nantenna report nano antenna energy harvest solar energy

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ABSTRACT

This research explores a new efficient approach for producing electricity from the

abundant energy of the sun. A nantenna electromagnetic collector (NEC) has been designed,

prototyped, and tested. Proof of concept has been validated. The NEC devices target mid-

infrared wavelengths, where conventional photovoltaic (PV) solar cells are inefficient and

where there is an abundance of solar energy. The initial concept of designing NEC was based

on scaling of radio frequency antenna theory. This approach has proven unsuccessful by

many due to not fully understanding and accounting for the optical behaviour of materials in

the THz region. Also, until recent years the nanofabrication methods were not available to

fabricate the optical antenna elements. We have addressed and overcome both technology

barriers.

Several factors were critical in successful implementation of NEC including: 1)

frequency-dependent modeling of antenna elements; 2) selection of materials with proper

THz properties; and 3) novel manufacturing methods that enable economical large-scale

manufacturing. The work represents an important step toward the ultimate realization of a

low-cost device that will collect, as well as convert this radiation into electricity, which will

lead to a wide spectrum, high conversion efficiency, and low-cost solution to complement

conventional PVs.

Dept. of ECE,AIT,Bangalore

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TABLE OF CONTENTS

CHAPTER TITLE PAGE NO.

CHAPTER 1

Introduction

1.1 Nantenna Overview …..................…………...........…1

1.2 Solar radiation spectrum ……......................................2

1.3 Photovoltaic technology ..............................................4

1.4 Limitations of photovoltaic cells..................................5

1.5 Economical alternative to v.............................................6

1.6 Economics of nantennas ..............................................7

CHAPTER 2

Nantenna Operation

2.1 Theory of operation………………………..................8

2.2 Energy conversion methods ………………..............10

CHAPTER 3

Nantenna modeling

3.1 Analytical model – RLC circuit….....……................12

3.2 Frequency-dependent modeling….............................14


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CHAPTER 4

Validation of nec concept.................................................................15

CHAPTER 5

Manufacturing of a nantenna

5.1 Laboratory-scale Silicon Wafer Prototypes...............17

5.2 Roll-to-Roll Manufacturing.......................................18

5.3 Development of a Master Pattern...............................19

5.4 Prototype NEC Devices..............................................20

CHAPTER 6

Experimental measurements

6.1 Silicon Wafer Prototype ............................................21

6.2 Flexible Substrates NEC Prototype……………........22

CHAPTER 7

Applications......................................................................................24

CHAPTER 8

Limitations of nantennas..............................................................26

CHAPTER 9

Conclusions and Future work.....................................................27

REFERENCES.........................................................................................................28


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LIST OF FIGURES.

FIG No. TITLE PAGE No.

Figure1.1 Solar radiation spectrum............................................................................................2

Figure 1.2 Model of Photovoltaic cell........................................................................................5

Figure 2.1 Flow of THz currents to feed point of antenna Modelled with Ansoft FSS.............8

Figure 2.2 Typical electromagnetic radiation patterns of antenna.............................................9

Figure 2.3 Array of nantennas, printed in gold .......................................................................11

Figure 3.1 Square FSS element and its RLC circuit analog.....................................................13

Figure 3.2 Side view of NEC structure showing path of incident wave..................................14

Figure 4.1 Modeled spectral output of a NEC..........................................................................15

Figure 4.2 Field overlay plot of a square-spiral nantenna at infrared wavelengths.................16

Figure 5.1Fabrication process flow for a square-slot IR FSS..................................................17

Figure 5.2 SEM micrograph of a small portion of the completed IR NEC..............................18

Figure 5.3 Electron microscope image of master template......................................................19

Figure 5.4 Prototype FSS structure on flexible substrate.........................................................20

Figure 5.5 Nantenna sheet, stitched together from 18 coupons...............................................20

Figure 6.1 Energy collection validation using a spectral radiometer and a FTIR...................21

Figure 6.2 Experimental setup for thermal characterization of prototypes..............................22

Figure 6.3 Optical and graphical experimental results from thermal characterization............23


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Chapter 1

INTRODUCTION

1.1 Nantenna Overview

A nantenna (nanoantenna) is a nanoscopic rectifying antenna, an experimental

technology being developed to convert light to electric power. The concept is based on the

rectenna (rectifying antenna), a device used in wireless power transmission. A rectenna is a

specialized radio antenna which is used to convert radio waves into direct current electricity.

Light is composed of electromagnetic waves like radio waves, but of much smaller

wavelength. A nantenna is a very small rectenna (the size of a light wave), fabricated using

nanotechnology, which acts as an "antenna" for light, converting light into electricity. It is

hoped that arrays of nantennas could 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 nantenna devices have

been built, demonstrating only that energy conversion is possible.

A nantenna is an electromagnetic collector designed to absorb specific wavelengths

that are proportional to the size of the nantenna. Currently, Idaho National Laboratories has

designed a nantenna to absorb wavelengths in the range of 3–15 μm These wavelengths

correspond to photon energies of 0.08-0.4 eV. Based on antenna theory, a nantenna can

absorb any wavelength of light efficiently provided that the size of the nantenna is optimized

for that specific wavelength. Ideally, nantennas would be used to absorb light at wavelengths

between 0.4–1.6 μm because these wavelengths have higher energy than far-infrared (longer

wavelengths) and make up about 85% of the solar radiation spectrum.

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


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51% is absorbed by the surface or organic life and reradiated at around 10 um. The energy

reaching the earth in oth the visible and IR regions and the reradiated IR energy are

under-utilized by current technology.

1.2 SOLAR RADITION SPECTRUMR

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.

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).

Figure1.1 (Solar radiation spectrum)


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Solar radiation spectrum can be divided into five regions in increasing order of wavelengths:

1. 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.

2. 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.

3. 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.

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

is visible to the naked eye.

5. Infrared range that spans 700 nm to 10^6 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:

Infrared-A: 700 nm to 1,400 nm

Infrared-B: 1,400 nm to 3,000 nm

Infrared-C: 3,000 nm to 1 mm

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%.


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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.

1.3 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 photo diode. 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 photovoltaic's 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.


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Figure 1.2(Model of Photovoltaic cell)

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 Photovoltaic's or BIPV for

short. Solar PV power stations today have capacities ranging from10±60 MW although

proposed solar PV power stations will have a capacity of 150 MW or more.

1.4 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. band gap).


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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 band gap is converted into heat (via lattice

vibrations ² 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, multi junction 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.

1.5 ECONOMICAL ALTERNATIVE TO PV

We have developed an alternative energy harvesting approach based on nano antennas

that absorb the incident solar radiation. In contrast to PV, which are quantum devices and

limited by material band gaps, antennas rely on natural resonance and bandwidth of operation

as a function of physical antenna geometries.

The nano antennas 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 nano

technology have provided a pathway for large-scale fabrication of nano antennas.


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1.6 Economics of nantennas

Nantennas are cheaper than photo voltaic. While materials and processing of photo

voltaic are dear (currently the cost for complete photovoltaic modules is in the order of $40

per square foot and declining.), Steven Novack estimates the current cost of the nantenna

material itself as around fifty cents to one dollar per square foot.

With proper processing techniques and different material selection, he estimates that

the overall cost of processing, once properly scaled up, will not cost much more. His

prototype was a one foot by two foot sheet of plastic, which contained only 60 cents of gold,

with the possibility of downgrading to a material such as aluminium, copper, or silver.

The prototype used a silicon substrate due to familiar processing techniques, but any

substrate could theoretically be used as long as the ground plane material adheres properly.


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Chapter 2

Nantenna Operation

2.1 THEORY OF OPERATION

Nantenna is designed in such a technique that it 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 modelling illustrates the current flow is toward the antenna feed point. In a

balanced antenna, the feed point is located at the point of lowest impedance. Figure 2.1 was

acquired from modelling the electromagnetic properties of an infrared spiral antenna. The e-

field is clearly concentrated at the center feed point. This provides a convenience point to

collect energy and transport it to other circuitry for conversion.

Figure 2.1 (Flow of THz currents to feed point of antenna. Red represents highest

concentrated E field. Modelled with Ansoft HFSS.)


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Antennas have electromagnetic radiation patterns, which allow them to exhibit gain

and directionality and effectively collect and concentrate energy, as illustrated in Figure 2.2.

Figure 2.2 (Typical electromagnetic radiation patterns of antenna.)

The nantenna 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 2.2, 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 feed point. Theoretical

efficiency is improved by the enhanced radiation capture area of the antenna.


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2.2 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 frequency rectification.

This requires embedding a rectifier diode element into the antenna structure. One possible

embodiment is metal-insulator-insulator-metal (MIIM) tunnelling-diodes.

The MIIM device 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 tunnelling 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 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

figure(2.3).


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Figure 2.3 (An array of nantennas, printed in gold and imaged with a scanning electron

microscope. The deposited wire is roughly a thousand atoms thick. A flexible panel of

interconnected nantennas may one day replace heavy, expensive solar panels.)


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Chapter 3

NANTENNA 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. The details of this code are discussed in . Ohio State’s PMM

serves as the analysis engine for the INL ‘design and modeling’ methods.

3.1 Analytical Model – RLC Circuit

To model NEC (Nantenna electromagnetic collector) 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 3.1.


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Figure 3.1 (Square FSS element and its RLC circuit analog)

The electrical behaviour 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 behaviour . 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.

Current designs incorporate an antenna layer, a dielectric standoff layer, and a ground

plane (Figure 3.2).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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Figure 3.2 (Side view of NEC structure showing path of incident wave.)

3.2 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 materials.

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,ἧ, 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 behaviour at

thermal wavelengths. It was demonstrated that the models accurately predicted the physics of

NEC energy absorption. This provides visualization of infrared thermal behaviour that cannot

be directly measured and supports rapid prototyping of nano-structure devices.


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Chapter 4

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 4.1).

Figure 4.1 (Modeled spectral output of a NEC. Proof of concept

based on loop antenna structure, with a 10um resonance.)


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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 half power 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 . 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 4.2) was acquired.

Figure 4.2 (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.


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Chapter 5

MANUFACTURING OF A NANTENNA

5.1 Laboratory-scale Silicon Wafer Prototypes

The original proof-of-concept was prototyped using standard semiconductor

integrated circuit fabrication techniques as outlined in Figure 5.1. Prototype fabrication was

performed at University of Central Florida under subcontract.

Electron beam lithography (EBL) was employed for fabrication of arrays of loop

antenna metallic structures on dielectric and semiconductor substrates.

Figure 5.1 (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.


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However, the throughput and cost limitations of EBL do not support large scale

manufacturing. The completed structure is shown in Figure 5.2.

Figure 5.2 (SEM micrograph of a small portion of the completed IR NEC. The entire IR FSS

covers 40 square millimeters and is composed of 9.76·10^8 elements.)

5.2 Precursors to Roll-to-Roll Manufacturing Nano-forming 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.

Micro Continuum, 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.


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It is recognized that continuous large-scale production of complex multi-layer

functional devices possessing nano scale 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 nano scale devices on flexible substrates.

5.3 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.

Figure 5.3 ( Electron microscope image of master template.)

5.4 Prototype NEC Devices

Processes have been developed to form nantennas onto polyethylene (PE) in a stamp

and repeat process. Prototype NEC structures have been fabricated onto flexible substrates.


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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 (Figures 5.4 & 5.5).

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 5.4 (Prototype FSS structure on flexible substrate)

Figure 5.5 (Nantenna sheet, stitched together from 18 coupons.)


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Chapter 6

EXPERIMENTAL MEASUREMENTS

6.1 Silicon Wafer Prototype

Experimental measurements were performed on a prototype consisting of a 1.0 cm^2 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 band pass 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 .

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.

Figure 6.1 (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


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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.

6.2 Flexible Substrates 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 6.2).

Initial results demonstrate proof of selective thermal energy collection.

Figure 6.2 (Experimental setup for thermal characterization of

prototypes.)

Figure 6.3 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.


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Figure 6.3 (Optical and graphical experimental results from

thermal characterization.)

This behaviour will be further optimized to increase the collection efficiency of

thermal radiation in the 10 um solar spectrums.


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Chapter 7

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/m^2 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, 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.


NANTENNA 2012-2013

Nanoantenna is used by biochemists to dramatically enhance sensitivity in detecting

the presence of a molecule on a surface.

In optoelectronics, ordered arrays of closely spaced metal nano particles enable the

guiding of electromagnetic energy in sub wavelength-sized devices.

Another application of the nano antennas is to create more compact, faster circuits and

computers that use packets of light, i.e. photons(which travel much faster than

electrons),instead of electrons for carrying signals. Such photonic circuits could be

used for a new type of sensitive sensors which detect tiny traces of chemicals and

biological materials, making them useful for applications including analyzing a

patient's DNA for medical diagnostics, monitoring air quality for pollution control

and detecting dangerous substances for homeland security.

In addition, with new types of materials, it may be possible to accomplish better

performance than all existing materials, in terms of making images and manipulating

light. If Light can be manipulate and guide by employing the plasmonic nonmaterials,

all the basic fundamental properties of electronic circuits can be basically simulate by

the replace of photons.

Nanoantenna can be used in the field of optical imaging. Although several clever

techniques such as dark field, phase contrast, and interference contrast are provided,

all these methods are using the imaging mechanism that the object can only be “seen”

when photons originating from it reach the detector. The shortcut of this is that these

methods depend sensitively on the intensity, phase, and polarization of light.

The spectral modifications of a nano antenna could serve as the signal to probe the

near-field optical contrast of the sample.


NANTENNA 2012-2013

Chapter 8

LIMITATIONS OF NANTENNAS

As previously stated, one of the major limitations of nantennas 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 nantennas, more advances are necessary to operate efficiently at higher frequencies.

Another disadvantage is that current nantennas 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 only used for research purposes when extremely fine

resolutions are needed for minimum feature size (typically, on the order of nanometers).

However, photolithographic techniques have advanced to where it is possible to have

minimum feature sizes on the order of tens of nanometers, making it possible to produce

nantennas by means of photolithography.


NANTENNA 2012-2013

Chapter 9

CONCLUSIONS AND FUTURE WORK

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, near-infrared 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 technology along with others when contemplating efforts and resources for solar energy.

REFERENCES


NANTENNA 2012-2013

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