[email protected] ENGR-45_Lec-13_Optical_Properties.ppt 1 Bruce Mayer, PE Engineering-45:...

[email protected] • ENGR-45_Lec-13_Optical_Properties.ppt1

Bruce Mayer, PE Engineering-45: Materials of Engineering

Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engineering 45

OpticalOpticalPropertiesProperties



Learning Goals – Optical PropsLearning 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



Properties of Solid MaterialsProperties 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



Material PropertiesMaterial 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



ElectroMagnetic RadiationElectroMagnetic 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



The EM SpectrumThe 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: 0.7-1000 µm– Not Visible; carries “sensible”

energy (heat)



EM Radiation QuantifiedEM Radiation Quantified All EM Waves

Travel at the Speed of Light, c

c is a Universal Constant with a value of 300 Mm/s (186 000 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)

001 c



EM Radiation QuantifiedEM Radiation Quantified The Wavelength

and Frequency of EM waves are related thru c

• Where WaveLength in

meters per cycle Frequency in

Hertz (cycles/sec)

c

EM radiation has a Wave↔Particle Duality

The Energy, E, of a Light Particle

chE h • Where h

Planck’s Constant (6.63x10-34 J-s)

h is the PHOTON Energy



EM-Solid InteractionEM-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



EM-Solid Interaction contEM-Solid Interaction cont Mathematically

An Energy Balance on the Solid:• E-in = E-reflected +

E-absorbed + E-transmitted

TAR IIII 0

• Where all the IK are Intensities in W/sq-m

Now Divide E-Balance Eqn by I0

TAR 1



EM-Solid Interaction cont.2EM-Solid Interaction cont.2

• Transparent → – T >> A+R

– Light Not Scattered

• Translucent→– T > A+R

– Light Scattered

0I RI

TI

AI• Where:

– R REFLECTANCE (IR/I0)

– A ABSORBANCE (IA/I0)

– T TRANSMITTANCE (IT/I0)

Using R, A, T, Classify EM-Solid Behavior• Opaque → T = 0

TAR 1



Metals – Optical AbsorptionMetals – Optical Absorption Metals Interact with Light Thru QUANTIZED Photon

Absorption by Electrons

Energy of electron

Incident photon

filled states

unfilled states

E = h required

Io of E

nergy h

Metals have Very Closely Spaced e- Energy Levels

• Thus Almost ALL incident Photons are ABSORBED within about 100 nm of the surface



Metals – Optical ReflectionMetals – 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

re-emitted photon from material surface

Energy of electron

filled states

unfilled states

E

IR “conducting” electron

• Thus Outgoing Light Looks About the Same as Incoming Light → High Reflectance



Light Absorbtion/ReflectionLight Absorbtion/Reflection

Amount of NON-Reflected Light Absorbed by a Matl

For normally incident light passing into asolid having an index of refraction n:

e0IIT = absorption coefficient, cm-1

= sample thickness, cm = NonReflected incident

light intensity = transmitted light intensity

0I

TI

2

1

1tyreflectivi

s

s

n

nR



Metals - ColorsMetals - Colors Metals also ABSORB

Some Photons• Dissipated as heat

Metals that Absorb few, orin 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-RedC

u B

ar

Sn

-Plated

Cu

Bar



Total TransmissionTotal Transmission

Combining External and Internal Reflection, along with Beer’s Absorbtion Yields the TOTAL Transmission Eqn

eRIIT2

0 1



Total-T ExampleTotal-T Example For the Situation at

Right Determine the thickness, d77, that will produce a total Transmittance of 77%

From Tab 21.1 Find Pyrex ns = 1.47

Next find R using Eqn (21.13)

13 mm

0I 086.0 IQuartzPyrex

23 mm

%621.3

147.1

147.1

1

122

R

n

nR

s

s



Total-T ExampleTotal-T Example Recall total

Transmission Eq

Now Solve for β13 mm


23 mm

eRIIT2

0 1

e

RI

IT2

0 1

20 1

lnRI

IT

20

1ln

R

IIT



Total-T ExampleTotal-T Example Thus

Solving Total-T Eqn for the length Then d77

13 mm


23 mm

meter350.3

2303621.01

86.0ln 2

mm

20

1ln

R

IIT mm 0.56

mm00335.0

03621.01

77.0ln

77

277

d

d



NonMetals – Selective Absorb.NonMetals – Selective Absorb. In The Case of Materials with “Forbidden” Gaps in the

Band Structure, Absorption Occurs only if h>Egap

For TheseMaterials there is Very little ReEmission• The Material Color

Depends on the Width of the BandGap

incident photon energy h

Energy of electron

filled states

unfilled states

Egap

Io

blue light: h 3.3 ev

red light: h 1.8 ev



Color Cases – BandGap MatlsColor 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



NonMetal ColorsNonMetal 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),

CdS

• Red/yellow/orange is transmitted and gives it this color



NonMetal Colors cont.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

40

60

70

80

50

0.3 0.5 0.7 0.9

Transm

itta

nce

(%

)

Ruby

sapphire

wavelength, c/)(m)



Wavelength vs. Band GapWavelength 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:

mE

mE

hc

g

g

131eV11 Sifor :note

851J/eV10x601eV670

m/s10 x 3sJ10x62619

834

..

.).)(.(

))(.(

max

max



Light RefractionLight Refraction When Light Encounters a Matter-Containing

Environment, it SLOWS DOWN Due to Interaction with Electrons

+no

transmitted light

transmitted light +

electron cloud distorts

Define the INDEX of REFRACTION, n

vOr

Matlin Light of SpdVacuumin Light of Spd

cn

n



Light Refraction contLight 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 :

2211 sinsin nn



Refraction PhysicsRefraction Physics Recall Thus n

v

c n

Now the relations for v and c

001 c

1 v• Where ε & µ are

respectively the Permittivity & Permeability of the Material

Now Recall

rr

c n

00v

Most Matls are NOT magnetic → µr 1 • So

r n e.g. Germanium

• n = 3.97 → n2 = 15.76 r = 16.0 (very close)



Application Application Luminescence Luminescence Based on EM Induced e− excitation, and then

Relaxation with Broad-Spectrum h Emission

e.g. fluorescent lamps

emitted light

h1+ h2+...

Energy of electron

filled states

unfilled states

Egap

Re-emissionOccurs

IncidentRadiation

h0

ElectronExcitation

Energy of electron

filled states

unfilled states

Egap

UV radiation

coating e.g.; -alumina,

doped w/ Europium

“white” light

glass



Application Application PhotoConduction 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

e.g. Cadmium Sulfide

semi conductor:

Energy of electron

filled states

unfilled states

Egap

+

-A. No incident radiation:

little current flow

Incident radiation

Energy of electron

filled states

unfilled states

EgapConducting e-

+

-B. Incident radiation: Increased current flow



Application Application Si Solar Cell Si Solar Cell Recall The

PN Junction

Operation for Si Cell:• An incident PHOTON produces

HOLE-ELECTRON pair.• Typically 0.5-0.7 V potential

– Theoretical Max = 1.1 V (Egap).

• Current INCREASES with INCREASED Light INTENSITY– Need to Minimize Reflectance

n-type Si

p-type Sip-n junction

B-doped Si

Si

Si

Si SiB

hole

P

Si

Si

Si Si

conduction electron

P-doped Si

n

p

+ E

-



Application – Heat MirrorApplication – 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



Application – Heat Mirror contApplication – Heat Mirror cont A Perfect Heat Mirror

Would • Transmit 100% of EM

radiation (light) in the visible 350-700 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

http://www.cerac.com/pubs/cmn/cmn6_4.htm



All Done for TodayAll Done for Today

TheSolar

Spectrum



WhiteBoard WorkWhiteBoard Work

Derive Eqns

• 21.18d

T eI I 0

– Thick, Strongly Absorbing Medium of thickness d

• 21.19 dT eRI I 2

0 1

– Weakly Absorbing (transparent) medium with Reflection, R, and thickness d



Heat MirrorHeat MirrorHot 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 spectrumand transmit heat (infrared). At any specified angle ofincidence, average reflectance is more that 95% from450 to 675 nm. Transmission is more than 85% from800 to 1200 nm.

Date post:	30-Dec-2015
Category:	Documents
Upload:	nathaniel-hodges
View:	214 times
Download:	1 times

[email protected] ENGR-45_Lec-13_Optical_Properties.ppt 1 Bruce Mayer, PE Engineering-45:...

Documents