Bab Xi. Sifat 0ptik

7/30/2019 Bab Xi. Sifat 0ptik

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Based on: B.S. Mitchell,An introduction to materials

engineering and science for chemical and materials engineers

pp644-659


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Optical devices

Examples: mirrors, lenses, beam splitters, photovoltaic devices


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Optical Properties of Materials

Interaction of electromagnetic radiation (light) with a material

Absorption

Reflection

Transmission

1 =IA

/I0

+IR

/I0

+IT/I0

1 = A + R + TI0 = IA + IR + IT

Total Intensity / Initial IntensityI0

Suatu material tidak dapat sekaligus bersifat highly

absorptive, reflective and transmissive

Absorptivity

Reflectivity

Transmissivity


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Optical Properties of Metals and Alloys

Shininess and inability to transmit visible light indicates

high absorption

high reflection

Incident beam Transmitted beam

Reflected beam

I0

IR= I0R

I0(1 - R) I 0(1 - R)e-l

I0R(1 - R)e-l

I0(1 - R)2e-l

l

I0- IRIncident beam Transmitted beam

Reflected beam

I0

IR= I0R

I0(1 - R) I 0(1 - R)e-l

I0R(1 - R)e-l

I0(1 - R)2e-l

l

I0- IR

linear absorption coefficient

dx

dt

dI

(up to R= 1)

and Rmenentukan bagaimana cahaya berinteraksi

dengan material


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Reflectance and color

UV

Vis

IR

(m)

R

(Hz)

Ag

Au

Cu

Ni

Au

Ag

Cu

Ni

Empty electronic states above occupied levels

Light absorption:

- e- promotion

- decay

- reemission

- reflectivity

- dependent on

frequency

- Electronic effects


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Reflectance and color - Surface texture

Incident light

Specular reflection

Diffuse reflection

True surfacetopography

Average surface

Incident light

Specular reflection

Diffuse reflection

True surfacetopography

Average surface

Specular reflection

- smooth or mirror-like surface

Rough surface

Incident light I0

I = I0cos

Rough surface

Incident light I0

I = I0cos

Diffuse reflectance

- rough surfaces- at all angles

- I= I0cos


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Photoelectric effecth

h=hc

W

Vacuum level

KE

V = 0

h

h=hc

W

Vacuum level

KE

V = 0

Release of electrons due to absorption of light energy

- potential energy barrier for surface electrons is finite

- critical energy for release: = W- Ef = hc

- below c: no ejection of photoelectrons

- characteristic measure


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Photoelectric effect Na

K

Rb

Cs

Wavelength ()

Relelectronemission

Na

K

Rb

Cs

Wavelength ()Wavelength ()

Relelectronemission

Relelectronemission

Photoelectric emission depending on wavelength

- optimal emission at c

- below c: insufficient energy

- above c : decrease of electronic excitations efficiency

c

chc

h


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Electromagnetic spectrum

0.4 m

0.5 m

0.6 m

0.7 m

Visible spectrum

wavelength

Wavelength (m)Energy (eV)

Frequency (Hz)

0.4 m

0.5 m

0.6 m

0.7 m

Visible spectrum

wavelength

0.4 m

0.5 m

0.6 m

0.7 m

Visible spectrum

wavelength

Wavelength (m)Energy (eV)

Frequency (Hz)


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Optical Properties of ceramics and glasses

Refractive index n

- velocity of light in vacuum: c= 299 792 458 m/s

- velocity of light in any other medium: v(v< c)

- refractive index n = c/v

- c can be related to 0 and 0

- v can be related to and 00

1

c

1v

)1(

r

n

rn - ceramics posess small susceptibilities:


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Refractive index

Values between 1 and 4

- air: 1.003- silicate glasses: 1.5 to 1.9

- solid oxide ceramics: 2.7

Dependent on structure-type and packing geometry- glasses and cubic crystals: n is independent of direction

- other crystal systems: n larger in closed-packed directions

- SiO2: glass= 1.46, tridymite = 1.47, cristobaltite = 1.49

quartz = 1.55


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Cristalline silicate vs glass

TEM images

Quartz

Addition of large ions

(Pb, Ba) to

SiO2 structures

increases n significantly


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Refractive index

Mechanical distortions of isotopic glasses changes n

- tensile stress: lowern normal to direction of applied stress- compression: highern normal to direction of applied stress

n dependent on frequency of light

Dense flint glass

Light flint glass

Borosilicate glass

Wavelength (m)

Refractiveindex

Hydrogen F Hydrogen C

Sodium D

Dense flint glass

Light flint glass

Borosilicate glass

Wavelength (m)

Refractiveindex

Hydrogen F Hydrogen C

Sodium D

d

dnDispersion


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Reflection and refractionIncident beam Reflected beam

Vacuum or air

Glass

Refracted beam

i i

r

Incident beam Reflected beam

Vacuum or air

Glass

Refracted beam

i i

r

n can be expressed with theangles of incidence and

refraction

r

in

sin

sin

n can be used to describe reflectivity R

2

2

0 )1(

)1(

n

n

I

IR R

n and Rvary with wavelength


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Absorbance and color

Non-reflected light can be transmitted or absorbed

Absorption process is a function of energy (wavelength)Absorption: fractional change of light intensity dx

dt

dI

Absorption coefficient is a material property

and a function of the wavelength

k4

Absorption of photon: excitation of electron from valence to

conduction band. Only if photon energy > band gap

Magnitude of band gap determines if the material

- does not absorb (transparent)

- absorbs certain wavelength (opaque)

gEh


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Penyerapan intrinsik

Kekuatan dissipasi per volume dalam dielektrik

ditunjukan dengan :

2

oo

''

edc

2

oacV E)k(2

1E

2

1P

dx

dIPv

intensitas cahaya pada medium yang mempunyai indek bias

dinyatakan sebagai :

2

cEnI

2

0o

Ic

I2dI a

)xexp(

I

Ia

o

T

dimana x = panjang optic dan a = konstanta penyerapan =c

2


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Absorban oleh ion pengotor

Icdx

dIiBL

)cix(expI

IBL

O

T

)cix(expII BLO

T

Ion pengotor dari material dapat menyerap cahayasecara selektif pada panjang gelombang spesifik(khromophor ).

BL adalah konstanta yang tergantung pada pengotor dan

medium/ koefisien penyerapan linear/ koefisien pemadaman


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Absorbance and color

Absorption of certainwavelength results in

colorRuby

Sapphire

Al2O3 with traces

of Ti3+ Cr3+

(m)

Transmittance(

%)

Ruby

Sapphire

Al2O3 with traces

of Ti3+ Cr3+

(m) (m)

Transmittance(

%)

Transmittance(

%)

Ruby: Corundum structure

with point defects of Cr3+

Filled valence band

Empty conduction band

Generating color in ceramics:

Addition of transition elements

with incomplete d band fillingV, Cr, Mn, Fe, Co, Ni


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Light scattering in solids

Pore (n=1)

Incident light ray

Scattered light rayGlass or ceramic

(n>1)

Pore (n=1)

Incident light ray

Scattered light rayGlass or ceramic

(n>1)

Some inherently transparent materialsappear milky: translucency

Scattering

- Pores (npore < nsolid)

- second-phase particles (SnO2) (n2nd phase > nsolid)


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Summary

Absorption, Transmission, Reflection Origin of colors in metals and ceramics

Refractive index

Light scattering

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