Optical rectenna

History

Robert Bailey, along with James C. Fletcher, received a patent ( US 3760257 ) 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 “type described by Javin (sic) 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 even be visible to DC ^[6]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. ^[7] Marks’ device showed substantial improvements in efficiency over Bailey’s device. ^[8] In 1996, Guang H. Lin reported resonant light absorption by a nanostructure fabricated and rectified of light with frequencies in the visible range. ^[8] In 2002, ITN Energy Systems, Inc. published a report on their work on optical antennas coupled with high frequency diodes . ITN set up an optical rectenna array with single digit efficiency. Although they have been unsuccessful, the results have been better understood.^[5]

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. ^[9] Vertical arrays of multiwall carbon nanotubes (MWCNTs) grown on metal-coated substrates were coated with insulating aluminum oxide and aided by a metal electrode layer. The small dimensions of the nanotubes act as an antenna, 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 %. ^[9] Nonetheless, optical rectenna research is ongoing. Future efforts have been undertaken to improve the understanding of alternative materials by manipulating the MWCNTs to promote conduction at the interface, and reduce resistances within the structure.

Theory

The theory behind optical rectennas is essentially the same as for traditional (radio or microwave) rectennas . 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 can be used to power an external load. The resonant frequency of antennas in the field of frequency and probability of linearity. ^[5] The wavelengths in the solar spectrum range from approximately 0.3-2.0 μm. ^[5] Thus, in order for a rectifying antenna to be an efficient electromagnetic collector in the solar spectrum, it needs to be in 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 world’s leading cross-sectional areas of the wire, leading to an increase in resistance. This effect is also known as the ” skin effect “. From a specific device perspective, the characteristics of the product would still be ommic, even though Ohm’s law, in its generalized vector form, is still valid.

Another complication of scaling is that the diodes used in large scale can not operate at THz. ^[4] The large loss in power is a result of the capacitance junction (also known as parasitic capacitance) found in pn junction diodes and Schottky diodes, which can only operate at frequencies less than THz. ^[5] 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 diodesare used. Unlike Schottky diodes, MIM diodes are not affected by parasitic capacitance because they work on the basis of electron tunneling . Because of this, MIM diodes have been shown to be effective at frequencies around 150 THz . ^[5]

Advantages

One of the biggest claims of optical perfection 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 calculating efficiency in the rectenna is based on the application of the Carnot efficiency of solar collectors. The Carnot efficiency , η, is given by

Where T _cold is the temperature of the cooler body and T _hot is the temperature of the body warmer. In order to be an efficient energy conversion, the temperature difference between the two bodies must be significant. RL Baileyclaims that they are not limited by Carnot efficiency, whereas photovoltaics are. However, he does not provide any argument for this claim. Furthermore, when the same assumptions are used to obtain solar cells, the theoretical efficiency of single junction solar cells is also greater than 85%.

The most apparent advantage optical rectennas have the semiconductors 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 it is different from the 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. ^[4]

Limitations and disadvantages

As previously stated, one of the major limitations of optical rectennas is the frequency at which they operate. The high frequency of light in the ideal range of wavelengths makes the use of typical Schottky diodes impractical. Although MIM diodes show promising features for use in optical rectennas, more are needed to operate at higher frequencies. ^[10]

Another disadvantage is that current optical rectennas are produced using electron beam ( e-beam ) lithography. This process is slow and relatively expensive because parallel processing is not possible with e-beam lithography. Typically, e-beam lithography is used only for research purposes when extremely small resolutions are required for minimum feature size (typically, on the order of nanometers). However, photolithographic techniques have made it possible to have minimal feature sizes on the order of nanometers, making it possible to produce rectennas by means of photolithography. ^[10]

Production

After the proof of concept was completed, laboratory-scale silicon wafers were fabricated using standard semiconductor integrated circuit manufacturing techniques. E-beam lithography was used to fabricate the arrays of loop antenna metallic structures. The optical antenna consists of three parts: the ground plane, the optical resonance cavity, and the antenna. The antenna absorbs the electromagnetic wave, the ground plane acts to reflect the light back towards the antenna, and the optical resonance cavity bends and concentrates the light back towards the antenna via the ground plane. ^[4] This work did not include production of the diode.

Lithography method

Idaho National Labs used the following steps to fabricate their optical antenna arrays. A metallic ground plane was deposited on a bare silicon wafer, followed by a sputter deposited amorphous silicon layer. The depth of the deposit is a quarter of a wavelength. A thin manganese film along with a selective frequency has been deposited to act as an antenna. Resist was applied and patterned via electron beam lithography. The gold film was selectively etched and the resist was removed.

Roll-to-roll manufacturing

In moving up to a greater production scale, laboratory processing steps such as the use of electron beam lithography are slow and expensive. Therefore, a roll-to-roll manufacturing method has been developed using a new manufacturing technique based on a master pattern. This master pattern in effect mechanically “stamps” on the inexpensive flexible substrate and causes the metallic loop elements seen in the laboratory processing steps. The master template fabricated by Idaho National Laboratories consists of approximately 10 billion antenna elements on an 8-inch round silicon wafer. Using this semi-automated process, Idaho National Labs has produced a number of 4-inch square coupons. These coupons were combined to form a broad flexible sheet of antenna arrays. This work did not include production of the diode component.

Atomic layer layer

Researchers at the University of Connecticut are using a technique called selective area atomic layer that is capable of producing them reliably and at industrial scales. ^[11] Research is ongoing to tune them to the optimal frequencies for visible and infrared light.

Proof of principle

The principle of optical antennas for optical antennas (OSCs) started with a 1 cm ² silicon substrate. The device has been tested using infrared light with a range of 3 to 15 μm . The peak emissivity is at a center of 6.5 μm and reaches an emissivity of 1. An emissivity of 1 means the antenna absorbs all of the photons of a specific wavelength (in this case, 6.5 μm) that are incident upon the device. ^[12]Comparing the experimental spectrum to the modeled spectrum, the experimental results are in agreement with the theoretical expectations (Figure 5). In some areas, the antenna had a lower emissivity than the theoretical expectations, but in other areas, namely at around 3.5 μm, the device absorbed more light than expected.

After a proof of concept on a stiff silicon substrate, the experiment was replicated on a flexible polymer-based substrate. The target wavelength for the flexible substrate was set to 10 μm. Initial tests show that the antenna design can be translated into a polymer substrate.

The proof of principle of optical antennas can be realized in radio frequency range. High permittivity dielectric particles can be used to simulate silicon behavior at optical frequencies. This allows to perform the experiment at microwave in order to predict antenna behavior at optics.

Economics of optical antennas

Optical antennas (by itself, omitting the crucial diode and other components) are cheaper than photovoltaics (if efficiency is ignored). While materials and processing of photovoltaics are expensive (currently the cost for complete photovoltaic modules is in the order of 430 USD / m ² in 2011 and declining. ^[13] ), Steven Novack estimates the current cost of the antenna material itself as around 5 – 11 USD / m ² in 2008. ^[14] With proper processing techniques and different material selection, it is estimated that the overall cost of processing, or properly scaled up, will not cost much more. His prototype was 30 x 61 cm of plastic, which contained only0.60 USD of gold in 2008, with the possibility of downgrading to a material such as aluminum , copper , gold silver . ^[15] The prototype used in the process of substrate processing, but any substrate could not be used properly.

Future research and goals

In an interview on National Public Radio Talk of the Nation, Dr. Novack claimed that it would be possible to use cell phones and even cool homes. Novack claimed to be of greater benefit in the room and can be used to further the room. (Other scientists have disputed this, saying it would violate the second law of thermodynamics . ^[16] [17] )

Improving the diode is an important challenge. There are two challenging requirements: Speed ​​and nonlinearity. First, the diode must have sufficient speed to rectify visible light. Second, unless the incoming light is extremely intense, the diode needs to be extremely nonlinear (much higher forward current than reverse current), in order to avoid “reverse-bias leakage”. An assessment for solar energy collection found, to get high efficiency, the diode would need a (dark) current much lower than 1μA at 1V reverse bias. ^[18] This assessment assumed (optimistically) that the antenna was a directional antenna arraypointing directly at the sun; a rectenna that collects light from the whole sky, like a typical silicon solar cell does, would need the reverse-biased to be even lower still, by orders of magnitude. The diode simultaneously needs a high forward-bias, related to impedance-matching to the antenna.

There are special diodes for high speed (eg, the metal-insulator-metal tunnel diodes discussed above), and there are special diodes for high nonlinearity, but it is quite difficult to find a diode that is outstanding in both respects at once.

To improve the carbon nanotube-based rectenna efficiency:

• Low work function: A large work function (WF) difference between the MWCNT is needed to maximize the asymmetry diode, which lowers the turn-on voltage required to induce a photoresponse. The WF of carbon nanotubes is 5 eV and the WF of the calcium top layer is 2.9 eV, giving a total work function difference of 2.1 eV for the MIM diode.
• High transparency: Ideally, the top electrode layers should be transparent to allow incoming light to reach the MWCNT antennae.
• Low electrical resistance: Improving device conductivity increases the rectified power output. But there are other impacts of resistance on device performance. Ideal impedance matching between the antenna and diode enhances rectified power. Lowering structure resistances also increases the cutoff frequency, which in turn increases the effective bandwidth of rectified frequencies of light. The current attempt to use calcium in the top layer results in high resistance to calcium oxidizing rapidly.

Researchers currently hope to rectify which can convert around 50% of the antenna’s absorption into energy. ^[14] Another focus of research to be able to properly upscale the process to mass-market production. New materials will need to be tested and tested that will be easily achieved with a roll-to-roll manufacturing process. Future goals will be made to manufacture flexible substrates to create flexible solar cells.

See also

• Graphene antenna
• nanoantenna
• nanolaser

References