1
Optical Properties of Solids
Laser
Non-linear optical properties
Photoluminescence
Optical Materials
Non-linear optical materials play an important
role in data transmission and storage
LiNbO3, KTiOPO4, KH2PO4
Laser materials
semiconductors and laser host crystals
Phosphors
for displays and imaging
Interactions Between Photons and Materials
Atomic and Electronic Interactions
Electronic Polarization ? interaction with
electron cloud induces electronic polarization?
energy lost in absorption
slows light down (seen as refraction)
Electron Transitions ? electrons excited to
higher unoccupied states?
cannot remain there indefinitely ?Emission
optical characteristics of a material relate to the
absorption and emission of electromagnetic
radiation
When Light Meets a Solid… .
Total intensity equals:
Transmitted
Absorbed
Reflected
T+A+R = 1
(fraction transmitted, absorbed and
reflected = 1)
Classes of Optical Materials
Materials are often characterized by what
happens to light
transparent ?? mostly transmitted
translucent ?? transmitted diffusely
opaque ?? no transmission
shiny ?? mostly reflected
Optical Properties of Semiconductors
Absorption
Eg
longest wavelength absorption to promote e-
corresponds to Eg
Eg is energy between “HOMO ”of valence band and
“LUMO”of conduction band
Emission
Eg
2
Optical Properties & Band Structure
Reflection
Scattering at an interface between two materials
with different n (index of refraction)
Absorption
Electron excitation to defect levels in the band
gap
Electron excitation across the band gap
Metal
filled
empty
Insulator
filled
empty
filled
empty
Semiconductor
Valence
Band
Conduction
Band
Electrochromic WO3 Thin
Films for Smart Windows
Uses: mirrors, displays,
rechargeable solid state
batteries, pH-sensitive
electrochemical transistors or
displays, chemical sensors,
solar cells, selective oxidation
catalyst, …
e- into
Conducting
Bond of WVIO3
M+ into hole
Electrochromic Film:
Multilayer stacks that behave like batteries
Visible indication of their electrical charge
Fully charged: opaque
Partially charged: partially transparent
Fully discharged: transparent
Electrochromic glass is an energy-saving component for buildings
that can change color on command. It works by passing low-voltage
electrical charges across a microscopically-thin coating on the glass
surface, activating a electrochromic layer which changes color from
clear to dark. The electric current can be activated manually or by
sensors which react to light intensity. Glass darkening reduces
solar transmission into the building. When there is little sunlight,
the glass brightens, so that the need for the artificial light is
minimized.
Electrochromic Glass
OFF ON
Why the Color Change?
WO3
Wide band
gap insulator
MxWO3
Narrow band gap
semiconductor
x(M+ + e-) M
xWO3
Metallic
x(M+ + e-)
VB
[O2-(2ppi)]
CB
[W6+(d0)]
Localized VB
[W5+(d1)]
Delocalized
VB [W5+(d1)]
W5+ + W6+ fi W6+ + W5+
COLORS in Solid Materials
COLORS!
Color is the result of the combination of
wavelengths that are transmitted
Absorbed radiation can be reemitted as excited
electrons drop back into original positions ? not
necessarily the same frequency as that absorbed
Specific impurities can introduce electron levels
within the band-gap - leads to color
e.g. Al2O3 - very pure - single crystal - colorless
add Cr2O3 to Al2O3 will become deep red - RUBY
Impurities in Ceramics
Corundum:Al2O3
Ruby, sapphire, topaz,
amethyst
Colorless in pure form
Impurities result in
different colors
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Impurities
§ Visible light would normally transmit
completely, but impurities can yield color
Eg > 3.1 eV
Opacity and Translucency
§ Most ceramics are insulators (Large Eg) ?
Why are they usually opaque and white?
Why Are Most Ceramics Opaque? ??
Internal reflectance and transmittance
Grain boundaries
anisotropic n (different grains at different
orientations)
both refraction and reflection occur
Two phase materials cause scattering
when there is a difference in n, the greater the
difference, the greater the scattering
e.g. porosity is very effective in scattering light
When particles are about the average
wavelength of visible light, all light wavelengths
are refracted, yielding a whitish, opaque color.
Why are Metals Shiny?
Metals Eg = 0 eV
All light with λ above X-ray wavelengths absorbed by
continuous unoccupied states above Ef.
Light is reemitted with exact energy of absorption as
electrons fall back into lowest state. Metals appear
reflective as the light we see is reemitted.
emissionabsorption
Ef
Applications of Optical Phenomena
Luminescence
Materials which are capable of absorbing energy
(light, heat, electron beam) then re-emitting
visible light
Time of delay between absorption of energy and
reemission varies
fluorescence (less than one second)
phosphorescence (greater than one second)
Television!
LED’s
la·ser n.
a device that utilizes the natural oscillations
of atoms or molecules between energy levels
for generating coherent electromagnetic
radiation usually in the ultraviolet, visible, or
infrared regions of the spectrum
l(ight) a(mplification by) s(timulated) e(mission of) r(adiation)
Applications of Optical Phenomena
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Laser??“Light Amplification by
Stimulated Emission of Radiation”
Coherent, high intensity light beams
Electron transitions are initiated by an
external stimulus (as opposed to
spontaneous emission)
e.g. Ruby laser (Ruby is 0.05%Cr2O3 in
Al2O3)
Cr provides electrons states for single
wavelength emission
The First Ruby Laser
The ruby laser is the first type of
laser actually constructed, first
demonstrated in 1960 by T. H.
Maiman. It is the symbol of
naissance of laser techniques.
Typical Setup of a Laser
Pump process
Optical
feedback
Optical
feedback
Output coupling
Optical amplification
(Optical gain) The light from a typical laser emerges in an extremely thin
beam with very little divergence. The high degree of
collimation arises from the fact that the cavity of the laser has
very nearly parallel front and back mirrors which constrain the
final laser beam to a path which is perpendicular to those
mirrors. The back mirror is made almost perfectly reflecting
while the front mirror is about 99% reflecting, letting out about
1% of the beam. This 1% is the output beam which you see. But
the light has passed back and forth between the mirrors many
times in order to gain intensity by the stimulated emission of
more photons at the same wavelength. If the light is the
slightest bit off axis, it will be lost from the beam.
Lasers: Light Amplification by
Stimulated Emission of Radiation
Shedding Some Light
How the Laser Works
The laser in it’s non-lasing stateThe flash tube fires light at the ruby rod. The light excites
the atoms.
Flash Tube
Partially
Reflective
MirrorMirrored Surface
Atoms
become excited
Emitted Light
Some of
these atoms
emit photons.
Shedding Some Light
How the Laser Works Cont.
Some of these photons run in a direction parallel to the
ruby's axis, so they bounce back and forth off the mirrors.
As they pass through the crystal, they stimulate emission in
other atoms.
Monochromatic, single-phase, columnated
light leaves the ruby through the half-silvered mirror ?
laser light!
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The laser is an example of a technology that was
developed long before its applications were ever imagined.
Forty-five years ago Arthur Schawlow and Charles
Townes could have had no possible way of knowing the
profound effects this technology would have on society or
the widespread applications it would have in a variety of
industries. The impact of this invention is usually not
realized because many consider laser technology a
futuristic idea depicted in Hollywood movies or science
fiction books. Most people don’t recognize that laser
technology is already present as an integral part of our
daily lives, allowing us to listen to CD’s, watch DVD’s, and
play computer games. Additionally, lasers are becoming
increasingly visible in medicine in ophthalmologic,
cosmetic, and general surgery.
Flashes of Brilliance
The History of the Laser
“A splendid light has dawned on me”
–Albert Einstein
In 1917 Einstein published ideas on stimulated emission of
radiation. These ideas laid the basic foundation for the
invention of the laser years later. While investigating what
is now known as the photo-electric effect, Einstein noted a
statistical tendency which caused photons (particles of
light), to want to move together. In addition, emitted
photons displayed a sort of snowball effect, once emitted,
photons stimulated other atoms to emit more photons.
Einstein was also able to prove that these emitted photons
all traveled in the same direction and with the same
frequency as the original photon.
Flashes of Brilliance
From Maser to Laser
AL Schawlow CH Townes
The laser is credited as being
invented in 1958 by Charles H.
Townes and Arthur L.
Schawlow. Townes coined the
term “laser”with help from his
students. The main
differentiating factor between
the two devices is that the
laser uses light waves as
opposed to the microwaves
utilized by the maser.
Laser = + +Einstein’sTheories Right typeof atoms ReflectingMirrors
One important application of semiconductor
devices is in lasers which are intense sources of
single frequency electromagnetic radiation
* lasers exploit an important process known
as stimulated emission in which the emission of
a photon at a given frequency stimulates the
emission of other photons at the same
frequency
? an important property of stimulated
emission is that the resulting photons emitted
in this process form a highly coherent beam
Lasers
An example of stimulated emission
in an electronic transition between
two levels in an atom
a photon whose energy matches
exactly the energy difference of the
two levels stimulates the electron in
the higher energy state to undergo a
transition to the lower energy state
in order to conserve energy, two
photons are emitted in this process
and these photons are highly
coherent
the photons may be used to induce
stimulated emission between the
same two levels in other atoms
giving rise to an avalanche effect
PHOTON
PHOTON
PHOTON
Ruby Laser
The ruby laser is the first type of laser actually
constructed, first demonstrated in 1960 by T. H. Maiman.
The ruby mineral (corundum) is aluminum oxide with a
small amount(about 0.05%) of chromium which gives it its
characteristic pink or red color by absorbing green and blue
light.
The ruby laser is used as a pulsed laser, producing red
light at 694.3 nm. After receiving a pumping flash from the
flash tube, the laser light emerges for as long as the excited
atoms persist in the ruby rod, which is typically about a
millisecond.
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earth
A pulsed ruby laser was
used for the famous laser
ranging experiment
which was conducted
with a corner reflector
placed on the Moon by
the Apollo astronauts.
This determined the
distance to the Moon
with an accuracy of about
15 cm.
Pumping Levels for Ruby Laser
Neodymium-YAG Laser
An example of a solid-state laser, the neodymium-YAG uses
the Nd3+ ion to dope the yttrium-aluminum-garnet (YAG) host
crystal to produce the triplet geometry which makes population
inversion possible. Neodymium-YAG lasers have become very
important because they can be used to produce high powers.
Such lasers have been constructed to produce over a kilowatt of
continuous laser power at 1065 nm and can achieve extremely
high powers in a pulsed mode.
Neodymium-YAG lasers are used in pulse mode in laser
oscillators for the production of a series of very short pulses for
research with femtosecond time resolution.
Helium-Neon LaserThe most common and
inexpensive gas laser, the
helium-neon laser is usually
constructed to operate in the
red at 632.8 nm. It can also be
constructed to produce laser
action in the green at 543.5
nm and in the infrared at
1523 nm.
One of the excited levels of
helium at 20.61 eV is very
close to a level in neon at
20.66 eV, so close in fact that
upon collision of a helium and
a neon atom, the energy can
be transferred from the
helium to the neon atom.
Laser Applications
Science
investigation of properties of gases, fluids, solids, plasma,
BEC (Bose-Einstein condensate), ignition of nuclear fusion,
find gravitational waves
Precision measurements
future atomic clocks, navigation, geophysics, astrophysics
Material processing
welding, cutting, drilling, surface hardening, from micro to
nano
Biology and medical
diagnosis & therapy (retina, cancer… ), DNS-structure, cell-
content
Information technology
data transmission, optical computing, laser display, optical
storage
Lasers & Industry
The Cutting Edge
The advantages of Laser technology over
mechanical processes are substantial: The cuts are
more precise and reduce raw material losses.
Laser technology has taken the jewelry industry
The superior cutting accuracy and precision which
have contributed to it’s success as a medical tool are
also highly desirable for this industry.
Laser welding can be automated for high-precision
tasks. The very high cutting speed makes it possible
to produce "haute couture" jewelry at industrial
prices
The process is cleaner
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Examples of Laser Applications
Precision
Global
monitoring
Energy
Science
earth
Medical analysis
Micro-chips
Ranging
Communications
More Examples
Lasers & Medicine
Going where no man has gone before
Advantages for the Laser as a Medical Cutting
Tool
qGreater accuracy of incisions
qLasers can be inserted inside the body with
little risk or discomfort
qIncisions can be guided by computers
qThe laser is extremely precise, and can be
tuned to work on a micro-level, barely visible to
the human eye
Lasers & Medicine
What is Laser Surgery?
The goal in laser eye surgery is to reshape the cornea,
changing the focal point of the eye. Ideally the focal
point is changed so it focuses perfectly on the retina.
If you are
nearsighted, the
image comes into
focus before it hits
your retina
If you are
farsighted, the
image doesn't come
into focus before it
hits your retina
In a good eye the
image is focused
on the retina
Lasers & Entertainment
The Light and the dark side
One of the most popular applications of laser technology, the
Compact Disc Player, marked a revolution in digital video
and sound technology.
Lasers & Entertainment
How a CD Works
The CD Player works by
using a laser beam to
determine the lengths of
a series of tiny ridges
inside a compact disk.
Inside a CD Player
The music is digitally
encoded in the ridge lengths
which are measured by the
reflected laser light.
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mechanics: 5$
batteries: 5$
display etc. 5$
electronics: 5$
profit:10$
Lasers = enabling technology
5 Ct-laser: enables clean sound and data storage
Can You See the Light?
Dentists use
laser drills
Bad eyesight can be
corrected by optical
surgery using lasers
CD -Audio is
read by a laser
Tattoo removal is
done using lasers
CD -Rom discs are
read by lasers
Laser pointers
can enhance
presentations Bar codes in
grocery stores are
scanned by lasers
Video game systems such as
PlayStation 2 utilize lasers
DVD players read
DVD ’s using lasers
Airplanes are
equipped with
laser radar
Military and Space
aircraft are equipped
with laser guns
Laser tech. is used in printers,
copiers, and scanners
Financial Position of Lasers
Dow-Jones
0
2
4
6
8
10
1997 1998 1999 2000 2001
manufacturing & medical applications
data storage & telecombillion $
2002
Lasers in Science and Engineering
0
100000
200000
300000
1950 1960 1970 1980 1990 2000 2010
Physics
Engineering
1st laser
Number of publications
now
P = εχεE (For linear medium )
Where ε is permitivity of medium and χε is electric
susceptibility
P = ε(χ1 E + χ2 E2 + χ3 E3 +… .) (For nonlinear
medium)
Where χ1 is linear susceptibility tensor, χ2 is
quadratic tensor (3X3X3 matrix) and χ3 is cubic
tensor (cubic matrix with 3X3 matrix at each
lattice point).
Nonlinear Optical Effect
linear
nonlinear
P
E
Second Harmonic
Generation
1961 Franken found the second harmonic generation
which generate a new field ?? nonlinear optics.
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Polarizers
Lenses
w
SHG Experimental Setup
Second Harmonic Generation
KDP crystal
ωRuby laser
694 nm
2ω 347 nm
Ti:Sapphire laser
l=700 -920nm
tp=100fs
Pav=400mW
RG-Filter
Sample
Rotation Table
UG-Filter
Iris
PMT
Prism
2w @ 3eV
Second Harmonic Generator of
Ba2NaNb5O15
Frequency Doubling / Tripling etc.
Proustite
crystal
ω3 = ω2+ω1
0.96 μm
Nd3+:YAG
1.06 μm
CO2 laser
10. 6 μm
Not all frequencies are generated; phase
matching condition is required
Assumption of linearity of the optical medium ?
Optical properties are independent of light intensity,
such as refractive index, absorption coefficient, etc;
Principle of superposition holds!
The frequency of light cannot be altered by passing
through a medium.
Light cannot interact with light!
At high intensity light ? nonlinearity phenomena
observed!
Nonlinearity originates from the interaction of light
via the medium only (not free space)! Presence of light
modified the medium ? in turn modify another optical
field or even the original field itself.
Nonlinear Optics
Laser beam enters quartz crystal as red light and emerges
as blue light (a second order NLO effect: second harmonic
generation)
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Nonlinear Optics Produces
Many Exotic Effects
Sending infrared light into a
crystal yielded this display of
green light:
Nonlinear optics allows us to
change the color of a light beam,
to change its shape in space and
time, to switch
telecommunications systems,
and to create the shortest
wavelength light ever made by
Man.
Why Do Nonlinear-optical Effects Occur?
Recall that, in normal linear optics, a light wave
acts on a molecule, which vibrates and then emits
its own light wave that interferes with the original
light wave.
We can also imagine this
process in terms of the
molecular energy levels, using
arrows for the photon energies:
Why Do Nonlinear-optical Effects Occur?
(Continued)
Now, suppose the irradiance is high enough that many
molecules are excited to the higher-energy state. This state
can then act as the lower level for additional excitation. This
yields vibrations at all frequencies corresponding to all
energy differences between populated states.
The invention of lasers led to
the discovery of interesting
nonlinear optical phenomena
in inorganic as well as organic
materials. Nonlinear optical
effects in organic materials
were reported about three
decades ago and the
importance of novel materials
was realized through theory
models and synthesis. The
extraordinary growth and
development of NLO
materials during the past
decade has made photonic
technologies an indispensable
part of our daily life as we
enter the 21st century, the
"INFORMATION AGE".
Nonlinear Optics Effect and NLO Materials
Nonlinear optics describes many interactions
where the intensity of the optical wave changes
the optical properties of a material.
NLO materials can be used to double the
frequency of laser radiation Useful as short
wavelength.
Lasers can be difficult to make short
wavelength radiation better for information
storage and transmission.
Nonlinear optic (NLO) crystals are used for
harmonic generation, including frequency
doubling (SHG), tripling (3HG), frequency
mixing; OPO(Optical Parametric Oscillator) and
OPA(Optical Parametric Amplifier). Combining
harmonic generation and frequency mixing, we
can generate the 4th, 5th, or even 15th
harmonics from the original laser output.
Pulsed Laser Oscillator
The Neodymium-YAG laser consists of a rod of
the material which can be pumped by a flash lamp
at a rate of about 15 Hz. The output consists of an
envelope of pulses which can be tuned for
optimization by adjusting the mirrors, adjusting
the prisms to change optical pathlength, adjusting
the crystal in the acoustic-optic modulator, and
adjusting the frequency of the modulator.
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NLO Crystals
KDP (KH2PO4), DKDP (KD2PO4) and ADP
(NH4H2PO4)， α-Lithium Iodate (α-LiIO3)
β-Barium Borate (β-BaB2O4 or BBO)， Lithium
Triborate (LiB3O5 or LBO)， Cesium Lithium Borate
(CsLiB6O10 or CLBO)
Potassium Titanyl Phosphate (KTiOPO4 or KTP)，
Potassium Titanyl Arsenate (KTiOAsO4 or KTA)
Potassium Pentaborate (KB5O8?4H2 O or KB5).
Tellurium Dioxide (TeO2 or Paratellurite), Lead
Molybdate (PbMoO4 or PM)
Silver Thiogallate (AgGaS2) and Silver Selenogallate
(AgGaSe2), Mercury Thiogallate （ HgGa2S4） ,
Lithium Thioindate (LiInS2 or LIS)
Zinc-Germanium Diphosphide (ZnGeP2), Gallium
Selenide (GaSe)
Bismuth Silicon Oxide (Bi12SiO20 or BSO) and
Bismuth Germanium Oxide (Bi12GeO20 or BGO)
Lithium Niobate (LiNbO3 or LNB) and Lithium
Tantalate (LiTaO3 or LTA)
Lithium Tetraborate (Li2B4O7)
Barium Nitrate Ba (NO3)2
NLO Crystals
Lithium Iodate
α-Lithium Iodate (α-LiIO3) crystal is an
uniaxial non-linear crystal with high nonlinear
optical coefficients and wide transparency range.
It is used for frequency doubling of the low and
medium power.
(a)IO3- structure
(b)LiIO3 structure
From second to forth harmonic generations of the
fundamental laser emission in the range from 690 to
2000nm
Optical parametric oscillation, obtaining the tuned
radiation in the ranges from 800 to 4000nm
Frequency multiplication and mixing in transparency
crystal range from 280 to 5500nm
Measurement of parameters of ultra -short laser
pulses including of the single ones
Visualization of IR radiation to obtain the object
image by non-linear optical methods
Applications of Lithium Iodate
BBO (b-Barium Borate)
§β-Barium Borate (β-BaB2O4 or BBO) is an
excellent optical nonlinear crystal. It exhibits broad
phase matching range, high nonlinearity (about 6
times more than that of KH2PO4), high optical
damage threshold, good mechanical and
temperature stability. This trigonal uniaxial
crystal possesses wide transparency range from
190 nm up to 2500 nm.
BBO (b-Barium Borate) Structure
β?Barium Borate (Low
temperature phase) :
a＝ 12.547?， c＝ 12.736?，
Every cell have 6
[Ba3(B3O6)2] molecules, thus
total 12 (B3O6)3- planer ring.
This structure result from
the order stacking of (B3O6)3-
group．
α?Barium Borate: (High
temperature phase),
transition temperature is
925± 5°C. Center of
Symmetry, no NLO effect.
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Applications:
§For harmonic generation (SHG, THG, 4HG,
5HG) of Nd:YAG laser, SHG, THG of Ti:
Sapphire, Alexandrite lasers
§For tunable solid state lasers using OPO
(pumped by 355, 532 or 1064 nm)
Main Properties:
§Transparency range, 190 –2500nm
§Point group: 3m
BBO (b-Barium Borate)
