An optical rectenna is a rectenna (rectifying antenna) that works with visible or infrared light. A rectenna is a circuit containing an antenna and a diode, which turns electromagnetic waves into direct current electricity. While rectennas have long been used for radio waves or microwaves, an optical rectenna would operate the same way but with infrared or visible light, turning it into electricity.
While traditional (radio- and microwave) rectennas are fundamentally similar to optical rectennas, it is vastly more challenging in practice to make an optical rectenna. One challenge is that light has such a high frequency—hundreds of terahertz for visible light—that only a few types of specialized diodes can switch quickly enough to rectify it. Another challenge is that antennas tend to be a similar size to a wavelength, so a very tiny optical antenna requires a challenging nanotechnology fabrication process. A third challenge is that, being very small, an optical antenna typically absorbs very little power, and therefore tend to produce a tiny voltage in the diode, which leads to low diode nonlinearity and hence low efficiency. Due to these and other challenges, optical rectennas have so far been restricted to laboratory demonstrations, typically with intense focused laser light producing a tiny but measurable amount of power.
Nevertheless, it is hoped that arrays of optical rectennas could eventually be an efficient means of converting sunlight into electric power, producing solar power more efficiently than conventional solar cells. The idea was first proposed by Robert L. Bailey in 1972. As of 2012, only a few optical rectenna devices have been built, demonstrating only that energy conversion is possible. It is unknown if they will ever be as cost-effective or efficient as conventional photovoltaic cells.
The term nantenna (nano-antenna) is sometimes used to refer to either an optical rectenna, or an optical antenna by itself. In 2008 it was reported that Idaho National Laboratories designed an optical antenna to absorb wavelengths in the range of 3–15 μm. These wavelengths correspond to photon energies of down to . Based on antenna theory, an optical antenna can absorb any wavelength of light efficiently provided that the size of the antenna is optimized for that specific wavelength. Ideally, antennas would be used to absorb light at wavelengths between because these wavelengths have higher energy than far-infrared (longer wavelengths) and make up about 85% of the solar radiation spectrum (see Figure 1).
History Robert Bailey, along with James C. Fletcher, received a patent () in 1973 for an "electromagnetic wave energy converter". The patented device was similar to modern day optical rectennas. The patent discusses the use of a diode "type described by [Ali Javan] in the IEEE Spectrum, October, 1971, page 91", to whit, a 100 nm-diameter metal cat's whisker to a metal surface covered with a thin oxide layer. Javan was reported as having rectified 58 THz infrared light. In 1974, T. Gustafson and coauthors demonstrated that these types of devices could rectify even visible light to DC current Alvin M. Marks received a patent in 1984 for a device explicitly stating the use of sub-micron antennas for the direct conversion of light power to electrical power. Marks's device showed substantial improvements in efficiency over Bailey's device. In 1996, Guang H. Lin reported resonant light absorption by a fabricated nanostructure and rectification of light with frequencies in the visible range. In 2002, ITN Energy Systems, Inc. published a report on their work on optical antennas coupled with high frequency diodes. ITN set out to build an optical rectenna array with single digit efficiency. Although they were unsuccessful, the issues associated with building a high efficiency optical rectenna were better understood.
In 2015, Baratunde A. Cola's research team at the Georgia Institute of Technology, developed a solar energy collector that can convert optical light to DC current, an optical rectenna using carbon nanotubes,. Vertical arrays of multiwall carbon nanotubes (MWCNTs) grown on a metal-coated substrates were coated with insulating aluminum oxide and altogether capped with a metal electrode layer. The small dimensions of the nanotubes act as antennae, capable of capturing optical wavelengths. The MWCNT also doubles as one layer of a metal-insulator-metal (MIM) tunneling diode. Due to the small diameter of MWCNT tips, this combination forms a diode that is capable of rectifying the high frequency optical radiation. The overall achieved conversion efficiency of this device is around 10−5 %. Nonetheless, optical rectenna research is ongoing.
The primary drawback of these carbon nanotube rectenna devices is a lack of air stability. The device structure originally reported by Cola used calcium as a semitransparent top electrode because the low work function of calcium (2.9 eV) relative to MWCNTs (~5 eV) creates the diode asymmetry needed for optical rectification. However, metallic calcium is highly unstable in air and oxidizes rapidly. Measurements had to be made within a glovebox under an inert environment to prevent device breakdown. This limited practical application of the devices.
Cola and his team later solved the challenges with device instability by modifying the diode structure with multiple layers of oxide. In 2018 they reported the first air-stable optical rectenna along with efficiency improvements.
The air-stability of this new generation of rectenna was achieved by tailoring the diode's quantum tunneling barrier. Instead of a single dielectric insulator, they showed that the use of multiple dissimilar oxide layers enhances diode performance by modifying diode tunneling barrier. By using oxides with different electron affinities, the electron tunneling can be engineered to produce an asymmetric diode response regardless of the work function of the two electrodes. By using layers of Al2O3 and HfO2, a double-insulator diode (metal-insulator-insulator-metal (MIIM)) was constructed that improved the diode's asymmetric response more than 10-fold without the need for low work function calcium, and the top metal was subsequently replaced with air-stable silver.
Future efforts will be focused on improving the device efficiency by investigating alternative materials, manipulating the MWCNTs and the insulating layers to encourage conduction at the interface, and reduce resistances within the structure.
Another approach was presented in 2022 by Proietti Zaccaria Remo and collaborators at the Italian Institute of Technology, based on a top-down solution where the antenna and the rectifier were merged together in a single plasmonic-based solution. The proposed rectenna was tested at 1064 nm, both in single and matrix format, achieving an efficiency of 10−3 %. Furthermore, both experimental and theoretical analyses were conducted at 780 nm, with positive results suggesting that plasmonic-based rectennas could potentially serve as a viable approach for achieving a high-performing rectenna in the visible range. Although this solution represents a significant step forward, several challenges remain that prevent optical rectennas from reaching the 1% efficiency threshold required for practical applications. Key obstacles include achieving optical resonance regardless of the incident radiation conditions (e.g., angle of incidence) and improving the rectifier performance.
Theory
The theory behind optical rectennas is essentially the same as for traditional (radio or microwave) antennas. Incident light on the antenna causes electrons in the antenna to move back and forth at the same frequency as the incoming light. This is caused by the oscillating electric field of the incoming electromagnetic wave. The movement of electrons is an alternating current (AC) in the antenna circuit. To convert this into direct current (DC), the AC must be rectified, which is typically done with a diode. The resulting DC current can then be used to power an external load. The resonant frequency of antennas (frequency which results in lowest impedance and thus highest efficiency) scales linearly with the physical dimensions of the antenna according to simple microwave antenna theory. The wavelengths in the solar spectrum range from approximately 0.3-2.0 μm. Thus, in order for a rectifying antenna to be an efficient electromagnetic collector in the solar spectrum, it needs to be on the order of hundreds of nm in size.
Because of simplifications used in typical rectifying antenna theory, there are several complications that arise when discussing optical rectennas. At frequencies above infrared, almost all of the current is carried near the surface of the wire which reduces the effective cross sectional area of the wire, leading to an increase in resistance. This effect is also known as the "skin effect". From a purely device perspective, the I-V characteristics would appear to no longer be ohmic, even though Ohm's law, in its generalized vector form, is still valid.
Another complication of scaling down is that diodes used in larger scale rectennas cannot operate at THz frequencies without large loss in power. The large loss in power is a result of the junction capacitance (also known as parasitic capacitance) found in p-n junction diodes and Schottky diodes, which can only operate effectively at frequencies less than 5 THz. The ideal wavelengths of 0.4–1.6 μm correspond to frequencies of approximately 190–750 THz, which is much larger than the capabilities of typical diodes. Therefore, alternative diodes need to be used for efficient power conversion. In current optical rectenna devices, metal-insulator-metal (MIM) tunneling diodes are used. Unlike Schottky diodes, MIM diodes are not affected by parasitic capacitances because they work on the basis of electron tunneling. Because of this, MIM diodes have been shown to operate effectively at frequencies around .
Advantages
One of the biggest claimed advantages of optical rectennas is their high theoretical efficiency. When compared to the theoretical efficiency of single junction solar cells (30%), optical rectennas appear to have a significant advantage. However, the two efficiencies are calculated using different assumptions. The assumptions involved in the rectenna calculation are based on the application of the Carnot efficiency of solar collectors. The Carnot efficiency, η, is given by
where Tcold is the temperature of the cooler body and Thot is the temperature of the warmer body. In order for there to be an efficient energy conversion, the temperature difference between the two bodies must be significant. claims that rectennas are not limited by Carnot efficiency, whereas photovoltaics are. However, he does not provide any argument for this claim. Furthermore, when the same assumptions used to obtain the 85% theoretical efficiency for rectennas are applied to single junction solar cells, the theoretical efficiency of single junction solar cells is also greater than 85%.
The most apparent advantage optical rectennas have over semiconductor photovoltaics is that rectenna arrays can be designed to absorb any frequency of light. The resonant frequency of an optical antenna can be selected by varying its length. This is an advantage over semiconductor photovoltaics, because in order to absorb different wavelengths of light, different band gaps are needed. In order to vary the band gap, the semiconductor must be alloyed or a different semiconductor must be used altogether.
Limitations and disadvantages