Opt ical Prop erties
Transcript of Opt ical Prop erties
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CHAPTER 19:
OPTICAL PROPERTIES
ISSUES TO ADDRESS...
• What happens when light shines on a material?
• Why do materials have characteristic colors?
• Why are some materials transparent and other not?
• Optical applications:--luminescence
--photoconductivity--solar cell
--optical communications fibers
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LIGHT INTERACTION WITH SOLIDS
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• Incident light is either reflected, absorbed, ortransmitted:
Incident: I o
Reflected : IR Absorbed : IA
Transmitted : IT
Io = IT + I A + IR
• Optical classification of materials:
Transparent
Transluscent
Opaque
Adapted from Fig. 21.10, Callister
6e. (Fig. 21.10 is by J. Telford,with specimen preparation by P.A.
Lessing.)
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TRANSMITTED LIGHT: REFRACTION
• Transmitted light distorts electron clouds.
+
no
transmitted
light
transmitted
light +
electroncloud
distorts
• Result 1: Light is slower in a material vs vacuum.
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Index of refraction (n) = speed of light in a vacuumspeed of light in a material
MaterialLead glass
Silica glass
Soda-lime glass
QuartzPlexiglas
Polypropylene
n2.1
1.46
1.51
1.551.49
1.49
--Adding large, heavy ions (e.g., lead
can decrease the speed of light.
--Light can be
"bent"
• Result 2: Intensity of transmitted light decreaseswith distance traveled (thick pieces less transparent!)
Selected values from Table 21.1,
Callister 6e.
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OPTICAL PROPERTIES OF
METALS: ABSORPTION• Absorption of photons by electron transition:
• Metals have a fine succession of energy states.
• Near-surface electrons absorb visible light.
Energy of electron
I n c i d e n t p h
o t o n
Planck 뭩 constant
(6.63 x 10 -34 J/s)
freq.
of
incident
light
filled states
unfilled states
∆E = hν required!
Io o f e n e r
g y h ν
Adapted from Fig. 21.4(a), Callister 6e.
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OPTICAL PROPERTIES OF
METALS: REFLECTION• Electron transition emits a photon.
Adapted from Fig. 21.4(b), Callister 6e.
Energy of electron
filled states
unfilled states
∆E
IR 밹onducting?electron
re-emitted
photon frommaterial surface
• Reflectivity = IR/Io is between 0.90 and 0.95.
• Reflected light is same frequency as incident.
• Metals appear reflective (shiny)!4
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Photo Device
ompact Disk
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APPLICATION LUMINESCENCE
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APPLICATION: LUMINESCENCE
• Process: Energy of electron
filled states
unfilled states
Egap
re-emission
occurs
Adapted from Fig. 21.5(a), Callister 6e. Adapted from Fig. 21.5(a), Callister 6e.
electron
transition occurs
Energy of electron
filled states
unfilled states
Egapincident
radiation emittedlight
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• Ex: fluorescent lamps
UV
radiation
coating
e.g., β-alumina
doped
w/Europium
뱖hite?lightglass
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SELECTED ABSORPTION: NONMETALS
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• Absorption by electron transition occurs if hν > Egap
• If Egap < 1.8eV, full absorption; color is black (Si, GaAs)
• If Egap > 3.1eV, no absorption; colorless (diamond)
• If Egap in between, partial absorption; material hasa color.
Adapted from Fig. 21.5(a), Callister 6e.
Energy of electron
filled states
unfilled states
Egap
Io
blue light: h ν= 3.1eV
red light: h ν= 1.7eV
incident photon
energy hν
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COLOR OF NONMETALS
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• Color determined by sum of frequencies of --transmitted light,
--re-emitted light from electron transitions.
• Ex: Cadmium Sulfide (CdS)-- Egap = 2.4eV,
-- absorbs higher energy visible light (blue, violet),
-- Red/yellow/orange is transmitted and gives it color.
• Ex: Ruby = Sapphire (Al2O3) + (0.5 to 2) at% Cr 2O3
-- Sapphire is colorless(i.e., Egap > 3.1eV)
-- adding Cr 2O3 :• alters the band gap
• blue light is absorbed
• yellow/green is absorbed
• red is transmitted• Result: Ruby is deep
red in color.
40
60
70
80
50
0.3 0.5 0.7 0.9
T r a n s m i t t a
n c e ( % )
Ruby
sapphire
wavelength, λ (= c/ν)(µm)
Adapted from Fig. 21.9, Callister 6e. (Fig. 21.9
adapted from "The Optical Properties of Materials" by A. Javan, Scientific American, 1967.)
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SUMMARY
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SUMMARY
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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.
• Non-Metals:--may have full (Egap < 1.8eV) , no (Egap > 3.1eV), or
partial absorption (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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Display
RT
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Display
PDP
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Display
FED
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Display
VFD
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Photo Device
Laser Diode
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Display
LED
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Display
OLED
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Photo Device
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Optical Fiber
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APPLICATION: FIBER OPTICS
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• Design with stepped index of refraction (n):core: silica glassw/higher n
cladding : glassw/lower n
∆n enhances
internal reflection i n
t e n
s i t y
time
input pulse
broadened!
i n t e n
s i t y
time
out put pulsetotal internal reflection
shorter pathlonger paths
• Design with parabolic index of refraction
Adapted from Fig. 21.19, Callister 6e. (Fig. 21.19 adapted from S.R. Nagel, IEEE
Communications Magazine, Vol. 25, No. 4, p. 34, 1987.)
core: Add gradedimpurity distrib.to make n higher in
core center
cladding : (as before)
total internal reflection
shorter, but s lower pathslonger, but faster paths
i n t e n s i t y
time
input pulse
i n t e n s i t y
time
out put pulse
less
broadening!
• Parabolic = less broadening = improvement!
Adapted from Fig. 21.20, Callister 6e. (Fig. 21.19 adapted from S.R. Nagel, IEEE
Communications Magazine, Vol. 25, No. 4, p. 34, 1987.)
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APPLICATION: PHOTOCONDUCTIVITY
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• Description:
• Ex: Photodetector (Cadmium sulfide)
Incidentradiation
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
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Photo Device
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Photo Detector
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APPLICATION: SOLAR CELL
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• p-n junction:
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• Operation:--incident photon produces hole-elec. pair.
--typically 0.5V potential.
--current increases w/light intensity.
n-type Si
p -type Si p-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 Si p-n junction
light
+-
++ +
---
creation of
hole-electron
pair
• Solar powered weather station:
polycrystalline SiLos Alamos High School weather
station (photo courtesy
P.M. Anderson)
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Laboratory for Advanced MaterialsProcessingDepartment of Chemical Engineering
Cross sectional view of TFT
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POSTECH
Pohang University of Science and Technology
S/D
ITO
Gate
Pass’n
Inter-insulator
Gate OxideN+ poly-Si
Poly-Si
Glass
Glass
S/D
N+ a-Si
Pass’n
a-Si
Gate nitride Gate
ITO
(a) poly-Si TFT
(b) a-Si TFT
(c) MOSFET
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