21.2 - ANNOUNCEMENTS Download: Web Chapter 21 (& Ch...

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Unformatted text preview: ANNOUNCEMENTS Download: Web Chapter 21 (& Ch 20) from BbVista Reading: Chapter 21, All Sections Week 10 Recitations: No Quiz… Problems: 21.4, 21.14, 21.16, 21.18, 21.19, 21.23, 21.D1 Review items for the Final Exam, Friday, June 4 Make sure your RI has an accurate record of your attendance…esp if you attended sections other than your scheduled one Final Exam: Friday, June 11, 1:00-3:00 pm Accommodations: LeBow 348; 1:00-4:00 (1.5X); 1:00-5:00 Noon (2X) Chapter 21 - 20 ANNOUNCEMENTS Final Exam Coverage: Chapters 17, 18, 20, 21 + some topics from previous chapters/earlier material… Style: Like Midterms…1 x multipart, short answer/circle the answer question + 5 recitation problem style Qs, plus one 15-point (~9%) bonus Q Example/sample exam posted on BbVista Keys to Success: Read textbook & posted lecture notes Read your own lecture notes Practice solving problems…Recitation, Example & other Take advantage of TA and RI office hours Chapter 21 - 21 ANNOUNCEMENTS Course Evaluations: Opening…soon! Closing, 2-3 weeks later Please complete an evaluation of the course, the recitation instructors…either before of after the final exam etc. o Your feedback provides us with useful information which we use to make changes in the future Chapter 21 - 22 Selective Absorption: Semiconductors • Absorption by electron transitions occurs if h > Egap e- from VB excited across band gap into vacant states in CB Free e- in CB + hole in VB Electron energy Blue ( ~0.4 m) light: Eg(max) = hc/ min = h = 3.1 eV unfilled states Red ( ~0.7 m) light: Eg(min) = hc/ max = h = 1.8 eV Egap Incident photon energy h Io filled states Adapted from Fig. 21.5(a), Callister 7e. • If Egap <1.8 eV, all photons absorbed VB/CB; opaque (Si, GaAs) • If Egap >3.1 eV, no photons absorbed; transparent (diamond) • If Egap in between, some photons absorbed; material colored Chapter 21 - 23 Wavelength vs. Band Gap e.g.: What is the minimum wavelength ( min) absorbed by Ge? For Ge, Eg = 0.67 eV min hc (6.62 x 10 34 J s)(3 x 108 m/s) = = Eg (0.67eV)(1.60 x 10 19 J/eV) Note : for Si E g = 1.1 eV c 1.85 μm 1.13 μm If donor (or acceptor) impurity states also available (which lie within the band gap) these provide other absorption frequencies/ wavelengths Chapter 21 - 24 Light Absorption & Transmission • Intensity of absorbed radiation = fn. (medium + path length) • Intensity of transmitted (non-absorbed) light: decreases with distance x light traverses in material/medium IT =e I0 IT ln = I0 ( = linear absorption coefficient (mm-1), a characteristic of the material, which varies with x = distance measured from incident surface into material x x ) 2 IT = I0 1- R e (See Example Problem 21.1) x I0 = intensity @ front surface x = thickness = absorption coefficient R = reflectance IT = transmitted intensity @ rear surface Chapter 21 - 25 Color of Nonmetals • Color determined by sum of frequencies/wavelengths of: transmitted light fraction of light re-emitted from electron VB-CB transitions • e.g.: Cadmium Sulfide (CdS) Eg = 2.4 eV absorbs higher energy visible light (blue, violet) red/yellow/orange is transmitted and gives it color - Sapphire is colorless (i.e., Eg > 3.1 eV) - Adding Cr2O3: • • • • • e- adds states in B-G blue/violet light absorbed yellow/green absorbed red is transmitted Result: Ruby is deep red in color Transmittance (%) • e.g.: Ruby = Sapphire (Al2O3) + (0.5 to 2) at.% Cr2O3 80 sapphire 70 ruby 60 50 40 0.3 wavelength, 0.5 0.7 (= c/ )(μm) 0.9 Adapted from Fig. 21.9, Callister 7e. (Fig. 21.9 adapted from "The Optical Properties of Materials" by A. Javan, Scientific American, 1967.) Chapter 21 - 26 Opacity & Translucency in Insulators • Depends on internal reflectance and transmittance • Semicrystalline or polycrystalline materials may appear translucent or opaque due to multiple scattering events (reflection, refraction) inside material • Polycrystalline: – scattering at grain boundaries translucent – internal porosity scatters light more opaque • Semicrystalline: – density of crystals higher than amorphous materials speed of light is lower - scatters light - can cause significant loss of light – Common in amorphous polymers – e.g.: LDPE milk cartons – cloudy – Polystyrene – clear – essentially no crystals Chapter 21 - 27 Single Crystal Polycrystalline & dense Polycrystalline & porous Chapter 21 - 28 Applications: Luminescence • Luminescence – re-emission of light by a material: – material absorbs light @ 1 frequency & re-emits at another (lower) frequency (different color) Conduction Band Eg How stable is the trapped state? • If very stable (long-lived = >10-8 s)… phosphorescence • If less stable (<10-8 s)…fluorescence trapped states e.g: Glow-in-the-dark toys: – Charge them up by exposing them to light – Re-emit over time…phosphorescence Eemission activator e.g.: Fluorescent lamps: level – glass tube, coated internally Valence Band – UV from Hg glow discharge – coating fluoresces, emits vis. light Chapter 21 - 29 Photoluminescence Hg uv electrode electrode • Glow discharge between electrodes excites Hg in lamp to higher energy level • e- falls back emitting UV light (i.e., tanning lamp) • Coat inner surface with material that absorbs in UV, re-emits (fluoresces) in visible: Ca10F2P6O24 with 20% of F- replaced by Cl• Adjust color by doping with metal cations (tungstates, silicates) Sb3+ Blue Mn2+ Orange-Red Chapter 21 - 30 Cathodoluminescence • Used in CRT TV sets: – Bombard phosphor coating with electron beam – Excite phosphor e- to high energy state – Relaxed by emitting photons (visible) ZnS (Ag+ & Cl-) (Zn, Cd) S + (Cu++Al3+) Y2O2S + 3% Eu blue green red • Note: light emitted is random in phase & direction – i.e., noncoherent Chapter 21 - 31 LASER Light • Is non-coherent (out of phase) light a problem? – diverges – can’t keep tightly collimated • How could we get all the light in phase? (i.e. coherent) – LASERS: • • • • • Light Amplification by Stimulated Emission of Radiation • Involves a process called population inversion of energy states Chapter 21 - 32 LASER Light Production • “Pump” the (solid) lasing material (single crystal Al2O3 w. ~0.05% Cr3+ ions to provide e- states) to an excited state: – e.g., by Xenon flash lamp (non-coherent light source) Fig. 21.13, Callister 7e. – e- of Cr3+ ions in ground state prior to pumping – 0.56 m photons from Xe excite e- from Cr3+ into higher energy states…2 possible decay paths… Chapter 21 - 33 Population Inversion - Ruby Laser • 2 decay paths for excited e- to return to ground state: – directly (lost) or via metastable state (EM)…reside for up to 3 ms (long time) then spontaneously decay (MG) to ground state – large # metastable states occupied – spontaneous (MG) photon emission triggers avalanche of emission from e- in metastable state – some photons // to long axis of rod TX thru partially-silvered mirror – others reflected – beam travels back & forth along rod Fig. 21.14, Callister 7e. – intensity , more emissions stimulated; intense, coherent, collimated laser beam Chapter 21 - 34 LASER Cavity “Tuned” cavity: • Stimulated Emission: – 1 photon induces the emission of another photon, in phase with the first – cascade produces very intense burst of coherent radiation • i.e., Pulsed laser • monochromatic ( = 0.6943 m) Fig. 21.15, Callister 7e. Chapter 21 - 35 Continuous Wave (CW) LASER • Can also use materials such as CO2 or yttriumaluminum-garnet (YAG) for LASERS • Set up standing wave in laser cavity: – tune frequency by adjusting mirror spacing • Uses of CW lasers: 1. 2. 3. 4. 5. 6. 7. Welding Drilling Cutting – laser carved wood, eye surgery Surface treatment Scribing – ceramics, etc. Photolithography – excimer laser Military - airborne laser weapons Chapter 21 - 36 Semiconductor LASER • • • • • • e.g. GaAs…layered S/C structure to confine e- & holes + laser beam = hc/Eg & is (0.4-0.7 m) Strong forward DC bias excites e- from VB to CB Excitation creates electron-hole pairs Some e- & holes spont. recombine, & can stimulate recomb’n. of other e--hole pairs, producing more photons of same and in-phase Monochromatic, coherent beam electron + hole excited state Adapted from Fig. 21.17, Callister 7e. neutral + h ground state photon of light Chapter 21 - 37 Uses of Semiconductor LASERs • #1 use = compact disk, DVD players, etc… – Color? - Red • Banks of semiconductor lasers used as flash lamps to “pump” other lasers • Communications: – fibers often turned to a specific frequency (typically in the blue) – only recently was this attainable Chapter 21 - 38 Chapter 21 - 39 Applications of Materials Science • New materials must be developed to make new & improved optical devices: – Organic Light Emitting Diodes (OLEDs) – White light semiconductor sources Fig. 21.12, Callister 7e. Reproduced by arrangement with Silicon Chip magazine.) • New semiconductors • Materials scientists (& many others) use lasers as tools • Solar cells Chapter 21 - 40 Solar Cells • Operation: • p-n junction: P-doped Si conductance Si electron Si P Si incident photons produce electron-hole pairs e- & holes drawn away from junction in opposite directions…external current flow typically ~0.5 V potential current increases w/light intensity light Si n-type Si p-n junction p-type Si n-type Si p-n junction p-type Si Si B - -- + + ++ • Solar powered weather station: Si hole creation of hole-electron pair Si Si B-doped Si polycrystalline Si Los Alamos High School weather station (photo courtesy P.M. Anderson) Chapter 21 - 41 Optical Fibers • Prepare high purity Si glass preform (see Chapter 13) • Draw preform to strong, flaw-free 5-100 μm Ø capillary fibers • ~ 60 μm plastic cladding applied to constrain light in core Fig. 21.20, Callister 7e. Fig. 21.18, Callister 7e. Chapter 21 - 42 Total Internal Reflection Occurs only when light travels from medium w. higher n to one with lower n, e.g. glass to air (not vice-versa) n > n’ i n sin = n sin n’(low) n (high) i i i i c i = incident angle = refracted angle c = critical angle occurs when i’ = 90° For I > c light is 100% internally reflected c Chapter 21 - 43 Example: Diamond in Air sin n = sin n sin c 1 = 2.41 2.41 sin 90 = 1 sin c c 1 = sin c = 24.5 • Fiber optic cables are clad in low n material for this reason Chapter 21 - 44 Optical Fiber Profiles Step-index Optical Fiber: ncladding < ncore Fig. 21.21, Callister 7e. Graded-index Optical Fiber: Impurities added to silica so n varies parabolically across Ø Fig. 21.22, Callister 7e. Chapter 21 - 45 SUMMARY • When light (em radiation) shines on a material, it may be: – Reflected, Absorbed and/or Transmitted • Optical classification: – transparent, translucent, opaque • Metals: – fine succession of available energy states above Ef results in absorption and reflection (re-emission) • Non-Metals: – may have full (Egap < 1.8 eV), no (Egap > 3.1 eV), or partial absorption (1.8 eV < Egap = 3.1 eV) – color determined by light wavelengths that are transmitted or re-emitted from electron VB/CB transitions – color may be changed by adding impurities which change the band gap (Eg) magnitude (e.g., Ruby) • Refraction: – speed of transmitted light varies between materials Chapter 21 - 46 ...
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