Summary

Description

Traditional photovoltaics are limited in two major ways: by the availability of light and the wavelengths that can be absorbed. Nighttime, seasons, and latitude limit the amount of radiation to absorb, and the available bandgap of materials for photovoltaic cells limits absorption to ?shortwave? radiation (near-IR wavelengths and shorter). However, the Earth absorbs incident sunlight and re-emits the energy as ?longwave? blackbody radiation, peaking in the 10 ?m wavelength range, known as ?earthglow? (as opposed to ?earthshine?, the visible light directly reflected by the Earth). Earthglow continues during nighttime, varying only about 25% during a 24-hour period. It is difficult to harvest with traditional photovoltaics, but not alternative radiant energy harvesting methods. An approach employing plasmonically-active nano-antennas offers a potential solution due to the tuneability of the resonant wavelength via antenna geometry. The ultimate goal of this project is to produce nano-antennas able to rectify infrared optical energy (terahertz frequencies), such as earthglow, into DC electrical current. The concept of nano-antennas has been around for many years, but due to fabrication difficulties, researchers have only recently begun to actually demonstrate them in the lab. Theoretically, single-junction photovoltaics have a light-to-electricity conversion efficiency limit of around 30%. Nano-antennas have a theoretical capture efficiency (incident light absorbed) of up to 95%, with lab demonstrations reaching 80%. The challenge facing researchers is converting this potential abundance of captured light efficiently into electricity, but nano-antennas promise a greater upper limit to energy production than present technology. Work on the energy harvesting project places NASA on the cutting edge of nano-antenna development. The design begins with an insulating substrate that is covered by a bottom electrode of metal film.