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Chapter 16: Optical Properties PowerPoint PPT Presentation


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Chapter 16: Optical Properties. ISSUES TO ADDRESS. • What happens when light shines on a material ?. • Why do materials have characteristic colors?. • Why are some materials transparent and others not?. • Optical applications: -- lasers -- optical communications fibers.

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Chapter 16: Optical Properties

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Chapter 16: Optical Properties

ISSUES TO ADDRESS...

• What happens when light shines on a material?

• Why do materials have characteristic colors?

• Why are some materials transparent and others not?

• Optical applications:

-- lasers

-- optical communications fibers


Optical Properties

Light has both particulate and wavelike properties

  • Photons

Important numbers:

Our Eyes Can ONLY see:

0.4 to 0.7 µm

1.65 eV to 3.1 eV

4.3x1014 to 7.5x1014 Hz


Light Interaction with Solids

Absorbed: IA

Reflected: IR

Transmitted: IT

Incident: I0

Scattered: IS

• Optical classification of materials:

Transparent

Adapted from Fig. 21.10, Callister 6e. (Fig. 21.10 is by J. Telford, with specimen preparation by P.A. Lessing.)

Translucent

Opaque

polycrystalline porous

polycrystalline dense

single crystal

• Incident light is either reflected, absorbed, or

transmitted:


Two main phenomena:Electronic Polarization+Electronic Transitions


electron

no

cloud

transmitted

+

transmitted

+

distorts

light

light

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

can decrease the speed of light.

--Light can be

"bent"

Refractive Index, n

• Transmitted light distorts electron clouds.

  • Note: n = f ()

    Typical glasses ca. 1.5 -1.7

    Plastics 1.3 -1.6

    PbO (Litharge) 2.67

    Diamond 2.41

• Light is slower in a material vs vacuum.

n = refractive index

Selected values from Table 21.1,

Callister 7e.


Index of Refraction


Relationship Between Dielectric and Optical Properties


For insulators

Question: Is n a function of frequency??

Answer ?????? 

As  approaches  n ?????


Origin is Electronic Polarizations


Convince

yourself

that in general

there is a

relationship

between

ion size

and n.


Light Absorption


Relationship betweenDielectric and Optical Properties


Band

gap!

Ionic

polariz.

Transparency

window


IR Adsorption Edge…


Optical Properties of Metals: Absorption

Energy of electron

unfilled states

DE= h required!

Incident photon

n

h

of energy

I

o

filled states

freq.

Planck’s constant

of

(6.63 x 10-34J/s)

incident

light

• Absorption of photons by electron transition:

• Metals have a fine succession of energy states.

• Near-surface electrons absorb visible light and

then re-emit it at same wavelength.


Scattering

  • In semicrystalline or polycrystalline materials

  • Semicrystalline

    • density of crystals higher than amorphous materials  speed of light is lower - causes light to scatter - can cause significant loss of light

  • Common in polymers

    • Ex: LDPE milk cartons – cloudy

    • Polystyrene – clear – essentially no crystals


Selected Absorption: Semiconductors

Energy of electron

unfilled states

blue light: h = 3.1 eV

red light: h = 1.7 eV

E

gap

I

o

filled states

• Absorption by electron transition occurs if hn > Egap

incident photon energy hn

Adapted from Fig. 21.5(a), Callister 7e.

• 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.


Wavelength vs. Band Gap

Example: What is the minimum wavelength absorbed by Ge?

Eg= 0.67 eV

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


Color

You need selective absorption. What is left over, plus what

is re-emitted by crystal is the color you see.

Two examples:

a) CdS Band gap = 2.4 eV.

What light will be absorbed? Red or blue?

What is not absorbed is what you see.

b) Sapphire vs Ruby

When you add impurities to an insulator you can create levels

in the band gap that in turn selectively absorb some frequencies

but not others giving rise to color.


Color of Nonmetals

80

sapphire

70

Transmittance (%)

ruby

60

50

l (= c/)(m)

wavelength,

40

0.3

0.5

0.7

0.9

• 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/yellow/orange is transmitted and gives it color.

• Ex:Ruby = Sapphire (Al2O3) + (0.5 to 2) at% Cr2O3

-- Sapphire is colorless

(i.e., Egap > 3.1eV)

-- adding Cr2O3 :

• introduces localized

levels in forbidden gap

• blue light is absorbed

• yellow/green is absorbed

• red is transmitted

• Result: Ruby is deep red in color.


Ligand Field Theory


Example: transition metal cation insapphire.

}


Scattering

2 limiting cases:

1) r << 

2) r >> 


Revisit 2 regimes:

1) r << 

2) r >> 


Like

a bat

outta hell


3 Examples

  • Milk

  • Paint….

  • Weather

    • Rain

    • Humid day in summer

    • Fog…


So why is the SKY blue???

Because of Rayleigh scattering


LASER Light

  • Is non-coherent light a problem? – diverges

    • can’t keep tightly collimated

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

    • LASERS

      • Light

      • Amplification by

      • Stimulated

      • Emission of

      • Radiation

  • Involves a process called population inversion of energy states


Population Inversion

  • What if we could increase most species to the excited state?

Fig. 21.14, Callister 7e.


X

X

X

X

X

X

X

Population Inversion

First

you

need

to

pump

electrons

into

conduction

band

Electrons

like

birds

on a

wire

Very rapid

Much slower

and all

together now!!

Fig. 21.14, Callister 7e.


LASER Light Production

  • “pump” the lasing material to the excited state

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

Fig. 21.13, Callister 7e.

  • If we let this just decay we get no coherence.


LASER Cavity

“Tuned” cavity:

  • Stimulated Emission

    • One photon induces the emission of another photon, in phase with the first.

    • cascades producing very intense burst of coherent radiation.

  • i.e., Pulsed laser

Fig. 21.15, Callister 7e.


Total Internal Reflectance

n > n’

n’(low)

n (high)


Example: Diamond in air

  • Fiber optic cables are clad in low n material for this reason.


Optical Fibers

  • prepare preform as indicated in Chapter 13

  • preform drawn to 125 m or less capillary fibers

  • plastic cladding applied 60 m

Fig. 21.20, Callister 7e.

Fig. 21.18, Callister 7e.


Optical Fiber Profiles

Step-index Optical Fiber

Graded-index Optical Fiber

Fig. 21.21, Callister 7e.

Fig. 21.22, Callister 7e.


SUMMARY

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

-- reflected, absorbed, scattered, and/or transmitted.

• Optical classification:

-- transparent, translucent, opaque

•Non-Metals:

-- may have full (Egap < 1.8eV) , no (Egap > 3.1eV), or

partial absorption (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 localized energy levels (e.g., Ruby)

• Refraction:

-- speed of transmitted light varies among materials.

-- refraction and reflection are due to electronic polarizations

-- The latter are proportional to the size of the ions.


  • From here on optional.


  • From here on optional..


Semiconductor LASER

  • Apply strong forward bias to junction. Creates excited state by pumping electrons across the gap- creating electron-hole pairs.

electron + hole  neutral + h

ground state

excited state

photon of light

Adapted from Fig. 21.17, Callister 7e.


Uses of Semiconductor LASERs

  • #1 use = compact disk player

    • Color? - red

  • Banks of these semiconductor lasers are used as flash lamps to pump other lasers

  • Communications

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

    • only recently was this a attainable


Applications of Materials Science

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

    • Organic Light Emitting Diodes (OLEDs)

    • White light semiconductor sources

  • New semiconductors

  • Materials scientists

    (& many others) use lasers as tools.

  • Solar cells


P-doped Si

conductance

Si

electron

creation of

Si

P

Si

hole-electron

light

pair

Si

-

-

-

n-type Si

-

p-n junction

+

n-type Si

p-type Si

+

+

+

p-n junction

P-type Si

Si

hole

B

Si

Si

Si

B-doped Si

Solar Cells

• p-n junction:

• Operation:

-- incident photon produces hole-elec. pair.

-- typically 0.5 V potential.

-- current increases w/light intensity.

• Solar powered weather station:

polycrystalline Si

Los Alamos High School weather

station (photo courtesy

P.M. Anderson)


Conduction band

trapped states

Eemission

Eg

activator level

Valence band

Luminescence

  • Luminescence – emission of light by a material

    • material absorbs light at one frequency & emits at another (lower) frequency.

  • How stable is the trapped state?

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

  • If less stable (<10-8 s) = fluorescence

  • Example: Glow in the dark toys. Charge them up by exposing them to the light. Reemit over time. -- phosphorescence


Hg

uv

electrode

electrode

Photoluminescence

  • Arc between electrodes excites mercury in lamp to higher energy 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


Cathodoluminescence

  • Used in T.V. set

    • Bombard phosphor with electrons

    • Excite phosphor to high state

    • Relaxed by emitting photon (visible)

      ZnS (Ag+ & Cl-)blue

      (Zn, Cd) S + (Cu++Al3+)green

      Y2O2S + 3% Eured

  • Note: light emitted is random in phase & direction

    • i.e., noncoherent


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 lasers

    • Welding

    • Drilling

    • Cutting – laser carved wood, eye surgery

    • Surface treatment

    • Scribing – ceramics, etc.

    • Photolithography – Excimer laser


Optical Properties of Metals: Reflection

• Electron transition emits a photon.

Energy of electron

IR

unfilled states

“conducting” electron

re-emitted photon from material surface

DE

filled states

Adapted from Fig. 21.4(b), Callister 7e.

• Reflectivity = IR/Io is between 0.90 and 0.95.

• Reflected light is same frequency as incident.

• Metals appear reflective (shiny)!


Metals

No damping - no restoring force

viz. free electrons in a metal

Polarization of all Nf free electrons


No restoring force,  and no friction, f = 0, then:

Polarization of all Nf free electrons

At low frequencies, the second term is > 1 and n is negative

which is the same as saying the wave is reflected.

At high frequencies, the second term < 1, and n is positive,

i.e. real but less than one.

Zeff is effective # of free electrons/atom


  • Example: Diamond

Reflectivity, R

  • Reflection

    • Metals reflect almost all light

    • Copper & gold absorb in blue & green => gold color


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