Chapter 16: Optical Properties

1. Chapter 16: Optical Properties ISSUES TO ADDRESS... • What happens when light shines on a material? • Why do materials have characteristic colors? • Why are some materials transparent and others not? • Optical applications: -- lasers -- optical communications fibers

4. Optical Properties Light has both particulate and wavelike properties • Photons Important numbers: Our Eyes Can ONLY see: 0.4 to 0.7 µm 1.65 eV to 3.1 eV 4.3x1014 to 7.5x1014 Hz

6. Light Interaction with Solids Absorbed: IA Reflected: IR Transmitted: IT Incident: I0 Scattered: IS • Optical classification of materials: Transparent Adapted from Fig. 21.10, Callister 6e. (Fig. 21.10 is by J. Telford, with specimen preparation by P.A. Lessing.) Translucent Opaque polycrystalline porous polycrystalline dense single crystal • Incident light is either reflected, absorbed, or transmitted:

7. Two main phenomena:Electronic Polarization+Electronic Transitions

8. electron no cloud transmitted + transmitted + distorts light light --Adding large, heavy ions (e.g., lead can decrease the speed of light. --Light can be "bent" Refractive Index, n • Transmitted light distorts electron clouds. • Note: n = f () Typical glasses ca. 1.5 -1.7 Plastics 1.3 -1.6 PbO (Litharge) 2.67 Diamond 2.41 • Light is slower in a material vs vacuum. n = refractive index Selected values from Table 21.1, Callister 7e.

9. Index of Refraction

10. Relationship Between Dielectric and Optical Properties

11. For insulators Question: Is n a function of frequency?? Answer ??????  As  approaches  n ?????

13. Origin is Electronic Polarizations

14. Convince yourself that in general there is a relationship between ion size and n.

15. Light Absorption

16. Relationship betweenDielectric and Optical Properties

17. Band gap! Ionic polariz. Transparency window

20. IR Adsorption Edge…

21. Optical Properties of Metals: Absorption Energy of electron unfilled states DE= h required! Incident photon n h of energy I o filled states freq. Planck’s constant of (6.63 x 10-34J/s) incident light • Absorption of photons by electron transition: • Metals have a fine succession of energy states. • Near-surface electrons absorb visible light and then re-emit it at same wavelength.

22. Scattering • In semicrystalline or polycrystalline materials • Semicrystalline • density of crystals higher than amorphous materials  speed of light is lower - causes light to scatter - can cause significant loss of light • Common in polymers • Ex: LDPE milk cartons – cloudy • Polystyrene – clear – essentially no crystals

23. Selected Absorption: Semiconductors Energy of electron unfilled states blue light: h = 3.1 eV red light: h = 1.7 eV E gap I o filled states • Absorption by electron transition occurs if hn > Egap incident photon energy hn Adapted from Fig. 21.5(a), Callister 7e. • If Egap < 1.8 eV, full absorption; color is black (Si, GaAs) • If Egap > 3.1 eV, no absorption; colorless (diamond) • If Egap in between, partial absorption; material has a color.

25. Wavelength vs. Band Gap Example: What is the minimum wavelength absorbed by Ge? Eg= 0.67 eV If donor (or acceptor) states also available this provides other absorption frequencies

26. Color You need selective absorption. What is left over, plus what is re-emitted by crystal is the color you see. Two examples: a) CdS Band gap = 2.4 eV. What light will be absorbed? Red or blue? What is not absorbed is what you see. b) Sapphire vs Ruby When you add impurities to an insulator you can create levels in the band gap that in turn selectively absorb some frequencies but not others giving rise to color.

27. Color of Nonmetals 80 sapphire 70 Transmittance (%) ruby 60 50 l (= c/)(m) wavelength, 40 0.3 0.5 0.7 0.9 • Color determined by sum of frequencies of -- transmitted light, -- re-emitted light from electron transitions. • Ex: Cadmium Sulfide (CdS) -- Egap = 2.4 eV, -- absorbs higher energy visible light (blue, violet), -- Red/yellow/orange is transmitted and gives it color. • Ex:Ruby = Sapphire (Al2O3) + (0.5 to 2) at% Cr2O3 -- Sapphire is colorless (i.e., Egap > 3.1eV) -- adding Cr2O3 : • introduces localized levels in forbidden gap • blue light is absorbed • yellow/green is absorbed • red is transmitted • Result: Ruby is deep red in color.

28. Ligand Field Theory

29. Example: transition metal cation insapphire. }

30. Scattering 2 limiting cases: 1) r <<  2) r >> 

31. Revisit 2 regimes: 1) r <<  2) r >> 

32. Like a bat outta hell

33. 3 Examples • Milk • Paint…. • Weather • Rain • Humid day in summer • Fog…

34. So why is the SKY blue??? Because of Rayleigh scattering

35. LASER Light • Is non-coherent light a problem? – diverges • can’t keep tightly collimated • How could we get all the light in phase? (coherent) • LASERS • Light • Amplification by • Stimulated • Emission of • Radiation • Involves a process called population inversion of energy states

36. Population Inversion • What if we could increase most species to the excited state? Fig. 21.14, Callister 7e.

37. X X X X X X X Population Inversion First you need to pump electrons into conduction band Electrons like birds on a wire Very rapid Much slower and all together now!! Fig. 21.14, Callister 7e.

38. LASER Light Production • “pump” the lasing material to the excited state • e.g., by flash lamp (non-coherent lamp). Fig. 21.13, Callister 7e. • If we let this just decay we get no coherence.

39. LASER Cavity “Tuned” cavity: • Stimulated Emission • One photon induces the emission of another photon, in phase with the first. • cascades producing very intense burst of coherent radiation. • i.e., Pulsed laser Fig. 21.15, Callister 7e.

40. Total Internal Reflectance n > n’ n’(low) n (high)

41. Example: Diamond in air • Fiber optic cables are clad in low n material for this reason.

42. Optical Fibers • prepare preform as indicated in Chapter 13 • preform drawn to 125 m or less capillary fibers • plastic cladding applied 60 m Fig. 21.20, Callister 7e. Fig. 21.18, Callister 7e.

43. Optical Fiber Profiles Step-index Optical Fiber Graded-index Optical Fiber Fig. 21.21, Callister 7e. Fig. 21.22, Callister 7e.

44. SUMMARY • When light (radiation) shines on a material, it may be: -- reflected, absorbed, scattered, and/or transmitted. • Optical classification: -- transparent, translucent, opaque •Non-Metals: -- may have full (Egap < 1.8eV) , no (Egap > 3.1eV), or partial absorption (1.8eV < Egap = 3.1eV). -- color is determined by light wavelengths that are transmitted or re-emitted from electron transitions. -- color may be changed by adding impurities which change localized energy levels (e.g., Ruby) • Refraction: -- speed of transmitted light varies among materials. -- refraction and reflection are due to electronic polarizations -- The latter are proportional to the size of the ions.

45. From here on optional.

46. From here on optional..

47. Semiconductor LASER • Apply strong forward bias to junction. Creates excited state by pumping electrons across the gap- creating electron-hole pairs. electron + hole  neutral + h ground state excited state photon of light Adapted from Fig. 21.17, Callister 7e.

48. Uses of Semiconductor LASERs • #1 use = compact disk player • Color? - red • Banks of these semiconductor lasers are used as flash lamps to pump other lasers • Communications • Fibers often turned to a specific frequency (typically in the blue) • only recently was this a attainable

50. Applications of Materials Science • New materials must be developed to make new & improved optical devices. • Organic Light Emitting Diodes (OLEDs) • White light semiconductor sources • New semiconductors • Materials scientists (& many others) use lasers as tools. • Solar cells