Date of Award
Doctor of Philosophy (PhD)
Electrical, Computer & Energy Engineering
Optical rectennas are antenna-coupled diode rectifiers that receive and convert optical-frequency electromagnetic radiation into DC output. Because classical rectennas working at microwave frequencies can achieve very high rectification efficiencies, rectennas working as solar cells were expected to have efficiencies significantly higher than conventional solar cells. By applying the theory of photon-assisted tunneling (PAT) to optical rectennas at solar intensities, I show that the power conversion efficiency of rectenna solar cells is fundamentally limited to the Trivich-Flinn efficiency limit of 44%. This unexpected result is the same as the Shockley-Queisser ultimate efficiency limit of conventional solar cells.
Is it possible to exceed this efficiency limit? I answer this question by showing that there is a correspondence between the quantum and classical operation of a rectenna. Such correspondence allows high frequency rectennas to operate in the same way as classical rectennas, and to potentially exceed the Trivich-Flinn efficiency limit. I propose two ways to achieve classical operation in optical rectennas.
Diode design is crucial for achieving high rectification efficiency. High-speed diodes, such as metal-insulator-metal (MIM) diodes, have insufficient asymmetry for harvesting low intensity radiation. I suggest steps to improve the characteristics of double insulator MIM diodes and calculate their power conversion efficiency using PAT theory. A simple figure of merit is provided to quickly assess the usefulness of MIM diodes in optical rectennas.
Although ineffective at visible frequencies, MIM diodes have RC time constants sufficient for operation at low terahertz frequencies, useful in applications including detection and high-speed electronics. I develop the steps to design MIM diodes at 1 THz, propose materials that will give the required current-voltage characteristics and RC time constant, perform electrical and optical measurements on fabricated devices, and suggest steps to improve their performance.
In contrast to MIM diodes, novel planar devices called geometric diodes have very low RC time constants and are capable of rectifying radiation up to 100 THz. I measure the infrared optical response of graphene-based geometric diodes, and demonstrate one of the best room-temperature detectors working at 28 THz.
Joshi, Saumil, "Performance limits of optical rectennas" (2015). Electrical, Computer & Energy Engineering Graduate Theses & Dissertations. 107.