optical-prop [compatibility mode]
TRANSCRIPT
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ISSUES TO ADDRESS...
What happens when light shines on a material?
Why do materials have characteristic colors?
Chapter 20: Optical Properties
Chapter 20 - 1
Optical applications:-- luminescence
-- photoconductivity
-- solar cell-- optical communications fibers
Why are some materials transparent and others not?
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Optical PropertiesLight has both particulate and wavelike properties
Photons - with mass
==hc
hE
Chapter 20 - 2
m/s)10x(3.00lightofspeedc
)sJ1062.6(constantsPlanck'
frequency
wavelength
energy
8
34
=
=
=
=
=
xh
E
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Refractive Index, n
vacuum)inlightof(velocityc
Transmitted light distorts electron clouds.
Light is slower in a material vs vacuum.
=
+no
transmittedlight
transmittedlight
+
electronclouddistorts
Chapter 20 - 3
Note: n= f ()
Typical glasses ca. 1.5 -1.7
Plastics 1.3 -1.6PbO (Litharge) 2.67
Diamond 2.41
medium)inlightof(velocityv
Selected values from Table 20.1
Callisters Materials Science andEngineering, Adapted Version.
--Adding large, heavy ions (e.g., leadcan decrease the speed of light.
--Light can be
"bent"
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Total Internal Reflectance
=
sin
sin
n
nn(low)
n(high)
n> n
'1
Chapter 20 - 4
1
c
anglecritical
anglerefracted
angleincident
=
=
=
c
i
i
reflectedinternallyislightfor
90whenoccurs
ci
ic
>
=
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Example: Diamond in air
sin
1
sin
90sin
1
41.2
sin
sin
==
=ccn
n
Chapter 20 - 5
Fiber optic cables are clad in low nmaterial for thisreason.
5.2441.2
sin == cc
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Incident light is either reflected, absorbed, ortransmitted: SRATo IIIII +++=
Light Interaction with Solids
Incident: I0
Absorbed: IA
Transmitted: IT
Reflected: IR
Chapter 20 - 6
Optical classification of materials:
From Fig. 20.10, Callisters MSEAdapted Version.
(Fig. 20.10 is by J. Telford, withspecimen preparation by P.A.Lessing.)
single
crystal
polycrystalline
dense
polycrystalline
porous
TransparentTranslucent
Opaque
Scattered: IS
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Absorption of photons by electron transition:
Optical Properties of Metals:
Absorption
Energy of electron
unfilled states
E= h required!
Chapter 20 - 7
Metals have a fine succession of energy states. Near-surface electrons absorb visible light.
From Fig. 20.4(a)
Callisters Materials Science andEngineering, Adapted Version.
Plancks constant
(6.63 x 10-34
J/s)
freq.of
incidentlight
filled statesIo
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Light Absorption
t
I
I = e
0 thicknesssamplecm][tcoefficienabsorptionlinear
1
===
t
Chapter 20 - 8
tI
ln0
=
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Optical Properties of Metals:
Reflection Electron transition emits a photon.
re-emitted
Energy of electron
unfilled states
E
IR conducting electron
Chapter 20 - 9
Reflectivity = IR/Iois between 0.90 and 0.95. Reflected light is same frequency as incident. Metals appear reflective (shiny)!
From Fig. 20.4(b)Callisters Materials Science and
Engineering, Adapted Version.
photon frommaterial surfacefilled states
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Reflectivity, R
Reflection
Metals reflect almost all light
Copper & gold absorb in blue & green => goldcolor
tyreflectivi1
2
=
=n
R
Chapter 20 -10
17.0141.2
141.22
=
+
=R
reflectedislightof%17
Example: Diamond
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Scattering
In semicrystalline or polycrystalline materials
Semicrystalline density of crystals higher than amorphous
materials speed of light is lower - causes light to
Chapter 20 - 11
sca er - can cause s gn can oss o g
Common in polymers
Ex: LDPE milk cartons cloudy
Polystyrene clear essentially no crystals
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Absorption by electron transition occurs if h > Egap
Selected Absorption: Semiconductors
incident photon
Energy of electron
unfilled statesblue light: h = 3.1 eV
red light: h = 1.7 eV
Chapter 20 -12
If Egap < 1.8 eV, full absorption; color is black (Si, GaAs)
If Egap > 3.1 eV, no absorption; colorless (diamond)
If Egap in between, partial absorption; material has a color.
From Fig. 20.5(a), Callisters MSE Adapted Version.
energy h
filled statesIo
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c =hc
=(6.62 x 1034 J s)(3 x 108m/s)
1.85 m
Wavelength vs. Band Gap
Eg= 0.67 eV
Example: What is the minimum wavelength absorbedby Ge?
Chapter 20 -13
g (0.67eV)(1.60 x 10
J/eV)
note : for Si Eg=1.1eV c1.13 m
If donor (or acceptor) states also available this provides otherabsorption frequencies
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Color determined by sum of frequencies of-- transmitted light,-- re-emitted light from electron transitions.
Ex: Cadmium Sulfide (CdS)-- Egap= 2.4 eV,-- absorbs higher energy visible light (blue, violet),
-- Red/ ellow/oran e is transmitted and ives it color.
Color of Nonmetals
Chapter 20 -14
Ex: Ruby = Sapphire (Al2O3) + (0.5 to 2) at% Cr2O3-- Sapphire is colorless
(i.e., Egap> 3.1eV)
-- adding Cr2O3 :
alters the band gap blue light is absorbed yellow/green is absorbed red is transmitted
Result: Ruby is deepred in color.
From Fig. 20.9, Callisters MSE Adapted Version.(Fig. 20.9 adapted from "The Optical Properties ofMaterials" by A. Javan, Scientific American, 1967.)
40
60
70
80
50
0.3 0.5 0.7 0.9
Transm
ittance(%)
ruby
sapphire
wavelength, (= c/)(m)
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Luminescence
Luminescence emission of light by a material
material absorbs light at one frequency & emits at
another (lower) frequency.
Conduction band
Chapter 20 -15
activatorlevel
Valence band
trappedstatesEg
Eemission
How stable is the trapped state?
If very stable (long-lived = >10-8 s) =phosphorescence
If less stable (
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PhotoluminescenceHg
uv
electrode electrode
Chapter 20 -16
Arc between electrodes excites mercury in lamp to higherenergy level.
electron falls back emitting UV light (i.e., suntan lamp).
Line inner surface with material that absorbs UV, emits visible
Ca10F2P6O24 with 20% of F- replaced by Cl -
Adjust color by doping with metal cations
Sb3+ blue
Mn2+ orange-red
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Cathodoluminescence
Used in T.V. set
Bombard phosphor with electrons
Excite phosphor to high state Relaxed by emitting photon (visible)
ZnS (Ag+ & Cl-) blue
Chapter 20 -17
n, + u++ + green
Y2O2S + 3% Eu red
Note: light emitted is random in phase & direction
i.e., noncoherent
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LASER Light
Is non-coherent light a problem? diverges
cant keep tightly columnated
How could we get all the light in phase? (coherent)
LASERS
Chapter 20 -18
Amplification by Stimulated
Emission of
Radiation
Involves a process called population inversion ofenergy states
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Population Inversion
What if we could increase most species to the excitedstate?
Chapter 20 -19
Fig. 20.14Callisters Materials
Science and Engineering,Adapted Version.
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LASER Light Production
pump the lasing material to the excited state
e.g., by flash lamp (non-coherent lamp).
Chapter 20 -20
If we let this just decay we get no coherence.
Fig. 20.13Callisters MaterialsScience and Engineering,
Adapted Version.
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LASER Cavity
Tuned cavity:
Stimulated Emission
One photon induces theemission of anotherphoton, in phase with the
Chapter 20 -21
.
cascades producing veryintense burst of coherentradiation.
i.e., Pulsed laser
Fig. 20.15Callisters MaterialsScience and Engineering,
Adapted Version..
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Continuous Wave LASER
Can also use materials such as CO2 or yttrium-aluminum-garret (YAG) for LASERS
Set up standing wave in laser cavity
tune frequency by adjusting mirror spacing.
Uses of CW lasers1. Welding
Chapter 20 -22
2. Drilling3. Cutting laser carved wood, eye surgery
4. Surface treatment
5. Scribing ceramics, etc.
6. Photolithography Excimer laser
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Apply strong forwardbias to junction.Creates excited state
by pumping electronsacross the gap-creating electron-holepairs.
Semiconductor LASER
Chapter 20 -23
electron + hole neutral + h
excited state ground state
photon oflight
From Fig. 20.17Callisters MaterialsScience and Engineering,
Adapted Version.
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Uses of Semiconductor LASERs
#1 use = compact disk player
Color? - red
Banks of these semiconductor lasers are used asflash lamps to pump other lasers
Communications
Chapter 20 -24
Fibers often turned to a specific frequency(typically in the blue)
only recently was this a attainable
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Applications of Materials Science
New materials must be developed to make new &improved optical devices.
Organic Light Emitting Diodes (OLEDs) White light semiconductor sources
Chapter 20 -25
New semiconductors
Materials scientists
(& many others) use lasers as tools.
Solar cells
. .Callisters MSE Adapted Version.
(Reproduced byarrangement with Silicon Chipmagazine.)
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Solar Cells p-njunction: Operation:
-- incident photon produces hole-elec. pair.-- typically 0.5 V potential.-- current increases w/light intensity.
P
Si
Si
Si Si
conductance
electron
P-doped Si
n-type Si-
light
----
creation ofhole-electronpair
Chapter 20 -26
Solar powered weather station:
polycrystalline SiLos Alamos High School weatherstation (photo courtesyP.M. Anderson)
n-type Si
P-type Sip-njunction
B-doped Si
Si
Si
Si SiB
hole
p-type Si-
+
++ +
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Optical Fibers
prepare preform as indicated in Chapter 13
preform drawn to 125 m or less capillary fibers
plastic cladding applied 60 m
Chapter 20 -27
Fig. 20.20, Callisters MSE
Adapted Version.
Fig. 20.18, Callisters MSEAdapted Version.
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Optical Fiber Profiles
Step-index Optical Fiber
Chapter 20 -28
Graded-index Optical FiberFig. 21.21, Callister 7e.
Fig. 20.22
Callisters MaterialsScience and Engineering,Adapted Version.
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When light (radiation) shines on a material, it may be:-- reflected, absorbed and/or transmitted.
Optical classification:
-- transparent, translucent, opaque Metals:-- fine succession of energy states causes absorption
and reflection.
SUMMARY
Chapter 20 -29
Non-Metals:-- may have full(Egap< 1.8eV) , no(Egap> 3.1eV), orpartialabsorption (1.8eV < Egap= 3.1eV).
-- color is determined by light wavelengths that are
transmitted or re-emitted from electron transitions.-- color may be changed by adding impurities which
change the band gap magnitude (e.g., Ruby)
Refraction:
-- speed of transmitted light varies among materials.
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Core Problems:
ANNOUNCEMENTS
Reading:
Chapter 20 -30
Self-help Problems: