Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege

1. Engineering 45 OpticalProperties Bruce Mayer, PE Licensed Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

2. Learning Goals – Optical Props • Learn How Light and Solid Materials Interact • Why materials have characteristic colors • Why some materials transparent and others not • Optical applications: • Luminescence • Photoconductivity • Solar Cell • Optical Fiber Communications

3. Properties of Solid Materials • Mechanical: Characteristics of materials displayed when forces are applied to them. • Physical: Characteristics of materials that relate to the interaction of materials with various forms of energy. • Chemical: Material characteristics that relate to the structure of a material. • Dimensional: Size, shape, and finish

4. Material Properties Chemical Physical Mechanical Dimensional Composition Melting Point Tensile properties Standard Shapes Microstructure Thermal Toughness Standard Sizes Phases Magnetic Ductility Surface Texture Grain Size Electrical Fatigue Stability Corrosion Optical Hardness Mfg. Tolerances Crystallinity Acoustic Creep Molecular Weight Gravimetric Compression Flammability

5. ElectroMagnetic Radiation • Energy associated with Light, Radio Signals, X-rays and Others is Transmitted as ElectroMagnetic (EM) Radiation (EMR) • Electromagnetic radiation Transmits energy in the form of a Sinusoidal wave Which Contains ELECTRICAL & MAGNETIC Field-Components • The EM waves Travel in Tandem, and are perpendicular to • Each Other • The Direction Of Propagation

6. The EM Spectrum • EM Waves Cover a Wide Range of WAVELENGTHS, , and FREQUENCIES,  • : miles→femtometers • “Light” is generally divided into Three Segments • UltraViolet: 0.001→0.35 µm • NOT Visible, High in Energy • Visible: 0.35→0.7 µm • A VERY Small Slice of the EM spectrum • InfraRed: 0.7-1000 µm • Not Visible; carries “sensible” energy (heat)

7. All EM Waves Travel at the Speed of Light, c c is a Universal Constant with a value of 300 Mm/s (186 000 miles/sec) c is related to the Electric & Magnetic Universal Constants EM Radiation Quantified • Where (Recalling From Previous Lectures) • 0  ELECTRIC Permittivity of Free Space (a vacuum) • µ0  MAGNETIC Permeability of Free Space (a vacuum)

8. The Wavelength and Frequency of EM waves are related thru c EM Radiation Quantified • EM radiation has a Wave↔Particle Duality • The Energy, E, of a Light Particle • Where •   WaveLength in meters per cycle •   Frequency in Hertz (cycles/sec) • Where h  Planck’s Constant (6.63x10-34 J-s) • h is the PHOTON Energy

9. Consider EM Radiation with Intensity I0 (in W/m2) Impinging on a Solid EM-Solid Interaction • The EM-Solid interaction Alters the incident Beam by 3 possible Phenomena • The EM Beam can be • Reflected • Absorbed • Transmitted

10. EM-Solid Interaction cont • Mathematically • Where all the IK are Intensities in W/sq-m • Now Divide E-Balance Eqn by I0 • An Energy Balance on the Solid: • E-in = E-reflected + E-absorbed + E-transmitted

11. EM-Solid Interaction cont.2 • Where: • R  REFLECTANCE (IR/I0) • A  ABSORBANCE (IA/I0) • T  TRANSMITTANCE (IT/I0) • Using R, A, T, Classify EM-Solid Behavior • Opaque → T = 0 • Transparent → • T >> A+R • Light Not Scattered • Translucent→ • T > A+R • Light Scattered

12. Energy of electron unfilled states D = h E required Incident photon of Energy h I o filled states Metals – Optical Absorption • Metals Interact with Light Thru QUANTIZED Photon Absorption by Electrons • Metals have Very Closely Spaced e- Energy Levels • Thus Almost ALL incident Photons are ABSORBED within about 100 nm of the surface

13. Energy of electron IR unfilled states “conducting” electron re-emitted photon from material surface D E filled states Metals – Optical Reflection • The Absorbed Energy is ReEmitted by e- “falling” back to Lower Energy states • Since Metals have Very Closely Spaced e- Energy Levels The Light is emitted at many ’s • Thus Outgoing Light Looks About the Same as Incoming Light → High Reflectance

14. Metals - Colors • Metals also ABSORB Some Photons • Dissipated as heat • Metals that Absorb few, orin broad-spectrum, reflect “WHITE” Light and Appear Silvery • Some Metals absorb Preferentially, and the Reflected Light is Colored due the absence of the Absorbed light • e.g., Cu Absorbs in the Violet-Blue; leaving Reflected light rich in Orange-Red Cu Bar Sn-Plated Cu Bar

15. Energy of electron unfilled states blue light: h 3.3 ev red light: h 1.8 ev incident photon energy hn E gap I o filled states NonMetals – Selective Absorb. • In The Case of Materials with “Forbidden” Gaps in the Band Structure, Absorption Occurs only if h>Egap • For TheseMaterials there is Very little ReEmission • The Material Color Depends on the Width of the BandGap

16. Color Cases – BandGap Matls • Egap < 1.8 eV • ALL Visible Light Absorbed; Solid Appears Gray or Black in Color • e.g., Si with Egap = 1.1 eV • Egap > 3.3 eV • NO Visible Light Absorbed; Solid Appears Clear and Transmissive • e.g., Diamond Egap = 5.45 eV, SiO2 Egap = 8-9 eV • 1.8 eV < Egap < 3.3 eV • Some Light is absorbed and Material has a color

17. Color determined by sum of frequencies transmitted light re-emitted light from electron transitions e.g., Cadmium Sulfide (CdS) Egap = 2.4eV Absorbs higher energy visible light (blue, violet), NonMetal Colors • Red/yellow/orange is transmitted and gives it this color • CdS

18. Ex: Ruby = Sapphire (Al2O3) + 0.5-2 at% Cr2O3 Sapphire is colorless (i.e., Egap > 3.1eV) adding Cr2O3 alters the band gap blue light is absorbed yellow/green is absorbed NonMetal Colors cont. • red is transmitted • Result: Ruby is deep Red in color

19. Wavelength vs. Band Gap • Example: What is the maximum wavelength absorbed by Ge? • Find Ge BandGap: Eg = 0.67 eV • Thus Need Ephoton = hc/λmax≥ Eg • Use the Photon Energy Eqn:

20. When Light Encounters a Matter-Containing Environment, it SLOWS DOWN Due to Interaction with Electrons electron no cloud transmitted transmitted + + distorts light light Light Refraction • Define the INDEX of REFRACTION, n

21. The slowing of light in a Non-Vacuum Medium Results in Refraction, or Bending of the light Path Light Refraction cont • Light Refracts per Snell’s Law :

22. Recall Refraction Physics • Thus n • Now the relations for v and c • Most Matls are NOT magnetic → µr  1 • So • Where ε& µ are respectively the Permittivity & Permeability of the Material • Now Recall • e.g. Germanium • n = 3.97 → n2 = 15.76 • r = 16.0 (very close)

23. Based on EM Induced e− excitation, and then Relaxation with Broad-Spectrum h Emission Energy of electron Energy of electron unfilled states unfilled states Incident Radiationh0 E gap E gap emitted lighth1+ h2+... filled states filled states Re-emissionOccurs ElectronExcitation glass coating UV “white” light e.g.; -alumina, doped w/ Europium radiation Application  Luminescence • e.g. fluorescent lamps

24. h Absorption by NO-Junction SemiConductors results in the Elevation of an e- to the Conduction Band Where it Can Carry an E-Field Driven Current + + Energy of electron Energy of electron unfilled states unfilled states semi Incident Conducting e- E gap conductor: radiation E gap filled states filled states - - A. No incident radiation:little current flow B. Incident radiation: Increased current flow Application  PhotoConduction • e.g. Cadmium Sulfide

25. Recall The PN Junction P -doped Si conduction Si electron Si P Si Si n-type Si p-n junction p -type Si n Si hole + E - B Si Si Si p B-doped Si Application  Si Solar Cell • Operation for Si Cell: • An incident PHOTON produces HOLE-ELECTRON pair. • Typically 0.5-0.7 V potential • Theoretical Max = 1.1 V (Egap). • Current INCREASES with INCREASED Light INTENSITY • Need to Minimize Reflectance

26. Natural SunLight is Very Pleasant However, In Sunny Climes Windows that Admit Visible Light ALSO transmit InfraRed EM radiation that Heats the Building; increasing AirConditioning costs Soln → “Heat” Mirror Window Application – Heat Mirror

27. A Perfect Heat Mirror Would Transmit 100% of EM radiation (light) in the visible 350-700 nm Wavelength range Reflect 100% of EMR over 700 nm Heat Mirror Windows are Constructed from thin-film coated “window glass” Application – Heat Mirror cont • HM Film Stack → dielectric / metal / dielectric (D/M/D) • e.g., 300Å TiO2 / 130Å Ag / 300Å TiO2 http://www.cerac.com/pubs/cmn/cmn6_4.htm

28. All Done for Today TheSolarSpectrum

29. WhiteBoard Work • Derive Eqns • 21.18 • Thick, Strongly Absorbing Medium of thickness d • 21.19 • Weakly Absorbing (transparent) medium with Reflection, R, and thickness d

30. Heat Mirror Hot Miror (Heat Reflecting) What - These "hot mirror" filters transmit the visible spectrum and reflect the infrared. At any specified angle of incidence, the average transmission is more than 93% from 425 to 675 nm. The average reflectance of our standard Hot Mirror is more than 95% from 750 to 1150 nm. Extended Hot Mirror: The average reflectance is more than 90% from 750 to 1600 nm. Long IR Hot Mirror The average reflectance is more than 90% from 1700 to 3000 nm Cold Mirror (Heat Transmitting) These "cold mirror" filters reflect the visible spectrumand transmit heat (infrared). At any specified angle ofincidence, average reflectance is more that 95% from450 to 675 nm. Transmission is more than 85% from800 to 1200 nm.