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Engineering 45 Optical Properties Bruce Mayer, PE Licensed Electrical & Mechanical Engineer [email protected] Engineering-45: Materials of Engineering 1 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt 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 Engineering-45: Materials of Engineering 2 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt 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 Engineering-45: Materials of Engineering 3 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Material Properties Chemical Composition Microstructure Phases Grain Size Corrosion Crystallinity Molecular Weight Flammability Physical Melting Point Thermal Magnetic Electrical Optical Acoustic Gravimetric Engineering-45: Materials of Engineering 4 Mechanical Dimensional Tensile properties Toughness Ductility Fatigue Hardness Standard Shapes Standard Sizes Surface Texture Stability Mfg. Tolerances Creep Compression Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt 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 Engineering-45: Materials of Engineering 5 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt 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) Engineering-45: Materials of Engineering 6 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt EM Radiation Quantified 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 Engineering-45: Materials of Engineering 7 c 1 0 0 • Where (Recalling From Previous Lectures) – 0 ELECTRIC Permittivity of Free Space (a vacuum) – µ0 MAGNETIC Permeability of Free Space (a vacuum) Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt EM Radiation Quantified The Wavelength and Frequency of EM waves are related thru c c • Where – WaveLength in meters per cycle – Frequency in Hertz (cycles/sec) Engineering-45: Materials of Engineering 8 EM radiation has a Wave↔Particle Duality The Energy, E, of a Light Particle E h hc • Where h Planck’s Constant (6.63x10-34 J-s) h is the PHOTON Energy Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt EM-Solid Interaction Consider EM Radiation with Intensity I0 (in W/m2) Impinging on a Solid The EM-Solid interaction Alters the incident Beam by 3 possible Phenomena • The EM Beam can be – Reflected – Absorbed – Transmitted Engineering-45: Materials of Engineering 9 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt EM-Solid Interaction cont Mathematically I 0 I R I A IT • Where all the IK are Intensities in W/sq-m An Energy Balance on the Solid: • E-in = E-reflected + E-absorbed + E-transmitted Engineering-45: Materials of Engineering 10 Now Divide E-Balance Eqn by I0 1 R AT Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt EM-Solid Interaction cont.2 1 R AT • Where: – R REFLECTANCE (IR/I0) – A ABSORBANCE (IA/I0) – T TRANSMITTANCE (IT/I0) Using R, A, T, Classify EM-Solid Behavior • Opaque → T = 0 Engineering-45: Materials of Engineering 11 IR I0 IA IT • Transparent → – T >> A+R – Light Not Scattered • Translucent→ – T > A+R – Light Scattered Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Metals – Optical Absorption Metals Interact with Light Thru QUANTIZED Photon Absorption by Electrons Metals have Very Closely Spaced eEnergy Levels Io Energy of electron unfilled states DE = h required filled states • Thus Almost ALL incident Photons are ABSORBED within about 100 nm of the surface Engineering-45: Materials of Engineering 12 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Metals – Optical Reflection The Absorbed Energy is ReEmitted by e- “falling” back to Lower Energy states Since Metals have Very Closely Spaced eEnergy Levels The Light Energy of electron is emitted at many ’s IR re-emitted photon from material surface • Thus Outgoing Light Looks About the Same as Incoming Light → High Reflectance Engineering-45: Materials of Engineering 13 unfilled states “conducting” electron DE filled states Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Light Absorbtion/Reflection Amount of NON-Reflected Light Absorbed by a Matl IT I0 e = absorption coefficient, cm-1 = sample thickness, cm I 0 = NonReflected incident light intensity IT = transmitted light intensity For normally incident 2 light passing into a Rreflectivi ty ns 1 n 1 solid having an s index of refraction n: Engineering-45: Materials of Engineering 14 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Cu Bar Metals also ABSORB Some Photons • Dissipated as heat Metals that Absorb few, or in broad-spectrum, reflect “WHITE” Light and Appear Silvery Sn-Plated Cu Bar Metals - Colors 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 Engineering-45: Materials of Engineering 15 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Total Transmission Combining External and Internal Reflection, along with Beer’s Absorbtion Yields the TOTAL Transmission Eqn IT I 0 1 R e 2 Engineering-45: Materials of Engineering 16 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Total-T Example For the Situation at Right Determine the thickness, d77, that will produce a total Transmittance of 77% From Tab 21.1 Find Pyrex ns = 1.47 Next find R using Eqn (21.13) Engineering-45: Materials of Engineering 17 Quartz Pyrex 0.86I 0 I0 13 23 mm mm 2 ns 1 1.47 1 R ns 1 1.47 1 R 3.621% Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt 2 Total-T Example Recall total Transmission Eq IT I 0 1 R e Quartz Pyrex 0.86I 0 I0 2 Now Solve for β 13 23 mm mm IT e 2 IT I 0 I 0 1 R ln 2 1 R IT ln 2 I 0 1 R Engineering-45: Materials of Engineering 18 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Total-T Example Quartz Pyrex Thus 0.86 23mm ln 2 1 0.03621 3.350 meter Solving Total-T Eqn for the length IT I 0 ln 2 1 R Engineering-45: Materials of Engineering 19 0.86I 0 I0 13 23 mm mm Then d77 0.00335 0.77 d 77 ln 2 mm 1 0.03621 d 77 56.0 mm Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt NonMetals – Selective Absorb. In The Case of Materials with “Forbidden” Gaps in the Band Structure, Absorption Occurs only if h>Egap Energy of electron blue light: h 3.3 ev unfilled states red light: h 1.8 ev For These Materials there is Very little ReEmission incident photon energy h Io Egap filled states • The Material Color Depends on the Width of the BandGap Engineering-45: Materials of Engineering 20 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt 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 Engineering-45: Materials of Engineering 21 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt NonMetal Colors Color determined by sum of frequencies • transmitted light • re-emitted light from electron transitions • Red/yellow/orange is transmitted and gives it this color e.g., Cadmium Sulfide (CdS) • Egap = 2.4eV • Absorbs higher energy visible light (blue, violet), Engineering-45: Materials of Engineering 22 CdS Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt NonMetal Colors cont. Ex: Ruby = Sapphire (Al2O3) + 0.5-2 at% Cr2O3 • red is transmitted Result: Ruby is deep Red in color • Sapphire is colorless (i.e., Egap > 3.1eV) adding Cr2O3 • alters the band gap • blue light is absorbed • yellow/green is absorbed Engineering-45: Materials of Engineering 23 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt 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: max hc (6.62x10 34 J s )(3 x 108 m/s ) 1.85m 19 E g (0.67eV )(1.60x10 J/eV ) note : for Si E g 1.1eV max 1.13 m Engineering-45: Materials of Engineering 24 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Light Refraction When Light Encounters a Matter-Containing Environment, it SLOWS DOWN Due to Interaction with Electrons no transmitted light + transmitted light electron cloud distorts + Define the INDEX of REFRACTION, n n Spd of Light in Vacuum Spd of Light in Matl c Or n v Engineering-45: Materials of Engineering 25 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Light Refraction cont The slowing of light in a Non-Vacuum Medium Results in Refraction, or Bending of the light Path Light Refracts per Snell’s Law : n1 sin 1 n2 sin 2 Engineering-45: Materials of Engineering 26 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Refraction Physics Recall c n v Now the relations for v and c v 1 • Where ε & µ are respectively the Permittivity & Permeability of the Material Now Recall c 1 0 0 Engineering-45: Materials of Engineering 27 Thus n c n r r v 0 0 Most Matls are NOT magnetic → µr 1 • So n r e.g. Germanium • n = 3.97 → n2 = 15.76 • r = 16.0 (very close) Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Application Luminescence 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 Radiation h0 Egap Egap emitted light h1+ h2+... filled states Electron Excitation e.g. fluorescent lamps Re-emission Occurs glass coating e.g.; -alumina, doped w/ Engineering-45: Materials of Engineering Europium 28 filled states UV radiation “white” light Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Application PhotoConduction 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 conductor: Egap Incident radiation filled states filled states - A. No incident radiation: little current flow e.g. Cadmium Sulfide Engineering-45: Materials of Engineering 29 Conducting e- Egap - B. Incident radiation: Increased current flow Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Application Si Solar Cell Recall The PN Junction P-doped Si conduction Si electron Si P Si • 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 Si n-type Si p-njunction p-type Si – Need to Minimize Reflectance n Si B +E- Si hole Si Si B-doped Si Engineering-45: Materials of Engineering 30 Operation for Si Cell: p Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Application – Heat Mirror 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 Engineering-45: Materials of Engineering 31 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt Application – Heat Mirror cont 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 HM Film Stack → dielectric / metal / dielectric (D/M/D) Heat Mirror Windows • e.g., 300Å TiO2 / are Constructed from 130Å Ag / 300Å TiO2 thin-film coated “window glass” http://www.cerac.com/pubs/cmn/cmn6_4.htm Engineering-45: Materials of Engineering 32 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt All Done for Today The Solar Spectrum Engineering-45: Materials of Engineering 33 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt WhiteBoard Work Derive Eqns • 21.18 IT I 0e d – Thick, Strongly Absorbing Medium of thickness d • 21.19 I T I 0 1 R 2 e d – Weakly Absorbing (transparent) medium with Reflection, R, and thickness d Engineering-45: Materials of Engineering 34 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt 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 spectrum and transmit heat (infrared). At any specified angle of incidence, average reflectance is more that 95% from 450 to 675 nm. Transmission is more than 85% from 800 to 1200 nm. Engineering-45: Materials of Engineering 35 Bruce Mayer, PE [email protected] • ENGR-45_Lec-13_Optical_Properties.ppt