plasmonics
TRANSCRIPT
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
2 Vivek Singh 327/08 ECE B1
Fig. 1 Practical visualization of Plasmons
PLASMONICS
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
3 Vivek Singh 327/08 ECE B1
Fig. 2 Electron Cloud Shifting
PLASMONICS
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
4 Vivek Singh 327/08 ECE B1
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.
5 Vivek Singh 327/08 ECE B1
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
6 Vivek Singh 327/08 ECE B1
Fig. 4 Nanoscale antenna
Fig. 5Localized surface plasmons
PLASMONICS
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
7 Vivek Singh 327/08 ECE B1
Fig. 7 Plasmon Excitation
Fig. 6 Surface Plasmon Excitation
PLASMONICS
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.
8 Vivek Singh 327/08 ECE B1
Fig. 7 Operating speed of data transporting and processing system
PLASMONICS
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 :
9 Vivek Singh 327/08 ECE B1
Fig. 9Dielectric-loaded surface plasmon polariton waveguides
PLASMONICS
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.
10 Vivek Singh 327/08 ECE B1
Fig. 10Schematic representation of the operation of a PSTM
PLASMONICS
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
11 Vivek Singh 327/08 ECE B1
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
12 Vivek Singh 327/08 ECE B1
Fig. 11 Structure of Graphene
PLASMONICS
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
13 Vivek Singh 327/08 ECE B1
Fig. 12 Plasmonic Solar Cells
PLASMONICS
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
14 Vivek Singh 327/08 ECE B1
Fig. 13 Silvernano-particles
PLASMONICS
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
15 Vivek Singh 327/08 ECE B1
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
16 Vivek Singh 327/08 ECE B1
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.
17 Vivek Singh 327/08 ECE B1
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.
18 Vivek Singh 327/08 ECE B1
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.”
19 Vivek Singh 327/08 ECE B1
PLASMONICS
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!
20 Vivek Singh 327/08 ECE B1
Fig. 14 Plasmonic Therapy of Cancer
PLASMONICS
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
21 Vivek Singh 327/08 ECE B1
Fig 15 Cloaking Device
PLASMONICS
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
22 Vivek Singh 327/08 ECE B1
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.
23 Vivek Singh 327/08 ECE B1
PLASMONICS
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.
24 Vivek Singh 327/08 ECE B1
Fig. 16 Advanced Plasmonics
PLASMONICS
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
25 Vivek Singh 327/08 ECE B1