1. 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

2. Optical Properties Light has both particulate and wavelike properties
Photons - with mass

3. Refractive Index, n • Transmitted light distorts electron clouds.+ no
transmitted
light
electron
cloud
distorts
• Light is slower in a material vs vacuum.
n = refractive index
--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
Selected values from Table 21.1,
Callister 7e.

4. Total Internal Reflectance
n > n’
n’(low)
n (high)

5. Example: Diamond in air
Fiber optic cables are clad in low n material for this reason.

6. Light Interaction with Solids
• Incident light is either reflected, absorbed, or
transmitted:
Incident: I0
Absorbed: IA
Transmitted: IT
Scattered: IS
Reflected: IR
• Optical classification of materials:
Adapted from Fig , Callister 6e. (Fig is by J. Telford, with specimen preparation by P.A. Lessing.)
single crystal
polycrystalline dense
polycrystalline porous
Transparent
Translucent
Opaque

7. Optical Properties of Metals: Absorption
• Absorption of photons by electron transition:
Energy of electron
Incident photon
Planck’s constant
(6.63 x J/s)
freq. of
incident
light
filled states
unfilled states
DE = h required! I o
of energy h n
Adapted from Fig. 21.4(a), Callister 7e.
• Metals have a fine succession of energy states.
• Near-surface electrons absorb visible light.

8. Light Absorption

9. Optical Properties of Metals: Reflection
• Electron transition emits a photon.
re-emitted photon from material surface
Energy of electron
filled states
unfilled states DE IR
“conducting” electron
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)!

10. Reflectivity, R Reflection Metals reflect almost all light
Copper & gold absorb in blue & green => gold color
Example: Diamond

11. Scattering In semicrystalline or polycrystalline materials
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

12. Selected Absorption: Semiconductors
• Absorption by electron transition occurs if hn > Egap
Energy of electron
filled states
unfilled states E
gap I o
blue light: h = 3.1 eV
red light: h = 1.7 eV
incident photon energy hn
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.

13. Wavelength vs. Band Gap
Example: What is the minimum wavelength absorbed by Ge?
Eg = 0.67 eV
If donor (or acceptor) states also available this provides other absorption frequencies

14. 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.
• 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 deep
red in color.
Adapted from Fig. 21.9, Callister 7e. (Fig adapted from "The Optical Properties of Materials" by A. Javan, Scientific American, 1967.) 40 60 70 80 50
0.3
0.5
0.7
0.9
Transmittance (%)
ruby
sapphire
wavelength,
l (= c/)(m)

15. Luminescence Luminescence – emission of light by a material
material absorbs light at one frequency & emits at another (lower) frequency.
activator level
Valence band
Conduction band
trapped states Eg
Eemission
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

16. Photoluminescence Hg uv 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
Sb blue
Mn orange-red

17. Cathodoluminescence Used in T.V. set Bombard phosphor with electrons
Excite phosphor to high state
Relaxed by emitting photon (visible)
ZnS (Ag+ & Cl-) blue
(Zn, Cd) S + (Cu++Al3+) green
Y2O2S + 3% Eu red
Note: light emitted is random in phase & direction
i.e., noncoherent

18. 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

19. Population Inversion
What if we could increase most species to the excited state?
Fig , Callister 7e.

20. LASER Light Production
“pump” the lasing material to the excited state
e.g., by flash lamp (non-coherent lamp).
Fig , Callister 7e.
If we let this just decay we get no coherence.

21. 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 , Callister 7e.

22. 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 lasers
Welding
Drilling
Cutting – laser carved wood, eye surgery
Surface treatment
Scribing – ceramics, etc.
Photolithography – Excimer laser

23. electron + hole  neutral + h
Semiconductor LASER
Apply strong forward bias to junction. Creates excited state by pumping electrons across the gap- creating electron-hole pairs.
electron + hole  neutral + h
excited state
ground state
photon of light
Adapted from Fig , Callister 7e.

24. 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
checkout scanner

25. Applications of Materials Science
New materials must be developed to make new & improved optical devices.
Organic Light Emitting Diodes (OLEDs)
White light semiconductor sources
New semiconductors
Materials scientists
(& many others) use lasers as tools.
Solar cells
Fig , Callister 7e. Reproduced by
arrangement with Silicon Chip magazine.)

26. Solar Cells • p-n junction: • Operation:
-- incident photon produces hole-elec. pair.
-- typically 0.5 V potential.
-- current increases w/light intensity.
n-type Si
P-type Si
p-n junction
B-doped Si Si B
hole P
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 Si
Los Alamos High School weather
station (photo courtesy
P.M. Anderson)

27. Optical Fibers prepare preform as indicated in Chapter 13
preform drawn to 125 m or less capillary fibers
plastic cladding applied 60 m
Fig , Callister 7e.
Fig , Callister 7e.

28. Optical Fiber Profiles
Step-index Optical Fiber
Graded-index Optical Fiber
Fig , Callister 7e.
Fig , Callister 7e.

29. 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.

30. ANNOUNCEMENTS
Reading:
Core Problems:
Self-help Problems: