1. Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu
Engineering 45
Optical Properties
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer

2. Learning Goals – Optical Props
Learn How Light and Solid Materials Interact
Why materials have characteristic colors
Why some materials transparent and others not
Optical applications:
Luminescence
Photoconductivity
Solar Cell
Optical Fiber Communications

3. Properties of Solid Materials
Mechanical: Characteristics of materials displayed when forces are applied to them.
Physical: Characteristics of materials that relate to the interaction of materials with various forms of energy.
Chemical: Material characteristics that relate to the structure of a material.
Dimensional: Size, shape, and finish

4. Material Properties Chemical Physical Mechanical Dimensional
Composition Melting Point Tensile properties Standard Shapes
Microstructure Thermal Toughness Standard Sizes
Phases Magnetic Ductility Surface Texture
Grain Size Electrical Fatigue Stability
Corrosion Optical Hardness Mfg. Tolerances
Crystallinity Acoustic Creep
Molecular Weight Gravimetric Compression
Flammability

5. ElectroMagnetic Radiation
Energy associated with Light, Radio Signals, X-rays and Others is Transmitted as ElectroMagnetic (EM) Radiation (EMR)
Electromagnetic radiation Transmits energy in the form of a Sinusoidal wave Which Contains ELECTRICAL & MAGNETIC Field-Components
The EM waves Travel in Tandem, and are perpendicular to
Each Other
The Direction Of Propagation

6. The EM Spectrum
EM Waves Cover a Wide Range of WAVELENGTHS, , and FREQUENCIES, 
: miles→femtometers
“Light” is generally divided into Three Segments
UltraViolet: 0.001→0.35 µm
NOT Visible, High in Energy
Visible: 0.35→0.7 µm
A VERY Small Slice of the EM spectrum
InfraRed: µm
Not Visible; carries “sensible” energy (heat)

7. EM Radiation Quantified
All EM Waves Travel at the Speed of Light, c
c is a Universal Constant with a value of 300 Mm/s ( miles/sec)
c is related to the Electric & Magnetic Universal Constants
Where (Recalling From Previous Lectures)
0  ELECTRIC Permittivity of Free Space (a vacuum)
µ0  MAGNETIC Permeability of Free Space (a vacuum)

8. EM Radiation Quantified
The Wavelength and Frequency of EM waves are related thru c
EM radiation has a Wave↔Particle Duality
The Energy, E, of a Light Particle
Where
  WaveLength in meters per cycle
  Frequency in Hertz (cycles/sec)
Where h  Planck’s Constant (6.63x10-34 J-s)
h is the PHOTON Energy
Solar Photon-Flux (photons/sq-m-s is important in solar cell design calculations

9. EM-Solid Interaction
Consider EM Radiation with Intensity I0 (in W/m2) Impinging on a Solid
The EM-Solid interaction Alters the incident Beam by 3 possible Phenomena
The EM Beam can be
Reflected
Absorbed
Transmitted
Solar Photon-Flux (photons/sq-m-s is important in solar cell design calculations

10. EM-Solid Interaction cont
Mathematically
Where all the IK are Intensities in W/sq-m
Now Divide E-Balance Eqn by I0
An Energy Balance on the Solid:
E-in = E-reflected + E-absorbed + E-transmitted
Solar Photon-Flux (photons/sq-m-s is important in solar cell design calculations

11. EM-Solid Interaction cont.2
Where:
R  REFLECTANCE (IR/I0)
A  ABSORBANCE (IA/I0)
T  TRANSMITTANCE (IT/I0)
Using R, A, T, Classify EM-Solid Behavior
Opaque → T = 0
Transparent →
T >> A+R
Light Not Scattered
Translucent→
T > A+R
Light Scattered
Solar Photon-Flux (photons/sq-m-s is important in solar cell design calculations

12. Metals – Optical Absorption
Energy of electron
Incident photon
filled states
unfilled states D E
= h
required I o
of Energy h
Metals – Optical Absorption
Metals Interact with Light Thru QUANTIZED Photon Absorption by Electrons
Metals have Very Closely Spaced e- Energy Levels
Thus Almost ALL incident Photons are ABSORBED within about 100 nm of the surface

13. Metals – Optical Reflection
re-emitted photon from material surface
Energy of electron
filled states
unfilled states D E IR
“conducting” electron
Metals – Optical Reflection
The Absorbed Energy is ReEmitted by e- “falling” back to Lower Energy states
Since Metals have Very Closely Spaced e- Energy Levels The Light is emitted at many ’s
Thus Outgoing Light Looks About the Same as Incoming Light → High Reflectance

14. Metals - Colors Metals also ABSORB Some Photons
Dissipated as heat
Metals that Absorb few, or in broad-spectrum, reflect “WHITE” Light and Appear Silvery
Some Metals absorb Preferentially, and the Reflected Light is Colored due the absence of the Absorbed light
e.g., Cu Absorbs in the Violet-Blue; leaving Reflected light rich in Orange-Red
Cu Bar
Sn-Plated Cu Bar

15. NonMetals – Selective Absorb.
incident photon energy hn
Energy of electron
filled states
unfilled states E
gap I o
blue light: h 3.3 ev
red light: h 1.8 ev
NonMetals – Selective Absorb.
In The Case of Materials with “Forbidden” Gaps in the Band Structure, Absorption Occurs only if h>Egap
For These Materials there is Very little ReEmission
The Material Color Depends on the Width of the BandGap

16. Color Cases – BandGap Matls
Egap < 1.8 eV
ALL Visible Light Absorbed; Solid Appears Gray or Black in Color
e.g., Si with Egap = 1.1 eV
Egap > 3.3 eV
NO Visible Light Absorbed; Solid Appears Clear and Transmissive
e.g., Diamond Egap = 5.45 eV, SiO2 Egap = 8-9 eV
1.8 eV < Egap < 3.3 eV
Some Light is absorbed and Material has a color

17. NonMetal Colors Color determined by sum of frequencies
transmitted light
re-emitted light from electron transitions
e.g., Cadmium Sulfide (CdS)
Egap = 2.4eV
Absorbs higher energy visible light (blue, violet),
Red/yellow/orange is transmitted and gives it this color
CdS

18. NonMetal Colors cont. Ex: Ruby = Sapphire (Al2O3) + 0.5-2 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

19. Wavelength vs. Band Gap
Example: What is the maximum wavelength absorbed by Ge?
Find Ge BandGap: Eg = 0.67 eV
Thus Need Ephoton = hc/λmax ≥ Eg
Use the Photon Energy Eqn:

20. Light Refraction
When Light Encounters a Matter-Containing Environment, it SLOWS DOWN Due to Interaction with Electrons
transmitted
light +
electron
cloud
distorts + no
transmitted
light
Define the INDEX of REFRACTION, n

21. Light Refraction cont
The slowing of light in a Non-Vacuum Medium Results in Refraction, or Bending of the light Path
Light Refracts per Snell’s Law :

22. Refraction Physics Recall Thus n Now the relations for v and c
Most Matls are NOT magnetic → µr  1 So
Where ε & µ are respectively the Permittivity & Permeability of the Material
Now Recall
e.g. Germanium
n = 3.97 → n2 = 15.76
r = 16.0 (very close)

23. Application  Luminescence
emitted light h1+ h2+...
Energy of electron
filled states
unfilled states E
gap
Re-emission Occurs
Incident
Radiation h0
Electron Excitation
Energy of electron
filled states
unfilled states E
gap UV
radiation
coating
e.g.; -alumina, doped w/ Europium
“white” light
glass
Application  Luminescence
Based on EM Induced e− excitation, and then Relaxation with Broad-Spectrum h Emission
e.g. fluorescent lamps

24. Application  PhotoConduction
Incident
radiation
Energy of electron
filled states
unfilled states E
gap
Conducting e- + -
B. Incident radiation: Increased current flow
Application  PhotoConduction
h Absorption by NO-Junction SemiConductors results in the Elevation of an e- to the Conduction Band Where it Can Carry an E-Field Driven Current
semi
conductor:
Energy of electron
filled states
unfilled states E
gap + -
A. No incident radiation: little current flow
e.g. Cadmium Sulfide

25. Application  Si Solar Cell
Recall The PN Junction
Operation for Si Cell:
An incident PHOTON produces HOLE-ELECTRON pair.
Typically V potential
Theoretical Max = 1.1 V (Egap).
Current INCREASES with INCREASED Light INTENSITY
Need to Minimize Reflectance
n-type Si p
-type Si
p-n
junction
B-doped Si B
hole P
conduction
electron
-doped n p
+ E -
hv generates Minority carriers, e.g. h+ in n-type. The h+ that make it to the jcn are then “collected” (swept downhill) by E-field and contribute to current flow
n-type => POS ion cores
P-type => NEGATIVE Ion Cores

26. Application – Heat Mirror
Natural SunLight is Very Pleasant
However, In Sunny Climes Windows that Admit Visible Light ALSO transmit InfraRed EM radiation that Heats the Building; increasing AirConditioning costs
Soln → “Heat” Mirror Window

27. Application – Heat Mirror cont
A Perfect Heat Mirror Would
Transmit 100% of EM radiation (light) in the visible nm Wavelength range
Reflect 100% of EMR over 700 nm
Heat Mirror Windows are Constructed from thin-film coated “window glass”
HM Film Stack → dielectric / metal / dielectric (D/M/D)
e.g., 300Å TiO2 / 130Å Ag / 300Å TiO2

28. The Solar Spectrum All Done for Today
".. indeed, far and away the most prolific writer in the history of the subject" writes Howard Eves in An Introduction to the History of Mathematics. Euler's contribution to mathematics is represented here by a few of the notations conventionalized by him or in his honor. Around the world, these are read, written, and spoken thousands of times every day:
e for the base of the natural logarithm (a.k.a. "the calculus number") a, b, c for the sidelengths of a triangle ABC f(x) for functional value R and r for the circumradius and inradius of a triangle sin x and cos x for values of the sine and cosine functions i for the imaginary unit, the "square root of -1" capital sigma for summation. capital delta for finite difference.
Euler grew up near Basel, Switzerland, and studied at an early age under Johann Bernoulli. He finished studies at the University of Basel when only 15 years old. From 1727 to 1741, Euler worked in St. Perersburg, Russia, and then moved to the Akademie in Berlin. In 1766 he returned to St. Petersburg, where he remained.

29. WhiteBoard Work Derive Eqns 21.18 21.19
Thick, Strongly Absorbing Medium of thickness d
21.19
Weakly Absorbing (transparent) medium with Reflection, R, and thickness d

30. Hot Miror (Heat Reflecting)
Heat Mirror
Hot Miror (Heat Reflecting)
What - These "hot mirror" filters transmit the visible spectrum and reflect the infrared. At any specified angle of incidence, the average transmission is more than 93% from 425 to 675 nm. The average reflectance of our standard Hot Mirror is more than 95% from 750 to 1150 nm.
Extended Hot Mirror: The average reflectance is more than 90% from 750 to 1600 nm.
Long IR Hot Mirror The average reflectance is more than 90% from 1700 to 3000 nm
Cold Mirror (Heat Transmitting)
These "cold mirror" filters reflect the visible spectrum and transmit heat (infrared). At any specified angle of incidence, average reflectance is more that 95% from 450 to 675 nm. Transmission is more than 85% from 800 to 1200 nm.