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Document By
SANTOSH BHARADWAJ REDDY
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1. Abstract:
With the increasing quest for
transporting large amounts of data at a fast
speed along with miniaturization, both
electronics and photonics are facing
limitations. Photonic components such as
fiber-optic cables can carry a lot of data
but are bulky compared to electronic
circuits. Electronic components such as
wires and transistors can be incredibly
small but carry less data.
A problem holding back the progress
of computing is that with mismatched
capacities and sizes, the two technologies
are hard to combine in a circuit. Photonic
components such as fiber optic cables can
carry a lot of data but are bulky compared
to
electronic
circuits.
Electronic
components such as wires and transistors
carry less data but can be incredibly small.
Researchers can cobble them together, but
a single technology that has the capacity of
photonics and smallness of electronics
would be the best bridge of all.
Researchers are pioneering such a
technology
called
‘plasmonics’.
Plasmonics, also called ‘light on a wire’,
would allow transmission of data at optical
frequencies along the surface of a tiny
metallic wire, despite the fact that the data
travels in the form of electron density
distributions rather than photons.
2. Introduction:
What is plasmonics?
Plasmonics
refers
to
the
investigation,
development
and
applications of enhanced electromagnetic
properties of metallic nanostructures. The
term plasmonics is derived from
plasmons, which are the quanta associated
with longitudinal waves propagating in
matter through the collective motion of
large numbers of electrons. These
plasmons travel at the speed of light and
are created when light hits a metal at a
particular angle, causing waves to
propagate through electrons near the
surface. Plasmonics exploits nanoscale
structural transformations which are
supported by rigorous numerical analysis.
Plasma is a medium with equal
concentration of positive and negative
charges, of which at least one charge type
is mobile. In a solid, the negative charges
of the conduction electrons (i.e., electron
gas) are balanced by an equal
concentration of positive charge of the ion
cores. A plasma oscillation in a metal is a
collective longitudinal excitation of the
conduction electron gas against a
background of fixed positive ions with a
plasma frequency.
Those plasmons that are confined to
surfaces and which interact strongly with
light are known as surface plasmons.
Surface plasmons are dense waves of
electrons—bunches of electrons passing a
point regularly—along the surface of a
metal. Plasmons have the same
frequencies and electromagnetic fields as
light, but their sub-wave-length size means
that they take up less space. Plasmonics,
then, is the technology of transmitting
these light-like waves along nanoscale
wires. With every wave, we can, in
principle, carry information.
exponentially decay into both media.
These unique interface waves result from
the special dispersion characteristics of
metals. Surface plasmons are very
sensitive to the properties of the materials
on which they propagate. What
distinguishes surface plasmons from
‘regular’ photons is that they have a much
smaller wavelength at the same frequency.
An interface between a dielectric and a
metal can support a surface plasmon. The
short-wavelength surface plasmons enable
the fabrication of nanoscale optical
integrated circuits, in which light can be
guided, split, filtered, and even amplified
using plasmonic integrated circuits that are
smaller than the optical wavelength. At a
given frequency, the surface plasmon
wavelength is strongly dependent on the
metal thickness. Surface Plasmons cannot
couple
directly
to
free-space
electromagnetic radiation of the same
energy because they travel too slowly,
their associated wavevector being too
large to satisfy conservation of energy and
momentum.
Surface plasmons:
The
existence
of
‘surface
plasmons’ in metallic structures opens up
a multitude of new possibilities for
photonic devices as they allow the
propagation and concentration of
light in subwavelength spaces. A
surface plasmon is a coherent electron
oscillation that propagates along the
interface together with an electromagnetic
wave. I.e., the interface between a
conductor and an insulator is where
surface plasmons propagate; bound to the
surface
between
the
two,
they
3. How do they work?
In plasmonics, light is first
converted into plasmons, which then
propagate in a metallic surface, but with a
wavelength smaller than the original light;
the plasmons could then be processed with
their own two-dimensional optical
components and later plasmons could be
turned back into light or into electric
signals. Light can be localized and
manipulated in appropriately designed
metallic and metallodielectric nanoparticle
array structures. In particular, interesting
phenomena occur near the plasmon
frequency where optical extinction is
resonantly enhanced and at the plasma
frequency where the real part of the
dielectric constant changes sign. Due to
their high reflection and absorption
coefficients, metal structures have been
generally overlooked as elements to guide,
focus and switch light at visible and
infrared wavelengths. However at the
nanoscale the intriguing guiding and
refractive properties of metal structures
can be realized since the metal
components become semitransparent due
to their small size.
Local
arrays
of
electronic
transistors would carry out the switching
necessary for computation, but when a lot
of data needs an express lane to travel
from one section of a chip to another;
electronic bits could be converted to
plasmon waves, sent along a plasmonic
wire and converted back to electronic bits
at their destination. When a light source is
placed close to a metal, it can excite a
surface plasmon through a near-field
interaction. When light of specific
frequency strikes a plasmon that oscillates
at a compatible frequency, the energy from
light is harvested by the plasmon,
converted into electrical energy that
propogates through the nanostructure and
eventually converted back into light.
And with a light-emitting diode
embedded in a plasmonic structure,
surface plasmons can be electrically
excited. Such surface plasmons may serve
as an alternative to overcome the
information bottlenecks presented by
electrical interconnects in integrated
circuits. Coupling to surface plasmons can
also enhance the extraction efficiency of
light from LEDs. Ordered arrays of
nanoparticles can possess even further
enhanced field intensities as a result of
plasmon coupling between adjacent
particles. By varying nanoparticle shape or
geometry, the surface plasmon resonance
frequency can be tuned over a broad
spectral range.
For example, a HeNe laser, whose
free-space emission wavelength is 633 nm,
can excite a surface plasmon at a Si/Ag
interface with a wavelength of only 70 nm.
When the laser frequency is tuned very
close to the surface plasmon resonance,
surface plasmon wavelengths in the
nanometer range can be achieved.
4. Benefits are many…
The big advantage of plasmons is
that you can make the devices the same
size as electrical components but give
them the speed of photons.
Plasmon waves are of particular
interest because these are at optical
frequencies. The higher the frequency of
the wave, the more the information we can
transport. Optical frequencies are about
100,000 times greater than the frequency
of today’s electronic microprocessors.
The key is using a material with a
low refractive index, ideally negative, such
that the incoming electromagnetic energy
is reflected parallel to the surface of the
material and transmitted along its length as
far as possible. There exists no natural
material with negative refractive index,
so nanostructured material must be
used to fabricate effective plasmonic
devices. For this reason, plasmonics is
frequentl
associated
with
nanotechnology.
Plasmonics describes how ultrasmall
metallic structures of various shapes
capture and manipulate light and provides
a practical design tool for nanoscale
optical components. The fact that light
interacts with nanostructures overcomes
the belief held for more than a century that
light waves couldn’t interact with anything
smaller than their own wavelengths.
Research has shown that nanoscale
objects can amplify the focus light in ways
scientists never imagined. The ‘how’ of
this involves plasmons—ripples of waves
in the ocean of electrons flowing across
the surface of metallic nanostructures. The
type of plasmon that exists on a surface is
directly related to its geometric structure.
“The plasmonic waveguides allow
light to be very tightly
Localized and this is a feasible way of
developing efficient optical ‘wires’ and
chips with the required level of
integration
similar
to
modern
electronic chips.” The scaling of optical
devices and components to their ultimate
size
limits
will
require
that
electromagnetic energy be guided on a
scale below the diffraction limit and that
information be guided around sharp
corners with nanometer-scale radii of
curvature. Plasmon waveguides are
periodic chain arrays of metal
nanoparticles which can localize light in
guided modes whose size is a few
percent of the optical wavelength. Such
waveguides can enable efficient power
transfer around sharp corners and may
form the basis for nanoscale all-optical
switches.
Metallic
nanoparticles
have
distinctly different optical characteristics
than surface plasmons at planar interfaces.
Planar waveguides and photonic crystal
structures
are
being
intensively
investigated as primary solutions for
integrated photonic devices. Nanoparticles
show strong optical resonances, again
because of their large free-electron
density. Arrays of metal nanoparticles can
also be used as miniature optical
waveguides. These nanoparticle array
waveguides provide confinement of light
within ~50 nm along the direction of
propagation, a 100-fold concentration
compared to dielectric waveguides. There
is a vast array of plasmonic concepts still
waiting to be explored, with applications
spanning (bio-) sensing, optical storage,
solid-state lighting, interconnects, and
waveguides.
5. R&D so far...
The field of plasmonics, which has
existed only for a few years, has already
attracted researchers from the industry and
government. Studying the way light
interacts with metallic nanostructures will
make it easier to design new optical
materials and devices from the bottom up,
using metal particles of specifically
tailored shapes. One primary 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 research shows that the
equations that determine the frequencies of
plasmons and complex nanoparticles are
almost identical to the quantum
mechanical equations that determine the
energies of protons in atoms and
molecules
called
‘plasmon
hybridization’.
Just as quantum mechanics allows
scientists to predict the properties of
complex molecules, research shows that
the properties of plasmons in complex
metallic nanostructures can be predicted in
a simple manner. The findings are
applicable not only to nanoshells but also
to nanoscale waveguides and other
nanophotonic structures.
The ultimate goal of R&D is to
demonstrate plasmonics in action on a
standard silicon chip and make working
plasmonic components. The next step will
be to integrate the components with an
electronic chip to demonstrate plasmonic
data generation, transport and detection.
Plasmons are generated when, under
the right conditions, light strikes a metal.
The electric field of the light jiggles the
electrons in the metal to the light's
frequency, setting off density waves of
electrons. Plasmon waves behave on
metals much like light waves behave in
glass, meaning that plasmonic engineers
can employ all the ingenious tricks that
photonic engineers use to cram more data
down a cable—such as multiplexing, or
sending multiple waves. Meanwhile,
because plasmonic components can be
crafted from the same materials that the
chipmakers use today, engineers are
hopeful that they can make all the devices
needed to route light around a processor or
other kind of chip. These would include
plasmon sources, detectors and wires as
well as splitters and even transistors.
6. Limitations do exist!!
The potential of plasmonics right
now is mainly limited by the fact that
plasmons can typically travel only
several millimeters before they peter
out. Chips, meanwhile, are typically about
a centimeter across, so plasmons can’t yet
go the whole distance. For sending data
even longer distances, the technology
would need even more improvement. The
key is using a material with a low
refractive index, ideally negative, such that
the incoming electromagnetic energy is
reflected parallel to the surface of the
material and transmitted along its length as
far as possible. There exists no natural
material with a negative refractive index,
so nanostructured materials must be used
to fabricate effective plasmonic devices.
For this reason, plasmonics is frequently
associated with nanotechnology.
The distance that a plasmon can
travel before dying out is a function of
several aspects of the metal. But for
optimal transfer through a wire of any
metal, the surface of contact with
surrounding materials must be as smooth
as possible and the metal should not have
impurities.
For most wavelengths of visible
light, aluminium allows plasmons to travel
farther than other metals such as gold,
silver and copper. It is somewhat ironic
that aluminium is the best metal to use
because the semiconductor industry
recently dumped aluminium in favour of
copper—the better electrical conductor—
as its wiring of choice. Of course, it may
turn out that some kind of alloy will have
even better plasmonic properties than
either aluminium or copper.
Another classic semiconductor issue
that the researchers will have to address is
‘heat’. Chipmakers are constantly striving
to ensure that their electronic chips don’t
run too hot. Plasmonics also will
generate some heat, but the exact amount
is not yet known. Even if plasmonics runs
as hot as electronics, it will still have the
advantage of a higher data capacity in the
same space.
7. Promising applications:
The study of plasmonics is one of
the fastest growing fields in optics because
it could prove useful for a wide range of
applications in biological sensing,
microelectronics, chemical detection,
medical technology and others. They have
been proposed as a means of high
resolution lithography and microscopy due
to their extremely small wavelengths. Both
of these applications have seen successful
demonstrations in the lab environment.
Finally, surface plasmons have the unique
capacity to confine light to very small
dimensions which could enable many new
applications. The sensitivity of surface
plasmons to the properties of materials has
lead to their use to measure the thickness
of mono layers on colloid films, such as
screening and quantifying protein binding
events.
“Before all-plasmonic chips are
developed, plasmonics will probably be
integrated with conventional silicon
devices. Plasmonic wires will act as high
bandwidth freeways across the busiest
areas of the chip”.
Plasmons have a variety of potential
uses. Plasmon wires can be much thinner
than conventional wires, and could support
much higher frequencies, so plasmons
have been considered as a means of
transmitting information on computer
chips. Applications of surface plasmons in
solid-state lighting and lasing are just
appearing, but it may be that traffic lights
are composed of surface plasmon LEDs in
a few years time. Surface-plasmon-based
sensors find uses in gas sensing,
biological environments such as
immuno-sensing and electrochemical
studies. The ability to achieve locally
intense fields has many possible
applications, including increasing the
efficiency of LEDs, (bio-) sensing, and
nanolithography.
Plasmon printing is a new
approach to lithographic printing that takes
advantage of the resonantly enhanced
optical intensity in optical nearfield of
metallic nanoparticles, and that could
enable printing of deep subwavelength
features using conventional photoresist
and simple visible/ultraviolet light sources.
Plasmonics has also been used in
biosensors. When a particular protein or
DNA molecule rests on the surface of a
plasmon-carrying metallic material, it
leaves its characteristic signature in the
angle at which it reflects the energy.
Currently the biggest application for
plasmons is in gold-coated glass
biosensors, which detect when particular
proteins or DNA are present - the biomatter changes the angle at which light
hitting the surface produces the most
intense plasmons.
Nanolithography is a term used to
describe a number of techniques for
creating incredibly small structures. The
sizes involved are on the order of tens of
nanometers (nm). One common method of
nanolithography, used particularly in the
creation of microchips, is known as
photolithography. This technique is a
parallel method of nanolithography in
which the entire surface is drawn on in a
single moment. Photolithography is
limited in the size it can reduce to,
however, because if the wavelength of
light used is made too small the lens
simply absorbs the light in its entirety.
In the field of chemical sensing,
plasmonics offers the possibility of new
technologies that will allow doctors, antiterror squads and environmental experts to
detect chemicals in quantities as small as a
single molecule.
8. What’s the future hold?
The production of light in a
standard
CMOS
process
is
a
‘fundamentally disruptive force’ in the
semiconductor industry. Light emission
technologies will transform the current
semiconductor integrated circuit market
for decades to come. By bridging the
growing gap between the massive
computing power of leading-edge
microprocessors and the ability to quickly
move data on and off chip, Applied
Plasmonics’ technology will support both
current and future generations of
integrated circuits.
With speeds more then a
thousand times faster than conventional
on-chip wiring, Applied Plasmonics’
optical-based technology will support
data rates substantially higher than
those achieved by current copper
interconnect technologies used in
today’s integrated circuits. Applied
Plasmonics’ technology enables these
massive data rates through light emission
devices that are directly interfaced to and
between integrated circuits, printed circuit
boards, and local area networks. This
massive increase in available bandwidth
will create new and unprecedented
applications by bringing optics-based data
communications to the chip and “desktop”. In addition, there is also the potential
to use light for future intra-chip transport
of signals and clocks within a single
CMOS silicon integrated circuit. This can
possibly “tame” one of the most
demanding aspects of modern chip design.
9. Building
Devices:
Plasmon
Enabled
In addition to the fundamental
plasmon
breakthroughs,
Applied
Plasmonics has accomplished significant
technical milestones relating to its light
emittingtechnologies.
Emitter devices:-Designed, fabricated, and tested lightemitting plasmon devices.
-Shown that different and multiple modes
can be achieved in one device at the
same time.
-Built multiple frequency devices on one
chip at the same time, and in the same
layer.
-Showed that devices can be built that
emit selective frequencies of light.
-Tested emitter switching speeds at
130MHzMEM, limited only by readily
available, commercial, detectors.
-Developed
a
comprehensive
understanding
of
theory/design/test/fabrication for its
light emitting devices.
-Developed frequency-related design
rules for development, design,
and
manufacturing of its light emitting
technologies.
10. Conclusions:
Since 2001, there has been an
explosive growth of scientific interest in
the role of plasmons in optical phenomena
including guided-wave propagation and
imaging at the subwavelength scale,
nonlinear spectroscopy and ‘negative
index’ metamaterials. The unusual
dispersion properties of metals near the
plasmon resonance enables excitation of
surface modes and resonant modes in
nanostructures that access a very large
range of wavevectors over a narrow
frequency range, and accordingly,
resonant plasmon excitation allows for
light localization in ultra-small volumes.
This feature constitutes a critical design
principle for light localization below the
free space wavelength and opens the path
to truly nanoscale plasmonic optical
devices. This principle, combined with
quantitative electromagnetic simulation
methods and a broad portfolio of
established and emerging nanofabrication
methods creates the conditions for
dramatic scientific progress and a new
class
of
subwavelength
optical
components.
The production of light
emission technologies in a standard
CMOS process offers the promise to
transform the semiconductor integrated
circuit market for decades to come.
Applied Plasmonics technology will
support both current and future
generations of microprocessors and will
reduce the growing gap between the
ever-increasing computing power of
leading-edge microprocessors and the
inability to quickly move data on and
off chip.
11. References:
1.
2.
EFY magazines..
www.sciencedaily.com
www.wisegeek.com
Etc….
Document By
SANTOSH BHARADWAJ REDDY
Email: [email protected]
Engineeringpapers.blogspot.com
More Papers and Presentations available on above site