D. K. Kotter S. D. Novack Idaho National Laboratory, 2025 Fremont Avenue, Idaho Falls, ID 83415 W. D. Slafer MicroContinuum, Inc., 57 Smith Place, Cambridge, MA 02138 P. J. Pinhero 1 Department of Chemical Engineering, University of Missouri, Columbia, MO 65211 e-mail: pinherop@missouri.edu Theory and Manufacturing Processes of Solar Nanoantenna Electromagnetic Collectors The research described in this paper explores a new and efficient approach for producing electricity from the abundant energy of the sun, using nanoantenna (nantenna) electro- magnetic collectors (NECs). NEC devices target midinfrared wavelengths, where conven- tional photovoltaic (PV) solar cells are inefficient and where there is an abundance of solar energy. The initial concept of designing NECs was based on scaling of radio frequency antenna theory to the infrared and visible regions. This approach initially proved unsuccessful because the optical behavior of materials in the terahertz (THz) region was overlooked and, in addition, economical nanofabrication methods were not previously available to produce the optical antenna elements. This paper demonstrates progress in addressing significant technological barriers including: (1) development of frequency-dependent modeling of double-feedpoint square spiral nantenna elements, (2) selection of materials with proper THz properties, and (3) development of novel manu- facturing methods that could potentially enable economical large-scale manufacturing. We have shown that nantennas can collect infrared energy and induce THz currents and we have also developed cost-effective proof-of-concept fabrication techniques for the large-scale manufacture of simple square-loop nantenna arrays. Future work is planned to embed rectifiers into the double-feedpoint antenna structures. This work represents an important first step toward the ultimate realization of a low-cost device that will collect as well as convert this radiation into electricity. This could lead to a broadband, high conversion efficiency low-cost solution to complement conventional PV devices. DOI: 10.1115/1.4000577 Keywords: nantenna, frequency selective surfaces, nanoscale modeling, nanofabrication, nanoimprinting, roll-to-roll manufacturing, rectenna 1 Introduction Full spectrum incident and reflective reemitted electromag- netic EM radiation originating from the sun provides a constant energy source to the earth. Approximately 30% of this energy is reflected back to space from the atmosphere, atmospheric gases absorb 19% and reradiated to the earth’s surface in the mid-IR range 7–14 m and 51% is absorbed by the surface or organic life and reradiated at around 10 m 1. The energy reaching the earth in both the visible and IR regions and the reradiated IR energy are under-utilized by current technology. Several approaches have been used to successfully harvest en- ergy from the sun and conversion of solar energy to electricity using photovoltaic PV cells is the most common. An alternative to photovoltaics is the optical rectenna, which is a combination of a rectifier and a receiving antenna. The initial rectenna concept was demonstrated for microwave power transmission by Ray- theon Co. in 1964 2. This work illustrated the ability to capture electromagnetic energy and convert it to dc power at efficiencies approaching 84% 3. Since then, much research has been per- formed to extend the concept of rectennas to the infrared and visible regime for solar power conversion, and progress has been made in the fabrication and characterization of metal-insulator- metal diodes for use in infrared rectennas 4,5. It has been dem- onstrated that optical antennas can couple electromagnetic radia- tion in the visible in the same way as radio antennas do at their corresponding wavelengths 6. A major technical challenge continues to be the development of economical manufacturing methods for large-scale fabrication of nanoantenna-based solar collectors. In addition, research is re- quired to improve the efficiency of rectification of antenna- induced terahertz THz currents to a usable dc signal, and mate- rial properties and behavior of antennas/circuits in the THz solar regions need to be further characterized. 1.1 Limitations of Photovoltaic Technology. Traditional p-n junction solar cells are the most mature of the solar energy har- vesting technologies. The basic physics of energy absorption and carrier generation is a function of the material characteristics and corresponding electrical properties i.e., bandgap. A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at 6000 K, and as such, much of the solar ra- diation reaching the earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell but the difference in energy between these photons and the silicon bandgap is converted into heat via lattice vibrations, i.e., phonons rather than into usable electrical energy. For a single-junction cell this sets an upper ef- ficiency of 31% 7. The current research path of implementing complex, multijunc- tion PV designs does not appear to be a cost-effective solution to overcoming PV efficiency limitations. Even optimized PV mate- rials require direct perpendicular to the surface sunlight for op- timum efficiency and are only operational during daylight hours. 1 Corresponding author. Contributed by the Solar Energy Division of ASME for publication in the JOUR- NAL OF SOLAR ENERGY ENGINEERING. Manuscript received September 9, 2008; final manuscript received September 20, 2009; published online January 5, 2010. Assoc. Editor: Aldo Steinfeld. Journal of Solar Energy Engineering FEBRUARY 2010, Vol. 132 / 011014-1 Copyright © 2010 by ASME Downloaded 05 Jan 2010 to 128.206.19.76. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm