Materials Science and Engineering Ch 21 OPTICAL PROPERTIES Chapter 21. OPTICAL PROPERTIES

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Introduction to

Materials Science and Engineering Ch 21 OPTICAL PROPERTIES Chapter 21. OPTICAL PROPERTIES

¾ What happens when light shines on a material?W ppe s w e g s es o e ?

¾ Why do materials have characteristic colors?

¾ Why are some materials transparent and others not?

¾ Optical applications:

√ L i

√ Luminescence

√ Display

√ S l ll

√ Solar cell

√ Photoconductivity

√ L

√ Laser

√

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Introduction

l P l’

¾ Optical Properties - A material’s response to exposure to electromagnetic radiation, particularly to visible light.

¾ Light is energy, or radiation, in the form of waves or particles called photons that can be emitted from a material.

¾ The important characteristics of the photons — energy E, wavelength λ, and frequency ν — are related by the equation:

E hν hc

= = λ

C = ν λ

(Fig. 21-1)

ε Dielectric constant

(Fig. 18-34)

μ Magnetic permeability

(6)

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Electromagnetic Spectrum

(Fig. 21-2)

( g )

390 nm - 720 nm

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Light Interaction with Solids

¾ Incident light is either reflected, absorbed, or transmitted.

I = I + I + I

Reflected: IR Absorbed: IA

Transmitted: IT

I₀ = I_T + I_A + I_R T + A + R = 1

Incident: Io

Transmitted: IT

Transmissivity (I_T/I₀) Absorptivity (I_A/I₀) Absorptivity (I_A/I₀) Reflectivity (I_R/I₀)

(Eq. 21-4)

¾ Optical classification of materials

translucent

( q )

transparent

opaque opaque

(Fig. 21-10)

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Light Interaction with Solids

Reflection Absorption Transmission Refraction

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Isolated Atoms for Photon Absorption

(Fig 21-3)

Electron transitions (Fig. 21 3)

Ab i & i i

- Absorption & emission - Discrete, specific energy

- Short stay in an excited

_ y

state, and decay back into its ground state.

__

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Absorption in Metals

¾ Absorption of photons by electron transition.

Energy of electron Energy of electron n

unfilled states

Incident photon

ΔE = hν required!

rgy hν

Inc

Planck constant 뭩 freq.

f

filled states

Io _{of e}^ner^gy

(6.63 x 10-34 J/s) of

incident

light (Fig. 21-4)

¾ Metals have almost continuously available empty electronic states which permit electron transitions electronic states, which permit electron transitions.

¾ Near-surface electrons absorb visible light.

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Reflection in Metals

¾ Electron transition emits a photon.

Energy of electron

unfilled states

IR 밹onducting?electron

ΔE

onducting?electron 밹

Re-emitted photon from

filled states

photon from

material surface

¾ Reflectivity (IR/I0) is approximately 0.90 - 0.95.

¾ Reflected light has almost the same frequency as incident.

¾ Metals are opaque & highly reflective (shiny).

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Optical Properties of Metals

(Fig 21 4)

- Reemit in the form of visible light of similar wavelength.

(Fig. 21-4)

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Optical Properties of Nonmetals

refractive index

¾ Refraction

- refractive index

sin i (snell's law)

n = ( )

sin

- wavelength dependent r

(dispersion)

v εμ

- ^vac _r _r

mat o o

n v

v

εμ ε μ

= = ε μ = ____

_

n ≅ ε_r for nonmagnetic μr ≅1 (E 21 9)

(Eq. 21-9)

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Optical Properties of Nonmetals

¾ Polarizability (skip)

Fig. 18-34 어려워요 어려워요 중요해요

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Optical Properties of Nonmetals

¾ Dispersion

(Page W57) (Page W57)

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Optical Properties of Nonmetals

¾ Absorption:

valence-band to conduction-band transition

( b d )

(energy band structure)

(Fig. 21-5) electron-hole

generation generation

electron-hole recombination recombination

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Optical Properties of Nonmetals

¾ Absorption

- Valence-band to conduction-band transitionValence band to conduction band transition can take place only if the photon energy is greater than that of the bandgap energy E greater than that of the bandgap energy E_g.

or hc

h

ν

> E_g or > E_g ^{(Eq 21-14)}

h

ν

E E

>

λ

>

- for visible light

(Eq. 21 14)

0.7

μ

m (=1.8 eV) ~ 0.4

μ

m (=3.1 eV)

- E_g less than 1.8 eV - all visible light absorb - opaque 1 8 eV < E < 3 1 eV partial absorption color

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

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Selected Absorption: Nonmetals

¾ Absorption by electron transition occurs if hν > Egap

unfilled states blue light: hν= 3.1eV

red light: hν= 1 7eV

Egap red light: hν= 1.7eV

incident photon

400 nm = 3.1 eV 700 nm = 1.8 eV

filled states

Io

incident photon energy hn

¾ If E_gap < 1.8 eV, full absorption of visible light Æ color is black

√ Si (1.12 eV), GaAs (1.42 eV)

¾ If E_gap > 3.1 eV, no absorption Æ transparent & colorless

√ Diamond (5 6 eV)

√ Diamond (5.6 eV)

¾ If E in between, partial absorption - material has a color.

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Optical Properties of Nonmetals

¾ Absorption

Electrically active defects (or impurities).

(Fig. 21-6)

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Optical Properties of Nonmetals

¾ C l Å i i i l l l i hi h f bidd b d

¾ Color Å impurities - electron level within the forbidden bandgap

¾ ex) sapphire (Al₂O₃) – colorless (E_gap > 3.1eV)

√ Ruby (0.5 to 2% Cr₂O₃ doped Al₂O₃) - red color (adding Cr₂O₃ to sapphire)

- Alters the band gap, blue/yellow light is absorbed, and red is transmitted. Æ Ruby is deep red in color.

(Fig. 21-9)

- 2009-12-14

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Application: Luminescence

Luminescence: Light emission in the visible spectrum

accompanying the absorption of other forms acco pa y g t e abso pt o o ot e o s of energy (high energy photon or electrons)

photoluminescence or electroluminescence - photoluminescence or electroluminescence.

Fl E i i f l t ti di ti

Fluorescence: Emission of electromagnetic radiation

occurs typically in much less than one second (down to ~10^-8 s) of an excitation event.

Phosphorescence: Emission of electromagnetic radiation over an extended period of time after the p

excitation event is over. (sec. 21-11)

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Luminescence

A fluorescent lamp is a type of lamp that uses electricity to excite mercury vapor in argon or neon gas, resulting in a plasma that produces short-wave ultraviolet light. This

light then causes a phosphor to fluoresce, producing light then causes a phosphor to fluoresce, producing visible light.

(skip)

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Light Emitting Diode (LED)

(sec. 21-12)

(Fig. 21-11)

A forward-bias voltage across the p-n junction can

d h t

produce photons.

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Electron-Hole Recombination of a p-n Junction

(skip)

Kittel, Solid State , Physics (Chapter 17).

Electron Energy

×

(Fig. 21-11)

30

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Photoconductivity

¾ Additional charge carriers are generated by photon-induced electron transitions:

Æ The resultant increase in conductivity is photoconductivity.

¾ D i i

(thermal energy = k_B T -- section 18-6)

+

¾ Description: +

Incident radiation semi

conductor:

Energy of electron unfilled states

Egap

unfilled states

Egapconducting electron

filled states

-

filled states

A. No incident radiation: -

little current flow B. Incident radiation:

increased current flow

¾ Ex: Photodetector (Cadmium sulfide)

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p-n Junction

Solar Cell

LED Solar Cell

LED

전기 빛

반도체 빛 반도체 전기

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Solar Cell

( ki )

¾ p-n junction:

¾ Operation:

- Incident photon produces hole-electron pairs.

Current increases with light intensity

(skip)

- Current increases with light intensity.

conductance Si electron

P-doped Si

creation of

P Si

Si Si

n-type Si p-n junction

light

-- - -

hole-electron pair

n-type Si p-type Si

p-n junction ^{p-type Si}

p-n junction +

+ + +

Si

Si B Si

hole ¾ Solar powered weather station:

B-doped SiSi

polycrystalline Si polycrystalline Si

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Solar Cell

교통신호등 교통신호등

마라도 유인등대

가리왕산(강원도 정선군)

해발 1,100M에 위치한 대피소용 건물 일본 교세라 본사빌딩

년간 44,435리터의 석유 절약효과

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Laser

^(skip)

¾ Light Amplification by Stimulated Emission of Radiation

√ Coherent beam - monochromatic

√ Collimation - pumping and population inversion

¾ Communication surgery machining welding heat treating CD’s

¾ Communication, surgery, machining, welding, heat treating, CD s, bar-code reading, hole piercing, ---

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GaAs Laser

¾ Because the surrounding p- and n-type GaAlAs layers have a higher energy gap and a lower index of refraction than GaAs the photons energy gap and a lower index of refraction than GaAs, the photons are trapped in the active GaAs layer.

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Solid State Ruby Laser

¾ Al O i l t l ( hi )

¾ Al₂O₃ single crystal (sapphire) with 0.05 wt. % Cr.

Fig. 21-13

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Laser

¾ Semiconductor laser Fig. 21-16

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Laser

¾ Semiconductor laser

Fig. 21-17

Because the surrounding p- and n- t G AlAs l s h hi h type GaAlAs layers have a higher bandgap and a lower index of

refraction than GaAs the photons refraction than GaAs, the photons are trapped in the active GaAs layer.

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Laser

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Fiber Optics and Data Transmission

Ph

(skip)

¾ Photonic communication

¾ Total internal reflection

¾ Core/cladding/coating

144 glass fiber carry three times

High purity silica glass 5 - 100 um

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Fiber Optics

¾ Step-index optical fiber design

core: silica glass

w/higher n input pulse total internal reflection output pulse

w/higher n

cladding: glass w/lower n

Δn enhances ntensity shorter path ntensity Δn enhances

internal reflection in

time broadened!

in time

shorter path longer paths

¾ Graded-index optical fiber design

core: Add graded input pulse total internal reflection output pulse

impurity distrib.

to make n higher in core center

ensity

p p

ensity

p p

cladding: (as before) shorter, but slower paths longer, but faster paths

inte

time inte

less time

broadening!

¾ graded-index Æ less broadening Æ improvement

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Summary

¾ When light (radiation) shines on a material it may be:

¾ When light (radiation) shines on a material, it may be:

√ Reflected, absorbed, and transmitted.

¾ Metals:

√ Fine succession of energy state causes absorption and reflection.

¾ Non Metals:

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

partial absorption (1 8 eV < E < 3 1 eV) partial absorption (1.8 eV < E_g < 3.1 eV)

√ Color is determined by light wavelengths that are

transmitted or re emitted from the electron transitions transmitted or re-emitted from the electron transitions.

¾ Problems from Chap 21 http://bp snu ac kr

¾ Problems from Chap. 21 http://bp.snu.ac.kr

Prob. 21-1 Prob. 21-2 Prob. 21-4 P b 21 7 P b 21 20 P b 21 23 Prob. 21-7 Prob. 21-20 Prob. 21-23

Materials Science and Engineering Ch 21 OPTICAL PROPERTIES Chapter 21. OPTICAL PROPERTIES

Introduction to