mse-21-optical properties%2861%29-2007-05-31

8/3/2019 MSE-21-Optical Properties%2861%29-2007-05-31

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Introduction to

Materials Science and EngineeringChapter 21. OPTICAL PROPERTIES

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

Laser

Optical communication fibers


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Contents

1 Electromagnetic RadiationElectromagnetic Radiation

Light Interactions with SolidsLight Interactions with Solids2

Optical Properties of MetalsOptical Properties of Metals3

Optical Properties of NonmetalsOptical Properties of Nonmetals4

5 ApplicationsApplications


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Index of refraction - Relates the change in velocity and direction

of radiation as it passes through a transparent medium (also knownas refractive index). Ratio of the velocity of light in vacuum to

the velocity of light in the material

Dispersion - Frequency dependence of the refractive index.

Linear absorption coefficient - Describes the ability of a material

to absorb radiation.

Absorption constant: the reciprocal of the absorptioncoefficient is a measure of how far the light will travel before

being reduced by a factor of exponential.

Penetration depth: the distance with 1/e reduction in intensity Reflectivity - The percentage of incident radiation that is

reflected.

Photoconduction - Production of a voltage due to the stimulation ofelectrons into the conduction band by light radiation.


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Luminescence - Conversion of radiation to visible light.

Fluorescence - Emission of light obtained typically within ~10-8

seconds.

Phosphorescence - Emission of radiation from a material after the

stimulus is removed.

Light-emitting diodes (LEDs) - Electronicp-njunction devices that

convert an electrical signal into visible light.

Electroluminescence - Use of an applied electrical signal to

stimulate photons from a material.

Laser - The acronym stands for light amplification by stimulatedemission of radiation. A beam of monochromatic coherent radiation

produced by the controlled emission of photons.

Thermal emission - Emission of photons from a material due toexcitation of the material by heat.


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Introduction Optical Properties - A materials response to exposure to

electromagnetic radiation, particularly to visible light.

Light is energy, or radiation, in the form of waves or particles

called photons that can be emitted from a material. The important characteristics of the photons energy E,

wavelength , and frequency are related by the equation:

hcE h

= =

0 electric permittivity of a vacuum0 Magnetic permeability of a vacuum

C =


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Electromagnetic Spectrum

400 nm - 700 nm


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Contents







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Light Interaction with Solids Incident light is either reflected, absorbed, or transmitted.

I0 = IT+ IA + IR

Incident: I o

Reflected : IRAbsorbed : IA

Transmitted : IT T+ A+ R= 1Transmissivity (IT/I0)

Absorptivity (IA/I0)

Reflectivity (IR/I0)

Optical classification of materials

translucenttransparent

opaque


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Light Interaction with Solids

ReflectionAbsorption

Transmission

Refraction

Absorptionindex

opaque

transparent


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Light Interaction with Solids

R: reflectance

X

exp( )oo

dI

dx I I xI = = : absorption coefficient

II0


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T, A, and R

green glasses

b l d &


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Interaction between electromagnetic radiation &atoms/ions/electrons

Polarizationelectronic ionic

Electron transitions


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Light Interaction with Solids Electronic polarization- Some of the radiation energy may be absorbed.- Light waves are retarded in velocity as they pass through

the medium (manifested as refraction).

Electron transitions

E h =

- Absorption & emission- Discrete, specific energy

- Short stay in an excited

state - decay back into itsground state


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Contents






b l


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Absorption in metals Absorption of photons by electron transition

Energy of electron

Incid

entph

oton

Planck constant

(6.63 x 10 -34 J/s)

freq.

of

incidentlight

filled states

unfilled states

E =hrequired!

Io ofenergy

h

Metals have a continuously available empty e states, which permit e transitions.

Near-surface electrons absorb visible light.

fl l


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Reflection in metals

Electron transition emits a photonEnergy of electron

filled states

unfilled states

E

IR onducting?electron

Re-emittedphoton frommaterial surface

Reflectivity = IR/I0 is between 0.90 and 0.95.

Reflected light has same frequency as incident.

Metals are opaque & highly reflective (shiny).

l P f l


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Optical Properties of Metals

- Reemit in the form of visible light of same wavelength

(below a metal plasmon energy)

- Reflectivity: 0.90 - 0.95

C t t


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Contents






O ti l P ti f N t l


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Optical Properties of Nonmetals

r

- refractive index

sin(snell's law)sin

- wavelength dependent

(dispersion)

-

for nonmagnetic 1

vacr r

mat o o

r

in

r

vn

v

n

=

= = =

Refraction

O ti l P ti f N t l


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Refraction

- Refraction is related to electronic polarization at the

relatively high frequencies for visible light. Electronic component of the dielectric constant may be

determined from the index of refraction measurements.

- Electronic polarization retard electromagnetic radiation

The greater the electronic polarization -> the slower the velocity

-> the greater the index of refraction

ex) soda-lime glass n = 1.5

90 wt.% PbO containing glass n = 2.1

O ti l P ti f N t l


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Optical Properties of Nonmetals Polarizability

O ti l P ti s f N m t ls


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Dispersion



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- antireflective coating-microscope, telescopeOptical Properties of Nonmetals

Antireflective coating for lenses and other opticalinstruments.



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Absorption- valence band-conduction band transition

(energy band structure)

electron-holegeneration

electron-holerecombination



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Absorption- Valence band-conduction band transition can take place

only if the photon energy is greater thanthat of the band gap energy Eg.

org ghc

h E E

> >

- for visible light

- Eg less than 1.8 eV - all visible light absorb - opaque

1.8 eV < Eg < 3.1 eV - partial absorption - color

0.7 (=1.8 eV) ~ 0.4 (=3.1 eV)m m

Selected Absorption: Nonmetals


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Selected Absorption: Nonmetals Absorption by electron transition occurs if h > Egap

Energy of electron

filled states

unfilled states

Egap

Io

blue light: h = 3.1eV

red light: h = 1.7eV

If Egap

< 1.8 eV, full absorption of visible light color is black

Si (1.12 eV), GaAs (1.42 eV)

If Egap > 3.1 eV, no absorption transparent & colorless

Diamond (5.6 eV) If Eg in between, partial absorption - material has a color

incident photon

energy hn

400nm = 3.1 eV

700nm = 1.8 eV



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AbsorptionImpurities or other electrically active defects

Absorption and Energy gap


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Absorption and Energy gap

metals

Dielectrics and intrinsic

semiconductors

Extrinsic (doped)semiconductors



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Absorption ' '

'exp( )

: intensity of nonreflected incident

radiation

4: absorption coefficient ( )

T o

o

I I xI

k

=

=

TIoI

x

'

T

'

o

ex) The fraction of nonreflected light that is transmitted through

a 200 mm thickness of glass is 0.98. Calculate the absorption

coefficient of this material.

1 I 1

=- ln( )=- ln(0.98)=1.01x I 200mm

solution

-4 -1

x10 mm



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Transmission

Transmitted Light: Refraction


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Transmitted Light: Refraction Transmitted light distorts electron clouds

+

no

transmitted

light

transmitted

light +

electroncloud

distorts

Light is slower in a material vs. vacuum.

Index of refraction (n) = speed of light in a vacuumspeed of light in a material

MaterialLead glassSilica glassSoda-lime glass

QuartzPlexiglasPolypropylene

n2.11.461.51

1.551.491.49

- Adding large, heavy ions (e.g., lead)

can decrease the speed of light.

- Light can be "bent."

Color of Materials


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Color determined by sum of frequencies of

Transmitted light

Re-emitted light from electron transitions



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p p

Color

As a consequence of selective absorption of specific wavelengthranges of light.

If absorption is uniform for all visible wavelength, the material

appears colorless (inorganic glass, diamond, sapphire). Selective absorption by electron excitation.

Example - Cadmium Sulfide (CdS)

Eg = 2.4 eV

absorb photons > 2.4 eV (blue-violet portion)

Re-radiate other wavelength

Red/yellow/orange is transmitted and gives it color



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p p Color impurities - electron level within the forbidden bandgap

ex) sapphire (Al2O3) colorless (Egap > 3.1eV)

ruby (0.5 to 2% Cr2O3 doped Al2O3) - red color

Adding Cr2O3 to sapphire:- Alters the band gap, blue light is absorbed, yellow/green is

absorbed, red is transmitted Ruby is deep red in color



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Color

impurities - transition or rare earth ions in

inorganic glasses

Blue color

1% cobalt oxide containing

silicate glass (Co2+

)



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p p

Opacity and translucency- Internal reflection and refraction- Scattering

- Polycrystalline - grain boundary

- Two phase materials with different refractive indices- Porosity with finely dispersed pores

porous alumina

fully dense polycrystalline

single crystal sapphire

Contents


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Application: Luminescence


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Luminescence: Light emission in the visible spectrum

accompanying the absorption of other forms

of energy (thermal, mechanical, chemical

or particles (high energy electrons)(photoluminescence, electroluminescence).

Fluorescence: Emission of electromagnetic radiation that

occurs within ~10-8 s of an excitation event.

Phosphorescence: Emission of electromagnetic radiation

over an extended period of time after theexcitation event is over.

Luminescence


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electron

transition occurs

Energy of electron

filled states

unfilled states

Egap

Energy of electron

filled states

unfilled states

Egap

re-emission

occurs

Process:

Ex: fluorescent lamps

UV

radiation

coating

e.g., -alumina

dopedw/Europium

glass

incident

radiation

emittedlight

Photo-luminescence (PL), Electro-L (EL)

White light

Luminescence(a)


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(a)

Luminescence occurs when

photons have a wavelength in the

visible spectrum.

(a) In metals, there is no energy

gap, so luminescence does not occur.

(b) Fluorescence occurs when there

is an energy gap.

(c) Phosphorescence occurs when

the photons are emitted over a

period of time, due to donor traps in

the energy gap.

(b)

(c)

Luminescence


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A fluorescent lamp is a type of lamp that uses electricity

to excite mercury vapor argon neonin or gas, resulting in a

plasma that produces short-wave ultraviolet light. This

light then causes a phosphor fluoresceto , producingvisible light.

Light Emitting Diode (LED)
http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Mercury_%28element%29http://en.wikipedia.org/wiki/Vaporhttp://en.wikipedia.org/wiki/Argonhttp://en.wikipedia.org/wiki/Neonhttp://en.wikipedia.org/wiki/Plasma_%28physics%29http://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Phosphorhttp://en.wikipedia.org/wiki/Fluorescencehttp://en.wikipedia.org/wiki/Lighthttp://upload.wikimedia.org/wikipedia/en/f/fb/FluorescentLight.jpghttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Fluorescencehttp://en.wikipedia.org/wiki/Phosphorhttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Plasma_%28physics%29http://en.wikipedia.org/wiki/Neonhttp://en.wikipedia.org/wiki/Argonhttp://en.wikipedia.org/wiki/Vaporhttp://en.wikipedia.org/wiki/Mercury_%28element%29http://en.wikipedia.org/wiki/Electricity


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A forward-bias voltage across thep-njunction can

produce photons.

Photoconductivity


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Additional charge carriers can be generated by photon-induced e

transition in which light is absorbed.

The resultant increase in conductivity is photoconductivity.

Incident

radiation

semi

conductor:

Energy of electron

filled states

unfilled states

Egap

+

-A. No incident radiation:

little current flow

Energy of electron

filled states

unfilled states

Egap

conducting

electron

+

-B. Incident radiation:

increased current flow

Description:

Ex: Photodetector (Cadmium sulfide)

Photoconduction & Solar Cell


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Photoconduction in semiconductors

involves the absorption of a stimulus

by exciting es from the valence band

to the conduction band.

Rather than dropping back to the

valence band to cause emission, the

excited electrons carry a charge

through an electrical circuit.

A solar cell takes advantage of this effect.Operation is the reverse of that for LED.

p-nJunction Solar Cell


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p

Solar Cell


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Operation:- incident photon produces hole-e pair- typically 0.5 V potential

- current increases with light intensity

p-njunction:

n-type Si

p-type Sip-n junction

B-doped Si

Si

Si

Si SiB

hole

P

Si

Si

Si Si

conductance

electron

P-doped Si

n-type Si

p-type Sip-n junction

light

+-

++ +

---

creation of

hole-electron

pair

Solar powered weather station:

polycrystalline Si

Solar Cell


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( ) 1,100M

44, 435

Laser


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Light Amplification by Stimulated Emission of Radiation

Coherent beam - monochromatic

Collimation - pumping and population inversion

Communication, surgery, machining, welding, heat treating, CDs,

bar-code reading, hole piercing, ----------

GaAs Laser
http://en.wikipedia.org/wiki/Image:Military_laser_experiment.jpghttp://en.wikipedia.org/wiki/Image:Starfield_Optical_Range_-_sodium_laser.jpg


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Because the surroundingp-and n-type GaAlAs layers have a higherenergy gap and a lower index of refraction than GaAs, the photons

are trapped in the active GaAs layer.

Solid State Ruby Laser Al O i l t l ( hi )


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Al2O3 single crystal (sapphire)

with 0.05 wt% Cr

Laser


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1. The laser in its non-lasing state

2. The flash tube fires and injects

light into the ruby rod.

The light excites atoms in the ruby.

3. Some of these atomsemit photons

Laser4 Some of these photons run in a


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4. Some of these photons run in a

direction parallel to the rubys axis, sothey bounce back and forth off the

mirrors. As they pass through the

crystal, they stimulate emission in

other atoms

5. Monochromatic, single-phase,

colliminated light

leaves the ruby through the

half-silvered mirror.

-- laser light!

Laser Semic nduct r laser


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Semiconductor laser

Laser Semiconductor laser


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Because the surroundingp- and n-typeGaAlAs layers have a higher energy

and a lower index of refraction than

GaAs, the photons are trapped in theactive GaAs layer.

Semiconductor laser

Laser


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Fiber Optics and Data Transmission Photonic communication


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Photonic communication

Total internal reflection

Core/cladding/coating

High purity silica glass 5 - 100 um

144 glass fibercarry three times

Fiber Optics

t i d ti l fib d i


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step-index optical fiber designcore: silica glassw/higher n

cladding : glassw/lower n

n enhancesinternal reflection

intensity

time

input pulse

broadened!

intensity

time

out put pulsetotal internal reflection

shorter pathlonger paths

graded-index optical fiber designcore: Add gradedimpurity distrib.to make n higher incore center

cladding : (as before)

total internal reflection

shorter, but s lower pathslonger, but faster paths

intens

ity

time

input pulse

intens

ity

time

out put pulse

less

broadening!

graded-index less broadening improvement

Summary When light (radiation) shines on a material, it may be:


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Reflected, absorbed and transmitted.

Optical classification:

Transparent, translucent, or opaque

Metals:

Fine succession of energy state causes absorption and reflection.

Non-Metals:

May have full (Eg < 1.8 eV) , no(Eg > 3.1 eV), or

partialabsorption (1.8 eV < Eg < 3.1 eV) Color is determined by light wavelengths that are

transmitted or re-emitted from electron transitions.

Color may be changed by adding impurities that the band structure. (e.g.,

Ruby)

Problems from Chap. 21 http://ep.snu.ac.kr

Prob. 21-1 Prob. 21-2 Prob. 21-4 Prob. 21-7

Prob. 21-12 Prob. 21-20 Prob. 21-23 Prob. 21-28
http://ep.snu.ac.kr/http://ep.snu.ac.kr/

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