1. OPTICAL PROPERTIES
K L University
Department of Physics

2. SESSION 15

3. Contents Introduction Optical Reflectance Optical Absorbance
Optical Refraction
Optical Scattering
Exciton binding energy
Raman effect in crystals

4. Introduction Optical property:
Classical concept- electromagnetic radiation - wavenature - electric and magnetic field components - perpendicular to each other and also to the direction of propagation.
Ex: Light, heat (or radiant energy), radio waves and x-rays are all forms of electromagnetic radiation.

5. The electric field component of the wave should interact with electrons electrostatically

6. The spectrum of electromagnetic radiation, including wavelength ranges for the various colors in the visible spectrum.

7. The speed of light c, in vacuum,
Quantum-mechanical perspective - radiation, rather than consisting of waves - packets of energy - Photons.
The energy of a single photon is

8. Light Interactions with Solids
Incident light is reflected, absorbed, scattered, and/or transmitted:
solid
Air
Incident: I0
Absorbed: IA
Transmitted: IT
Scattered: IS
Reflected: IR

9. The photons may give their energy to the material (absorption);
Photons give their energy, but photons of identical energy are immediately emitted by the material (reflection);
Photons may not interact with the material structure (transmission);
During transmission photons are changes in velocity (refraction).

10. Absorption Reflection Transmission Where T = Transmissivity = IT/I0
A = Absorptivity = IA/I0
R = Reflectivity = IR/I0

11. Classification of Optical Materials
1.Transparent Materials
2.Translucent Materials
3.Opaque Materials

12. Materials that are capable of transmitting light with relatively little absorption and reflection are TRANSPARENT.
TRANSLUCENT materials are those through which light is transmitted diffusely; that is, light is scattered within the interior, to the degree that objects are not clearly distinguishable when viewed through a specimen of the material.
Materials that are resistant to the transmission of visible light are termed OPAQUE.

13. single-crystal material (sapphire), which is transparent; a polycrystalline and fully dense (nonporous) material, which is translucent; and a polycrystalline material that contains approximately 5% porosity, which is opaque

14. The interaction of EM radiation with solid materials: At Atomic Level.
Electronic Polarization – Two Consequences
A. Some of light is absorbed
B.The velocity of light is reduced in the medium - Leads to the concept of “Refraction” or “Refractive index”
Electronic Transitions
The Absorption or Emission of EM Radiation may involve electron transition from one energy state to another energy state

15. The optical phenomena that occur within solid materials involve interactions between the electromagnetic radiation and atoms, ions, and/or electrons. Two of the most important of these interactions are
Electronic Polarization
For the visible range of frequencies, this electric field interacts with the electron cloud surrounding each atom within its path in such a way as to induce electronic polarization, or to shift the electron cloud relative to the nucleus of the atom with each change in direction of electric field component.
Electron Transitions
For an electron transition, change in energy equals the product of Planck’s constant and the frequency of radiation absorbed (or emitted) E = hν

16. Optical Properties
Refraction
Reflection
Absorption
Transmission

17. Optical Properties - Metals Absorption
When photons are directed at metals, their energy is used to excite electrons into unoccupied states. Thus metals are opaque to the visible light.
Metals are, transparent to high end frequencies i.e. x-rays and γ-rays.

18. Reflectivity = IR /I0 is between 0.90 and 0.95.
Reflection
Reflectivity = IR /I0 is between 0.90 and
Metals are opaque and highly reflective.
Color of reflected light depends on wavelength distribution. 18

19. Metal surfaces appear shiny.
Most of absorbed light is reflected at the same wavelength
Small fraction of light may be absorbed.
Example: The metals copper and gold –red-orange and yellow
Al and Silver – reflective nature.

20. A bright silvery appearance when exposed to white light indicates that the metal is highly reflective over the entire range of the visible spectrum
Copper and gold appear red-orange and yellow, respectively, because some of the energy associated with light photons having short wavelengths is not reemitted as visible light.

21. Optical Properties – Non-Metals
Non-metallic materials are transparent to visible light.
In addition to reflection and absorption, refraction and transmission is to be considered.
Refraction

22. Refraction Light is transmitted into the interior of transparent materials experiences a decrease in velocity as a result bent at the interface
n = index of refraction + no
transmitted
light
electron
cloud
distorts

23. -Light can be “bent” as it passes through a transparent prism

24. Material
Refractive Index
Typical glasses
1.5 – 1.7
Plastics
1.3 – 1.6
PbO (Litharge)
2.67
Diamond
2.41

25. Reflection
The reflectivity R represents the fraction of the incident light that is reflected at the interface,
If the light is normal (or perpendicular) to the interface, then.
When light is transmitted from a vacuum or air into a solid s, then

26. 26
Example: For Diamond n = 2.41
Reflection losses for lenses and other optical instruments are minimized significantly by coating the reflecting surface with very thin layers of dielectric materials such as magnesium fluoride (MgF2).

27. High reflectivity is desired in many applications including mirrors, coatings on glasses, etc.
9/19/2018

28. Absorption
Absorption of a photon of light may occur by the promotion or excitation of an electron from the nearly filled valence band, across the band gap, and into an empty state within the conduction band.

29. These excitations with the accompanying absorption can take place only if the photon energy is greater than that of the band gap Eg - that is, if E2
photon E1

30. Maximum possible band gap energy for absorption of visible light by valence band-to- conduction band electron transitions

31. Minimum possible band gap energy for absorption of visible light by valence band- to-conduction band electron transitions.

32. Of visible spectra absorbed by the materials
Only a portion of the visible spectrum is absorbed by materials having band gap energies between 1.8 and 3.1 eV; consequently, these materials appear colored.
Band gap energies
1.8 – 3.1 ev
Of visible spectra absorbed by the materials

33. 33
The amount of light absorbed by a material is calculated using Beer’s Law
 = absorption coefficient, mm-1  = sample thickness, cm = Non reflected incident light intensity = transmitted light intensity
Rearranging and taking the natural log of both sides of the equation leads to

34. Transmission
For an incident beam of intensity Io, that impinges on front surface of specimen of thickness l and absorption coeff. , the transmitted intensity at the back face IT is
R - Reflectance

36. Computations of Minimum Wavelength Absorbed36
Computations of Minimum Wavelength Absorbed
(a) What is the minimum wavelength absorbed by Ge, for which Eg = 0.67 eV?
Solution:
(b) Redoing this computation for Si which has a band gap of 1.1 eV
Note: the presence of donor and/or acceptor states allows for light absorption at other wavelengths.

37. 37
Scattering of Light
The radiation emanating from the oscillating electrons which travels in all direction is called the scattered radiation.
Blue color in the sunlight gets scattered more than other colors in the visible spectrum and thus making sky look blue.

38. Applications of Optical Phenomena38
Applications of Optical Phenomena
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

39. activator level
Valence band
Conduction band
trapped states Eg
Eemission

40. Phosphorescence:If residence time in trapped state is relatively long(>10-8s)
Fluorescence:For short residence times
(<10-8s) Ex: Sulphides, oxides, tungstates
Based on source for electron excitation, luminescence is three types:
photo-luminescence,
cathode luminescence and
electro-luminescence.

42. 42
Photoluminescence
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

43. 43
Examples
Here ultra-violet radiation from low-pressure mercury arc is converted to visible light by calciumhalo-phosphate phosphor(Ca10F2P6O24).
In commercial lamps, about 20% of F-ions are replaced with Cl-ions.
Antimony,Sb3+,ions provide a blue emission while manganese, Mn2+, ions provide an orange-red emission band.

44. Cathode-luminescence
Cathode-luminescence is produced by an energized cathode which generates a beam of high-energy bombarding electrons.
Ex.:Applications of this include electron microscope; cathode-ray oscilloscope;
color television screens.

45. SESSION 16

46. EXCITON
An exciton is a bound state of an electron and hole, which are attracted to each other by the electrostatic Coulomb force.
It is an electrically neutral quasiparticle that exists in insulators, semiconductors and in some liquids.

47. Exciton binding energy
The exciton is regarded as an elementary excitation of matter that can transport matter without transporting net electric charge.
Exciton binding energy
An electron and a hole bound together by their attractive coulomb interaction, the energy of the bound state is less than that of separated electron hole pair.

48. where Eex is the exciton binding energy
Valance Band
Conduction Band
Band Gap
Exciton Potential Energy
Exciton Levels
The energy of the photon involved in the excitation is
hν = Eg –Eex
where Eex is the exciton binding energy

49. Ya.I. Frenkel ( )
Sir N.F. Mott ( )

50. Mott-Wannier exciton is weakly bound, with an electron-hole separation
They are bound together at larger distances, greater than lattice constant.
Hoping of the exciton from one atom to another is not possible.
There are known as weekly bound excitons.

51. A tightly-bound or Frenkel exciton
They are bound together within a short distance; less than atomic radius
Hoping of the exciton from one atom to another is possible.
There are known as strongly bound excitons.

53. Raman effect

54. Contents Raman effect Raman scattering Stokes lines Anti-Stokes lines
Necessary condition to observe Raman effect
Importance of Raman effect

55. Raman scattering
elastic
Inelastic

57. Raman effect: Inelastic scattering of photons from matter (liquid,
gas and Solid).
Discovered in 1928 by
C.V. Raman and K.S. Krishnan in liquids
G. Landsberg and L.I. Mandelstam in crystals
C.V. Raman received Noble prize in 1930 for this
discovery.

58. Note: Rayleigh scattering is more intense than Raman scattering
Elastic in nature – No exchange of energy takes place during
collision.
Incident and scattered photons have same energy.
Raman scattering:
Inelastic in nature – exchange of energy takes place during
Energy may absorb or release
Stokes lines Material absorbs energy and the emitted photon has a lower energy than the absorbed photon (ʋ-ʋi )
Anti-Stokes lines Material loses energy and the emitted photon has a higher energy than the absorbed photon (ʋ+ʋi )
Note: Rayleigh scattering is more intense than Raman scattering

59. Scattering of photons from matter

63. Necessary condition to observe Raman scattering
Eo : Amplitude of electric field
 : frequency of electric field
q : displacement of atoms
P : dipole-moment
 : polarizability
o : polarizability at
equilibrium position

64. From above equations
First term describes an oscillating dipole that radiates light of
frequency ‘ʋ’ (Rayleigh scattering).
The second term gives the Raman scattering with frequency
ʋ+ʋi (anti-Stokes)
ʋ-ʋi (Stokes)
If is zero the second term vanishes. Thus, the
vibration is not Raman-active

65. To observe Raman effect polarizability should change during vibration

66. Applications: It gives the compositional and structural information of
different class of materials.
Phase transition in materials can be identified more
accurately than powder X-ray diffraction.
This principle is used in optical amplifiers.