key design factors (resonances, quality factors and radiation efficiencies). Using numerical simulations, we find that these results extend to hexagons and to squares. The principle differences between loops in the radio frequency region (RF) and in the IR/OR are due to dispersion in the loop material. This causes a scaling such that resonances eventually reach saturation; that is, closed loops made of the noble metals will not have their first fundamental resonance at frequencies above the IR. Closed loops, though, do have strong higher harmonic resonances with quality factors on the order of 2 to 5, and these can appear in the OR depending on the loop circumference. Such higher order resonances may be promising for light trapping in solar cells.