(Optical_Properties).pptx

Introduction to Materials Science and Engineering
Chapter 13: Optical Properties
Textbook Chapter 22

Content
1. Electromagnetic Radiation
2. Light Interactions with Solids
3. Optical Properties of Metals
4. Optical Properties of Non-metals
5. Applications

 Optical Property: a material’s response to exposure to electromagnetic
radiation, particularly to visible light.
 Light is energy, or radiation, in the form of waves or particles (energy quanta)
called photons that can be emitted from a material.
 Velocity, c, of electromagnetic radiation in a vacuum
8
c=3x10 m/s
1
: electric permittivity
: magnetic permeability
o o
o
o
c
 



Introduction

Introduction
 The important characteristics of the photons—their energy E,
wavelength λ, and frequency ν—are related by the equation
-34
h: plank's constant (=6.63x10 Js)
hc
E h

 
 Electromagnetic spectrum (wavelength or energy)
g-ray, X-ray, Ultraviolet, Visible, Infrared, microwave, Radio
Visible light: 0.4 m ~ 0.7 m

Light Interaction with Solids
 Incident light is either reflected, absorbed, or
transmitted: Io  IT IA IR
Optical classification of materials:
2 2
[J/m s=W/m ]
I I

Translucent definition, permitting light to
pass through but diffusing it so that persons,
objects, etc., on the opposite side are not
clearly visible

 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 its
ground state
Light Interaction with Solids

Optical Properties of Metals
- all frequencies of visible light absorbed
- opaque to visible light
- total absorption- less than 0.1mm
- re-emit in the form of visible light of the same wavelength
- reflectivity- 0.90~0.95

- Al, Ag: bright silvery
the composition of re-emitted
photons is approximately the
same as for the incident beam
- Cu, Au- red orange & yellow
some of the energy associated
with light photons having
short wavelength is not
re-emitted as visible light
Ag
Au
Optical Properties of Metals

r
- refractive index
sin
(snell's law)
sin
- wavelength dependent
(dispersion)
( )
-
for nonmagnetic 1
vac
r r
mat o o
r
i
n
r
v c
n
v
n

 
 
 

  
 
Refraction
Optical Properties of Nonmetals

 Refraction
- refraction is related to electronic polarization of the materials at
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 larger an atom or ion, 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

Polarizability

2
2 1
2
2 1
2 2
2
2 2
- Fresnel's formula for normal incidence
( )
( )
- from a vavuum or air to solid
-1 ( 1)
( ) or
1 ( 1)
- incidence angle dependent
- wavelength dependent
- ex) n: 1
R
o
s s
s s
I n n
R
I n n
s
n n k
R R
n n k

 

 
 
  
.5 1.9
R: 4 10%


 Reflection

 Absorption
- electronic polarization (important at frequency in the vicinity of
relaxation frequency of constituent atoms)
- valence band-conduction band transition (energy band structure)
electron-hole
generation
electron-hole
recombination

Absorption
- valence band-conduction band transition can take place only if the
photon energy is greater than the band gap energy Eg
or
1.24
g g
hc
h E E
hc eV m



 
 
- 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
 

 Absorption
- impurities or other electrically active defects assisted
- two photons
- one phonon + one photon

x
o
I T
I
' '
'
exp( )
: intensity of nonreflected incident
radiation
4
: absorption coefficient ( )
T o
o
I I x
I
k




 

 Absorption
'
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
β=- ln( )=- ln(0.98)=1.01
x I 200mm
solution
-4 -1
x10 mm

2
(1 ) e l
T o
I I R 

 
 Transmission

 Color
-as a consequence of selective absorption of specific wavelength ranges of light
- if absorption is uniform for all visible wavelength, the material appears colorless
(inorganic glass, diamond, sapphire)
- selective absorption by electron excitation
ex) CdS- Eg=2.4 eV
absorb photons > 2.4 eV (blue-violet portion)
reradiate other wavelength
consequently, take yellow-orange color

 Color
- impurities- electron level within the forbidden bandgap
- ex) sapphire- Al2O3- colorless
ruby- 0.5 to 2% Cr2O3 doped Al2O3- red color

 Opacity and translucency
- Internal reflection and refraction
- scattering
- polycrystalline- grain boundary
- two phase materials with different refractive indices
- porosity in the form of finely dispersed pores
porous alumina
fully dense polycrystalline
single crystal sapphire

 Luminescence – reemission of light by a material
 Material absorbs light at one frequency and reemits it at another
(lower) frequency.
 Trapped (donor/acceptor) states introduced by impurities/defects
• If residence time in trapped state is
relatively long (> 10-8 s)
-- phosphorescence
• For short residence times (< 10-8 s)
-- fluorescence
Example: Toys that glow in the dark.
Charge toys by exposing them to light.
Reemission of light over time—phosphorescence
Luminescence
activator level
Valence band
Conduction band
trapped states
Eg
Eemission

Hg atom
UV light
electrode electrode
• Arc between electrodes excites electrons in mercury atoms in the lamp to
higher energy levels.
• As electron falls back into their ground states, UV light is emitted
(e.g., suntan lamp).
• Inside surface of tube lined with material that absorbs UV and reemits
visible light
- For example, Ca10F2P6O24 with 20% of F-
replaced by Cl-
• Adjust color by doping with metal cations,
Sb3+ blue
Mn2+ orange-red
Photoluminescence

• Used in cathode-ray tube devices (e.g., TVs, computer monitors)
• Inside of tube is coated with a phosphor material
– Phosphor material bombarded with electrons
– Electrons in phosphor atoms excited to higher state
– Photon (visible light) emitted as electrons drop back into ground states
– Color of emitted light (i.e., photon wavelength) depends on composition of
phosphor
ZnS (Ag+
& Cl-
) blue
(Zn, Cd) S + (Cu++Al3+) green
Y2O2S + 3% Eu red
• Note: light emitted is random in phase & direction
– i.e., is noncoherent
Cathodoluminescence

• Description:
• Ex: Photodetector (Cadmium sulfide)
Application: Photoconductivity

 light amplification by stimulated emission of radiation
- coherent beam, - monochromatic
- collimation, - pumping and population inversion
Application: Laser

Application: Laser
 Luby laser

Application: Laser
 Semiconductor laser

Application: Laser
 Semiconductor laser
• Apply strong forward bias across
semiconductor layers, metal, an
d heat sink.
• Electron-hole pairs generated by
electrons that are excited across
band gap.
• Recombination of an electron-h
ole pair generates
a photon of laser light
electron + hole  neutral + hν
recombination ground state
photon of light

Other Applications of Optical Phenomena
• New materials must be developed to make new & improved
optical devices.
– Organic Light Emitting Diodes (OLEDs)
• More than one color available from a single diode
• Also sources of white light (multicolor)

Other Applications - Solar Cells
• p-n junction:
• Operation:
-- incident photon of light produces elec.-hole pair.
-- typical potential of 0.5 V produced across junction
-- current increases w/light intensity.
n-type Si
p-type Si
p-n junction
B-doped Si
Si
Si
Si Si
B
hole
P
Si
Si
Si Si
conductance
electron
P-doped Si
n-type Si
p-type Si
p-n junction
light
+
-
+
+ +
-
-
-
creation of
hole-electron
pair

high purity silica glass 5-100m
144 glass fiber, carry three times
Application: Optical fiber

Application: Optical fiber
- step index optical fiber design
- graded index optical fiber design
B2O3 or GeO2

• Light radiation impinging on a material may be reflected
from, absorbed within, and/or transmitted through
• Light transmission characteristics:
-- transparent, translucent, opaque
• Optical properties of metals:
-- opaque and highly reflective due to electron energy band
structure.
• Optical properties of non-Metals:
-- for Egap < 1.8 eV, absorption of all wavelengths of light radiation
-- for Egap > 3.1 eV, no absorption of visible light radiation
-- for 1.8 eV < Egap < 3.1 eV, absorption of some range of light
radiation wavelengths
-- color determined by wavelength distribution of transmitted light
• Other important optical applications/devices:
-- luminescence, photoconductivity, light-emitting diodes, solar
cells, lasers, and optical fibers
SUMMARY

(Optical_Properties).pptx

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