PLASMONICS

MODEL INSTITUTE OF ENGINEERING AND TECHNOLOGY

AICTE Approved & Affiliated to University of Jammu

ISO 9001:2000 Certified

Department of Electronics and Communication

SEMINAR REPORT

On

PLASMONICS

Submitted by:- Submitted to:-

Vivek Singh

TaruMahajan

E.C.E. ‘B1’

Lecturer, E.C.E. Department

327/08.

PREFACE

This Seminar report deals with the different aspects in the development of new

technology called Plasmonics.The term ‘plasmonics’ is derived from plasmons—quanta

associated with collective excitation of free electrons in metals.

This Seminar report provides the basic knowledge necessary to understand the

basic concept involved in the fields of plasmonics. This seminar report covers firstly the

introduction about plasmonics,itstechnology,disadvantages of present modes-electronics

and photonics and how plasmonics can bridge both these modes. The application part

PLASMONICS

where plasmonics can be used for making superfast computer and its use in many other

applications is also discussed.In the last, a brief conclusion and future aspects is

discussed.

The plan of this seminar report is to present the detailed information in simple

language. This seminar report is suitable for the self-study by engineers and scientists

who need to acquire the basic knowledge of Plasmonics.

VIVEK SINGH

ACKNOWLEDGEMENTS

In the preparation of this seminar report, I am grateful to the Principal and H.O.D.

of E.C.E. Dept. of MIET, and specially to Lect. TaruMahajan, E.C.E. Dept., who have

left no stone unturned for the successful completion of my seminar and other respected

faculty members.

My special dept. of gratitude to my grandfather Sh.HarnamSingh,father

Sh.RajinderSingh,motherSmt.TaraJamwal and sisterMs.Natasha and other respected

family members.

1 Vivek Singh 327/08 ECE B1

PLASMONICS

I have received help and encouragement for which I am deeply grateful to my

friends- Rajat Sharma,Rameshwar Sharma,Rahul Lakhanpuria, Sourab Sharma,

RajatBasotra, SahilDogra,Varinder Singh,Ankush Sharma and Zorawarsingh.

VIVEK SINGH

1. INTRODUCTION

Currently, communication systemsare

based on either electronicsor photonics.

However,with the quest for transporting

hugeamounts of data at a high speed alongwith

miniaturisation, both these technologiesare

facing limitations. Due totheir mismatched

capacities and sizes,it is very difficult to cobble

them to geta high bitrate with

miniaturisation.So researchers are pioneering

anew technology called ‘plasmonics.’Due to its frequency being approximatelyequal to


Fig. 1 Practical visualization of Plasmons

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that of light and abilityto interface with similar size electroniccomponents, plasmonics

can act as abridge between photonics and electronicsfor communication.

The term ‘plasmonics’ is derived from plasmons (Fig. 1)—quanta associated with

collective excitation of free electrons in metals.Plasmons are a physics phenomenon

based on the optical properties of metals;they are represented by the energy associated

with charge density waves propagating in matter through the motions of large number of

electrons.Whenlight falls on a metal, owing to the electric field component of light, the

conductionelectron cloud of the metal shifts and results in the deficiency of negative

charge on the opposite side. Due to coulomb attraction, the electron cloud rebouncesto its

original position, but owing to inertia it gets overshot resulting in a oscillation frequency

called surface plasmon resonance frequency, which is equal to the frequency of irradiated

light as shown in the fig 2. Electrons,in a metal,screen an electric field.Light of frequency

below the plasma frequency is reflected.Surface plasmons as shown in fig3are associated

with surface charge oscillations. These oscillations are also known as plasma

oscillations.These are rapid oscillations of the electron density in conducting media such

as plasmas or metals.Plasma is a state of matter similar to gas in which a certain portion

of particle are ionised. Heating a gas may ionise its molecules or atoms,thus turning into

plasma,which contain charged particles,positive ions and negative electrons.The presence

of a non-negligible no.of charge carriers makes the plasma electrically conductive so that

it responds strongly to electromagnetic fields.The frequency of plasma oscillations is

almost equal to that of light,optical frequency of today’s electronic microprocessors.So

light can be used to excite them on the surface of a material in localised regime.

The energy required to receive and send a surface plasmon pulse can be less than

for electric charging of a metallic wire. This could allow plasmons to travel along

nanoscale wires (called interconnects) carrying information from one part of a


Fig. 2 Electron Cloud Shifting

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microprocessor to another with a high bitrate.Plasmonic interconnects would be a great

boon for chip designers, who have been able to develop ever smaller and faster transistors

but have had a harder time building minute electronic circuits that can move data quickly

across the chip.Surface plasmons can be excited on a flat nano-film, nanostrip or other

shaped nanoparticles such as nanosphere, nanorod, nanocube and nanostar.When

nanoparticles are used to excite surface plasmons by light, these are known as localised

surface plasmons.Silver and gold are of particular interest due to their high field

enhancement and resonance wavelength lying in the visible spectral regime. The speed of

these surface plasmons is almost equal to that of light with wavelength of the order of

tens of nanometres.

2. Limitations of present modes

Presently, electronics plays an importantrole in communication. In

laboratories,though, photonics has started replacing electronics where a high data transfer

rate is required.

Electronics deals with the flow of charge (electrons). When the frequency of an

electronic pulse increases, the electronic device becomes hot and wires become very

loose. Hence by the principle of “the higher the frequency,the higher the data transfer

rate,” a huge amount of data cannot be transferred.On the other hand, when the size of an

electronic wire reduces, its resistance (inversely proportional to the cross-sectional area of

the wire)increases but the capacitance remains almost the same. This leads to time delay

effects.

In photonics, optical fibres (cylindricaldielectric/non-conducting waveguides)

are used. These transmit light along their axis by the process of total internal reflection.

The fibre consists of a core surrounded by a cladding layer, both of which are made of

dielectric materials. To confine the optical signal in the core, the refractive index of the

core must be greater than of the cladding. The lateral confinement size of the optical cable

is approximately half the wavelength of the light used signal passing through it and is

called diffraction limit.Although, thedata transportation rate is high in photonics,owing to

the diffraction limit, the size of optical fibre is in the order of hundreds of nanometres

much larger than the present-day nano-electronic devices.In the increasing quest for

transporting huge amount of data at high speed along with miniaturization, both


PLASMONICS

electronics and photonics are facing limitations. It is difficult to cobble them to obtain a

high bit rate along with miniaturization owing to their mismatched capacities and sizes.

Researchers are promoting plasmonics as the future of wave communication.The

confinement of light wave on the dimensions of metal below the diffraction limit forms a

major part of the application.

3. Plasmonics can bridge microscale photonics and nanoscale electronics

Based on the data presented above, it seems that the propagation lengths for

plasmonic waveguides are too short to propagate SPPs with high confinement over the

length of an entire chip (~1 cm). Although the manufacturability of long-range SPP

waveguides may well be straightforward within a CMOS foundry, it is unlikely that such

waveguides will be able to compete with well-established, low-loss,high-confinement Si,

Si3N4, or other dielectric waveguides.However, it is possible to create new capabilities

by capitalizing on an additional strongpoint of metallic nanostructures. Metal

nanostructureshave a unique ability to concentrate light into nanoscale volumes. This

capability has been employed to enhance a diversity of nonlinear optical phenomena. For

example,surface-enhanced Raman scattering (SERS) is widely used in the field of

biology. This technique makes use of the enhanced electromagnetic fields near metallic

nanostructures to study the structure and composition of organic and biological materials.

Enhancement factors on the order of 100 have been predicted and observed for spherical

particles. Even greater enhancements can be obtained near carefully engineered metal

optical antenna structures that basically resemble scaled-down versions of acar antenna.

Recently, such antennas have even enabled single molecule studies by SERS and white-

light supercontinuum generation.


PLASMONICS

Despite the numerous studies on antennas in the microwaveandoptical regimes,

their application to solve current issues in chip-scaleinterconnection has remained largely

unexplored. The fieldconcentrating abilities of optical antennas may serve to bridge the

large gap between microscale dielectric photonic devices and nanoscale electronics

(Fig.4). This diagram shows a detail of a chip on which optical signals are routed through

conventional dielectric optical waveguides. The mode size of such waveguides is

typically one or two orders of magnitude larger than the underlying CMOS electronics.

An antenna can be used to concentrate the electromagnetic signals from the waveguide

mode into a deep subwavelength metal/insulator/metal waveguide and inject it into a

nanoscale photodetector. The small size of the detector ensures a small capacitance, low-

noise, and high-speed operation. By using metallic nanostructures as a bridge between

photonics and electronics, we play to the strengths of metallic

nanostructures(concentrating fields and subwavelength guiding),dielectric waveguides

(low-loss information transport), and nanoscale electronic components (high-speed

information processing).

4. COMPONENTS OF PLASMONICS

There are two main components ofplasmonics: (i) surface plasmon (SP) polaritons

and (ii) localized surface plasmons (LSPs) (Fig.5). SPs are associated with surface charge

oscillation having frequency almost equal to light. The energy required to receive and

send a SP pulse can be less than that needed for the electric charging of a metallic wire.

This couldallow the plasmons to travel

along nanoscale wires (called

interconnects) to carrying information

from one part of a microprocessor to

another with high bit rate. Plasmonic

interconnects would be a great boom

for chip designers, who have been able


Fig. 4 Nanoscale antenna

Fig. 5Localized surface plasmons

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to develop ever smaller and faster transistors that can move data quickly across the chip.

Plasmon-based waveguides are not only a mode by which light can be guided on

nanoscales, but also promise a path for chip scale device integration. Here, we provide a

qualitative discussion on the factors that manage plasmon excitation by different methods

along with a brief description on some theoretical aspects of plasmonics. The article ends

with aconcise dialogue on promising applications of plasmonics in communication. It is

hopeful that this will inspire detailed study of plasmonic devices in the field

ofcommunication.

5. Surface plasmon excitation

Plasmonic structures can exert huge control over light waves at the nanoscale. As a result,

energy carried by plasmons allow for light localization in ultra-small volumes, far beyond

the diffraction limit.To generate the SPs, it is necessary to excite the metal – dielectric

interfaceas shown in the fig. 6which the dielectric constantof the metal is a function of

frequency and possesses a negative real part.The

plasmon losses are lower at the interface between a

thin metal film and a dielectric than inside the bulk

of the metal film because the field spreads into the

nonconductive materials,where there are no free

electrons to oscillate,and hence no energy

dissipation owing to collisions. This property naturally confines plasmons to the metallic

surface neighbouring the dielectric; in a sandwich with dielectric and metal layers.

6. Communication with plasmonics

Plasmonic structures can exert huge control over electromagnetic wavesat the

nanoscale. As a result, energycarried by plasmons allows for lightlocalisation in ultra

small volumes—far beyond the diffraction limit of light.To generate surface plasmons, it

isnecessary to excite the metal-dielectricinterface in which the dielectric constantof the

metal is a function of frequencyand negative. At the nanoscale,the electromagnetic (EM)

field of theEM wave displays the electron clouddue to its well coupling, which is not


Fig. 7 Plasmon Excitation

Fig. 6 Surface Plasmon Excitation

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possible in the case of bulk matter.Hence plasmonics is frequently associated with

nanotechnology.Investigators have found that by creatively designing the metaldielectric

interface, they can generate surface plasmons with the same frequency as the

electromagnetic wave but with much smaller wavelength.This phenomenon could allow

plasmons to travel along nanoscale wirescalled ‘interconnects’ in order to carry

information from one part of the microprocessor to another. Fig 7 shows different

operating speed of operating and processing system.

7. Methods

Plasmonic waveguides are gaining much attention owing to their abilityto operate in

various parts of the spectrum-ranging from visible to infrared region.A plasmon could

travel as far as several micrometres in the slot waveguide (dielectric core with metallic

cladding)—far enough to convey a signal from one part of a chip to another. The plasmon

slot waveguide squeezes the optical signal, shrinking its wavelength.Metallic nanowires

can provide lateral confinement of the mode below the diffraction limit. Nanowires have

larger attenuation than planer films but light transport over a distance of several microns

has been demonstrated.A chain of differently-shaped nanoparticles(such as spheres and

rods) can be used to transport EM waves from one nanoparticle to another via the near-

field electrodynamic interaction between them. If the second particle is situated in the

near field of theother and so on along the chain, EMenergy can be propagated within the

lateral size confinement less than the diffraction limit. In a chain of closelyspaced

nanostructures, the propagation distance depends upon the shape and nature of materials,

separation between them as well as the dielectric constant of the host medium.


Fig. 7 Operating speed of data transporting and processing system

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Optical regimes-applicable size

and speed scale-forplasmonic

and other devices. Plasmocom

team took a novel approach,

developing what they called

dielectric-loaded surface

plasmon polariton waveguides

(DLSPPW) as shown in fig 9. By patterning a layer of various polymer (polymethyl

methacrylate) dielectic onto gold film supported by a glass

substrate, they were able to achieve

waveguides that were only 500

nanometres in size while extending

the signal propagation.

Using this approach, the

researchers built a variety of

plasmonic devices, including low-

loss S bends, Y-splitters and a

waveguide ring resonator, a crucial

part of the add-drop multiplexers

(ADM) in optical networks that

combine and separate several streams of data into a single signal and vice versa.

8. Imaging :


Fig. 9Dielectric-loaded surface plasmon polariton waveguides

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In order to study the propagation of SPPs, a photon scanning tunneling microscope

was constructed (PSTM) by modifying a commercially available scanning near-field

optical microscope. PSTMs are the tool of choice for characterizing SPP propagation

along extended films as wellas metal stripe waveguide. Figure shows how a microscope

objective at the heart of our PSTM can be used to focus a laser beam onto a metal film at

a well-defined angle and thereby launch a SPP along the top metal surface

A sharp, metal-coated pyramidal tip (Figure 10b and 10c) is used to tap into the guided

SPP wave locally and scatter light toward a far-field detector. These particular tips have a

nanoscale aperture at the top of the pyramid through which light can be collected. The

scattered light is then detected with a photomultiplier tube. The signal provides a measure

of the local light intensity right underneath the tip and, by scanning the tip over the metal

surface, the propagation of SPPs can be imaged The operation of the PSTM can be

illustrated by investigating the propagation of SPPs on a patterned Au film (Figure 10d).

Here, a focused ion beam (FIB) was used to define a series of parallel grooves, which

serve as a Bragg grating to reflect SPP waves. Figure (10e) shows a PSTM image of a

SPP wave excited with a 780 nm wavelength laser and directed toward the Bragg grating.

The back reflection of the SPP from the grating results in the standing wave interference

pattern observed in the image. From this type of experiment the wavelength of SPPs can

be determined in a straightforward manner and compared to theory.


Fig. 10Schematic representation of the operation of a PSTM

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9. Close to market technology

While current commercial optical ring resonators have a radius of up to 300 micrometres,

the plasmonic demonstrator built by the Plasmocom team measured just five micrometres.

“The devices performed almost 100 percent as we had modelled them, and showed very

good characteristics overall,” Zayats says. “Such devices need to keep getting smaller if

we are to continue to see performance gains in new applications,” he adds.

Crucially, the Plasmocom technology can create plasmonic devices using existing

commercial lithographytechniques.

“Other groups of researchers have achieved similar or better propagation or smallerdevice

sizes but the processes they have used are often extremely complex and would be difficult

to replicate at an industrial scale,” Zayats explains. “Our technology may not be the

smallest... but it is closer to market.”

French chipmaker and project partner Silios Technologies is currently drawing up a

commercialisation plan, which may involve either producing plasmonic components itself

or licensing the Plasmocom technique to one of the big players in the industry.

Zayats notes that interest in the team’s work has been extensive within both academia and

industry, evidenced by the success of a workshop in June in Amsterdam attended by

representatives of several photonics and electronics firms, including NEC and Panasonic.

“I think that we will start to see this technology make its way into commercial

applications over the next five to ten years,” Zayats says. “A key breakthrough will be

using plasmonics for inter-chip communication, making it possible to transmit data

between one or more chips at optical speeds and eliminating a major bottleneck to faster

computers.”

10. APPLICATIONS:

10.1 Graphene:

On the one hand graphene, a single layer of carbon atoms (fig 11) in a honeycomb

pattern, can move electrons (electricity) very fast and efficiently. On the other hand

graphene is lousy at absorbing energy, specifically from sunlight; only about 3% is

absorbed. Sounds like graphene, a wonder material in many accounts, isn’t cut out for

solar cells or photonics (such as communication by light). Well by itself it’s not, but

graphene is such a tempting material that clever minds are set upon making it do all kinds


PLASMONICS

of things it doesn’t appear to do. In this case, among the clever minds are the two fellows

who won the Nobel Prize for their work with graphene, Andre Geim and

KostyaNovoselov plus their team at the University of Manchester, and Cambridge

University (UK). Their newest work, published in the journal Nature Communications

[ Strong plasmonic enhancement of photovoltage in graphene] advances the use of

graphene.

Their approach to making graphene part of a photonics system, where it contributes

higher speed transmission, is to put closely-spaced nanoscale metallic wires (nanowire)on

top of the graphene layer. The wires, called a plasmonic nanostructure, can take on a large

variety of shapes with exotic names such as nanoshells, nanomatryushkas, and nanorice.

The shapes (structures) of wire are significant because of what they do to incoming light

energy – they, in effect, bend, reflect and transform it so that, in this case, far more energy

is absorbed by the graphene layer. In fact, it boosts the absorption efficiency by about

twenty times, a rather remarkable figure.

This increase makes it realistic to look at the graphene-plasmonic nanostructure

combination as a potential material for use in all sorts of optoelectronics such as solar

cells and photodetectors for high-speed optical communications.

Whether this approach will ever be put to industrial use, that is, can be

manufactured in quantity, quality and competitive price is a big unanswered question; but

graphene solutions like this one have a big advantage. It might be called ‘concentrated

attention,’ that is, so many people are working on so many aspects of graphene

production and utilization that new techniques and processes appear with great regularity.

So it’s possible that even if graphene isn’t the best possible material, it may turn out to be

the one that is practical. This effect is seen all the time in the way in which industry has

used silicon and silicon chip manufacturing, where ‘limitations’ are constantly overcome

by brilliant new techniques. What’s at work is a critical mass of research, manufacturing

know-how, and a willing market that will pay for the improvements. Many, including

some of the world’s big electronics corporations, are betting thatgraphene will reach that

kind of critical mass.

10.2 Nanoparticle inspire solar cells


Fig. 11 Structure of Graphene

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As demand grows for greener power generation and energy conservation, how can

renewable technologies take on the might of goliaths of the fossil fuel industry? In the

case of thin-film solar cells, the weapon of choice comes in the diminutive form of

metallic nanoparticles. Thanks to a combination of the resonant plasmonic properties

of metallic nanoparticles with thin-film photovoltaic technology, a new generation of

plasmonic solar cell (fig 12) has evolved with similar performance to silicon cells but at

potentially a fraction of the cost. Today, plasmonic solar cells are emergingas promising

candidates amongst many solar energy

technologies spurring continuing research to

improve device performance.One leading

research group in thisarea is based at the Centre

for SustainableEnergy Systems at the Australian

NationalUniversity (ANU) who are working

alongside other principal groups led by

HarryAtwater and AlbertPolman at Caltech,

California, US and the FOM-Institute, AMOLF,

the Netherlands, respectively. The group at

ANU measured an enhanced photocurrent

attributed to the increased trapping of light

scattered into a thin-film silicon cell by silver

metal nanoparticles excited at their surface plasmon resonance. Now, leading scientists in

the field are looking to drive plasmonic solar cells out of the science of the small into the

next big thing in the photovoltaics industry.

10.2.1 A thin slice of the solar industry

The global photovoltaic market as a whole looks set to ride out the economic

downturn with a predicted growth hitting $2.4 bn in 2011 and $7.5 bn by 2015, according

to arecent report by NanoMarkets. In spite of this fact, photovoltaics will only outshine

existing methods of generating electricity if they can genuinely compete with

currentfossil fuel technologies in terms of cost and performance. This requires at the least

halving the price of current solar cells.Thin-film cells are made from a thin

semiconducting layer – usually of amorphous or polycrystalline silicon, cadmium

telluride or copper indium diselenide – deposited on a cheap glass, plastic or stainless

steel substrate.Now, researchers believe that thin films will succeed as alternative energy


Fig. 12 Plasmonic Solar Cells

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sources by eliminating the need for thick and expensive silicon wafers.

“The thickness of the thin-film silicon solar cell is only 1 or 2 μm compared with

the 200 μm for the wafer cells,” Kylie Catchpole, research fellow at the ANU, told OLE.

“That can dramatically reduce your materials

cost as it reduces the amount of high purity

semiconductor that you need.”However, while

thin-film silicon solar cells are a cheaper

alternative to silicon wafers the poor

absorption of near-bandgaplight remains a

severe limitation on their performance.

“When you decrease the thickness that

much, you also decrease the absorption,”

said Catchpole. “So for thin-film solar cells

you really need to increase the absorption.

For wafer-based solar cells there are already

quite good ways for increasing the absorption but not for thin-film solar cells”.In line

with this, the solar cells need tobe structured so that light remains trapped inside to

increase the absorption. For thin-film cells, the thickness range of a few microns is too

small to support surface texturingcommonly used in the wafer-based silicon cells where

pyramids in the range of2–10 μ m are etched into the surface. Thishas prompted several

research groups tolook to alternative methods, one of which was to use the scattered light

from the surfaceplasmon resonance of metallic nanoparticles on the surface of the thin-

film cell.

According to Catchpole a texture on the surface of the thin-film solar cell can also

reducethe maximum voltage produced by the cell through increased electron-

holerecombination at the surface. Metal nanoparticles remain independent of the

structureof the solar cell itself and so increase the absorption while leaving the

electricalperformance intact.

10.2.2 Silver takes first place

The optical properties of metal particleshave

been a subject of great interest in the last few decades,

especially with the potentialapplications of plasmonic

resonances in integrated optics and biosensing.At


Fig. 13 Silvernano-particles

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wavelengths near the plasmon resonance,metal nanoparticles are strong scatterersof light.

A plasmon arises from the collective oscillation of the free electrons in the metal particle.

For particles with diameters well below the wavelength of light, the absorption and

scattering cross-sections can be described by those of a point dipole. At the surface

plasmonresonance, the scattering cross-section is found to exceed the geometrical cross-

section of the particle, thereby increasing the amount of light scattered into the cell.

Noble metals are ideal for this purpose as they do not have many interband

transitions and do not absorb much light as a result. Significant enhancements in

photocurrent measurements have been found using noble metals such as silver or gold.

While the dielectric functions of silver and gold are reported to be very similar,the group

at ANU believes silver to be the better choice due to its lower absorption and lower cost.

“What you want is for the light to comein, scatter from the nanoparticle and gointo the

solar cell. You really don’t want the light to be absorbed in the metal particle itself,”

described Catchpole. “Silvernanoparticles(fig 13) is by far the best for that. Other metal

particles tend to absorb the light just because of their atomic structure.”While there are

many techniques and materials for plasmonic solar cell fabrication,the group at the ANU

uses borondopedsilicon solar cells and evaporates a layer of hemispherical silver

nanoparticles close to 100 nm in size on the surface.Starting with the silicon cell, an oxide

is grown on the surface in an oxygen furnace at high temperature. The metal

nanoparticles are then deposited on the thin-film silicon cells by vacuum evaporation.

This process initially involves evaporating a thin silver film onto the cell surface and then

heating the sample to 200 °C. Even though this is below the melting point of the metal,the

layer is thin enough so that little blobs form under surface tension. This creates roughly

evenly sized, evenly distributedparticles on the solar cell surface.In this way it is possible

to cover any desired area with these very tiny particles.This would otherwise be a very

difficult and expensive process were each individual particle to be made via

techniquessuch as electron beam lithography.This process also has the advantage of

having no effect on the electrical performance of the solar cell and has no influence on the

fabrication process of the solar cell itself (as metal evaporation is performed after the

thin-film solar cell is made).One of the main challenges that the group found, however,

was getting the nanoparticles close enough to the surface of the cell. “Putting the

nanoparticles extremelyvclose to the silicon surface turned out to be very important for

getting a good enhancement in the absoption,” described Catchpole. “A difference of 20

nm makes abig difference in this situation. You need to have the metal nanoparticles


PLASMONICS

really closeto the surface and so we have had to understand how the absorption

enhancement works to figure out what we really need to do with the

particles.”Evaporating the metal particles to within the desired 20 nm of the silicon

surface requires control over the thickness of the oxide layer grown on the cell surface.

This can be achieved through controlling the temperature or duration of the oxidation

process or by etching the oxide layer after it has been grown.

10.2.3 The future’s bright?

According to Catchpole, progress in plasmonic solar cells has recently been

dramatic thanks to a fuller understanding of plasmonics. “Plasmonics has become a big

field. It is now possible to make nanoscale particles and nanoscale type structures,and so

a lot of people have become interested in it. There has been work done to figure out what

happens at that scale,” she said.Research into plasmonic solar cells is rapidly

expanding,exploiting the benefits offered by plasmonics with those of thinfilm

technology.Fabricating thin-film solar cells uses a lot less material and can take place on

a very large scale – a big advantage for reducing the installation costs that form a

significant part of the whole cost of a solar system. One of the added advantages of using

metal nanoparticles is that they are generally applicable to any thin-film solar cell

irrespective of the underlying semiconductor be it a silicon or organic solar cell.“It’s

essentially all about cost in the solar industry. Whatever you can do to lower the cost, that

is what is going to winout in the end,” added Catchpole. “Thereare a number of things

that affect cost. Itcan be the efficiency of the cell or it can be the cost of the process, or

how fast you can do the process. But all of these things are headed towards the reduced

overall cost of the solar cells.”Research is ongoing into improving the performance,

which includes looking into how differences in particle size and shape influence the

photocurrent measurements.The group expects a coZayats and his team reported an

advance toward developing optical components for superfast computers and high-speed

Internet services, which they say could revolutionize data processing speeds by

transmitting information via light rather than through electric currents.

The scientists have designed an artificial material similar to a stack of nanoscale

rods that allows light beams to interact efficiently and change intensity — allowing

information to be sorted by beams of light and very high speeds, solving the difficulty of

light beams interacting with one other while they travel through a material.

This metamaterial reportedly could be incorporated into existing electron chips or

it could be used to build completely new all-optical chips that could revolutionize data


PLASMONICS

processing speeds. The scientists showed that closely spaced plasmonic gold nanorods

produced an ultrafast transmission change when they were illuminated with a low-energy

optical pulse The main discovery is that nanorod material exhibitsnonlocalityof the

optical response, which has an unusually strong linear dependence on incident light

intensity.

10.3 Superfast computers

However, to date, plasmonic properties have been limited to nanostructures that

feature interfaces between noble metals and dielectrics.

Now, researchers with the US Department of Energy’s Lawrence Berkeley National

Laboratory (Berkeley Lab) have shown that plasmonic properties can also be achieved in

the semiconductor nanocrystals known as quantum dots.

“We have demonstrated well-defined localised surface plasmon resonances arising

from p-type carriers in vacancy-doped semiconductor quantum dots that should allow for

plasmonic sensing and manipulation of solid-state processes in single nanocrystals,” says

Berkeley Lab director Paul Alivisatos, who led this research.

“Our doped semiconductor quantum dots also open up the possibility of strongly

coupling photonic and electronic properties, with implications for light harvesting,

nonlinear optics, and quantum information processing.”

The term ‘plasmonics’ describes a phenomenon in which the confinement of light

in dimensions smaller than the wavelength of photons in free space make it possible to

match the different length-scales associated with photonics and electronics in a single

nanoscale device.

Scientists believe that through plasmonics it should be possible to design

computer chip interconnects that are able to move much larger quantities of data much

faster than today’s chips.

It should also be possible to create microscope lenses that can resolve nanoscale

objects with visible light, a new generation of highly efficient light-emitting diodes and

supersensitive chemical and biological detectors.

There is even evidence that plasmonic materials can be used to bend light around

an object, making that object invisible.

The plasmonic phenomenon was discovered in nanostructures at the interfaces

between a noble metal, such as gold or silver, and a dielectric, such as air or glass.


PLASMONICS

Directing an electromagnetic field at such an interface generates electronic surface

waves that roll through the conduction electrons on a metal, like ripples spreading across

the surface of a pond that has been disturbed by a stone.

Just as the energy in an electromagnetic field is carried in a quantised particle-like

unit called a photon, the energy in such an electronic surface wave is carried in a

quantised particle-like unit called a plasmon.

The key to plasmonic properties is when the oscillation frequency between the

plasmons and the incident photons matches, a phenomenon known as localised surface

plasmon resonance (LSPR).

Conventional scientific wisdom has held that LSPRs require a metal

nanostructure, where the conduction electrons are not strongly attached to individual

atoms or molecules.

This has proved not to be the case.

Prashant Jain, a member of the research team, says:

“Our study represents a paradigm shift from metal nanoplasmonics as we’ve shown that,

in principle, any nanostructure can exhibit LSPRs so long as the interface has an

appreciable number of free charge carriers, either electrons or holes.”

“By demonstrating LSPRs in doped quantum dots, we’ve extended the range of

candidate materials for plasmonics to include semiconductors and we’ve also merged the

field of plasmonic nanostructures, which exhibit tunable photonic properties, with the

field of quantum dots, which exhibit tunable electronic properties.”

Jain and team members made their quantum dots from the semiconductor copper

sulfide, a material that is known to support numerous copper-deficient stoichiometries.

Initially, the copper sulfidenanocrystals were synthesised using a common hot injection

method.

While this yielded nanocrystals that were intrinsically self-doped with p-type

charge carriers, there was no control over the number of charge vacancies or carriers.

“We were able to overcome this limitation by using a room-temperature ion

exchange method to synthesise the copper sulfidenanocrystals,” Jain says. “This freezes

the nanocrystals into a relatively vacancy-free state, which we can then dope in a

controlled manner using common chemical oxidants.”

By introducing enough free electrical charge carriers via dopants and vacancies,

Jain and his colleagues were able to achieve LSPRs in the near-infrared range of the

electromagnetic spectrum.


PLASMONICS

The extension of plasmonics to include semiconductors as well as metals offers a

number of significant advantages, as Jain explains.

“Unlike a metal, the concentration of free charge carriers in a semiconductor can

be actively controlled by doping, temperature and/or phase transitions,” he says.

“Therefore, the frequency and intensity of LSPRs in dopable quantum dots can be

dynamically tuned. The LSPRs of a metal, on the other hand, once engineered through a

choice of nanostructure parameters, such as shape and size, is permanently locked in.”

Jain envisions quantum dots being integrated into a variety of future film and

chip-based photonic devices that can be actively switched or controlled, and also being

applied to such optical applications as imaging.

In addition, the strong coupling that is possible between photonic and electronic

modes in such doped quantum dots holds exciting potential for applications in solar

photovoltaics and artificial photosynthesis.

“In photovoltaic and artificial photosynthetic systems, light needs to be absorbed

and channelled to generate energetic electrons and holes, which can then be used to make

electricity or fuel,” Jain says.

“To be efficient, it is highly desirable that such systems exhibit an enhanced

interaction of light with excitons. This is what a doped quantum dot with an LSPR mode

could achieve.”

The potential for strongly coupled electronic and photonic modes in doped

quantum dots arises from the fact that semiconductor quantum dots allow for quantised

electronic excitations (excitons), while LSPRs serve to strongly localise or confine light

of specific frequencies within the quantum dot.

The result is an enhanced exciton-light interaction. Since the LSPR frequency can

be controlled by changing the doping level, and excitons can be tuned by quantum

confinement, it should be possible to engineer doped quantum dots for harvesting the

richest frequencies of light in the solar spectrum.

Quantum dot plasmonics also hold intriguing possibilities for future quantum

communication and computation devices.

“The use of single photons, in the form of quantised plasmons, would allow quantum

systems to send information at nearly the speed of light, compared with the electron speed

and resistance in classical systems,” Jain says.

“Doped quantum dots by providing strongly coupled quantised excitons and

LSPRs and within the same nanostructure could serve as a source of single plasmons.”


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Jain and others in the research group are now investigating the potential of doped

quantum dots made from other semiconductors, such as copper selenide and germanium

telluride, which also display tunable plasmonic or photonic resonances.

Germanium telluride is of particular interest because it has phase change

properties that are useful for memory storage devices.

“A long-term goal is to generalise plasmonic phenomena to all doped quantum

dots, whether heavily self-doped or extrinsically doped with relatively few impurities or

vacancies,” Jain says.

10.4 CURE FOR CANCER

Biochemists have engineered silica particles 100 nanometers wide which are covered in a

film of gold. These particles were injected into the bloodstream of the test subject, the

mice with a tumor. After discovering that this material is non-toxic, they also found that

these Nanoshells tended to embed in the tissues of the tumor instead of other cells, since

that is where more blood circulates due to its rapid growth. An infrared laser light was

then shone onto the tumor, resulting

in the plasmonic activity on the gold

shells of the silica particles. The

cancer tissues began to heat up from

37C to around 45C, where the

photothermalenergy killed the cancer

cells while leaving the surrounding

healthy cells unharmed (fig 14). All

signs of cancer on the mice was gone

within 10 days, while the control

subjects continued to be plagued by

the disease. Houston Nanospectra

Biosciences is currently requesting

permission to conduct clinical trials

of "nanoshell therapy" on cancer

patients; we are very close to finally

getting a real cure!


Fig. 14 Plasmonic Therapy of Cancer

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10.5 INVISIBILITY:

On a lighter note, plasmonics also allow the futuristic technology of invisibility.

Many physicists theorize that this is highly possible. The results, as for now, yields

invisibility for certain colours,or certain range of frequencies. To achieve total

invisibility, allfrequencies of the visible light must be covered; that will only take time.

The basic idea is to make the structure's refractive index equal to air's; it would not bend

or reflect light, like the classical ways of invisibility, but instead absorb the light. When

it's laminated with a material that produces optical gain, the increases in intensity would

offset the absorption losses, making the object invisible (in a certain selection of

frequencies, for now).Fig (15) shows the working of a cloaking device.

onics research

11. Plasmonic Researches

The possibility to confine light to the nanoscale and the ability to tune the

dispersion relation of light have evoked large interest and led to rapid growth of

plasmonic research. The parallel development of nanoscale fabrication techniques like

electron beam lithography and focused-ionbeam milling has opened up new ways to

structure metals’ surfaces and control surface plasmon polariton propagation and

dispersion at the nanoscale.In 2000, Mark L. Brongersma et al (and others) proposed that

EM energycould be transported below the diffraction limit with high efficiency and group

velocity greater than 0.1c along a wire of its characteristic length 0.1λ.In year 2002,

Maier et al experimentally observed the most efficient frequency for transport to be

3.19×1015 rad/sec with a corresponding group velocity of 4.0x106 m/s for longitudinal

mode of plasmon waveguide having an inter-particle distance of 75 nm. The achieved

bandwidth was calculated to be 1.4×1014 rad/sec. Dionne et al in year 2006 constructed

slot waveguides. Slot waveguides can support both transverse electric and transverse

magnetic photonic polarisation. The loss in slot waveguide can be minimised by using a


Fig 15 Cloaking Device

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low-refractive-index material; for example, a 100nm thick Ag/SiO2/Ag slab waveguide

sustains signal propagation up to 35 μm at wavelength of 840 nm. In 2007, Feng et al

observed that field localisation could be improved by introducing the partial dielectric

filling of the metal slot waveguide, which also reduces propagation losses. The channel in

metal surface waveguides supports surface plasmons at telecommunication wavelength

with very low loss (having propagation length of 100 μm) and well-confined guiding.

In this experiment, surface plasmons are guided along a 0.6μm wide and 1μm

deep triangular groove in gold material Thin metallic strips can support long-range

surface plasmons—a particular type of surface plasmon mode characterised by

electromagnetic fields mostly contained in the region outside of the metal, i.e., in

dielectric medium.

Jung et al in 2007 experimentally confirmed that long-range surface plasmons

could transfer data signal as well as the carrier light. In a demonstration, a 10Gbps signal

was transmitted over a thin metallic strip (14nm thick, 2.5μm wide and 4cm long gold

strip).

Furthermore, to reduce the propagation loss, Jin Tae Kim et al fabricated a low-

loss, long-range surface plasmon polariton waveguide in an ultravioletcurable acrylate

polymer having low refractive index and absorption loss. A 14nm thick and 3μm wide

metallic strip cladded in acrylate polymer material shows a loss of 1.72 dB/cm.

Rashid Zia et al obtained the numerical solution by using the full-vectorial magnetic field

finite-difference method for 55nm thick and 3.5nm wide strip on glass at a wavelength of

800 nm and noted that surface plasmons are supported on both sides of the strip and can

propagate independently.

Alexandra et al in year 2008 suggested that triangular metal wedge could guide

surface plasmons at telecommunication wavelength. It was experimentally observed that

1.43-1.52μm wavelength can propagate over a distance of about 120 μm with confined-

mode width of 1.3 μm along a 6μm high and 70.5º angled triangular gold wedge.

12. Future directions

In the field of plasmonics, studying the way light interacts with metallic

nanostructures will make it easier to design new optical material devices.One primary


PLASMONICS

goal of this field is to develop new optical components and systems that are of the same

size as today’s smallest integrated circuits and that could ultimately be integrated with

electronics on the same chip.The next step will be to integrate the components with an

electronic chip todemonstrate plasmonic data generation,transport and detection.Plasmon

waves on metals behave much like light waves in glass. That means engineers can use

techniques like multiplexing or sending waves.Plasmon sources, detectors and wires as

well as splitters and even plasmonsters can be developed. Applications mainly depend on

controlling thelosses and the cost of nanofabrication techniques. Enhanced and directed

emission of semiconductor luminescence(quantum dots) may well find commercial

application in plasmonassisted lighting in the near future.Finally, plasmonic nanocircuits

combine a high bandwidth with a high level of compaction and make plasmonic

components promising for all-optical circuits. Plasmonic wires will act as high-bandwidth

freeways across the busiest areas of the chip. Plasmons can ferry data along computer

chips.Plasmonic switches required for this are under development.Rotaxanes molecule is

being used for the purpose. Change in the shape of the molecule is the principleof this

molecular switch.

13. DISADVANTAGES

A major disadvantage of using metals in plasmonics and metamaterials is their

inherent absorption losses. Bringing the technology from the research labs to applications

requires that the losses be reduced considerably. On the other hand, plasmonic

nanostructures can be of considerable help in extracting light out of devices such as

organic light-emitting diodes (OLEDs).A serious obstacle to the widespread use of this

technology so far has been that plasmons tend to dissipate after only a few millimeters of

propagation, making them unusable on most computer chips. Under the EU-funded

Plasmacon project, a team of European researchers has reported they have now overcome

this obstacle, demonstrating the first commercially-viable plasmonics devices.

The researchers' approach was to develop a so-called "dielectric-loaded surface plasmon

polariton waveguide" (DLSPPW), a layer of dielectric that was patterned onto a gold film

with a glass substrate. Using this structure, they were able to achieve waveguides only

500 nanometres in size and extend the signal propagation, opening the way to further

advances.


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Unlike previous results obtained by other research groups, the technology

developed by the team can create plasmonic devices using existing and low-cost

commercial lithography techniques, and while some issues still need to be tackled, it

would seem that one of the main obstacles has just been overcome.

14. Challenges remaining

Despite many advances in the field ofplasmonics, several important open questions

and problems remain. For example, how can plasmons be efficiently excited with

nanoscale resolution?Surface plasmon polaritons are usually excited using far-field

optical techniques, which have a higher resolution than plasmonic phenomena under

investigation. However, for true nanoscale plasmonic studies, a surfaceplasmon-polariton

point source with nanoscale dimensions is required.“What are the fundamental processes

thatdetermine the losses of surface plasmon polaritons?” is another important question.

Practically, plasmon experiments are performed on poly-crystalline surfaces, and the

limits to the losses due to surface roughness,grain boundaries, etc. are not known.Surface

plasmons propagate along the chain of nanoparticles, but the losses are high. On the other

hand,propagation losses are low in the case of nanowires, which leaves open the

possibility of surface-plasmon optical devices.The dream of making all-plasmonic

devices requires further research. In order to realise advanced active circuits,there is a

need for active modulator and switching components operating at ultra-high bandwidth

and low power utilisation.To manipulate surface plasmon polaritons on a

surface,reflectors are needed. So far, macroscopic Bragg reflectors structured into the

surface have been used. For true nanoscale integration, nanoscale surface

plasmonpolariton mirrors are required. Oncethese are realised, nanoscale cavities to

confine surface plasmon polaritons can also be designed. The limits to the mode volume

and quality factor of plasmonic cavities are not yet known.Finally, the use of a particle

beamrather than a light beam to excite surface plasmon polaritons raises questions and

novel opportunities regarding the selectivity with which surface plasmon modes with

different

symmetry can be excited.


Fig. 16 Advanced Plasmonics

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REFERENCES

‘Plasmonics Promises Faster Communication’ by Jagmeet Singh in ‘Electronics

for you’ magazine.

1. ‘Fundamentals and applications of Plasmonics’ by Dr. Stephen Maier.

2. www.en.wikipedia.org

3. www.howstuffworks.com

4. www.motortrends.com

5. www.worldchanging.com

6. www.post-gazette.com

7. www.bnet.com

8. www.digitaldaily.allthingsd.com

9. www.singularityhub.com

10. www.future.wikia.com

11. www.asia.cnet.com

12. www.hackingtheuniverse.com

13. www.techpin.com


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