1. OPTICAL PROPERTIES
K L University
Department of Physics

2. Contents Introduction Optical Reflectance Optical Absorbance
Optical fibers
Snell’s law
Total internal reflection
Losses in optical fibers

3. 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.

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

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

6. 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

7. 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

8. 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).

9. 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

13. The interaction of EM radiation with solid materials:
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

14. Optical Properties
Absorption
Reflection

15. 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.

16. 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. 16

17. 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.

18. 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.

19. Optical Properties – Non-Metals
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

20. 20
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).

21. High reflectivity is desired in many applications including mirrors, coatings on glasses, etc.
11/24/2018

22. 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.

23. 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

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

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

26. 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

27. 27
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

28. Computations of Minimum Wavelength Absorbed28
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.

29. Applications of Optical Phenomena29
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

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

31. 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.

33. 33
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

34. OPTICAL FIBER

35. Optical Fiber construction
Engineering
Physics
Core : micro m
Cladd :125 – 200 micro m
Buffer :250 micro m
Jacket :above 250 micro m

36. PRINCIPLE :
Total
Internal
Reflection
Engineering
Physics

37. Three important angles in optical fiber
08/06/14
Three important angles
The reflection angle always equals the incident angle
Refraction Angle
Incident Angles
Reflection Angle
06/08/14 37 37

38. Index of Refraction n = c/v c = velocity of light in a vacuum
08/06/14
n = c/v
c = velocity of light in a vacuum
v = velocity of light in a specific medium
light bends as it passes from one medium to another with a different index of refraction
air, n is about 1
glass, n is about 1.4
Light bends away from normal - higher n to lower n
Light bends in towards normal - lower n to higher n 38

39. Snell’s Law
08/06/14
The angles of the rays are measured with respect to the normal.
n1sin 1=n2sin 2
Where
n1 and n2 are refractive index of two materials
1and 2 the angle of incident and refraction respectively 39

40. Snell’s Law
08/06/14
The amount light is bent by refraction is given by Snell’s Law: n1sin1 = n2sin2
Light is always refracted into a fiber (although there will be a certain amount of Fresnel reflection)
Light can either bounce off the cladding (TIR) or refract into the cladding 40

41. Losses in Optical Fibers 5. Signal distortion Losses
1.Absorptrion Losses
2.Scattering Losses
3.Bending Losses
4. Coupling Losses
5. Signal distortion Losses

42. Thank you