Optical Properties of Materials: Understanding Reflection, Absorption, and Transmission, Lecture notes of Mechanical Engineering

Download Optical Properties of Materials: Understanding Reflection, Absorption, and Transmission and more Lecture notes Mechanical Engineering in PDF only on Docsity! ISSUES TO ADDRESS... • What happens when light shines on a material? 1 • Why do materials have characteristic colors? • Optical applications: --luminescence --photoconductivity --solar cell --optical communications fibers • Why are some materials transparent and other not? CHAPTER 19: OPTICAL PROPERTIES 2 • Incident light is either reflected, absorbed, or transmitted: Incident: Io Reflected: IR Absorbed: IA Transmitted: IT Io IT  IA  IR • Optical classification of materials: Transparent Transluscent Opaque Adapted from Fig. 21.10, Callister 6e. (Fig. 21.10 is by J. Telford, with specimen preparation by P.A. Lessing.) LIGHT INTERACTION WITH SOLIDS 5 • Absorption by electron transition occurs if h > Egap • If Egap < 1.8eV, full absorption; color is black (Si, GaAs) • If Egap > 3.1eV, no absorption; colorless (diamond) • If Egap in between, partial absorption; material has a color. Adapted from Fig. 21.5(a), Callister 6e. SELECTED ABSORPTION: NONMETALS Energy of electron filled states unfilled states Egap Io blue light: h= 3.1eV red light: h= 1.7eV incident photon energy h 6 • Color determined by sum of frequencies of --transmitted light, --re-emitted light from electron transitions. • Ex: Cadmium Sulfide (CdS) -- Egap = 2.4eV, -- 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 : • alters the band gap • blue light is absorbed • yellow/green is absorbed • red is transmitted • Result: Ruby is deep red in color. 40 60 70 80 50 0.3 0.5 0.7 0.9 Tr a n sm it ta n ce ( % ) Ruby sapphire wavelength,  c/)(m) Adapted from Fig. 21.9, Callister 6e. (Fig. 21.9 adapted from "The Optical Properties of Materials" by A. Javan, Scientific American, 1967.) COLOR OF NONMETALS 7 • Transmitted light distorts electron clouds. + no transmitted light transmitted light + electron cloud distorts • Result 1: Light is slower in a material vs vacuum. Index of refraction (n) = speed of light in a vacuum speed of light in a material Material Lead glass Silica glass Soda-lime glass Quartz Plexiglas Polypropylene n 2.1 1.46 1.51 1.55 1.49 1.49 --Adding large, heavy ions (e.g., lead can decrease the speed of light. --Light can be "bent" • Result 2: Intensity of transmitted light decreases with distance traveled (thick pieces less transparent!) Selected values from Table 21.1, Callister 6e. TRANSMITTED LIGHT: REFRACTION 10 • p-n junction: • Operation: --incident photon produces hole-elec. pair. --typically 0.5V potential. --current increases w/light intensity. n-type Si p-type Si p-n junction B-doped Si Si Si Si SiB hole P Si Si Si Si conductance electron P-doped Si n-type Si p-type Si p-n junction light + - ++ + --- crea io of hole-electron pair • Solar powered weather station: polycrystalline Si Los Alamos High School weather station (photo courtesy P.M. Anderson) APPLICATION: SOLAR CELL 11 • Design with stepped index of refraction (n): core: silica glass w/higher n cladding: glass w/lower n n enhances internal reflection in te n si ty time input pulse broadened! in te n si ty time output pulsetotal internal reflection shorter path longer paths • Design with parabolic index of refraction core: Add graded impurity distrib. to make n higher in core center cladding: (as before) total internal reflection shorter, but slower paths longer, but faster paths in te n si ty time input pulse in te n si ty time output pulse less broadening! • Parabolic = less broadening = improvement! Adapted from Fig. 21.19, Callister 6e. (Fig. 21.19 adapted from S.R. Nagel, IEEE Communications Magazine, Vol. 25, No. 4, p. 34, 1987.) Adapted from Fig. 21.20, Callister 6e. (Fig. 21.19 adapted from S.R. Nagel, IEEE Communications Magazine, Vol. 25, No. 4, p. 34, 1987.) APPLICATION: FIBER OPTICS 12 • When light (radiation) shines on a material, it may be: --reflected, absorbed and/or transmitted. • Optical classification: --transparent, translucent, opaque • Metals: --fine succession of energy states causes absorption and reflection. • 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 the band gap magnitude (e.g., Ruby) • Refraction: --speed of transmitted light varies among materials. SUMMARY