CHAPTER 7:
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
1
LIGHT INTERACTION WITH SOLIDS
• Incident light is either reflected, absorbed, or
transmitted:
Io = IT + IA + IR
Reflected: IR
Absorbed: IA
Transmitted: IT
Incident: Io
• 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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1
OPTICAL PROPERTIES OF METALS:
ABSORPTION
• Absorption of photons by electron transition:
ton
Energy
ho of electron
p
t unfilled states
enrgy∆E=hνrequired!
d
i
e
Adapted from
Fig.
hν 21.4(a), Callister 6e.
nc n
Io I of efreq. filled states
Planck’s
constant
-34
(6.63
J.s)
x 10of
incident
lightstates.
• Metals have a fine succession of energy
• Near-surface electrons absorb visible light.
3
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 6e.
• Reflectivity = IR/Io is between 0.90 and 0.95.
• Reflected ray is the same frequency as incident
ray.
• Metals appear reflective (shiny appearance)
LUSTER
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2
SELECTED ABSORPTION: NONMETALS
• Absorption by electron transition occurs if hν > Egap
Energy of electron
unfilled states
blue light: hν= 3.1eV
red light: hv= 1.7eV
incident photon
energy hν
Egap
1eV=1.6x10 -19 J
Io
filled states
Adapted from Fig. 21.5(a), Callister 6e.
• 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 has
a color.
5
COLOR OF NONMETALS
• Color determined by sum of the frequencies of
1.transmitted light,
2.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.
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 deep
red in color.
Transmittance (%)
• Ex:
80
sapphire
70
Ruby
60
50
40
0.3
wavelength, λ (= c/ν)(µm)
0.5
0.7
0.9
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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3
TRANSMITTED LIGHT: REFRACTION
• Transmitted light distorts electron clouds.
no
transmitted
light
transmitted
light
+
+
electron
cloud
distorts
• Result 1: Light is slower in a material vs vacuum.
Index of refraction (n) =
speed of light in a vacuum
speed of light in a material
Material
Lead glass
Silica glass
Soda-lime glass
Quartz
Plexiglas
Polypropylene
--Adding large, heavy ions (e.g., lead
can decrease the speed of light.
--Light can be
"bent"
n
2.1
1.46
1.51
1.55
1.49
1.49
Selected values from Table 21.1,
• Result 2: Intensity of transmitted light decreases
Callister 6e.
with distance traveled (thick pieces less transparent!)
7
APPLICATION: LUMINESCENCE
• Process:
Energy of electron
Energy of electron
unfilled states
unfilled states
incident
radiation
Egap
Egap
filled states
emitted
light
electron
transition occurs
Adapted from Fig. 21.5(a), Callister 6e.
• Ex: fluorescent lamps
glass
filled states
re-emission
occurs
Adapted from Fig. 21.5(a), Callister 6e.
“white” light
coating
UV
e.g., β-alumina radiation
doped
w/Europium
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4
APPLICATION: PHOTOCONDUCTIVITY
• Description:
+
+
Energy of electron
Energy of electron
unfilled states
unfilled states
semi
conductor:
Incident
radiation
Egap
conducting
electron
Egap
filled states
filled states
-
-
A. No incident radiation:
little current flow
B. Incident radiation:
increased current flow
• Ex: Photodetector (Cadmium sulfide)
9
APPLICATION: SOLAR CELL
• 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 0.5V potential.
--current increases with light intensity.
Si
Si
B-doped Si
polycrystalline Si
Los Alamos High School weather
station (photo courtesy
P.M. Anderson)
10
5
APPLICATION: FIBER OPTICS
• Design with stepped index of refraction (n):
time
intensity
cladding: glass
w/lower n
∆n enhances
internal reflection
input pulse total internal reflection output pulse
intensity
core: silica glass
w/higher n
shorter path
longer paths
time
broadened!
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.)
intensity
intensity
• Design with parabolic index of refraction
core:
Add
graded
input
total
pulse
internal
reflection
out
put
pulse
impurity
to make ndistrib.
higher in
core center
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.)
shorter,
slower
but
cladding
: (as before)
paths
time
time paths
longer, but faster
• Parabolic = less broadening = improvement!
less
broadening!
11
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.
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