Date of Award

Spring 11-17-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Garret Moddel

Second Advisor

Steven George

Third Advisor

Wounjhang Park

Fourth Advisor

Sean Shaheen

Fifth Advisor

Bart Van Zeghbroeck

Abstract

Infrared optical rectennas absorb electromagnetic radiation in a micro-antennas and rectify the AC signal with high speed diodes. These devices have applications in detection and energy harvesting. Metal-insulator-metal (MIM) diodes provide an excellent option for the high speed diode required for optical rectification, but are limited by poor coupling efficiency to the antenna due to their capacitive nature. One option to improve the antenna/diode coupling efficiency is the traveling-wave diode (TWD). By changing the diode to an MIM transmission line structure that acts like a distributed rectifier, the impedance seen by the antenna becomes the input impedance of the transmission line rather than the capacitive impedance of the lumped-element diode. The germanium shadow mask is an effective method for fabricating TWD rectennas and has several benefits. First, this technique requires only a single lithography step, which helps reduce processing time. The critical feature sizes are controlled by angled metal evaporations at a higher resolution than the lithography used pattern the shadow mask. Finally, it keeps contaminates such as photoresist away from the sensitive MIM diode junction and results in high diode yield. A simplified TWD geometry is modeled in COMSOL. Since linear FEM solvers cannot handle a nonlinear I(V ) characteristic, the rectification is calculated as part of the post processing. The fabricated devices are experimentally measured with infrared illumination from a CO2 laser with an automated measurement system. The measured system responsivites are as high as 471 μA/W, which is within a factor of ten of a commercially available HgCdTe photodiode detector. With additional development, the TWD is expected to surpass this existing technology. Despite the coupling efficiency improvements of more than three orders of magnitude for the TWD compared to the lumped-element, the overall improvement is limited. This limitation is because in the TWD there is an additional loss mechanism: the propagation of the surface plasmon along the MIM interface. The thin insulator required to support electron tunneling leads to very high field confinement and lossy plasmonic propagation. In a lumped-element rectenna, once the AC power enters the diode, all of it is available to be rectified. Alternatively, in a TWD, since all the rectification does not occur at the same location, the field that couples to the diode must still propagate along the MIM interface, and this propagation loss becomes costly. Because of this limitation, the traveling-wave rectenna probably cannot achieve efficiencies high enough for practical energy harvesting in the current configuration. However, the work in this thesis has shown that the coupling efficiency limitations of MIM rectennas can be circumvented through careful engineering of the diode input impedance as seen by the antenna. This means that capacitance compensation works, and high coupling efficiencies are possible. Infrared optical rectennas with some alternative, low-loss, compensation structure show promise as energy harvesters.

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