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Chapter 21: 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 others not?
• Optical applications:
-- luminescence
-- photoconductivity
-- solar cell
-- optical communications fibers
Chapter 21 - 1
Optical Properties
Light has both particulate and wavelike properties
– Photons - with mass
E  h 
hc

E  energy
  wavelength
  frequency
h
 Planck' s constant (6.62 x10 34 J  s)
c
 speed of light (3.00 x 108 m/s)
Chapter 21 - 2
Refractive Index, n
• Transmitted light distorts electron clouds.
no
transmitted
light
+
transmitted
light
+
electron
cloud
distorts
• Light is slower in a material vs vacuum.
c (velocity of light in vacuum)
n = refractive index 
v (velocity of light in medium)
--Adding large, heavy ions (e.g., lead
can decrease the speed of light.
--Light can be
"bent"
• Note: n = f ()
Typical glasses ca.
Plastics
PbO (Litharge)
Diamond
1.5 -1.7
1.3 -1.6
2.67
2.41
Selected values from Table 21.1,
Callister 7e.
Chapter 21 - 3
Total Internal Reflectance
n > n’
1'
n’(low)
n (high)
n sin 

n sin 
i  incident angle
i  refracted angle
c
1
c  critical angle
c occurs when i  90
for i  c light is internally reflected
Chapter 21 - 4
Example: Diamond in air


n
sin 
2.41
sin 90
1




n
sin 
1
sin c
sin c
sin c
1

2.41
c  24.5
• Fiber optic cables are clad in low n material for this
reason.
Chapter 21 - 5
Light Interaction with Solids
• Incident light is either reflected, absorbed, or
transmitted: Io  IT  I A  IR  IS
Reflected: IR
Absorbed: IA
Transmitted: IT
Incident: I0
Scattered: IS
• Optical classification of materials:
Transparent
Translucent
Opaque
single
crystal
polycrystalline
dense
Adapted from Fig. 21.10, Callister
6e. (Fig. 21.10 is by J. Telford,
with specimen preparation by P.A.
Lessing.)
polycrystalline
porous
Chapter 21 - 6
Optical Properties of Metals:
Absorption
• Absorption of photons by electron transition:
Energy of electron
unfilled states
E = h required!
Io
Planck’s constant
(6.63 x 10-34 J/s)
filled states
freq.
of
incident
light
Adapted from Fig. 21.4(a), Callister 7e.
• Metals have a fine succession of energy states.
• Near-surface electrons absorb visible light.
Chapter 21 - 7
Light Absorption
I
 t
e
I0
  linear absorption coefficient [] cm1
t  sample thickness
I 
ln      t
 I0 
Chapter 21 - 8
Optical Properties of Metals:
Reflection
• Electron transition emits a photon.
Energy of electron
IR
re-emitted
photon from
material surface
unfilled states
“conducting” electron
E
filled states
Adapted from Fig. 21.4(b), Callister 7e.
• Reflectivity = IR/Io is between 0.90 and 0.95.
• Reflected light is same frequency as incident.
• Metals appear reflective (shiny)!
Chapter 21 - 9
Reflectivity, R
• Reflection
– Metals reflect almost all light
– Copper & gold absorb in blue & green => gold
color
2
 n  1
R 
  reflectivi ty
 n  1
2
• Example: Diamond
 2.41  1 
R 
  0.17
 2.41  1
 17% of light is reflected
Chapter 21 - 10
Scattering
• In semicrystalline or polycrystalline materials
• Semicrystalline
– density of crystals higher than amorphous
materials  speed of light is lower - causes light to
scatter - can cause significant loss of light
• Common in polymers
– Ex: LDPE milk cartons – cloudy
– Polystyrene – clear – essentially no crystals
Chapter 21 - 11
Selected Absorption: Semiconductors
• Absorption by electron transition occurs if h > Egap
Energy of electron
blue light: h = 3.1 eV
unfilled states
red light: h = 1.7 eV
incident photon
energy h
Io
Egap
filled states
Adapted from Fig. 21.5(a), Callister 7e.
• 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.
Chapter 21 - 12
Wavelength vs. Band Gap
Example: What is the minimum wavelength absorbed
by Ge?
Eg = 0.67 eV
hc (6.62 x 1034 J s)(3 x 10 8 m/s)
c 

 1.85 m
19
Eg
(0.67eV)(1.60 x 10
J/eV)
note : for Si
Eg  1.1 eV
c  1.13 m
If donor (or acceptor) states also available this provides other
absorption frequencies

Chapter 21 - 13
Color of Nonmetals
• 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/yellow/orange is transmitted and gives it color.
-- 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 deep
red in color.
Transmittance (%)
• Ex: Ruby = Sapphire (Al2O3) + (0.5 to 2) at% Cr2O3
80
sapphire
70
ruby
60
50
40
0.3
wavelength,  (= c/)(m)
0.5
0.7
0.9
Adapted from Fig. 21.9, Callister 7e. (Fig. 21.9
adapted from "The Optical Properties of Materials" by
A. Javan, Scientific American, 1967.)
Chapter 21 - 14
Luminescence
• Luminescence – emission of light by a material
– material absorbs light at one frequency & emits at
another (lower) frequency.
Conduction band
Eg
trapped
states
Eemission
activator
level
Valence band
How stable is the trapped state?
• If very stable (long-lived = >10-8 s) =
phosphorescence
• If less stable (<10-8 s) = fluorescence
Example: glow in the dark toys.
Charge them up by exposing them to
the light. Reemit over time. -phosphorescence
Chapter 21 - 15
Photoluminescence
Hg
uv
electrode
electrode
• Arc between electrodes excites mercury in lamp to higher
energy 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
Chapter 21 - 16
Cathodoluminescence
• Used in T.V. set
– Bombard phosphor with electrons
– Excite phosphor to high state
– Relaxed by emitting photon (visible)
ZnS (Ag+ & Cl-)
(Zn, Cd) S + (Cu++Al3+)
Y2O2S + 3% Eu
blue
green
red
• Note: light emitted is random in phase & direction
– i.e., noncoherent
Chapter 21 - 17
LASER Light
• Is non-coherent light a problem? – diverges
– can’t keep tightly columnated
• How could we get all the light in phase? (coherent)
– LASERS
•
•
•
•
•
Light
Amplification by
Stimulated
Emission of
Radiation
• Involves a process called population inversion of
energy states
Chapter 21 - 18
Population Inversion
• What if we could increase most species to the excited
state?
Fig. 21.14, Callister 7e.
Chapter 21 - 19
LASER Light Production
• “pump” the lasing material to the excited state
– e.g., by flash lamp (non-coherent lamp).
Fig. 21.13, Callister 7e.
– If we let this just decay we get no coherence.
Chapter 21 - 20
LASER Cavity
“Tuned” cavity:
• Stimulated Emission
– One photon induces the
emission of another
photon, in phase with the
first.
– cascades producing very
intense burst of coherent
radiation.
• i.e., Pulsed laser
Fig. 21.15, Callister 7e.
Chapter 21 - 21
Continuous Wave LASER
• Can also use materials such as CO2 or yttriumaluminum-garret (YAG) for LASERS
• Set up standing wave in laser cavity –
– tune frequency by adjusting mirror spacing.
• Uses of CW lasers
1.
2.
3.
4.
5.
6.
Welding
Drilling
Cutting – laser carved wood, eye surgery
Surface treatment
Scribing – ceramics, etc.
Photolithography – Excimer laser
Chapter 21 - 22
Semiconductor LASER
• Apply strong forward
bias to junction.
Creates excited state
by pumping electrons
across the gapcreating electron-hole
pairs.
electron + hole  neutral + h
excited state
ground state
photon of
light
Adapted from Fig. 21.17,
Callister 7e.
Chapter 21 - 23
Uses of Semiconductor LASERs
• #1 use = compact disk player
– Color? - red
• Banks of these semiconductor lasers are used as
flash lamps to pump other lasers
• Communications
– Fibers often turned to a specific frequency
(typically in the blue)
– only recently was this a attainable
Chapter 21 - 24
Applications of Materials Science
• New materials must be developed to make new &
improved optical devices.
– Organic Light Emitting Diodes (OLEDs)
– White light semiconductor sources
Fig. 21.12, Callister 7e.
Reproduced by
arrangement with Silicon
Chip magazine.)
• New semiconductors
• Materials scientists
(& many others) use lasers as tools.
• Solar cells
Chapter 21 - 25
Solar Cells
• p-n junction:
• Operation:
P-doped Si
conductance
Si
electron
Si P Si
Si
B
creation of
hole-electron
pair
-
- -
+
+ + +
• Solar powered weather station:
Si
Si
light
n-type Si
p-n junction
p-type Si
n-type Si
p-n junction
P-type Si
hole
-- incident photon produces hole-elec. pair.
-- typically 0.5 V potential.
-- current increases w/light intensity.
Si
Si
B-doped Si
polycrystalline Si
Los Alamos High School weather
station (photo courtesy
P.M. Anderson)
Chapter 21 - 26
Optical Fibers
• prepare preform as indicated in Chapter 13
• preform drawn to 125 m or less capillary fibers
• plastic cladding applied 60 m
Fig. 21.20, Callister 7e.
Fig. 21.18, Callister 7e.
Chapter 21 - 27
Optical Fiber Profiles
Step-index Optical Fiber
Graded-index Optical Fiber
Fig. 21.21, Callister 7e.
Fig. 21.22, Callister 7e.
Chapter 21 - 28
SUMMARY
• 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.
Chapter 21 - 29
ANNOUNCEMENTS
Reading:
Core Problems:
Self-help Problems:
Chapter 21 - 30