Editing Rectenna (section)

==Optical rectennas==
{{main|Optical rectenna}}

In principle, similar devices, scaled down to the proportions used in [[nanotechnology]], can be used to convert [[light]] directly into electricity.  This type of device is called an ''[[optical rectenna]]'' (or "nantenna").<ref>{{cite journal |author-first1=Asha |author-last1=Sharma |author-first2=Virendra |author-last2=Singh |author-first3=Thomas L. |author-last3=Bougher |author-first4=Baratunde A. |author-last4=Cola |date=2015-10-09 |url=https://zenodo.org/record/851685 |journal=Nature Nanotechnology |volume=10 |issue=12 |pages=1027–1032 |title=A carbon nanotube optical rectenna |doi=10.1038/nnano.2015.220 |pmid=26414198 |bibcode=2015NatNa..10.1027S}}</ref><ref>
{{cite press release |date=2015-09-28 |title=First optical rectenna -- combined rectifier and antenna -- converts light to DC current |website=EurekAlert! |url=http://www.eurekalert.org/pub_releases/2015-09/giot-for092315.php}}</ref><ref>Patent application {{patent|WO|2014063149}} relates.</ref> Theoretically, high efficiencies can be maintained as the device shrinks, but to date efficiency has been limited, and so far there has not been convincing evidence that rectification has been achieved at optical frequencies. The [[University of Missouri]] previously reported on work to develop low-cost, high-efficiency optical-frequency rectennas.<ref>{{cite press release |url=http://munews.missouri.edu/news-releases/2011/0516-new-solar-technology-could-break-photovoltaic-limits/ |title=New solar technology could break photovoltaic limits |publisher=[[University of Missouri]] |date=2011-05-16}}</ref> Other prototype devices were investigated in a collaboration between the [[University of Connecticut]] and [[Penn State Altoona]] using a grant from the [[National Science Foundation]].<ref>{{cite press release |url=http://today.uconn.edu/blog/2013/02/uconn-professors-patented-technique-key-to-new-solar-power-technology/ |title=UConn Professor's Patented Technique Key to New Solar Power Technology |date=2013-02-04 |author-first=Colin |author-last=Poitras}}</ref> With the use of [[atomic layer deposition]] it has been suggested that conversion efficiencies of solar energy to electricity higher than 70% could eventually be achieved.

The creation of successful optical rectenna technology has two major complicating factors: 

# Fabricating an antenna small enough to couple optical wavelengths. 
# Creating an ultra-fast diode capable of rectifying the high frequency oscillations, at frequency of ~500&nbsp;THz.

Below are a few examples of potential paths to creating diodes that would be fast enough to rectify optical and near-optical radiation.

A promising path towards creating these ultrafast diodes has been in the form of "[[geometric diode]]s".<ref>{{Cite book |author-last=Zhu |author-first=Z. |title=Rectenna Solar Cells |publisher=Springer |date=2013 |location=New York, USA |pages=209–227}}</ref> Graphene geometric diodes have been reported to rectify [[terahertz radiation]].<ref>{{Cite journal |author-last1=Zhu |author-first1=Zixu |author-last2=Joshi |author-first2=Saumil |author-last3=Grover |author-first3=Sachit |author-last4=Moddel |author-first4=Garret |date=2013-04-15 |title=Graphene geometric diodes for terahertz rectennas |journal=Journal of Physics D: Applied Physics |volume=46 |issue=18 |pages=185101 |doi=10.1088/0022-3727/46/18/185101 |bibcode=2013JPhD...46r5101Z |s2cid=9573157 |issn=0022-3727}}</ref> In April 2020, geometric diodes were reported in [[silicon]] [[Nanowire|nanowires]].<ref>{{Cite journal |author-last1=Custer |author-first1=James P. |author-last2=Low |author-first2=Jeremy D. |author-last3=Hill |author-first3=David J. |author-last4=Teitsworth |author-first4=Taylor S. |author-last5=Christesen |author-first5=Joseph D. |author-last6=McKinney |author-first6=Collin J. |author-last7=McBride |author-first7=James R. |author-last8=Brooke |author-first8=Martin A. |author-last9=Warren |author-first9=Scott C. |author-last10=Cahoon |author-first10=James F. |date=2020-04-10 |title=Ratcheting quasi-ballistic electrons in silicon geometric diodes at room temperature |url=https://www.science.org/doi/10.1126/science.aay8663 |journal=Science |language=en |volume=368 |issue=6487 |pages=177–180 |doi=10.1126/science.aay8663 |issn=0036-8075 |pmid=32273466 |bibcode=2020Sci...368..177C |s2cid=215550903|url-access=subscription }}</ref> The wires were shown experimentally to rectify up to 40&nbsp;GHz, that result was the limit of the instrument used, and the wires theoretically may be able to rectify signals in the terahertz region as well.