NanophotonicsAn Overview

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• Nanophotonics-An Overview
• NSF-RISE Workshop
• July 9-14, 2007
• Anup Sharma
• Department of Physics
• Alabama AM University
• Anup.sharma_at_aamu.edu

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What is Nanophotonics?Science of Light-Matter
Interaction at Nanometer scale (lt 1 micron to 1
nm)
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Examples of Nanophotonics

• Iridescent colors on butterfly wings are due to
  Photonic-Crystals. i.e. Stacks of Nanoscale
  Gratings

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Emission of Semiconductor (CdS, InAs, InP, CdSe)
Nanospheres depends on the size
Used for Bioimaging and Fabrication of
Quantum-Well Lasers
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Metal Nanoparticles Enhance Raman Scattering
Signals by several orders of magnitude
This Effect is called Surface Enhanced Raman
Scattering (SERS)
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Nanophotonics can be divided into three parts

• 1. Nanoscale confinement of Radiation

Example Near-Field confinement of radiation by
squeezing light through nanoscale apertures
Used for Near-Field Microscopy to resolve below
the Far-Field Diffraction Limit. Also used for
Near-Field Optical/UV Lithography.
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2. Nanoscale confinement of Matter

• Nanoscale Cyrstals are used for
• (i) Optical Upconversion of radiation in
  Rare-Earth Nanocrystals.
• (ii) Size-dependent Emission properties of
  nanoscale semiconductor crystals (Quantum dots or
  Q-dots)
• Metal nanoparticles and nanotips used in Surface
  Enhanced Raman Spectroscopy
• Photonic Bandgap Crystals and Photonic Bandgap
  Fibers (Photonic Fibers) involve periodic
  variation of dielectric constant over
  wavelength-scale.
• Applications Fabrication of MOEMS (Micro Opto
  Electro Mechanical Systems), Micro-Optics, i.e.
  Microlasers, Directional Couplers between
  waveguides, Biophotonic Chips etc.

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3. Nanoscale Photoprocesses

• Examples Nanoscale Lithography, Fabrication of
  Nanoscale Structures, Nanoscale Optical Memories

Foundations for Nanophotonics Basic Equations
describing propagation of photons in dielectrics
has some similarities to propagation of electrons
in crystals
Similarities between Photons and Electrons
Wavelength of Light,
Wavelength of Electrons,
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Maxwells Equations for Light
Eigenvalue Wave Equation
Describes the allowed frequencies of light
Schrodingers Eigenvalue Equation for Electrons
Describes allowed Energies of Electrons
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Free Space Solutions
Photon Plane Wave
Electron Plane Wave
Interaction Potential in a Medium Propagation of
Light affected by the Dielectric Medium
(refractive index) Propagation of Electrons
affected by Coulomb Potential
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Propagation through Classically Forbidden Zones

• Photon tunneling through classically forbidden
  zones. E and B fields decay exponentially.
  k-vector imaginary.

Electron Wavefunction decays exponentially in
forbidden zones
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Confinement of Light and Electrons

• Confinement of Light results in field variations
  similar to the confinement of Electron in a
  Potential Well. For Light, the analogue of a
  Potential Well is a region of high
  refractive-index bounded by a region of lower
  refractive-index.

Microscale Confinement of Light
Nanoscale Confinement of Electrons
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Differences between Light and Electron Waves

• Electron Momentum generally bigger than photon
  momentum and so wavelength of light generally lt 1
  micron while wavelength of electrons generally lt
  1 nm.
• Light wave is described by a vector field
  described by E and B while electron wavefunction
  is scalar
• Photons satisfy Bose-Einstein statistics while
  Electrons are Fermions

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Free-Space Propagation

• Free space propagation of both electrons and
  photons can be described by Plane Waves.
• Momentum for both electrons and photons, p
  (h/2p)k
• For Photons, k (2p/?) while for Electrons, k
  (2p/h)mv
• For Photons, Energy E pc (h/2p)kc while for
  Electrons,

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Electronic and Photonic Crystals
Similar to the periodic electron-crystal lattice,
one can fabricate photonic-crystal lattice. The
refractive index varies with a much larger period
of around 200 nm.
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Nanoscale Optical Interactions

• This involves interaction of light with matter
  over nanoscale distances. Some examples

Light propagating in a medium of high refractive
index can be totally internally reflected at the
interface with a lower refractive index.
Applications
Existence of an evanescent wave was first
demonstrated by Newton. Light is transmitted not
only at the point of contact but also
through the neighboring regions due to
penetration evanescent field through thin air
film
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Excitation of Surface Plasmon Resonance by
Evanescent-Tail
Surface plasmon is a wave at the interface of a
metal and a dielectric film. This is a widely
used technique for Biosensing and instruments
using this technique are available commercially.
To generate a Surface Plasma Wave, the diagram
shown below is used.
For Surface Plasma Wave traveling in z-direction,
the condition to excite it optically is,
reflected light intensity is seen. Thickness of
metal film is 40-50 nm. By coating the dielectric
film with biological antibodies, this is a widely
used technique for sensing antigens, proteins etc.
When the resonance condition is met, a sharp
decrease of the internally
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Nanoscale Confinement of Light Near-Field
Microscopy for Sub-Wavelength Resolution
In Far-Field Microscopy,
This can be overcome with Near-Field Techniques
by having nanoscale apertures or by using
aperture-less techniques which enhance light
interaction over nanoscale dimensions with the
use of nanoscale tips, nanospheres etc. The idea
of using sub-wavelength aperture to improve
optical resolution was first proposed by Synge in
a letter to Einstein in 1928. These ideas were
implemented into optics much later in 1972 Ash
and Nicholls
Schematic set-ups for Near-Field Scanning Optical
Microscope (NSOM)
Aperture-less Technique Near-Field around
Nano-Tip
Near-field light decays over a distance of 50 nm
from aperture.
Tapered Optical-Fiber
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Theoretical Results for Near-Field Nanoscopic
Interactions
Light Diffraction by Sub-Wavelength (a / ?) lt 1
Circular Aperture
Only the terms with (1/r) contribute to net
average radiation intensity when flux over a
spherical surface is evaluated. Other terms give
evanescent near-field radiation over distances
less than (?/2p).
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Aperture-less Near-Field Microscopy
Effect of Evanescent Coupling on radiative
life-time of a dipole is shown below
In the aperture-less technique of microscopy, a
metal tip of diameter lt 50 nm is used to
enhance inelastic scattering like Raman,
fluorescence, nonlinear phenomena. For small
distances between the tip and sample substrate
(lt50 nm), this enhancement is due to the
near-field component of light diffracted by the
tip.
The radiative decay rate for the dipole is
calculated as the dipole is translated along the
x-axis for a fixed distance (D) between the two
substrates. This is shown below.
Limitation of small throughput of apertures can
be overcome. Enhances inelastic light scattering
from sample by generating surface plasma in the
metals
Above calculations clearly show that
sub-wavelength resolution is due to coupling of
the evanescent field to the environment. A.
Rahmani et. al., Phys. Rev. A 56, 3245 (1997)

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Nanoscale Confinement of Matter or
Quantum-Confined Materials
Quantum-confined materials refer to structures
which are constrained to nanoscale lengths in
one, two or all three dimensions. The length
along which there is Quantum confinement must be
small than de Broglie wavelength of electrons for
thermal energies in the medium.
de Broglie Wavelength,
Thermal Energy, E
For T 10 K, the calculated ? in GaAs is 162 nm
for Electrons and 62 nm for Holes
For effective Quantum-confinement, one or more
dimensions must be less than 10 nm. Structures
which are Quantum-confined show strong effect on
their Optical Properties. Artificially created
structures with Quantum-confinement on one, two
or three dimensions are called, Quantum Wells,
Quantum Wires and Quantum Dots respectively.
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Quantum-Confined Materials
Nanoscale Confinement in 1-Dimension results in a
Quantum Well
Quantization of energy into discrete levels has
applications for fabrication of new solid-state
lasers. Two or more Quantum wells side-by-side
give rise to Multiple Quantum Wells (MQM)
structure.
Motion is confined only in the Z-direction. For
electrons and holes moving in the Z-direction in
low bandgap material, their motion can be
described by Particle in a Box. If the depth of
Potential Well is V, for energies EltV, we can
write,
At 300 K, The band gap of GaAs is 1.43 eV while
it is 1.79 eV for AlxGa1-xAs (x0.3). Thus the
electrons and holes in GaAs are confined in a 1-D
potential well of length L in the Z-direction.
n 1, 2, 3,..
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1-D Confinement Quantum-Well
Energy-levels
Wave-functions in a Semi-conductor Quantum-Well
Efficiency of a Quantum-Well Laser depends on the
density of states First let us find the
density of states in a bulk semiconductor no
confinement
For electrons in conduction-band
dN is proportional to
This represents a sphere in momentum-space of
radius R
(2mE)1/2
The number of states dN between energy E and EdE
is proportional to the volume of shell between R
and RdR
Density, D(E)dN / dE
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Density of States for Quantum-Confinement
Density of States
Quantum Well 1D Confinement
Due to 1-D confinement, the number of continuous
energy states in the 2-D phase space satisfy
Quantum Wire 2D Confinement
2D confinement in X and Z directions. For wires
(e.g. of InP, CdSe). with rectangular
cross-section, we can write
Quantum Dot 3D Confinement
For a cubical box with the discrete energy levels
are given by
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Manifestations of new optical effects due to
Quantum Confinement
Size Dependence of Optical Properties In general,
confinement produces a blue shift of the
band-gap. Location of discrete energy levels
depends on the size and nature of confinement.
Increase of Oscillator Strengths This implies
increase of optical transition probability. This
happens anytime the energy levels are squeezed
into a narrow range, resulting in an increase of
energy density. The oscillator strengths increase
as the confinement increases from Bulk to Quantum
Well to Quantum Wire to Quantum Dot.
New Intraband Transitions Confinement produces
sub-bands within the conduction and valence
bands, enabling intraband optical transitions
which are not allowed in bulk. These IR
transitions have applications to making new
Quantum Cascade Lasers and also detectors.
Oscillator strengths increase as the width of
Quantum Well decreases.
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Quantum Dots
The most important optical feature of these
structures is that absorption/emission spectra
shifts to shorter wavelengths as the size becomes
smaller. The luminescence spectra for InAs, InP
and CdSe Quantum Dots is shown below.
Likewise, Quantum-Dot Quantum Well refers to
alternate layers of high and low bandgap
semiconductors. Covering the surface of a Quantum
Dot reduces non-radiative decay of electrons
close to the surface and thus enhances
luminescence intensity.
Core-Shell Quantum Dot refers to a Quantum-Dot
surrounded by a shell of higher band-gap
semiconductor.
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Quantum Confined Lasing Structures
Semiconductor Laser is the best known application
of quantum confined structures. Size of laser is
around 100µm x 100µm x100µm and lasing
wavelength can be tailored by the choice of the
gain medium between 400 to 1600 nm.
Single Quantum Well (SQW) lasers employ a much
thinner (lt10 nm) gain medium. Due to discrete
energy levels, the threshold current for lasing
is smaller, 0.5 mA as compared to 20 mA for
double heterostructure laser (DHL). Line Widths
are narrower, each mode can be lt 10 MHz. It can
be modulated at higher frequencies.
First continuous wave semiconductor diode laser
was a Double Heterostructure Laser. It was
demonstrated by Alferov in USSR and by Panish and
Hayashi in US.
Thickness of GaAs was greater than 100 micron.
So this is not a Quantum-Confined Laser
Edge-Emitting Diode Laser
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Quantum Cascade (QC) Laser
Operates within the sub-bands of the Conduction
band. It is different from other designs where
emission is due to electron-hole recombination.
Often called Unipolar Laser. In conventional
semiconductor lasers one electron can emit only
one photon as it combines with a hole. QC laser
is a Multiple Quantum Well (MQW). Discovered in
1996 (Appl. Phy. Lett. 68, 3680).
There could be 50 Quantum Wells in MQW geometry.
The barrier layer is very thin (1-3 nm) An
excited electron emits 25-75 photons as it
cascades down the ladder of sub-bands in the
Conduction Band. QC lasers have been demonstrated
for wavelengths between 3-20 micron. Useful for
sensing atmospheric pollution
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From Semi-Conductor Quantum-Dots to Metal
Nano-Particles
Under some conditions, one can resonantly excite
a surface plasma-wave on the interface of metal
and dielectric Surface Plasmon Resonance (SPR).
Applications of SPR has resulted in the field of
Plasmonics
Propagating SPR at Optical Frequency on a metal
nano-wire Light on a Wire
Science Daily (March 2005) Engineers Study
Whether Plasmonics, 'Light On A Wire,' Is
Circuitry Wave Of Future If data drove itself
around in cars, photonics would be a roomy
minivan and electronics would be a nimble coupe.
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..a single technology that
has the capacity of photonics and the smallness
of electronics would be the best bridge of all. A
new research group in Stanford's School of
Engineering is pioneering just such a technology
plasmonics..
A new Journal called plasmonics published by
Springer, beginning March 2006
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Plasmonics
Some Interesting Phenomena
? Interaction of light wave with metal
nanoparticles ? Generation of surface plasma
waves and dependence of plasmon resonance on
metal particle size and geometry ? Dependence
of plasmon resonance condition on the dielectric
adjacent to the metal film and its application
for sensing ? Enhancement of electromagnetic
field close to metal nanoparticles and its
application to spectroscopy like Surface Enhanced
Raman and Fluorescence ? Application of metal
nanotips for apertureless imaging ? Effects of
metal nanoshells on plasmon resonance ?
Propagation of high-frequency electromagnetic
waves along sub-wavelength-wide metal
waveguides ? Effect of metal surface on
radiative decay of molecules
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Excitation of Surface Plasmon Wave
Surface plasma waves can be generated optically
on bulk surface at the interface of metal and
dielectric. These are referred as Surface
Plasmons. The traveling wave is associated with a
wavevector k(sp). Special excitation geometry is
required to produce such a wave.
In nanoparticles, the surface plasmon wave is
localized (not traveling) and so no special
excitation geometry is required.
For Surface Plasma Wave traveling in z-direction,
the condition to excite it optically is,
Wavelengths which are absorbed by metal
nanoparticles and produce such a wave are called
Surface Plasmon Bands or Plasmon Bands.
Where, k(sp) is the wavevector of the surface
plasma wave and k(2p/?) that of the light wave.
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Surface Plasmon Resonance (SPR) on Metal
Nano-Particles
Most of the applications of such localized
plasmon waves is due to electromagnetic field
enhancement in the viscinity of the metal
nanoparticle surface. Light absorption by
nanoparticles takes place within a narrow range
of wavelengths. This resonance (SPR) depends on
size, shape and the nature of metal nanoparticle.
This is shown in the graph below for gold
nanoparticles.
SPR can be understood from dielectric properties
of metal nano-particles. This can be understood
in a simple way from Drudes Model for Dielectric
Constant in Metals.
Where m is electron mass and e is its charge. ?
is a damping constant and electric field
Solution are of form,
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SPR on Metal Nano-Particles
This gives
Where plasma frequency,
Real part of Dielectric Constant
Substituting in equation above, we get
As can be seen from above, real part of
e(?) can be negative
for
Polarization vector, P is
Calculated Plasma Wavelength for Silver is 137 nm
and for Copper 114 nm. Thus in the visible
region, Ree(?) is negative.
N is number density of electrons and ? is
susceptibility.
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SPR on Metal Nano-Particles
Surface Plasmon Resonance takes place for
frequency which satisfies
Constant ? (e2) describes damping of electron
motion. In bulk metals, this is largely due to
electron-electron and electron-phonon scattering.
However in metal nanoparticles, surface effects
dominate since electron motion is constrained by
the size of nanospheres. This damping is
inversely proportional to the size of sphere.
Thus the effect of size on plasmon resonance in
metal nanospheres is contained in ?.
Absorption of incident light by Metal Nanospheres
embedded in a Dielectric is given the Extinction
Coefficient
Where, ? is the wavelength of light, eh is the
dielectric constant of surrounding medium, N is
the number density of metal spheres and V its
volume, e1 and e2 are the real and imaginary
parts of the metal dielectric constant
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SPR on Metal Nano-Shells
Dielectric Core surrounded by Gold Shell
Another type of nanosphere is dielectric sphere
coated with a nanoshell of metals like gold. Core
materials like AuS and silica of radius between
30-250 nm and shell thickness of 10-30 nm.
Thinness of the shell results in a substantial
red-shift of plasmon resonance. Effective
dielectric constant of the dielectric medium with
embedded nanoparticles is given by
Here f is the volume fraction of nanoparticles, d
is the ratio of core volume to the volume of
particle and eh, es, ec are respectively the
dielectric constants of the surrounding medium,
the shell and the core. In general, as the
thickness decreases, the resonance shifts to
longer wavelength. This is due to increased
electron scattering and an increase in the
damping constant in metal dielectric constant. As
seen from equations above, the condition for
resonance is
Re(es) 2eh 0
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SPR on Metal Nano-Shells
In general, as the shell thickness decreases, the
resonance shifts to longer wavelengths.
Vial on the left has solid gold colloids. Others
have colloids with metal nano-shells with
decreasing thickness. Vial on left absorbs IR
Fabrication of Metal Nanoshells
For fabrication, the dielectric sphere is coated
with a layer of amines which binds 1-2 nm gold
colloids from suspension. This is followed by a
chemical treatment with HAuCl4 in the presence of
formaldehyde. This results in an additional layer
of gold.
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Applications of Plasmonics
Metal nanoshells have several potential uses. It
has been shown that a coating of these prevents
photo-oxidation of polymer semiconducting devices
if the resonance condition for nanoshells is at
the wavelength of maximum photo-oxidation.
Nanoshells thus act an an extinction filter
Nanoshells have also been used for whole blood
immunoassay. Nanoshells can be attached to
antibodies as shown below.
They form nanoshell dimers when they attach to
the antigen resulting in a change of plasmon
resonance condition. This can be monitored
optically.
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Applications of Plasmonics
Plasmonic Wave Guiding
Light on a Wire
Waveguiding in traditional optical waveguides
involves structures which cannot be smaller than
?/2. Further, waveguiding along bent guides is
very lossy. The latter problem is overcome in
photonic bandgap structures. But the dimensions
of the structures are still limited by the
wavelength of light.
These surface excitations can be at any frequency
between UV and IR and so these waveguides combine
the less bulky nature of metal waveguides with
the high bandwidth of optical waveguides. Typical
size of metal nanosphere is 50 nm. Plasmon
excitations can travel over bent plasmonic
waveguides. A weakness of this technique is high
loss (6 dB/µm) and transmission has been
demonstrated over short ( 1 µm) distances. It is
an active area of research for future
applications.
The latter restriction can be overcome by
waveguiding of plasmonic excitation in closely
placed metal nanoparticles.
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Applications of Plasmonics
Electromagnetic field is enhanced locally on the
surface of metal particles. This enhancement is
especially strong at plasmon resonance.
Surface Enhanced Raman Scattering (SERS)
Aperture-Less Near-Field Microscopy
Local field enhancement due to surface plasmons
has been developed as a technique for
aperture-less near-field microscopy.
A nano-tip metal needle is used within the focal
volume of an excitation laser.
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Applications of Plasmonics
Surface Plasmon-Wave Bio-Sensor
Excitation of a plasmon wave at the interface
between a metal and dielectric films has been
used for sensing by evanescent wave spectroscopy.
The evanescent wave associated with plasmon wave,
penetrates the dielectric film to sense in the
medium around. Evanescent wave can be absorbed by
molecules being sensed. The technique shown above
is for biosensing by antibodies. As they
conjugate with specific antigens, the dielectric
constant of the film changes and this is
sensitively observed as an increase of internally
reflected light since the condition for plasmon
resonance is no longer met. This technique is
widely used in several commercially available
sensors.
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Photonic Crystals
In common Electronic Crystals like NaCl, the
periodicity is 1 nm. Photonic Crystals are
produced by periodically varying refractive index
in one, two or three dimensions. The period is
comparable to the wavelength of light. Thus the
field of Photonic Crystals can be looked upon as
Microphotonics. However, in order to fabricate
Photonic Crystals with micron-scale period, the
fabrication technique must have nano-scale
resolution. Thus it is appropriate to include
Photonic Crystals in our study of Nanophotonics.
Figures below show schematic representations of
1D, 2D and 3D Photonic Crystals.
2D Photonic Crystal Periodic variation of
refractive index in X and Y directions
3D Photonic Crystal Periodic variation of
refractive index in X, Y and Z directions
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Photonic Crystals
In Nature, parts of several living organisms have
Photonic Crystals in them. Iridescent colors of
butterfly wings and peacock feathers are due to
Photonic Crystals.
Photonic Band Gap crystals have several
photonics-related applications, including
microlasers, and waveguides/ waveguide-couplers,
photonic band-gap optical fibers with novel
dispersion characteristics etc.
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Similarities between Electronic and Photonic
Crystals
The most striking similarity is the Band-Gap
within the spectra of Electron and Photon Energies
Likewise, diffraction of light within a Photonic
Crystal is forbidden for a range of frequencies
which gives the concept of Photonic Band-Gap. The
forbidden range of frequencies depends on the
direction of light with respect to the photonic
crystal lattice. However, for a sufficiently
refractive-index contrast (ratio n1/n2), there
exists a Band-Gap which is omni-directional.
Solution of Schroedingers equation in a 3D
periodic coulomb potential for electron crystal
forbids propagation of free electrons with
energies within the Energy Band-Gap.
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Band-Gap in Photonic Crystals
Band-Gap frequencies when incident on the
photonic crystal will be not be transmitted but
be reflected/diffracted
Optical characteristics of Photonic Crystals can
best be understood by plotting the Dispersion
Curve, i.e. variation of frequency (?) of light
with components of its wave-vector (k). Similar
dispersion curves in Electronic Crystals, i.e.
variation of energy E with k of electrons reveal
the Electronic Band Gap.
Period of refractive index variation in Photonic
Crystals is taken as a. Figure shows dispersion
character of light in a bulk medium with a
uniform refractive index, n
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Theoretical Modeling of Photonic Crystals
Thus the incident and diffracted z-components of
wave-vector are shifted with respect to each
other by integral multiple of (2p/a). A special
case of diffraction is Bragg Diffraction, when
incident light is reflected by the photonic
crystal
1D Photonic Crystal
It is simplest to plot the dispersion curves for
1D Photonic Crystals. Incident light with a
wave-vector (k) is diffracted by the 1D photonic
crystal (period, a).
Incident light can be diffracted into many
possible directions as shown in the figure above.
Wave-vectors are related by (show as Home-Work)
Thus, whenever kz is a multiple of (p/a), the
incident wave is reflected back. It cannot
propagate in the photonic crystal and the
group-velocity of such a wave is zero.
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Dispersion in 1D Photonic Crystals
Deviation from the straight-line dispersion curve
of a uniform bulk medium is seen in the diagram.
To ensure Bragg reflection for kz N(p/a), the
curve becomes horizontal.
It is necessary to plot the above dispersion
curve only for values of kz between p/a and p/a.
All other values of kz can be got by diffraction
of waves with kz between p/a and p/a. The region
of kz between p/a and p/a is called the First
Brillouin Zone. There is a band of frequencies
that are forbidden for all possible values of kz.
This is the Band-Gap for the 1D Photonic Crystal.
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2D Photonic Crystals
Compared to 1D Photonic Crystal, it is relatively
more difficult to deduce the Dispersion Curves
for 2D and 3D Photonic Crystal. The software for
doing this is publicly available at
http//www.elec.gla.ac.uk/groups/opto/photoniccrys
tal/Software/SoftwareMain.htm
Dispersion curves for k for the boundary of the
shaded region are plotted in three parts In the
first part (? to M), k increases from 0 to ?M
along the same direction. In the second part, the
tip of k vector moves from M to K. In the third
part (? to K), k increases from 0 to ?K along the
same direction. For sufficiently high ratio of
refractive index (n1/n2), there exists a
Band-Gap.
Just like for 1D case, it is necessary only to
investigate dispersion curves for values of k
within the First Brillouin Zone.
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Properties of Photonic Crystals
Super Refraction or Super Prism Effect
Prism effect is related to the derivative (dn/d?)
of refractive index with wavelength. This
derivative can be made unusually large in
photonic crystals. This can be understood with
dispersion curves for 2D photonic crystals
mentioned earlier.
Prism effect refers to separation of colors by
refraction through a prism. This is related to
dispersion or variation of refractive index with
wavelength.
In a normal bulk medium like glass, dispersion
is small.
Photonic crystals can also exhibit an effective
negative refractive index as explained below.
This effect is seen in photonic crystals in the
microwave and visible regions of spectrum.
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Fabrication Techniques for Photonic Crystals
Fabrication of 1D and 2D Photonic Crystals
These are relatively easy to fabricate using UV
and Electron-Beam Lithography. Bragg Grating in
an optical fiber is a simple 1D Photonic Crystal.
It is fabricated by UV Lithography
Fabrication of 3D Photonic Crystals
1. Sedimentation of monodisperse colloidal
nanospheres of polystyrene or silica.
Fiber Bragg-Grating
2. Two-photon Lithography
Since two-photon absorption takes place only
close to the focal point of a laser, one can
Holographic UV Lithography with 244 nm UV light
fabricate structures deep within the volume of
the polymer matrix.
US Patent 6,873,762, awarded 2005
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Applications of Photonic Crystals
Photonic Components using 2D Photonic Crystals
2D Photonic crystals have a potential for
fabricating waveguides and related components for
integrated optics.
Unlike the conventional refractive planar
waveguides, these work by diffraction. Modes that
cannot propagate through the 2D photonic crystal
lattice (i.e. those within the band-gap) can
propagate easily within the waveguides, including
those waveguides which have a sharp bend at right
angle.
Microcavity Effect in 3D Photonic Crystals
Defects in a Photonic Crystal Lattice produce
microcavities which have size-dependent radiation
modes. Just like electronic crystal these produce
defect-states in the Band-Gap. Microcavity is
resonant for wavelength ?, satisfying,
D N(?/2), where N is an integer and D, the
characteristic size of the cavity. Microcavity
lasers have been fabricated by this technique.
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Photonic Crystal Optical Fibers
This is a special class of 2D photonic crystal
where the dimension of the medium perpendicular
to the crystal plane could be 100s of meters
long. These are known by several names Photonic
Crystal Fiber (Phillip Russell), Photonic Bandgap
Fiber, Holey Fiber, Microstructured Fiber and
Bragg Fiber.
Frequencies in the Band-gap propagate within the
fiber-core, which is like a defect in 2D photonic
crystal. Hollow-core fibers have several
advantages as compared to the conventional
solid-core fiber. While hollow-core photonic
band-gap fibers work by diffraction, solid-core
photonic band-gap fibers work by both internal
refraction as well as diffraction.
Since light travels in air, group velocity
dispersion can be zero for all wavelengths. By
filling the hollow-core with gas, the fiber can
be used as a very sensitive gas sensor.
In hollow-core, even those wavelengths can
propagate which have high loss in conventional
fibers. Propagation for 1000 m has been shown
for wavelengths like 1.5 micron and 10 micron
with an attenuation of around 1dB/km.
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Nanophotonics for Biotechnology and Nanomedicine
Bioimaging with fluorophore embedded Nanoparticles
Specific molecules like proteins are difficult to
image directly in a biological tissue.
Nanoparticles are used for enhancing the
contrast. To be able to use fluorophores like
dyes or rare-earth-ions for biological imaging or
as tracers they must be biologically benign. In
principle this can be accomplished by embedding
the fluorophores inside transparent shells of
glass etc. Being small, the nanoparticles can
flow with the bloodstream without interrupting
any process. The example below illustrates how
such nanoparticles can be used for bioimaging
tissues or cells with a specific protein (here
A).
The technique involves immobilizing antibodies
for protein A on the nanoparticle surface. These
nanoparticles will attach only to protein A.
Illumination with a light source of suitable
wavelength will reveal the site of protein A with
a characteristic fluorescence.
53
Bioimaging with Nanoparticles
Nanoparticles are also used for bioimaging by
non-optical techniques like Magnetic Resonance
Imaging (MRI), Radioactive Nanoparticles as
tracers to detect drug pathways or imaging by
Positron Emission Tomography (PET), and
Ultrasonic Imaging. For MRI, the magnetic
nanoparticles could be made of
oxide particles which are coated with some
biocompatible polymer. Newer Nanoparticle
Heterostructures have been investigated which
offer the possibility of imaging by several
techniques simultaneously. An example is Magnetic
Quantum Dot.
54
Targeted Therapy with Nanocapsules
This technique of therapy uses the same specific
antibody-antigen binding that is used for
bioimaging. Nanocapsules of drugs conjugated with
specific antibodies get attached to diseased
portions of tissues which express signature
proteins. Release of drug at the required site
minimizes side effects to healthy tissues.
The drug is encapsulated in a lipid layer on
nanoparticle which also has an antibody to
specifically target the site of interest. Since
lipids are natural constituents of cell
membranes, the drug is absorbed by cell by
lipid-exchange.
In one study, Nanoparticles were made of Human
Serum Albumin (HSA), which is the most abundant
blood protein. Drug Herceptin was attached to
nanoparticle. This drug binds to protein HER2
found in breast cancer tumors. HER2 multiplies
abnormally in cancerous cells. Herceptin thus
specifically targets HER2 with minimal side
effects on normal tissues. Radioactive
nanoparticles could be used for local exposure to
radiation.
55
Biomedical Applications of Nanoparticles
Elastic Properties of DNA
Biosensing with Nanoparticles
Strands of DNA can be attached to 500 nm beads at
the ends and held by laser-tweezers as shown
below.
Surface of nanoparticles are conjugated with
antibodies for biosensing.
Attachment of antibodies to antigens can lead to
aggregation. This changes the Plasmon Resonance
properties of the nanoparticle which can be
observed by Extinction Spectrum. Alternately,
sensing can lead to detection of antigens by SERS
spectrum.
DNA is in an aqueous medium. Refractive index of
beads is greater than that of the liquid medium.
The beads get trapped at the beam-waists of
focused laser beams. By moving the laser
tweezers, the DNA can be stretched. Forces
involved in stretching DNA can be measured to an
accuracy of few pico-Newtons.
56
Micro/Nano UV Lithography of Biological Substrates

• Outline of Presentation
• History of Lithography
• Examples of Biological Substrates
• What can Lithography Accomplish?
• IV. Case Studies Four Specific Examples from
  our Research

57
I. HISTORY OF LITHOGRAPHY Invented by Alois
Senefelder in Germany in 1798 Based originally on
the chemical repellence of oil and water.

Designs were drawn with greasy (Hydrophobic)
crayon on specially prepared limestone
(Hydrophilic).
The stone was moistened with water, which the
stone accepts in areas not covered by the crayon.
An oily ink, applied with a roller, adheres only
to the drawing and is repelled by the wet parts
of the stone.
The print is then made by pressing paper against
the inked drawing.
58
I. LITHOGRAPHY Drawing patterns on substrates

• Examples of UV Lithography
• Development of Microchips
• on Silicon Wafers
• 2. Fabrication of MEMS Devices
• Role of UV
• UV induces chemical changes in
• the substrate/photoresist
• UV Wavelength limits the minimum size of patterns
• UV Wavelengths for Lithography
• near UV (380200 nm)
• far or vacuum UV (20010 nm
• FUV or VUV)
• extreme UV (131 nm EUV or XUV).

59
II. Biological Substrates

• Examples of Substrates used to immobilize
  biological molecules
• Biomembranes for Protein immobilization

Model biomembrane ExamplePhosphatidylcholine
(Egg PC)

• C. K. Yee et. al., Adv. Mater. 2004, 16, 1184
• Sharma et. al., Optics Letters 2005, 30, 501
• Cooper et. al. Anal Biochem. 2000, 277, 196-205.
  Sensor for Cholera Toxin
• Swanson et. al., United States Patent 6893814
  Influenza Sensor

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II. Biological Substrates
2. Biocompatible Polymers for immobilizing DNA,
tissue fragments. Example Poly-Amino acids
(Poly-L-Lysine) film coated on glass
substrate It attaches ionically to the
phosphate group of the DNA backbone. Subsequent
UV-light exposure is used to cross-link
oligonucleotides to the Polymer surface. Erie
Scientific Poly-L-Lysine slides Used in
fabrication of DNA Microarrays G. C. Vivian et.
al., Nature Genetics supplement, 1999, 21, 15-19.
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II. Biological Substrates

• 3. Micro/Nanoporous polymer films for
  encapsulation of biomolecules
• Example Nanoporous poly(methylsilsesquioxane)
  (PMSSQ) thin films on Silicon substrate
• Porous Sol-Gel Silica glasses

Matrix
Antibody
Proteins, enzymes, antibodies, whole cells have
been embedded within sol-gel glasses. They retain
their bioactivity and remain accessible to
external reagents. Sol-gel glasses are optically
transparent, so it is possible to couple optics
and bioactivity to make biosensors.
D. Jiang et. Al Anal Chem. 2003, 75, 4578-84.
sensor for hyaluronan and laminin (liver fibrosis
markers) Livage et al J. Phys. Condens. Matter
2001, 13, R673-R691 - Sol-Gel glass   H. C. Kim
et. al., Nanoletters 2004, 7, 1169-1174
Photopatterned Nanoporous Media
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63
III. UV Lithography of Biological Substrates

• Why Lithography of Biological Substrates?
• Development of High Density Microarrays for DNA
  and Protein Sensors

High throughput, Multianalyte Microarray Biochip
Sensor
Current technology limits the size of each spot
to 10-100 micron UV lithography promises spot
size of 100 nm, i.e. 10,000 higher density

• P. Blanchard, Genetic Engineering 1998, 20,
  111-123
• L. Demers, Genetic Engineering News 2003, 23,
• Spotting accomplished by ultrasharp pen tips as
  in AFM
• R. D. Piner et. al Science 1999, 283, 661-663

64

• UV Patterning on Biosubstrates
• Examples
• Development of monolithic biochips.
• Fabrication of Photonic Components
• like Waveguides and Gratings
• S. Huang et al J. Micromech. Microeng. 2005, 15,
  2235-2242
• A. Sharma et. al., Optics Letters 2005, 30, 501
• UV-activated Hydrophobic Hydrophilic
• Conversion Fabrication of Micro/Nanowells
• A. Hozumi et al Langmuir. 2002, 18, 9022-9027

65

• Fabrication of Bioelectronics BioMEMS
• Examples
• Fabrication of Microfluidic
• Channels
• Fabrication of nano-gap electrodes
• conduction properties of DNA
• J. Lee et. al., MRS, 2002, pg. 185

66
Lithography on Polybutadiene thin-films
Microarray of Hydrophilic Wells on Polybutadiene
thin-films using lithographic masks
Effect of UV Makes Substrate Hydrophilic
Hydrophilic Dye (Rhodamine 6G) sticks to areas
exposed to UV
Spot Size 100 µm x 100 µm DNA sensor chips
involve immobilizing oligonucleotides on an array
of hydrophilic wells separated by hydrophobic
barriers Prevents cross-contamination
67
Lithography on Polybutadiene thin-films
Sputtered Gold attaches only to areas not exposed
to UV
Spot Size 100 µm x 100 µm Gold-coated surfaces
are widely used to immobilize biomolecules for
sensing-related applications. A common technique
involves an intermediate thiol self-assembled
monolayer (SAM) on gold substrate. Gold provides
a bio-inert shield against a substrate that could
denature proteins.
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Lithography on Poly-L-Lysine thin-films
UV
Hydrophilic Substrate Mask Opaque Grid
Polystyrene and Gold Micro/Nano spheres stick to
surface not exposed to UV
Gold
Polystyrene
300 nm 10 micron
300 nm
Nanoporous substrates with controlled
porosity
69
Lithography on Poly-L-Lysine thin-films
UV
Polystyrene Wall Hydrophilic Well
Polystyrene Microspheres stick to surface not
exposed to UV
300 nm
10 micron
Ridged Hydrophilic Wells
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Interferometric (Maskless) UV Lithography
Spacing between Interference Fringes
?UV 244 nm ? 45 deg ? 170 nm
71
Interferometric (Maskless) UV Lithography
Fabrication of Microarrays
Done by Sequential Fabrication of Two
Perpendicular Gratings
Minimum Spot-Size in Array with 244 nm UV 200nm
X 200 nm
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INTERFEROMETRIC UV LITHOGRAPHY OF POLYMER
SUBSTRATES
Lithography on Polybutadiene thin-films
Grating Period 900 nm
Array of Nanowells
Dimension of wells 500 nm x 500 nm x 30 nm
73

• UV Lithography of Model Biomembrane (Phospholipid
  Bilayer) in Aqueous Phase

Example Bilayer of Phosphatidylcholine (NBD-Egg
PC)
Stack of Phospholipid Bilayers
74
UV Lithography of Membrane in Aqueous Phase
C. K. Yee et. al., Adv. Mater. 2004, 16, 1184
75
INTERFEROMETRIC LITHOGRAPHY OF MODEL MEMBRANE
76
Mechanism for Lithographic Membrane Grating
Formation

• Gradient Force
• 2. UV Induced Photodissociation

Grating Formation
77
Detection of Membrane-Grating Growth
Solid supported membrane bilayer floats on a
layer of hydration
UV intermittently on for 0.5 seconds and off for
40 seconds
A. Sharma et. al., Optics Letters 2005, 30, 501
78
Mechanism for Lithographic Grating Formation
The Membrane shows elastic behavior As the UV is
blocked, it relaxes
79
Elastic Relaxation of Biomembrane
Solid supported membrane bilayer floats on a
layer of hydration
Effect of humidity on grating relaxation
80
BIOSENSING WITH MEMBRANE GRATING
81
BIOSENSING WITH MEMBRANE MICRO-ARRAY
Transduction Mechanism Fluorescence