mse-21-optical properties%2861%29-2007-05-31
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Introduction to
Materials Science and EngineeringChapter 21. OPTICAL PROPERTIES
What happens when light shines on a material?
Why do materials have characteristic colors?
Why are some materials transparent and others not?
Optical applications:
Luminescence
Photoconductivity
Solar cell
Laser
Optical communication fibers
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Contents
1 Electromagnetic RadiationElectromagnetic Radiation
Light Interactions with SolidsLight Interactions with Solids2
Optical Properties of MetalsOptical Properties of Metals3
Optical Properties of NonmetalsOptical Properties of Nonmetals4
5 ApplicationsApplications
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Index of refraction - Relates the change in velocity and direction
of radiation as it passes through a transparent medium (also knownas refractive index). Ratio of the velocity of light in vacuum to
the velocity of light in the material
Dispersion - Frequency dependence of the refractive index.
Linear absorption coefficient - Describes the ability of a material
to absorb radiation.
Absorption constant: the reciprocal of the absorptioncoefficient is a measure of how far the light will travel before
being reduced by a factor of exponential.
Penetration depth: the distance with 1/e reduction in intensity Reflectivity - The percentage of incident radiation that is
reflected.
Photoconduction - Production of a voltage due to the stimulation ofelectrons into the conduction band by light radiation.
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Luminescence - Conversion of radiation to visible light.
Fluorescence - Emission of light obtained typically within ~10-8
seconds.
Phosphorescence - Emission of radiation from a material after the
stimulus is removed.
Light-emitting diodes (LEDs) - Electronicp-njunction devices that
convert an electrical signal into visible light.
Electroluminescence - Use of an applied electrical signal to
stimulate photons from a material.
Laser - The acronym stands for light amplification by stimulatedemission of radiation. A beam of monochromatic coherent radiation
produced by the controlled emission of photons.
Thermal emission - Emission of photons from a material due toexcitation of the material by heat.
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Introduction Optical Properties - A materials response to exposure to
electromagnetic radiation, particularly to visible light.
Light is energy, or radiation, in the form of waves or particles
called photons that can be emitted from a material. The important characteristics of the photons energy E,
wavelength , and frequency are related by the equation:
hcE h
= =
0 electric permittivity of a vacuum0 Magnetic permeability of a vacuum
C =
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Electromagnetic Spectrum
400 nm - 700 nm
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Contents
1 Electromagnetic RadiationElectromagnetic Radiation
Light Interactions with SolidsLight Interactions with Solids2
Optical Properties of MetalsOptical Properties of Metals3
Optical Properties of NonmetalsOptical Properties of Nonmetals4
5 ApplicationsApplications
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Light Interaction with Solids Incident light is either reflected, absorbed, or transmitted.
I0 = IT+ IA + IR
Incident: I o
Reflected : IRAbsorbed : IA
Transmitted : IT T+ A+ R= 1Transmissivity (IT/I0)
Absorptivity (IA/I0)
Reflectivity (IR/I0)
Optical classification of materials
translucenttransparent
opaque
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Light Interaction with Solids
ReflectionAbsorption
Transmission
Refraction
Absorptionindex
opaque
transparent
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Light Interaction with Solids
R: reflectance
X
exp( )oo
dI
dx I I xI = = : absorption coefficient
II0
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T, A, and R
green glasses
b l d &
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Interaction between electromagnetic radiation &atoms/ions/electrons
Polarizationelectronic ionic
Electron transitions
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Light Interaction with Solids Electronic polarization- Some of the radiation energy may be absorbed.- Light waves are retarded in velocity as they pass through
the medium (manifested as refraction).
Electron transitions
E h =
- Absorption & emission- Discrete, specific energy
- Short stay in an excited
state - decay back into itsground state
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Contents
1 Electromagnetic RadiationElectromagnetic Radiation
Light Interactions with SolidsLight Interactions with Solids2
Optical Properties of MetalsOptical Properties of Metals3
Optical Properties of NonmetalsOptical Properties of Nonmetals4
5 ApplicationsApplications
b l
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Absorption in metals Absorption of photons by electron transition
Energy of electron
Incid
entph
oton
Planck constant
(6.63 x 10 -34 J/s)
freq.
of
incidentlight
filled states
unfilled states
E =hrequired!
Io ofenergy
h
Metals have a continuously available empty e states, which permit e transitions.
Near-surface electrons absorb visible light.
fl l
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Reflection in metals
Electron transition emits a photonEnergy of electron
filled states
unfilled states
E
IR onducting?electron
Re-emittedphoton frommaterial surface
Reflectivity = IR/I0 is between 0.90 and 0.95.
Reflected light has same frequency as incident.
Metals are opaque & highly reflective (shiny).
l P f l
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Optical Properties of Metals
- Reemit in the form of visible light of same wavelength
(below a metal plasmon energy)
- Reflectivity: 0.90 - 0.95
C t t
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Contents
1 Electromagnetic RadiationElectromagnetic Radiation
Light Interactions with SolidsLight Interactions with Solids2
Optical Properties of MetalsOptical Properties of Metals3
Optical Properties of NonmetalsOptical Properties of Nonmetals4
5 ApplicationsApplications
O ti l P ti f N t l
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Optical Properties of Nonmetals
r
- refractive index
sin(snell's law)sin
- wavelength dependent
(dispersion)
-
for nonmagnetic 1
vacr r
mat o o
r
in
r
vn
v
n
=
= = =
Refraction
O ti l P ti f N t l
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Optical Properties of Nonmetals
Refraction
- Refraction is related to electronic polarization at the
relatively high frequencies for visible light. Electronic component of the dielectric constant may be
determined from the index of refraction measurements.
- Electronic polarization retard electromagnetic radiation
The greater the electronic polarization -> the slower the velocity
-> the greater the index of refraction
ex) soda-lime glass n = 1.5
90 wt.% PbO containing glass n = 2.1
O ti l P ti f N t l
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Optical Properties of Nonmetals Polarizability
O ti l P ti s f N m t ls
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Optical Properties of Nonmetals
Optical Properties of Nonmetals
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Optical Properties of Nonmetals
Dispersion
Optical Properties of Nonmetals
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- antireflective coating-microscope, telescopeOptical Properties of Nonmetals
Antireflective coating for lenses and other opticalinstruments.
Optical Properties of Nonmetals
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Optical Properties of Nonmetals
Absorption- valence band-conduction band transition
(energy band structure)
electron-holegeneration
electron-holerecombination
Optical Properties of Nonmetals
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Optical Properties of Nonmetals
Absorption- Valence band-conduction band transition can take place
only if the photon energy is greater thanthat of the band gap energy Eg.
org ghc
h E E
> >
- for visible light
- Eg less than 1.8 eV - all visible light absorb - opaque
1.8 eV < Eg < 3.1 eV - partial absorption - color
0.7 (=1.8 eV) ~ 0.4 (=3.1 eV)m m
Selected Absorption: Nonmetals
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Selected Absorption: Nonmetals Absorption by electron transition occurs if h > Egap
Energy of electron
filled states
unfilled states
Egap
Io
blue light: h = 3.1eV
red light: h = 1.7eV
If Egap
< 1.8 eV, full absorption of visible light color is black
Si (1.12 eV), GaAs (1.42 eV)
If Egap > 3.1 eV, no absorption transparent & colorless
Diamond (5.6 eV) If Eg in between, partial absorption - material has a color
incident photon
energy hn
400nm = 3.1 eV
700nm = 1.8 eV
Optical Properties of Nonmetals
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Optical Properties of Nonmetals
AbsorptionImpurities or other electrically active defects
Absorption and Energy gap
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Absorption and Energy gap
metals
Dielectrics and intrinsic
semiconductors
Extrinsic (doped)semiconductors
Optical Properties of Nonmetals
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Optical Properties of Nonmetals
Absorption ' '
'exp( )
: intensity of nonreflected incident
radiation
4: absorption coefficient ( )
T o
o
I I xI
k
=
=
TIoI
x
'
T
'
o
ex) The fraction of nonreflected light that is transmitted through
a 200 mm thickness of glass is 0.98. Calculate the absorption
coefficient of this material.
1 I 1
=- ln( )=- ln(0.98)=1.01x I 200mm
solution
-4 -1
x10 mm
Optical Properties of Nonmetals
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Optical Properties of Nonmetals
Transmission
Transmitted Light: Refraction
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Transmitted Light: Refraction Transmitted light distorts electron clouds
+
no
transmitted
light
transmitted
light +
electroncloud
distorts
Light is slower in a material vs. vacuum.
Index of refraction (n) = speed of light in a vacuumspeed of light in a material
MaterialLead glassSilica glassSoda-lime glass
QuartzPlexiglasPolypropylene
n2.11.461.51
1.551.491.49
- Adding large, heavy ions (e.g., lead)
can decrease the speed of light.
- Light can be "bent."
Color of Materials
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Color determined by sum of frequencies of
Transmitted light
Re-emitted light from electron transitions
Optical Properties of Nonmetals
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p p
Color
As a consequence of selective absorption of specific wavelengthranges of light.
If absorption is uniform for all visible wavelength, the material
appears colorless (inorganic glass, diamond, sapphire). Selective absorption by electron excitation.
Example - Cadmium Sulfide (CdS)
Eg = 2.4 eV
absorb photons > 2.4 eV (blue-violet portion)
Re-radiate other wavelength
Red/yellow/orange is transmitted and gives it color
Optical Properties of Nonmetals
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p p Color impurities - electron level within the forbidden bandgap
ex) sapphire (Al2O3) colorless (Egap > 3.1eV)
ruby (0.5 to 2% Cr2O3 doped Al2O3) - red color
Adding Cr2O3 to sapphire:- Alters the band gap, blue light is absorbed, yellow/green is
absorbed, red is transmitted Ruby is deep red in color
Optical Properties of Nonmetals
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Color
impurities - transition or rare earth ions in
inorganic glasses
Blue color
1% cobalt oxide containing
silicate glass (Co2+
)
Optical Properties of Nonmetals
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p p
Opacity and translucency- Internal reflection and refraction- Scattering
- Polycrystalline - grain boundary
- Two phase materials with different refractive indices- Porosity with finely dispersed pores
porous alumina
fully dense polycrystalline
single crystal sapphire
Contents
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1 Electromagnetic RadiationElectromagnetic Radiation
Light Interactions with SolidsLight Interactions with Solids2
Optical Properties of MetalsOptical Properties of Metals3
Optical Properties of NonmetalsOptical Properties of Nonmetals4
5 ApplicationsApplications
Application: Luminescence
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Luminescence: Light emission in the visible spectrum
accompanying the absorption of other forms
of energy (thermal, mechanical, chemical
or particles (high energy electrons)(photoluminescence, electroluminescence).
Fluorescence: Emission of electromagnetic radiation that
occurs within ~10-8 s of an excitation event.
Phosphorescence: Emission of electromagnetic radiation
over an extended period of time after theexcitation event is over.
Luminescence
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electron
transition occurs
Energy of electron
filled states
unfilled states
Egap
Energy of electron
filled states
unfilled states
Egap
re-emission
occurs
Process:
Ex: fluorescent lamps
UV
radiation
coating
e.g., -alumina
dopedw/Europium
glass
incident
radiation
emittedlight
Photo-luminescence (PL), Electro-L (EL)
White light
Luminescence(a)
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(a)
Luminescence occurs when
photons have a wavelength in the
visible spectrum.
(a) In metals, there is no energy
gap, so luminescence does not occur.
(b) Fluorescence occurs when there
is an energy gap.
(c) Phosphorescence occurs when
the photons are emitted over a
period of time, due to donor traps in
the energy gap.
(b)
(c)
Luminescence
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A fluorescent lamp is a type of lamp that uses electricity
to excite mercury vapor argon neonin or gas, resulting in a
plasma that produces short-wave ultraviolet light. This
light then causes a phosphor fluoresceto , producingvisible light.
Light Emitting Diode (LED)
http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Mercury_%28element%29http://en.wikipedia.org/wiki/Vaporhttp://en.wikipedia.org/wiki/Argonhttp://en.wikipedia.org/wiki/Neonhttp://en.wikipedia.org/wiki/Plasma_%28physics%29http://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Phosphorhttp://en.wikipedia.org/wiki/Fluorescencehttp://en.wikipedia.org/wiki/Lighthttp://upload.wikimedia.org/wikipedia/en/f/fb/FluorescentLight.jpghttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Fluorescencehttp://en.wikipedia.org/wiki/Phosphorhttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Plasma_%28physics%29http://en.wikipedia.org/wiki/Neonhttp://en.wikipedia.org/wiki/Argonhttp://en.wikipedia.org/wiki/Vaporhttp://en.wikipedia.org/wiki/Mercury_%28element%29http://en.wikipedia.org/wiki/Electricity -
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A forward-bias voltage across thep-njunction can
produce photons.
Photoconductivity
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Additional charge carriers can be generated by photon-induced e
transition in which light is absorbed.
The resultant increase in conductivity is photoconductivity.
Incident
radiation
semi
conductor:
Energy of electron
filled states
unfilled states
Egap
+
-A. No incident radiation:
little current flow
Energy of electron
filled states
unfilled states
Egap
conducting
electron
+
-B. Incident radiation:
increased current flow
Description:
Ex: Photodetector (Cadmium sulfide)
Photoconduction & Solar Cell
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Photoconduction in semiconductors
involves the absorption of a stimulus
by exciting es from the valence band
to the conduction band.
Rather than dropping back to the
valence band to cause emission, the
excited electrons carry a charge
through an electrical circuit.
A solar cell takes advantage of this effect.Operation is the reverse of that for LED.
p-nJunction Solar Cell
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p
Solar Cell
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Operation:- incident photon produces hole-e pair- typically 0.5 V potential
- current increases with light intensity
p-njunction:
n-type Si
p-type Sip-n junction
B-doped Si
Si
Si
Si SiB
hole
P
Si
Si
Si Si
conductance
electron
P-doped Si
n-type Si
p-type Sip-n junction
light
+-
++ +
---
creation of
hole-electron
pair
Solar powered weather station:
polycrystalline Si
Solar Cell
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( ) 1,100M
44, 435
Laser
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Light Amplification by Stimulated Emission of Radiation
Coherent beam - monochromatic
Collimation - pumping and population inversion
Communication, surgery, machining, welding, heat treating, CDs,
bar-code reading, hole piercing, ----------
GaAs Laser
http://en.wikipedia.org/wiki/Image:Military_laser_experiment.jpghttp://en.wikipedia.org/wiki/Image:Starfield_Optical_Range_-_sodium_laser.jpg -
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Because the surroundingp-and n-type GaAlAs layers have a higherenergy gap and a lower index of refraction than GaAs, the photons
are trapped in the active GaAs layer.
Solid State Ruby Laser Al O i l t l ( hi )
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Al2O3 single crystal (sapphire)
with 0.05 wt% Cr
Laser
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1. The laser in its non-lasing state
2. The flash tube fires and injects
light into the ruby rod.
The light excites atoms in the ruby.
3. Some of these atomsemit photons
Laser4 Some of these photons run in a
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4. Some of these photons run in a
direction parallel to the rubys axis, sothey bounce back and forth off the
mirrors. As they pass through the
crystal, they stimulate emission in
other atoms
5. Monochromatic, single-phase,
colliminated light
leaves the ruby through the
half-silvered mirror.
-- laser light!
Laser Semic nduct r laser
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Semiconductor laser
Laser Semiconductor laser
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Because the surroundingp- and n-typeGaAlAs layers have a higher energy
and a lower index of refraction than
GaAs, the photons are trapped in theactive GaAs layer.
Semiconductor laser
Laser
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Fiber Optics and Data Transmission Photonic communication
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Photonic communication
Total internal reflection
Core/cladding/coating
High purity silica glass 5 - 100 um
144 glass fibercarry three times
Fiber Optics
t i d ti l fib d i
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step-index optical fiber designcore: silica glassw/higher n
cladding : glassw/lower n
n enhancesinternal reflection
intensity
time
input pulse
broadened!
intensity
time
out put pulsetotal internal reflection
shorter pathlonger paths
graded-index optical fiber designcore: Add gradedimpurity distrib.to make n higher incore center
cladding : (as before)
total internal reflection
shorter, but s lower pathslonger, but faster paths
intens
ity
time
input pulse
intens
ity
time
out put pulse
less
broadening!
graded-index less broadening improvement
Summary When light (radiation) shines on a material, it may be:
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Reflected, absorbed and transmitted.
Optical classification:
Transparent, translucent, or opaque
Metals:
Fine succession of energy state causes absorption and reflection.
Non-Metals:
May have full (Eg < 1.8 eV) , no(Eg > 3.1 eV), or
partialabsorption (1.8 eV < Eg < 3.1 eV) Color is determined by light wavelengths that are
transmitted or re-emitted from electron transitions.
Color may be changed by adding impurities that the band structure. (e.g.,
Ruby)
Problems from Chap. 21 http://ep.snu.ac.kr
Prob. 21-1 Prob. 21-2 Prob. 21-4 Prob. 21-7
Prob. 21-12 Prob. 21-20 Prob. 21-23 Prob. 21-28
http://ep.snu.ac.kr/http://ep.snu.ac.kr/