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Plasmons
Surface Plasmon Resonance
Plasmonic Effects and Applications
Introduction
During the last two decades many researches devoted to develop optical
sensors for the measurement of chemical and biological quantities. Large
variety of optical methods have been used in chemical and biosensors,
among them, surface Plasmon resonance. In these sensors, a desired
quantity is determined by measuring the refractive index, absorbance and
fluorescence properties of analyte molecules.
What is Plasmon? The quanta of waves produced by collective effects of
large numbers of electrons in matter when the electrons are disturbed from
equilibrium.
In other word, the quantum of Plasma Oscillation is called Plasmon
The name plasmon derives from the physical plasma as a state of matter in
which the atoms are ionized. At the lowest densities this means an ionized
gas, or classical plasma; but densities are much higher in a metal, or
quantum plasma, the atoms of a solid metal being in the form of ions. In
both types of physical plasma, the frequency of plasma-wave oscillation is
determined by the electronic density. In a quantum plasma the energy of the
plasmon is its frequency multiplied by Planck's constant, a basic relationship
of quantum mechanics.
• Metals provide the best evidence of plasmons, because they have a
high density of electrons free to move.
• Plasmons play a large role in the optical properties of metals. Light of
frequency below the plasma frequency is reflected, because the
electrons in the metal screen the electric field of the light. Light of
frequency above the plasma frequency is transmitted, because the
electrons cannot respond fast enough to screen it. In most metals, the
plasma frequency is in the ultraviolet, making them shiny (reflective)
in the visible range. On the other hand, some metals, such as copper,
have a plasmon frequency in the visible range, yielding their distinct
color.
• Again, the geometry of the metal film plays an important role in
plasmon frequency. For example gold, has plasmon frequency in the
deep ultraviolet, but geometric factors bring it close to the visible.
• In doped semiconductors, the plasma frequency is usually in the
infrared.
• The plasmon energy for most metals corresponds to that of an
ultraviolet photon. However, as mentioned above for some metals like
silver, gold, the alkali metals, and a few other materials, the plasmon
energy can be sufficiently low to correspond to that of a visible or
near-ultraviolet photon. This means there is a possibility of exciting
plasmons by light. If plasmons are confined upon a surface, optical
effects can be easily observed. In this case, the quanta are called
surface plasmons, and they have the bulk plasmon energy as an upper
energy limit.
• Surface plasmons are those plasmons that are confined to surfaces
and that interact strongly with light resulting in a polariton. They
occur at the interface of a material with a positive dielectric constant
with that of a negative dielectric constant (usually a metal or doped
dielectric).
• Surface plasmons were first proposed to explain energy losses by
electrons reflected from metal surfaces. Since then, numerous
experiments have involved coupling photons to surface plasmons.
Potential applications extend to new light sources, solar cells,
holography, Raman spectroscopy, microscopy, and sensors.
• Surface plasmons on a plane surface are non-radiative electromagnetic
modes, that is, SPs cannot be generated directly by light nor can they
decay spontaneously into photons. The origin of the non-radiative
nature of SPs is that the interaction between light and SPs cannot
simultaneously satisfy energy and momentum conservation. This
restriction can be circumvented by relaxing the momentum
conservation requirement by roughening or corrugating the metal
surface. Other method is to increase the effective wave vector (and
hence momentum) of the light by some means (discussed later).
Surface Plasmon Resonance
• The excitation of surface plasmons by light is denoted as a surface
plasmon resonance (SPR) for planar surfaces or localized surface
plasmon resonance (LSPR) for nanometer-sized metallic structures.
• Surface plasmons, also known as surface plasmon polaritons,
( coupling between photon and an excitation of a material) and are
surface electromagnetic waves that propagate parallel along a
metal/dielectric interface. For surface plasmons to exist, the complex
dielectric constants of the two media must be of opposite sign. This
condition is met in the IR-visible wavelength region for air/metal and
water/metal interfaces (where the real dielectric constant of a metal is
negative and that of air or water is positive). Typical metals that
support surface plasmons are silver and gold, but metals such as
copper, titanium, or chromium can also support surface plasmon
generation.
• Surface Plasmon resonance (SPR) is a non-destructive analysis
technique, which is used in the investigation of thin layers of
molecules upon a material surface. More specifically SPR is capable
of detecting changes in the refractive index (n) occurring near the
surface of a metal (within ~200nm). It is a physical process, which
occurs when plane polarized light hits a metal film under total internal
reflection conditions.
• When a light beam, traveling from a dense to a less dense medium,
strikes the surface of a prism this causes the light to bend towards the
interface plane. As depicted in the figure changing the angle of
incidence changes the resulting light until a critical angle is reached.
Upon reaching the critical angle all the incoming light is reflected
within the prism, this is referred to as total internal reflection (TIR).
Light is not generated during TIR, however the electrical field of the
photons extends approximately a quarter of the wave length beyond
the reflecting surface.
θt
ki
θi
θi
Transmitted
(refracted) light
kt
n2
n 1 > n2
kr
Evanescent wave
θc θc
θi >θc
TIR
Incident
light
Reflected
light
(a)
(b)
(c)
Light wave travelling in a more dense medium strikes a less dense medium. Depending on
the incidence angle with respect to θ c, which is determined by the ratio of the refractive
indices, the wave may be transmitted (refracted) or reflected. (a) θi < θc (b) θ i = θc (c) θi
> θ c and total internal reflection (TIR).
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
• When the prism is coated with a material with an infinitely high index
of refraction, past a certain critical angle, total internal reflection
occurs for all light reflected. In addition to this all light sent toward
the surface is reflected away from the surface. However, when light is
totally internally reflected off the inside of a prism there is a
probability that some of the light will exist outside the surface of the
prism. This light is called the evanescent wave.
• The prism described above is generally coated with a thin metal film
placed in contact with the base of the prism (usually the reflection
site), e.g. gold. The use of a metal sensing surface in SPR is critical
as this technique capitalizes upon the fact that metals contain electrons,
which behave as a continuous “sea” of charge. This "sea" of charge
can undergo charge-density oscillations, plasmons, at the surface of
the conductor, particularly at a surface in contact with an insulator.
Furthermore a molecular layer of interest can be coated onto the thin
metal film on the side opposite the prism.
• When a particular type of light (from the light source) strikes the
metal sensor, surface plasmon waves (SPW) are generated at the
interface between the conductive metal and the insulating molecular
layer. In addition to the generation of the SPWs, light is also reflected
off of the metal surface. As indicated earlier at TIR, all the energy
from the incident light wave will be transferred to the reflected light
wave. However, at a particular angle past the point of TIR, which
results in the SPR angle, a majority of the incidence light energy will
interact with the generated SPW’s. This results in a phenomenon
called resonance. At resonance, the reflected light intensity will be
minimal; this intensity corresponds with the SPR angle (the intensity
of the reflected light may be measured using the photo-detector.
• The SPR angle is dependent upon several factors, including:
properties of the metal film, the wavelength of the incident light and
the refractive index of the media on either side of the metal film i.e.
molecular layer in contact with the metal sensing surface; (the
refractive index is sensitive to temperature, therefore it is important to
perform the measurements at defined temperatures as well).
• The metal film used must have conduction band electrons capable of
resonating with the incoming light at a suitable wavelength. Metals
that satisfy this condition are silver, gold, copper, aluminum, sodium
and indium. In addition, the metal on the sensor surface must be free
of oxides, sulfides and should not react to other molecules on
exposure to the atmosphere or liquid. The thickness of the metal layer
is also of great importance. Above an optimum thickness the dip in
reflective light becomes shallow, and below an optimum thickness the
dip becomes broader; thus affecting the SPR angle.
Wavelength vs Reflectance
Techniques to Induce Surface Plasmon Resonance
• Several configurations of SPR devices exist, and serve as sensors.
These optical devices are capable of exciting the SPWs and are also
used to interrogate the SPR. The configurations that are known and
used today are the following:
• Surface plasmon resonance sensors using optical prism couplers
• Surface plasmon resonance sensors using grating couplers
• Surface plasmon resonance sensors using optical waveguides
• Surface plason resonance sensors based on optical fibers
Surface plasmon resonance sensors using optical prism couplers
– A very suitable geometry for sensors using attenuated total
reflection (ATR) is the Kretschmann Prism. The Kretschmann
prism is used to measure reactions on a sensor chip attached to
a prism. The apparatus consists of a sensor chip, a light source,
a light detector, and a prism also referred to as the Kretschmann
Prism.
-
• In order to promote evanescent waves, rather than coating the prism
with a material with a high index of refraction, a sensor chip is
attached to the prism with a thin layer of metal. In this scenario,
waves are present in the “sea of free electrons” in the metal. when the
plasmons have similar properties to that of the evanescent wave, the
two couple resulting in SPR. SPR uses energy, therefore the intensity
of the light which reflects back from the surface is less that that of the
incident on the surface. This intensity may be measured in order to
determine the occurrence of SPR.
• Furthermore when a sensor chip is fabricated such that it is capable of
changing the nature of its surface plasmon in the presence of an
analyte, the presence or concentration of this analyte may be
determined.
• Most sensors are operated in the following manner:
• “Monochromatic light is directed through the prism through a
range of angles which all cause total internal reflection.
• The sensor chip is coated with receptors to a specific analyte.
The concentration of the analyte present on the opposite surface
of the sensor chip modifies the resonant frequency of the
Surface Plasmon.
• The intensity of the reflected light vs. incident angle will have
a minimum that corresponds to the resonant frequency. From
the location and magnitude of this minimum the concentration
of the analyte can be determined.”
Surface plasmon resonance sensors using optical grating
• In this technique, the incident electromagnetic radiation is directed
towards a medium whose surface has a spatial periodicity (D) similar
to the wavelength of the radiation, for example a reflection diffraction
grating. The incident beam (red) is diffracted producing propagating
modes which travel away from the interface (blue) and evenescent
modes which exist only at the interface. The evenscent modes have
wavevectors parallel to the interface similar to the incident radiation
but with integer 'quanta' of the grating wavevector added or subtracted
from it. These modes couple to Surface Plasmons (green), which run
along the interface between the grating and the ambient medium.
Surface plasmon resonance sensors using optical waveguides
– The use of optical waveguides in SPR sensors provides
numerous attractive features such as a simple way to control the
optical path in the sensor system to suppress the effect of stray
light. The process of exciting an SPW in this configuration is
similar to that of the Kretschmann ATR coupler. A light wave
is guided by the waveguide and, entering the region with a thin
metal layer, it evanescently penetrates through the metal layer.
If the SPW and the guided mode are phase matched, the light
wave excites an SPW at the outer interface of the metal.
Theoretically, the sensitivity of waveguide-based SPR devices
is approximately the same as that of the corresponding ATR
configurations.
Light
Light
n2
Light
Light
n2
n1 > n2
A planar dielectric waveguide has a central rectangular region of
higher refractive index n 1 than the surrounding region which has
a refractive index n2 . It is assumed that the waveguide is
infinitely wide and the central region is of thickness 2a. It is
illuminated at one end by a monochromatic light source.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
High order mode
Light pulse
Low order mode
Broadened
light pulse
Cladding
Core
Intensity
Intensity
Axial
Spread, Δτ
t
0
Schematic illustration of light propagation in a slab dielectric waveguide. Light pulse
entering the waveguide breaks up into various modes which then propagate at different
group velocities down the guide. At the end of the guide, the modes combine to
constitute the output light pulse which is broader than the input light pulse.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
t
Surface plasmon resonance sensors based on optical fibers
• Optical fiber SPR probes present the highest level of miniaturization
of SPR devices, allowing for chemical and biological sensing in
inaccessible locations. The ability to transmit optical signals over a
long distance makes the use of optical fibers very attractive. Fiber
optic waveguides have a number of advantages over prism-based
sensors. They are inexpensive and can easily be used to make
disposable sensors for medical tasks. Fibers are also very small and
have no moving parts, giving them a much broader range than the
Kretschmann sensors and making multiple sensor arrays a possibility.
• A fiber optic SPR sensor is built using a large diameter (~400mm) and
multimode fiber. Cladding is removed from a portion of the fiber, and
a surface plasmon metal layer e.g. silver is deposited instead. The
length from which the cladding is removed is dependant upon the
diameter of the fiber, and determines the number of reflections
occurring at the surface plasmon metal interface. If the length is too
short, not enough coupling will occur. If the length is too long,
coupling will be very strong and the minimum coupling intensity will
be difficult to determine.
• When light enters a fiber at a specific angle, corresponding to a
specific mode, it will propagate through a multimode optical fiber.
Although modes are more of an energy distribution, in the fiber, they
can also be thought of as angles of total internal reflection as the light
bounces back and forth along the fiber. Light which enters the fiber at
larger angles (i.e. low-order modes) bounces back and forth at a slow
pace, whereas light which enter the fiber at a tighter angles (i.e.
higher-order modes) bounces back and forth a fast pace. At low-order
modes the energy is distributed in the fiber core, whereas the energy
for high-order modes spreads into the cladding, and beyond the
waveguide.
y
y
Cladding
φ
Core
n2 n1
z Fiber axis
r
n
The step index optical fiber. The central region, the core, has greater refractive
index than the outer region, the cladding. The fiber has cylindrical symmetry. We
use the coordinates r, φ, z to represent any point in the fiber. Cladding is
normally much thicker than shown.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Along the fiber
1
1, 3
3
(a) A meridional
ray always
crosses the fiber
axis.
Meridional ray
Fiber axis
2
2
1
2
1
Skew ray
Fiber axis
2
5
5
3
4
Ray path along the fiber
3
4
(b) A skew ray
does not have
to cross the
fiber axis. It
zigzags around
the fiber axis.
Ray path projected
on to a plane normal
to fiber axis
Illustration of the difference between a meridional ray and a skew ray.
Numbers represent reflections of the ray.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
• In order to achieve SPR sensing, as opposed to sweeping through a
range of coupling angles in the Kretschmann Prism, the fiber only
sweeps through a number of coupling wavelengths. The wavelengths
are interrogated, i.e. measuring the amount of each wavelength
leaving the fiber, using a broadband, multi-wavelength source e.g.
white light. Using a spectrophotometer it is then possible to
determine which wavelength coupled with the surface plasmon and
how much analyte (species being analyzed) is present.
Applications
• The SPR signal is directly dependent on the change of the refractive
index of the medium on the sensor side of the SPR surface.
• The spectra can be generated for a metal surface with and without a
coated molecular layer. Then, the shift in SPR angle between the two
can be quantified and used to calculate the thickness or refractive
index of the adhered molecules. SPR has proven useful in
determining both growth in the thickness of a molecular layer and loss
in thickness, even of a single monolayer.
• Along with its ability to determine the thickness of coated films, SPR
has also emerged as a technology in the area of sensors (e.g., for the
detection of physical quantities, chemicals and biological
purposes). Physical quantities (such as temperature and humidity) can
be deduced from changes in refractive index.
• Chemical sensing can use changes in refractive index to indicate
changing concentrations of molecules adhered to the metal surface (as
a result of chemical reactions). Biosensing can also use refractive
index changes to deduce the occurrence of binding interactions (such
as between antigens and antibodies). SPR also provides the important
advantage of being able to monitor reactions in real-time, without the
need to go through the often complicated process of labeling
molecules with fluorescent or radioactive probes.
• Like all surface analysis techniques, SPR has its limitations in terms
of sensitivity (the smallest amount of molecule detectable), resolution
(the smallest difference in SPR angle distinguishable) and sample
characteristics (geometry, thickness, etc.). However, this technique
still provides a remarkable variety of capabilities for the
characterization of reaction kinetics and thin film properties, with a
high degree of sensitivity.
• Most of the interesting SP-mediated effects happen when the metal
surface at which the SP is generated is covered with a dielectric thin
film. The presence of even very thin films measurably alters the
behavior of the SP reflectivity resonance -- typically shifting the
incident angle at which resonance occurs and broadening the
reflectivity dip. These effects can be used to make devices. For
example, if the film is electro-optically active, one can make an
optical modulator; chemical changes in the dielectric overlayer can be
used to make a chemical sensor.
• There are many areas of applications of SPR sensors. For instance
they are used for measurements of physical quantities, chemical
sensing, and biosensing. Because of the complexity of biological
systems and the number of possible interference to chemical
nanosensors, the need for added specificity in cellular analyses can
arise: nanobiosensors are then employed. Biological receptor
molecules (i.e., antibodies, enzymes, etc.) are used to provide added
specificity. The different types of bioreceptor molecules that have
been used for the fabrication of nanobiosensors include antibodies,
oligonucleotides, and enzymes, thereby allowing for the detection of a
wide array of analytes.
Applications in Technology
• Plasmons have been considered as a means of transmitting
information on computer chips, since plasmons can support much
higher frequencies (into the 100 THz range, while conventional wires
become very lossy in the tens of GHz.)
• They have also 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
laboratory environment.
• It is evident that deeply sub-wavelength focal spots cannot be formed
through conventional focusing using a lens system or microscope
objective. This is due, primarily, to the lack of high-index media at
visible frequencies. What if, however, one was able to achieve a high
effective index with conventional optical materials? That is the
potential of surface plasmon optics. By employing geometries of
conductors (such as metals or doped semiconductors) with dielectrics
(such as air or glass), modes at optical frequencies can be created with
effective indices of refraction that are orders of magnitude higher than
those of the constituent materials. In fact, these indices can be so high
as to create X-ray wavelengths (less than 10nm) with visible
frequencies.
• The reason surface plasmon modes can achieve anomalously high
wave-vectors at visible frequencies is because they are mediated by
electrons rather than free space optical fields.
• The ability to focus the optical field to deeply sub-wavelength
dimensions opens the door to an entirely new class of photonic
devices. If one could combine the imaging powers of X-ray
wavelengths with the economy and maturity of visible light sources,
one could greatly broaden the practical engineering toolbox. Imagine
focusing visible photons to spatial dimensions less than ten
nanometers. By doing so, electron beam microscopy is immediately
displaced by optical microscopy, replacing expensive electron beam
sources with inexpensive visible lasers. Beyond simple economics,
though, this achievement would allow for the nanoscale imaging of
living biological samples.
•
• Combining Plasmonics Effects and Photonic Crystals
• Photonic band structure refers to the modification of the propagation
properties of electromagnetic waves traveling through a periodically
modulated dielectric. The effects of scattering and interference of the
light by the periodic structure would result in a change in the
propagation of the waves. The alteration in the propagation properties
is particularly significant when the wavelength of the light is
approximately equal to the spacing between the dielectric structures.
In this regime photonic band gaps--frequency intervals in which no
photon modes are allowed--can be created for appropriately designed
dielectric arrays. The ability to create volumes of space in which no
photons of a given band of energies can exist has a number of
fundamental and applied consequences.
Application in Health Science
• Surface plasmon resonance is used by biochemists to detect the
presence of a molecule on a surface.
• SPR reflectivity measurements can be used to detect DNA or proteins
by the changes in the local index of refraction upon adsorption of the
target molecule to the metal surface. If the surface is patterned with
different biopolymers, the technique is denoted as Surface Plasmon
Resonance Imaging (SPRI).
•
For nanoparticles, localized surface plasmon oscillations can give rise
to the intense colors of solutions of plasmon resonance nanoparticles
and/or very intense scattering. Nanoparticles of noble metals exhibit
strong ultraviolet-Visible absorption bands that are not present in the
bulk metal. Shifts in this resonance due to changes in the local index
of refraction upon adsorption of biopolymers to the nanoparticles can
be used to detect biopolymers such as DNA or proteins.
• Areas of interest in this domain are for instance the
examination of protein-protein or protein-DNA
interactions, in order to detect conformation changes in
an immobilized protein. In addition to above mentioned,
biosensors may also be used to monitor the glucose levels
in diabetic patients. The system under study would be
based upon direct measurements of the reflection and
transmission spectra in the near infrared spectrum.