Nanoantenna

ABSTRACT

Researchers have devised an inexpensive way to produce plastic sheets containing billions of nanoantennas that collect heat energy generated by the sun and other sources. The technology, developed at the U.S. Department of Energy's Idaho National Laboratory, is the first step toward a solar energy collector that could be mass-produced on flexible materials. While methods to convert the energy into usable electricity still need to be developed, the sheets could one day be manufactured as lightweight "skins" that power everything from hybrid cars to iPods with higher efficiency than traditional solar cells. The nanoantennas also have the potential to act as cooling devices that draw waste heat from buildings or electronics without using electricity. The nanoantennas target mid-infrared rays, which the Earth continuously radiates as heat after absorbing energy from the sun during the day. In contrast, traditional solar cells can only use visible light, rendering them idle after dark. Infrared radiation is an especially rich energy source because it also is generated by industrial processes such as coal-fired plants.

The nanoantennas are tiny gold squares or spirals set in a specially treated form of polyethylene, a material used in plastic bags. While others have successfully invented antennas that collect energy from lower-frequency regions of the electromagnetic spectrum, such as microwaves, infrared rays have proven more elusive. Part of the reason is that materials' properties change drastically at high-frequency wavelengths. The researchers studied the behavior of various materials -- including gold, manganese and copper -- under infrared rays and used the resulting data to build computer models of nanoantennas. They found that with the right materials, shape and size, the simulated nanoantennas could harvest up to 92 percent of the energy at infrared wavelengths.

The nanoantennas' ability to absorb infrared radiation makes them promising cooling devices. Since objects give off heat as infrared rays, the nanoantennas could collect those rays and re-emit the energy at harmless wavelengths. Such a system could cool down buildings and computers without the external power source required by air-conditioners and fans.

But more technological advances are needed before the nanoantennas can funnel their energy into usable electricity. The infrared rays create alternating currents in the nanoantennas that oscillate trillions of times per second, requiring a component called a rectifier to convert the alternating current to direct current. Today's rectifiers can't handle such high frequencies. The nanoscale rectifier would need to be about 1,000 times smaller than current commercial devices and will require new manufacturing methods. Another possibility is to develop electrical circuitry that might slow down the current to usable frequencies.

If these technical hurdles can be overcome, nanoantennas have the potential to be a cheaper, more efficient alternative to solar cells.

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INTRODUCTION

Worldwide energy demands have increased by 40% over the last 20 years.1 Although thedeleterious effects of hydrocarbon-based power are becoming increasingly apparent, more than85% of the world’s power is still generated by combustion of fossil fuels.1 Clean renewablealternative energy sources are required to meet the demands, with direct solar-conversion devicesas leading candidates. The worldwide market for conventional photovoltaics (PV) has increasedat an annual rate of 20% over the last five years, and industry estimates suggest as much as 18billion watts per year could ship by 2020.1 To meet the increased demands for solar-conversiontechnologies, dramatic improvements are required in state-of-the-art PV technologies. Efficiencyimprovements and cost/complexity reduction are the main issues that need to be addressed tomeet these goals.

Traditional p-n junction solar cells are the most mature of the solar-energy-harvesting technologies. Although great improvements have been made in the last 20 years, energy absorption, carrier generation, and collection are all a function of the materials chemistry and corresponding electronic properties (i.e., bandgap). As a quantum device, the efficiency of PV is a function of, and therefore, ultimately fundamentally limited by, the bandgap and the match of the bandgap to the solar spectrum. For single-junction cells, this sets an upper efficiency limit of~30%. Even with complex multi-junction designs, the theoretical efficiency plateaus around 55% without excessive concentration of the incident radiation. Current state-of-the-art solar cells are ~20% efficient for single cells and ~30% efficient for multijunction systems.4 In the long term, the PV industry will require newer, higher efficiency technologies to improve performance and to meet the increasing demands of the solar power market.

Researchers have devised an inexpensive way to produce plastic sheets containing billions of nanoantennas that collect heat energy generated by the sun and other sources. The technology, developed at the U.S. Department of Energy's Idaho National Laboratory, is the first step toward a solar energy collector that could be mass-produced on flexible materials.

While methods to convert the energy into usable electricity still need to be developed, the sheets could one day be manufactured as lightweight "skins" that power everything from hybrid cars to iPods with higher efficiency than traditional solar cells, say the researchers, who report their findings Aug. 13 at the American Society of Mechanical Engineers 2008 2nd International Conference on Energy Sustainability in Jacksonville, Fla. The nanoantennas also have the potential to act as cooling devices that draw waste heat from buildings or electronics without using electricity.

As a means for capturing or converting the abundant energy from solar radiation, an antenna is the ideal device because it is an efficient transducer between free space and guided waves. In thecase of conventional PV cells, solar radiation is only absorbed if the photon energy is greater than the bandgap. Because the bandgap must also be tuned to minimize the excess energy lost to heat when the photon energy is significantly above the bandgap, a significant portion of the incident solar energy, up to 24%, is not absorbed by conventional PV. In contrast, an adequately designed antenna array can efficiently absorb the entire solar spectrum, with nearly 100% efficiency theoretically possible.

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Rather than generating single electron-hole pairs as in the PV, the electric field (E) from an incident electromagnetic radiation source will induce a time-changing current (i.e., wave of accelerated electric charge) in a conductor. Efficient collection of incident radiation is then dependent on resonance length scales and impedance matching of the antenna to the diode to prevent losses.

The antenna is used at a television, radio or cell phone to capture the aerial signals that make those devices practical. However, the amazing thing is if an antenna becomes in nanoscale, it has different power and abilities. A nanoantenna is a solar collection device based on rectifying antennas. Nanoantenna is a recent device, so it does not have old history. It is worthy to mention that, the idea of using antennas to collect solar energy was first proposed by Robert L. Bailey in 1972. Broca states that an antenna is a means for concentrating electromagnetic waves and defined it as" the vertical long-wire pole of an excitation source.” The other pole is considered to be grounded. He wrote that “the extremity of the antenna is a point of escape for electromagnetic energy” and that an antenna is also needed on the receiving end. In 1973, Robert Bailey, along with James C. Fletcher, received a patent for an “electromagnetic wave converter”. The patented device was similar to modern day nantenna devices. 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.

The nanoantenna, as the normal antenna, receives electromagnetic radiation. But the characteristic thing, nanoantenna receives higher frequency and shorter wavelength. Therefore, it can deal with light (frequency of light higher than frequency of radio wave). In fact, light is defined as the portion of electromagnetic radiation that is visible to the human eye, responsible for the sense of sight. Visible light has a wavelength in a range from about 380 or 400 nanometres to about 760 or 780 nm, with a frequency range of about 405 THz to 790 THz. It is worthily to mention that electromagnetic radiation has both electric and magnetic field components, which oscillate in phase perpendicular to each other (see figure 1) and perpendicular to the direction of energy propagation. The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths ( Frequency and wavelength has a versus relation). When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.

The nanoantenna consists of three main 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.

One of the main advantages of using nanoantennas is their high theoretical efficiency. For example, the theoretical efficiency of single junction solar cells (30%). In contrast, in the case of

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nanoantenna the theoretical efficiency will be greater than 85%. Another advantage is the most apparent advantage of nanoantennas have over semiconductor photovoltaics is that nanoantenna arrays can be designed to absorb any frequency of light. By simply varying the size of the nanoantenna in the array, the resonant frequency of the nanoantenna can be engineered to absorb a specific wavelength of light (resonance frequency scales approximately linearly with antenna size).

Fig.2: nanoantennas array for solar panel

The limitations of nanoantennas include nanoantennas are produced using electron beam (lithography). This process is slow and expensive because parallel processing is not possible with e-beam lithography. Operation frequency is one of the main limitations of nanoantennas. 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 nanoantennas, more advances are necessary to operate efficiently at higher frequencies.

The applications of nanoantenna include nanoantennas can provide us with a large energy source. The sun radiates a lot of infrared energy. Because of the small size of nanoantennas, which approximately as wide as 1/25 the diameter of a human hair, it absorbs energy in the infrared part of the spectrum, just outside the range of what is visible to the eye. Nanoantennas can absorb energy from both sunlight and the earth’s heat, with higher efficiency than conventional solar cells.

The nanoantenna has advantages, disadvantages and useful applications. In fact, currently, the largest problem is not with the antenna device, but with the electronic switches and amplifiers, so that, a bringing photonic switches and amplifiers to the reality is the concerned of the recent researches. Because of the electronic switches and amplifiers are unable to efficiently deal with frequencies which correspond to high-infrared to visible light . Therefore, a photonic system must be designed that can properly deal with the absorbed light without converted to the electric energy.

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SOLAR RADIATION SPECTRUM

Radiation from the sun sustains life on earth and determines climate. The energy flow within the sun results in a surface temperature of around 5800 K, so the spectrum of the radiation from the sun is similar to that of a 5800 K blackbody with fine structure due to absorption in the cool peripheral solar gas (Fraunhofer lines).

The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K. The Sun emits EM radiation across most of the electromagnetic spectrum. Although the Sun produces Gamma rays as a result of the Nuclear fusion process, these super high energy photons are converted to lower energy photons before they reach the Sun's surface and are emitted out into space, so the Sun doesn't give off any gamma rays to speak of. The Sun does, however, emit X-rays, ultraviolet, visible light , infrared, and even Radio waves. When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause damage to the skin known as sunburn or trigger an adaptive change in human skin pigmentation.

The spectrum of electromagnetic radiation striking the Earth's atmosphere is on the order of approximately 100 (i.e., 102, one hundred) to approximately 1,000,000 (i.e., 106, one million) nanometers (nm) (or equivalently, 1 millimeters (mm), one millimeter).

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This can be divided into five regions in increasing order of wavelengths:

Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence also invisible to the human eye). Owing to absorption by the atmosphere very little reaches the Earth's surface (Lithosphere). This spectrum of radiation has germicidal properties, and is used in germicidal lamps.

Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for the photochemical reaction leading to the production of the ozone layer.

Ultraviolet A or (UVA) spans 315 to 400 nm. It has been traditionally held as less damaging to the DNA, and hence used in tanning and PUVA therapy for psoriasis.

Visible range or light spans 380 to 780 nm. As the name suggests, it is this range that is visible to the naked eye.

Infrared range that spans 700 nm to 106 nm [1 (mm)]. It is responsible for an important part of the electromagnetic radiation that reaches the Earth. It is also divided into three types on the basis of wavelength:

o Infrared-A: 700 nm to 1,400 nm

o Infrared-B: 1,400 nm to 3,000 nm

o Infrared-C: 3,000 nm to 1 mm.

Full spectrum incident and reflective (readmitted) electromagnetic (EM) radiation originating from the sun provides a constant energy source to the earth. Approximately 30% of this energy is reflected back to space from the atmosphere, 19% is absorbed by atmospheric gases and reradiated to the earth’s surface in the mid-IR range (7-14 um), and 51% is absorbed by the surface or organic life and reradiated at around 10 um. The energy reaching the earth in both the visible and IR regions and the reradiated IR energy are under-utilized by current technology. Several approaches have been pursued to harvest energy from the sun. Conversion of solar energy to electricity using photovoltaic cells is the most common. An alternative to photovoltaics is the rectenna, which is a combination of a receiving antenna and a rectifier. The initial rectenna concept was demonstrated for microwave power transmission by Raytheon Company in 1964. This illustrated the ability to capture electromagnetic energy and convert it to DC power at efficiencies approaching 84%. Since then much research has been performed to extend the concept of rectennas to the infrared and visible regime for solar power conversion. Progress has been made in fabrication and characterization of metal-insulator-metal diodes for use in an infrared rectenna. The major technical challenges continue to be in developing economical manufacturing methods for large-scale fabrication of antenna-based solar collectors. Further research is required to improve the efficiency of rectification of antenna induced terahertz currents to a usable DC signal. The material properties and behavior of antennas/circuits in the THz solar regions need to be further characterized.

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PHOTOVOLTAIC TECHNOLOGY

Photovoltaics are best known as a method for generating electric power by using solar cells to convert energy from the sun into electricity. The photovoltaic effect refers to photons of light knocking electrons into a higher state of energy to create electricity. The term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the transduced light energy. Virtually all photovoltaic devices are some type of photodiode.

Solar cells produce direct current electricity from sun light, which can be used to power equipment or to recharge a battery. The first practical application of photovoltaics was to power orbiting satellites and other spacecraft, but today the majority of photovoltaic modules are used for grid connected power generation. In this case an inverter is required to convert the DC to AC. There is a smaller market for off-grid power for remote dwellings, boats, recreational vehicles, electric cars, roadside emergency telephones, remote sensing, and cathodic protection of pipelines.

model of a crystalline solar cell

Cells require protection from the environment and are usually packaged tightly behind a glass sheet. When more power is required than a single cell can deliver, cells are electrically connected together to form photovoltaic modules, or solar panels. A single module is enough to power an emergency telephone, but for a house or a power plant the modules must be arranged in multiples as arrays. Although the selling price of modules is still too high to compete with grid electricity in most places, significant financial incentives in Japan and then Germany, Italy, Greece and France triggered a huge growth in demand, followed quickly by production. A significant market has emerged in off-grid locations for solar-power-charged storage-battery

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based solutions. These often provide the only electricity available. The first commercial installation of this kind was in 1966 on Ogami Island in Japan to transition Ogami Lighthouse from gas torch to fully self-sufficient electrical power.

Due to the growing demand for renewable energy sources, the manufacture of solar cells and photovoltaic arrays has advanced dramatically in recent years. Photovoltaic production has been increasing by an average of more than 20 percent each year since 2002, making it the world’s fastest-growing energy technology. At the end of 2009, the cumulative global PV installations surpassed 21 GW. Roughly 90% of this generating capacity consists of grid-tied electrical systems. Such installations may be ground-mounted (and sometimes integrated with farming and grazing) or built into the roof or walls of a building, known as Building Integrated Photovoltaics or BIPV for short. Solar PV power stations today have capacities ranging from 10–60 MW although proposed solar PV power stations will have a capacity of 150 MW or more.

LIMITATIONS OF PHOTOVOLTAIC CELLS

Traditional p-n junction solar cells are the most mature of the solar energy harvesting technologies. The basic physics of energy absorption and carrier generation are a function of the materials characteristics and corresponding electrical properties (i.e. bandgap). A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon bandgap is converted into heat (via lattice vibrations — called phonons) rather than into usable electrical energy. For a single-junction cell this sets an upper efficiency of ~20%. The current research path of implementing complex, multijunction PV designs to overcome efficiency limitations does not appear to be a cost-effective solution. Even the optimized PV materials are only operational during daylight hours and require direct (perpendicular to the surface) sunlight for optimum efficiency.

ECONOMICAL ALTERNATIVE TO PV

We have developed an alternative energy harvesting approach based on nanoantennas that absorb the incident solar radiation. In contrast to PV, which are quantum devices and limited by material bandgaps, antennas rely on natural resonance and bandwidth of operation as a function of physical antenna geometries. The NECs can be configured as frequency selective surfaces to efficiently absorb the entire solar spectrum. Rather than generating single electron-hole pairs as in the PV, the incoming electromagnetic field from the sun induces a time-changing current in the antenna. Efficient collection of the incident radiation is dependent upon proper design of antenna resonance and impedance matching of the antenna. Recent advances in nanotechnology have provided a pathway for large-scale fabrication of nanoantennas.

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HISTORY OF NANOANTENNA

Robert Bailey, along with James C. Fletcher, received a patent in 1973 for an “electromagnetic wave converter”. The patented device was similar to modern day nanoantenna devices. 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 was the first to report 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 a nanoantenna array with single digit efficiency. Although they were unsuccessful, the issues associated with building a high efficiency nanoantenna were better understood. Research on nantennas is ongoing.

THEORY OF NANOANTENNA

The theory behind nantennas is essentially the same for rectifying 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 in the antenna circuit. To convert this into DC power, the AC current must be rectified, which is typically done with some kind of 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.

Figure 3. Image showing the skin effect at high frequencies. The dark region indicates electron flow where the lighter region indicates little to no electron flow.

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Because of simplifications used in typical rectifying antenna (rectenna) theory, there are several complications that arise when discussing nantennas. 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 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 nantenna 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 150 THz.

One of the biggest claimed advantages of nantennas is their high theoretical efficiency. When compared to the theoretical efficiency of single junction solar cells (30%), nantennas appear to have a significant advantage. However, the two efficiencies are calculated using different assumptions. The assumptions involved in the nantenna 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. R. L. Bailey claims that nantennas 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 nantennas are applied to single junction solar cells, the theoretical efficiency of single junction solar cells is also greater than 85%.

The most apparent advantage nantennas have over semiconductor photovoltaics is that nantenna arrays can be designed to absorb any frequency of light. By simply varying the size of the nantenna in the array, the resonant frequency of the nantenna can be engineered to absorb a specific wavelength of light (resonance frequency scales approximately linearly with antenna size). 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.

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THEORY OF OPERATION

We have designed nantenna elements that capture electromagnetic energy from naturally occurring solar radiation and thermal earth radiation. The size of the antenna is relative to the wavelength of light we intend to harvest. The basic theory of operation is as follows: The incident electromagnetic radiation (flux) produces a standing-wave electrical current in the finite antenna array structure. Absorption of the incoming EM radiation energy occurs at the designed resonant frequency of the antenna. When an antenna is excited into a resonance mode it induces a cyclic plasma movement of free electrons from the metal antenna. The electrons freely flow along the antenna generating alternating current at the same frequency as the resonance. Electromagnetic modeling illustrates the current flow is toward the antenna feedpoint. In a balanced antenna, the feedpoint is located at the point of lowest impedance.

Figure 1. Flow of THz currents to feedpoint of antenna. Red represents highest concentrated E field.

Figure 1 was acquired from modeling the electromagnetic properties of an infrared spiral antenna. The e-field is clearly concentrated at the center feedpoint. This provides a convenience point to collect energy and transport it to other circuitry for conversion.

Antennas have electromagnetic radiation patterns, which allow them to exhibit gain and directionality and effectively collect and concentrate energy, as illustrated in Figure 2.

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Figure 2. Typical electromagnetic radiation patterns of antenna

The nanoantenna radiation pattern displays angular reception characteristics, resulting in a wider angle of incidence exposure to thermal radiation than typical PV. Any flux from the sun that falls within the radial beam pattern of the antenna is collected. This property is a critical antenna characteristic that optimizes energy collection from the sun as it moves throughout the horizon. Thus, it may be possible to reduce the need for mechanical solar tracking mechanisms. It also provides designers another mechanism to increase the efficiency of antenna arrays through the expansion of the radial field. Antennas by themselves do not provide a means of converting the collected energy. This will need to be accomplished by associated circuitry such as rectifiers. As illustrated in Fig 2, the electrical size of the antenna (comprised of radiation beam pattern) is much larger than the physical size of the antenna. The virtual large surface area antenna focuses the electromagnetic energy onto the nano-sized energy conversion material fabricated at the antenna feedpoint. Theoretical efficiency is improved by the enhanced radiation capture area of the antenna

ENERGY CONVERSION METHODS

This research has demonstrated that infrared rays create an alternating current in the nantenna at THz frequencies. Commercial grade electronic components cannot operate at that switching rate without significant loss. Further research is planned to explore ways to perform high frequencyrectification. This requires embedding a rectifier diode element into the antenna structure. One possible embodiment is metalinsulator- insulator-metal (MIIM) tunneling-diodes. The MIIMdevice consists of a thin barrier layer and two dielectric layers (oxide) sandwiched between two metal electrodes with different work functions. The device works when a large enough field causes the tunneling of electrons across the barrier layers. A difference in the work function between the metal junctions produces non-linear effects resulting in high-speed rectification. The thinner the insulated layers become the higher the non-linear effects. If one thinks about the location of an electron at a certain time being a probability curve then theoretically this concept

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can be thought of as the shift of the probability curve representing the electron location toward the other side of the insulated material. The thinner the insulation material thus increases this shift in probability toward the outside of the insulated material creating an increase in the nonlinear response.

The output of the rectifiers can be dc-coupled together, allowing arrays of antennas to be networked together to further increase output power capacity. This is conceptually illustrated in Fig. 3.

PROOF OF CONCEPT THROUGH MODELING

Frequency Selective Surface (FSS) structures have been successfully designed and implemented for use in radio (RF) and microwave frequency applications. The classic laws of physics apply for adapting microwave applications to the infrared. Innovative INL research has further optimized FSS designs for operation in the Terahertz (THz) and infrared (IR) spectrums. It is recognized that several numerical analysis techniques can be employed for electromagnetic analysis. The basis for our research and development is the adaptation of the ‘Method of Moments’ technique, which is a numerical computational method of solving linear partial differential equations associated with electromagnetic fields. Ohio State University has implemented a method of moments based algorithm in a software product, termed Periodic Method of Moments (PMM), which was developed for designing military RF frequency selective surfaces. Ohio State’s PMM serves as the analysis engine for the INL ‘design and modeling’ methods.\

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ANALYTICAL MODEL – RLC CIRCUIT

To model NEC structures it is first necessary to understand the electrical equivalent circuit and basic theory of operation. The primary antenna structure studied in the initial design of an NEC is a periodic array of square-loop antennas. Its RLC circuit analog is shown in Figure 4. The electrical behavior of the structure is described as follows. The metal loops give inductance to the NEC as thermally-excited radiation induces current. The gaps between the metallic loops and the gap within the loop compose capacitors with a dielectric fill. A resistance is present because the antenna is composed of lossy metallic elements on a dielectric substrate. The resulting RLC circuit has a resonance “tuned” filter behavior. It is evident that the proper selection of element and substrate material is important and contributes to the RLC parameters. The electro-optical characteristics of the NEC circuit and the surrounding media have the effect of shaping and optimizing the spectral response.

Figure 4. Square FSS element and its RLC circuit analog.

Our current designs incorporate an antenna layer, a dielectric standoff layer, and a ground plane. (see figure 5). The standoff layer serves as an optical resonance cavity. The NEC-to ground plane separation acts as a transmission line that enhances resonance. The thickness of the standoff layer is selected to be a ¼ wavelength to insure proper phasing of the electromagnetic energy.

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NANTENNA STRUCTURE

FREQUENCY-DEPENDENT MODELING

The material properties of circuits in the infrared (IR) regime are not well characterized. It was necessary to adapt the PMM modeling software to account for the frequency-dependent optical properties of dielectric and antenna conductor material. The optical functions (n and k) of various materials that comprise NEC devices were determined using spectroscopic ellipsometry. These values characterize how a material responds to excitation by light of a given wavelength. One representation is the complex index of refraction,n~, where the real part n is the index and the imaginary part, k, is the extinction coefficient. The index, n, describes phase velocity of light in a material compared to propagation in a vacuum. The absorption of light is governed by the extinction coefficient, k. These quantities also determine the amount of light reflected and transmitted at an interface between two materials. This allows accurate simulation of antenna behavior at thermal wavelengths. It was demonstrated that the models accurately predicted thephysics of NEC energy absorption. This provides visualization of infrared thermal behavior that cannot be directly measured and supports rapid prototyping of nano-structure devices.

VALIDATION OF NEC CONCEPT

A periodic array of loop antennas was designed for resonance at 10um. This is the region of maximum thermal re-radiated emission from the earth’s absorption. All required antenna geometric and material properties were entered into the PMM model. The PMM model plots the reflection and transmission spectra for the electric field and the power spectra of the NEC antenna. The PMM tool was previously validated [by comparing experimental and modeled datasets. However, larger datasets for specific applications are required before deriving correlation and error bounds. It is anticipated that the research team will collect future datasets for the purpose of estimating model specific applications. The following emissivity plot was acquired to estimate NEC efficiencies at the 8-12 micron wavelength supporting the reradiated heat energy application (Figure 6).

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Ground plane - reflector

Dielectric resonance layer

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Figure 6. Modeled spectral output of a NEC. Proof of concept based on loop antenna structure, with a 10um resonance.

The output of the model is in units of emissivity versus wavelengths. It is referenced to a blackbody. A theoretical efficiency of 92% was demonstrated at a peak resonance of 10um. Efficiency is defined as the ratio of the power accepted by the antenna to the power emitted by the sun (reaching the earth’s surface) over a defined frequency range. The halfpower bandwidth of the antenna is 9.2um to 12um, easily covering the energy region of interest. Additional validation modeling was performed using the Ansoft HFSS tools [8]. Proof of principle of the ability to collect and concentrate thermal energy to a focal point was modeled using a complementary square spiral antenna. The following field overlay plot (Figure 7) was acquired.

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Figure 7. Field overlay plot of a square-spiral nantenna at infrared wavelengths

A field overlay plot is representative of the E-field [v/m] on a surface. The NEC antenna is the surface of interest. A linear frequency sweep of 1.0 THz to 40.0 THz, at 1 THz step size, was performed. The propagation of the e-field through the antenna structure was calculated as a function of different stimulus amplitude, frequency and phase. We further visualize how the field propagates throughout the volume of the NEC structure by animating the plot versus phase (time) when the NEC antenna is ‘excited’ by an incoming electromagnetic wave, representative of solar energy. It is demonstrated that surface currents flow and are concentrated into the antenna feed points.

PROOF OF CONCEPT THROUGH SMALL SCALE PROTOTYPE

Laboratory-Scale Silicon Wafer Prototypes

The original proof-of-concept was prototyped using standard semiconductor integrated circuit fabrication techniques [11] as outlined in Figure 8. Prototype fabrication was performed atUniversity of Central Florida under subcontract. Electronbeam lithography (EBL) was employed for fabrication of arrays of loop antenna metallic structures on dielectric and semiconductor substrates.

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Figure 8. Fabrication process flow for a square-slot IR FSS.

E-beam lithography provided a convenient way to systematically investigate dimensional, spacing, and geometrical effects in a controlled manner. This approach allowed for systematic trade studies. However, the throughput and cost limitations of EBL do not support large scale manufacturing. The completed structure is shown in Figure 9.

Figure 9. SEM micrograph of a small portion of the completed IR NEC. The entire IR FSS covers 40 square millimeters and is composed of 9.76·108 elements

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PRECURSORS TO ROLL-TO-ROLL MANUFACTURING

NANOFORMING PROCESS

Well-established FSS theory indicates that nano-scale antennas could be fabricated for use in any region of the EM spectrum. However, feature sizes required for an NEC required the use of electron beam lithography (EBL), x-ray lithography or some other ultra-high resolution fabrication means. These techniques are generally very slow and expensive. This has limited NEC’s to small sizes and has severely compromised their utility. However, due to recent advancements in ultra-high-resolution lithography and roll-to-roll (R2R) processing of flexible polymer films, low cost manufacturing is now possible. MicroContinuum, Inc. pioneered the use of sophisticated R2R technology for rapid, low-cost production of complex multilayer film structures. A key feature of this highly innovative manufacturing technique is that a "master pattern", which can be relatively expensive, is used to mechanically "print" the precision pattern (analogous to a printing plate in the graphic arts field) on an inexpensive flexible substrate. This pattern, in turn, is used to create the metallic loop elements of the NEC device over large areas. It is recognized that continuous large-scale production of complex multi-layer functional devices possessing nanoscale features is a monumental problem. Coupling this with a requirement for producing these devices on flexible, shape conforming substrates complicates this problem further. We have addressed these concerns by designing a multi-step roll-to-roll manufacturing process for continuous fabrication of these unique optical devices. This comprises the first demonstrated large-scale manufacture of multi-layer functional nanoscale devices on flexible substrates.

DEVELOPMENT OF A MASTER PATTERN

One of the most important aspects of fabricating the NEC is an understanding of the tolerances required to make structures behave like antennas at infrared wavelengths. Tolerances were derived through extensive modeling studies evaluating impacts of changing the geometric parameters including: antenna wire width/depth, antenna periodicity, gap size between adjacent antennas, etc. Many parameters are co-dependent and required use of optimization algorithms.

The resulting NEC geometry was incorporated into a master template (see Figure 10) fabricated on an 8-inch round silicon wafer. The template consists of approximately 10 billion antenna elements. The fidelity between the nantenna design and the replicated master template is excellent.

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Figure 10. Electron microscope image of master template

Prototype NEC DevicesProcesses have been developed to form nantennas onto polyethylene (PE) in a stamp and repeat process. Prototype NEC structures have been fabricated onto flexible substrates. Using this semi-automated process, we have produced a number of 4-inch square coupons that were tiled together to form sheets of NEC structures. (See Figures 11a and b) Prior to fully automated roll-to-roll scale up, all tooling and processes are being further developed and optimized using the semi-automated lab equipment.

Figure 11a. Prototype FSS structure on flexible substrate.

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Figure 11b. Nantenna sheet, stitched together from 18 coupons.

PROOF OF CONCEPT THROUGH EXPERIMENTAL MEASUREMENTS

Silicon Wafer Prototype

Experimental measurements were performed on a prototype consisting of a 1.0 cm2 array of loop nantennas fabricated on a silicon substrate. This rigid substrate prototype served as the precursor to current work on large-scale flexible substrate nantennas. The IR nantennas were designed to operate as a reflective bandpass filter centered at a wavelength of 6.5um. The spectral surface characteristics from 3 to 15um were studied using spectral radiometer and FTIR analysis methods. The prototype was elevated to a temperature of 200ºC and its spectral radiance spectrum was compared to blackbody emission at 200ºC. Maximum contrast is over 90% between emission near 4 μm and emission at resonance [10]. A further metric of IR FSS performance is its spectral emissivity. This ratio of IR FSS radiance to blackbody radiance is shown in Figure 12. The emissivity approaches unity at resonance, indicating the IR FSS radiates like a blackbody at a specific wavelength, with a steep drop off to shorter wavelengths and a more gradual drop off in emission to longer wavelengths.

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Figure 12. Demonstrated success in energy collection. Validated using a spectral radiometer and a FTIR.

The experimental spectrum closely correlates to the modeled spectrum. Peak resonance was achieved at the 6.5um wavelength. Resonant wavelengths are attainable by varying FSS parameters such as the standoff layer thickness and FSS element size and distribution. This basic design can be adapted to specific NEC implementations. Furthermore, out-of-band emission is 90% less than emission at resonance, making this IR FSS an excellent narrowband emissive energy concentrator or reflective filter.

FLEXIBLE SUBSTRATE NEC PROTOTYPE

Phase two prototypes, manufactured with manual methods using flexible polymer-based substrates, are in the process of being experimentally tested. Initial thermal characterization has been performed using high-resolution thermal cameras (Figure 13a). Initial results demonstrate proof of selective thermal energy collection. Figure 13b shows a circular NEC structure at the center of a metal plate. When excited by thermal energy the metal reference plate and the NEC structure have distinct emissivity contrasts, per the modeled design. This behavior will be further optimized to increase the collection efficiency of thermal radiation in the 10 um solar spectrum.

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Figure 13a. Experimental setup for thermal characterization of prototypes

Figure 13b. Optical and graphical experimental results from thermal characterization.

APPLICATIONS

Applications for this technology are very diverse. It is conceivable that nantenna collectors, combined with appropriate rectifying elements, could be integrated into the ‘skin’ of consumer electronic devices to continuously charge their batteries. Economical large-scale fabrication would support applications, such as, coating the roofs of buildings and supplementing the power grid. Due to the ability of integrating the nanostructures into poly materials it is possible that they could even be directly fabricated into polyester fabric.

The NEC devices can be optimized for collection of discrete bands of electromagnetic energy. Double-sided panels could absorb a broad spectrum of energy from the sun during the day, while the other side might be designed to take in the narrow frequency of energy produced from the earth's radiated heat or potentially residual heat from electronic devices. Available flux that reaches the earth’s surface in the .8-.9 um range is about 800w/m2 at its zenith. While visible light flux is dependent on cloud cover and humidity, incident light in this range is a constant during daylight hours. This technology may also support several emerging applications,

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including passive energy management products, such as building insulation, window coatings, and heat dissipation in small electronic consumer products, such as, computers. The nantennas are broadband collectors of energy with a tailorable spectral emission response. This in effect generates a frequency selective distribution of energy. This potentially will collect unwanted energy (residual or incident heat) and redistribute it at other innocuous wavelengths.

CONCLUSIONS AND FUTURE WORK

Finally, the nanoantenna has advantages, disadvantages and useful applications. In fact, currently, the largest problem is not with the antenna device, but with the electronic switches and amplifiers, so that, a bringing photonic switches and amplifiers to the reality is the concerned of the recent researches. Because of the electronic switches and amplifiers are unable to efficiently deal with frequencies which correspond to high-infrared to visible light. Therefore, a photonic system must be designed that can properly deal with the absorbed light without converted to the electric energy. What is being seen in the far sight of the horizon is a photonic processor. Both modeling and experimental measurements demonstrate that the individual nantennas can absorb close to 90 percent of the available in-band energy. Optimization techniques, such as, increasing the radial field size could potentially increase this efficiency to even higher percentages. More extensive research needs to be performed on energy conversion methods to derive overall system electricity generation efficiency. The circuits can be made from any of a number of different conducting metals. The nantennas can be formed on thin, flexible materials like polyethylene. Further laboratory evaluations of the flexible substrate NEC prototypes are planned. Manufacturing methods will continue to be refined to support roll-to-roll manufacturing of the nanostructures. Future work will focus on designing the nantenna structure for operation in other wavelengths. By further shaping the spectral emission of the NEC it may be possible to concurrently collect energy in the visible, nearinfrared and mid-infrared regions. This research is at an intermediate stage and may take years to bring to fruition and into the market. The advances made by our research team have shown that some of the early barriers of this alternative PV concept have been crossed and this concept has the potential to be a disruptive and enabling technology. We encourage the scientific community to consider this technologyalong with others when contemplating efforts and resources for solar energy.

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REFERENCES

References[1] Guy J. Consolmagno and Martha W. Schaefer, World's Apart: A Textbook in Planetary Sciences (1994) Englewood Cliffs, NJ: Prentice Hall.[2]http://www.mtt.org/awards/WCB's%20distinguished%20career.htm[3] http://www.kurasc.kyoto-u.ac.jp/plasma-group/sps/history2-e.html[4] I. Wilke, Y. Oppliger, W. Herrmann, F.K. Kneubuhl: Appl.Phys. A58, 329-341 (1994)[5] Subramanian Krishnan, Shekhar Bhansali, Kenneth Buckle, and Elias Stefanakos, “Fabrication and Characterization of Thin-film Metal-Insulator-Metal Diode for use in Rectenna as Infrared Detector”, Mater. Res. Soc. Symp.Proc. Vol 935.[6] Alda, J. Rico-García, J. López-Alonso,and G. Boreman, "Optical antennas for nano-photonic applications," Nanotechnology, vol. 16, pp. S230-4, 20[7] B. A. Munk, “Frequency Selective Surfaces: Theory and Design”. New York: Wiley, 2000, pp. 2–23.[8]Ansoft High Frequency Structure Simulator v10 User’s Guide, Ansoft Corporation, (2005)[9] L. W. Henderson, “Introduction to PMM, Version 4.0,” The Ohio State Univ., EletroScience Lab., Columbus, OH, Tech.Rep. 725 347-1, Contract SC-SP18-91-0001, Jul. 1993. [10] B. Monacelli, J. Pryor, B. Munk, D. Kotter, G. Boreman, “Infrared Frequency Selective Surfaces based on circuit-analogsquare loop design”. IEEE Transactions on antennas, Vol. 53, No.2 Feb 2005[11] B. Monacelli, J. Pryor, B. Munk, D. Kotter, G. Boreman,“Infrared Frequency Selective Surfaces:Square loop versus Square-Slot Element Comparison” AP0508-0657, Aug 2005

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