Department of Physics

1. Department of Physics OPTICAL PROPERTIES K L University

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 Absorbed: IA Reflected: IR Transmitted: IT Incident: I0 Scattered: IS Incident light is reflected, absorbed, scattered, and/or transmitted: solid Air

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 2.Translucent Materials 1.Transparent Materials 3.Opaque Materials

13. The interaction of EM radiation with solid materials: • Electronic Polarization – Two Consequences A. Some of light is absorbed B.Thevelocity 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. Reflection Reflectivity = IR/I0 is between 0.90 and 0.95. Metals are opaque and highly reflective. • Color of reflected light depends on wavelength distribution.

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. 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. 5/20/2017

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. 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.  = 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 The amount of light absorbed by a material is calculated using Beer’s Law

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

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

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. 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. Engineering Physics Optical Fiber construction Core : 5 -100 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 • Three important angles • The reflection angle always equals the incident angle Refraction Angle Incident Angles Reflection Angle 06/08/14 37

38. Index of Refraction • 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

39. Snell’s Law • 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

40. Snell’s Law • 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

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

42. Thank you