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7/28/2019 Optical-Prop [Compatibility Mode]

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ISSUES TO ADDRESS...

What happens when light shines on a material?

Why do materials have characteristic colors?

Chapter 20: Optical Properties

Chapter 20 - 1

Optical applications:-- luminescence

-- photoconductivity

-- solar cell-- optical communications fibers

Why are some materials transparent and others not?


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Optical PropertiesLight has both particulate and wavelike properties

Photons - with mass

==hc

hE

Chapter 20 - 2

m/s)10x(3.00lightofspeedc

)sJ1062.6(constantsPlanck'

frequency

wavelength

energy

8

34

=

=

=

=

=

xh

E


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Refractive Index, n

vacuum)inlightof(velocityc

Transmitted light distorts electron clouds.

Light is slower in a material vs vacuum.

=

+no

transmittedlight

transmittedlight

+

electronclouddistorts

Chapter 20 - 3

Note: n= f ()

Typical glasses ca. 1.5 -1.7

Plastics 1.3 -1.6PbO (Litharge) 2.67

Diamond 2.41

medium)inlightof(velocityv

Selected values from Table 20.1

Callisters Materials Science andEngineering, Adapted Version.

--Adding large, heavy ions (e.g., leadcan decrease the speed of light.

--Light can be

"bent"


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Total Internal Reflectance

=

sin

sin

n

nn(low)

n(high)

n> n

'1

Chapter 20 - 4

1

c

anglecritical

anglerefracted

angleincident

=

=

=

c

i

i

reflectedinternallyislightfor

90whenoccurs

ci

ic

>

=


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Example: Diamond in air

sin

1

sin

90sin

1

41.2

sin

sin

==

=ccn

n

Chapter 20 - 5

Fiber optic cables are clad in low nmaterial for thisreason.

5.2441.2

sin == cc


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Incident light is either reflected, absorbed, ortransmitted: SRATo IIIII +++=

Light Interaction with Solids

Incident: I0

Absorbed: IA

Transmitted: IT

Reflected: IR

Chapter 20 - 6

Optical classification of materials:

From Fig. 20.10, Callisters MSEAdapted Version.

(Fig. 20.10 is by J. Telford, withspecimen preparation by P.A.Lessing.)

single

crystal

polycrystalline

dense

polycrystalline

porous

TransparentTranslucent

Opaque

Scattered: IS


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Absorption of photons by electron transition:

Optical Properties of Metals:

Absorption

Energy of electron

unfilled states

E= h required!

Chapter 20 - 7

Metals have a fine succession of energy states. Near-surface electrons absorb visible light.

From Fig. 20.4(a)

Callisters Materials Science andEngineering, Adapted Version.

Plancks constant

(6.63 x 10-34

J/s)

freq.of

incidentlight

filled statesIo


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Light Absorption

t

I

I = e

0 thicknesssamplecm][tcoefficienabsorptionlinear

1

===

t

Chapter 20 - 8

tI

ln0

=


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Optical Properties of Metals:

Reflection Electron transition emits a photon.

re-emitted

Energy of electron

unfilled states

E

IR conducting electron

Chapter 20 - 9

Reflectivity = IR/Iois between 0.90 and 0.95. Reflected light is same frequency as incident. Metals appear reflective (shiny)!

From Fig. 20.4(b)Callisters Materials Science and

Engineering, Adapted Version.

photon frommaterial surfacefilled states


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Reflectivity, R

Reflection

Metals reflect almost all light

Copper & gold absorb in blue & green => goldcolor

tyreflectivi1

2

=

=n

R

Chapter 20 -10

17.0141.2

141.22

=

+

=R

reflectedislightof%17

Example: Diamond


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Scattering

In semicrystalline or polycrystalline materials

Semicrystalline density of crystals higher than amorphous

materials speed of light is lower - causes light to

Chapter 20 - 11

sca er - can cause s gn can oss o g

Common in polymers

Ex: LDPE milk cartons cloudy

Polystyrene clear essentially no crystals


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Absorption by electron transition occurs if h > Egap

Selected Absorption: Semiconductors

incident photon

Energy of electron

unfilled statesblue light: h = 3.1 eV

red light: h = 1.7 eV

Chapter 20 -12

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.

From Fig. 20.5(a), Callisters MSE Adapted Version.

energy h

filled statesIo


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c =hc

=(6.62 x 1034 J s)(3 x 108m/s)

1.85 m

Wavelength vs. Band Gap

Eg= 0.67 eV

Example: What is the minimum wavelength absorbedby Ge?

Chapter 20 -13

g (0.67eV)(1.60 x 10

J/eV)

note : for Si Eg=1.1eV c1.13 m

If donor (or acceptor) states also available this provides otherabsorption frequencies


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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/ ellow/oran e is transmitted and ives it color.

Color of Nonmetals

Chapter 20 -14

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 deepred in color.

From Fig. 20.9, Callisters MSE Adapted Version.(Fig. 20.9 adapted from "The Optical Properties ofMaterials" by A. Javan, Scientific American, 1967.)

40

60

70

80

50

0.3 0.5 0.7 0.9

Transm

ittance(%)

ruby

sapphire

wavelength, (= c/)(m)


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Luminescence

Luminescence emission of light by a material

material absorbs light at one frequency & emits at

another (lower) frequency.

Conduction band

Chapter 20 -15

activatorlevel

Valence band

trappedstatesEg

Eemission

How stable is the trapped state?

If very stable (long-lived = >10-8 s) =phosphorescence

If less stable (


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PhotoluminescenceHg

uv

electrode electrode

Chapter 20 -16

Arc between electrodes excites mercury in lamp to higherenergy 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


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Cathodoluminescence

Used in T.V. set

Bombard phosphor with electrons

Excite phosphor to high state Relaxed by emitting photon (visible)

ZnS (Ag+ & Cl-) blue

Chapter 20 -17

n, + u++ + green

Y2O2S + 3% Eu red

Note: light emitted is random in phase & direction

i.e., noncoherent


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LASER Light

Is non-coherent light a problem? diverges

cant keep tightly columnated

How could we get all the light in phase? (coherent)

LASERS

Chapter 20 -18

Amplification by Stimulated

Emission of

Radiation

Involves a process called population inversion ofenergy states


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Population Inversion

What if we could increase most species to the excitedstate?

Chapter 20 -19

Fig. 20.14Callisters Materials

Science and Engineering,Adapted Version.


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LASER Light Production

pump the lasing material to the excited state

e.g., by flash lamp (non-coherent lamp).

Chapter 20 -20

If we let this just decay we get no coherence.

Fig. 20.13Callisters MaterialsScience and Engineering,

Adapted Version.


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LASER Cavity

Tuned cavity:

Stimulated Emission

One photon induces theemission of anotherphoton, in phase with the

Chapter 20 -21

.

cascades producing veryintense burst of coherentradiation.

i.e., Pulsed laser

Fig. 20.15Callisters MaterialsScience and Engineering,

Adapted Version..


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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 lasers1. Welding

Chapter 20 -22

2. Drilling3. Cutting laser carved wood, eye surgery

4. Surface treatment

5. Scribing ceramics, etc.

6. Photolithography Excimer laser


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Apply strong forwardbias to junction.Creates excited state

by pumping electronsacross the gap-creating electron-holepairs.

Semiconductor LASER

Chapter 20 -23

electron + hole neutral + h

excited state ground state

photon oflight

From Fig. 20.17Callisters MaterialsScience and Engineering,

Adapted Version.


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Uses of Semiconductor LASERs

#1 use = compact disk player

Color? - red

Banks of these semiconductor lasers are used asflash lamps to pump other lasers

Communications

Chapter 20 -24

Fibers often turned to a specific frequency(typically in the blue)

only recently was this a attainable


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Applications of Materials Science

New materials must be developed to make new &improved optical devices.

Organic Light Emitting Diodes (OLEDs) White light semiconductor sources

Chapter 20 -25

New semiconductors

Materials scientists

(& many others) use lasers as tools.

Solar cells

. .Callisters MSE Adapted Version.

(Reproduced byarrangement with Silicon Chipmagazine.)


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Solar Cells p-njunction: Operation:

-- incident photon produces hole-elec. pair.-- typically 0.5 V potential.-- current increases w/light intensity.

P

Si

Si

Si Si

conductance

electron

P-doped Si

n-type Si-

light

----

creation ofhole-electronpair

Chapter 20 -26

Solar powered weather station:

polycrystalline SiLos Alamos High School weatherstation (photo courtesyP.M. Anderson)

n-type Si

P-type Sip-njunction

B-doped Si

Si

Si

Si SiB

hole

p-type Si-

+

++ +


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Optical Fibers

prepare preform as indicated in Chapter 13

preform drawn to 125 m or less capillary fibers

plastic cladding applied 60 m

Chapter 20 -27

Fig. 20.20, Callisters MSE

Adapted Version.

Fig. 20.18, Callisters MSEAdapted Version.


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Optical Fiber Profiles

Step-index Optical Fiber

Chapter 20 -28

Graded-index Optical FiberFig. 21.21, Callister 7e.

Fig. 20.22

Callisters MaterialsScience and Engineering,Adapted Version.


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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.

SUMMARY

Chapter 20 -29

Non-Metals:-- may have full(Egap< 1.8eV) , no(Egap> 3.1eV), orpartialabsorption (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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Core Problems:

ANNOUNCEMENTS

Reading:

Chapter 20 -30

Self-help Problems:

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