The Electromagnetic Properties of Materials

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Harvey Davidson
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1 The Electromagnetic Properties of Materials Electrical conduction Metals Semiconductors Insulators (dielectrics) Superconductors Magnetic materials Ferromagnetic materials Others Photonic Materials (optical) Transmission of light Photoactive materials Photodetectors and photoconductors Light emitters: LED, lasers

2 The Optical Properties of Materials: Photonic Materials Beauty: one-half of the earliest materials science Pottery glazes(the origin of metals), paints and cosmetics Jewelry - the development of metals and metalworking Information Window glass Optical fibers (rapidly replacing copper wire) Light The electric light LEDs and Lasers Photodetectors and photoconductors Power Photovoltaics (solar cells) Laser power transmission (welding, surface treatments)

3 The Optical Properties of Materials: Photonic Materials Optical means the whole electromagnetic spectrum From radio waves to γ-rays Can be regarded as Waves in space Particles with quantized energies Light as waves Refraction and reflection at an interface (windows, light pipes, solarium) Absorption and scattering (optical fibers) Diffraction (x-ray and electron crystallography) Light as particles Transmission and absorption Photodetectors and photoconductors: switches, photocopiers Photoemitters: LEDs and lasers

4 Electromagnetic Waves in Free Space E H Wave carries electric and magnetic fields Oriented perpendicular to the direction of propagation Wave: Particle: E = E 0 exp[ i( kx ωt) ] k = 2π λ ω = 2πν ω k = νλ = c (λ = wavelength) (ν = frequency) c = speed ε = hν = ω

5 The Electromagnetic Spectrum - ray µm violet x-ray ultraviolet visible infrared microwave log[frequency(hz)] log[energy(ev)] log[wavelength(m)] Å 1 nm 1 µm 1 mm 1 m 0.5 µm 0.6 µm 0.7 µm blue green yellow orange red radio km Visible light: λ ~ µm E ~ ev

6 Light as a Wave Propagation through free space at velocity, c When light enter a material, it is Refracted Reflected Attenuated incident transmitted reflected

7 Refraction and Reflection at an Interface: Normal Incidence incident reflected Inside material: E = E 0 exp[ i(kx ωt)] k = nk o λ = λ 0 n v = ω k = c n transmitted Refraction: Wave drags charges Friction slows propagation Index of refraction (n) Property governing refraction Related to dielectric constant: n = Depends on frequency (dispersion) n = n(ω) = ε(ω) ε

8 Refraction at an Interface Φ 2 d Φ 1 n 2 n 1 Snells Law n 1 sinφ 1 = n 2 sinφ 2 Light bends toward low-n region The critical angle Light cannot exist region 1 if n φ 2 > φ c = sin 1 1 n 2 Principle of light pipe Optical fiber confines light by reflection

9 Reflection at an Interface Normal incidence from n 1 to n 2 Δn reflection Intensity thrown back incident n 1 reflected n 2 transmitted Reflected intensity R = I r I i = (n 2 n 1 ) 2 (n 2 + n 1 ) 2 Transmitted intensity T = I t I i =1 R = Note: depends on Δn Not transparency 4n 2 (n 1 + n 2 ) 2

10 Exploiting the Light as a Wave: Examples Optical fibers Transparent pipes that transmit light Note that light need not be visible GaAs systems operate in the infrared Greenhouses and solar heaters Glass containers that let light in, Then trap its energy for heat

Optical Fibers Require Small diameter to minimize surface loss Perfect cylinder to minimize surface scattering Exceptional purity to suppress absorption Exceptional uniformity to suppress Rayleigh

11 Optical Fibers Require Small diameter to minimize surface loss Perfect cylinder to minimize surface scattering Exceptional purity to suppress absorption Exceptional uniformity to suppress Rayleigh scattering Gradient fibers Rays that reflect from surface travel farther than rays on-axis Loss of coherence and information Want gradient in n such that n lower on outside Rays that reflect from surface move faster Can adjust n with solute additions

12 Propagation of Light: Attenuation I incident reflected transmitted x I T = I 0 exp ηx [ ] I T is gradually attenuated Mechanisms of attenuation Absorption Rayleigh scattering Mechanisms of absorption Conduction electrons Phonons Electronic transitions Valence Core

13 Attenuation: Rayleigh Scattering Light scatters from heterogeneities Density fluctuations Chemical heterogeneities Defects and second-phase particles Only recently is it possible to produce clear, uniform glass As through a glass - darkly

14 Absorption: Insulator or Semiconductor conduction band E Ionic transitions valence band x E Optical phonons Absorption by Optical phonons (solar panels) Ionic transitions (color) Band transitions (photoconductivity) Core transitions (x-ray spectroscopy)

15 Absorption: The Greenhouse Effect conduction band E Ionic transitions valence band x E Optical phonons Absorption by Optical phonons (solar panels) Ionic transitions (color) Band transitions (photoconductivity) Core transitions (x-ray spectroscopy)

16 The Solarium and Solar Heater glass earth Mechanism is glass transparent in the visible Opaque in the infrared Sunlight enters Rays are absorbed and re-emitted in the infrared Re-emitted rays cannot penetrate glass Solar energy is trapped inside

17 Diffraction œ œ d Waves reflected from successive planes Destructive interference unless nλ = 2dsinθ Bragg s Law strong intensity peak (Bragg s Law) Pattern of diffraction peaks identifies crystal structure Use x-rays or electrons with λ of a few Å

18 Electron Diffraction of Intercritically Tempered Steel Electron microscopy Diffraction pattern Combined analysis Photograph Diffraction pattern Peaks from crystal planes Pattern identifies phases Ex.: bcc and fcc Fe present Bright field microstructure Diffraction pattern shows phases Dark field locates phases Image diffraction spot

19 Light as a Particle: Photons Transparency and color Photodetectors Photoconductors Photoelectronics Photocopiers Photoemitters Phosphors Light-emitting diodes (LED) Lasers

20 Light as a Particle: Photons Transparency and color Photodetectors Photoconductors Photoelectronics Photocopiers Photoemitters Phosphors Light-emitting diodes (LED) Lasers

21 Transparency and Color e - e - EG E I ED EG intrinsic excitation extrinsic excitations ionic excitations hˆ hˆ intrinsic extrinsic Materials are opaque to all radiation for which ћω>e G All materials with E G < about 2.5 ev are opaque to visible light Radiation with hν=e i is also absorbed For all internal excitations (donors, acceptors, ionic excitations) Leads to colored or dimmed light

22 Light as a Particle: Photons Transparency and color Photodetectors Photoconductors Photoelectronics Photocopiers Photoemitters Phosphors Light-emitting diodes (LED) Lasers

23 Photoconductivity e - e - conduction band conduction band Ó intrinsic excitation extrinsic excitations E valence band EG E Ed valence band EG k direct gap k indirect gap Light of suitable wavelength excites carriers σ = neµ (n = steady state density from optical excitations) Semiconductor or insulator becomes metal Note two kinds of semiconductor direct gap produces carriers at E G indirect gap excitation requires phonons at E<E d - messy behavior

24 Photoconductors E + e e e e e e e e e - Ó G x Light creates current Electric eye circuits Photodetecters (need multiple conductors to detect frequency) Photoelectronic transistors Switch on when light on Illumination plays the part of positive voltage at the base

25 Photocopiers ÎV ÎV ÎV (a) (b) (c) Charge photoconductor plate Reflect light from page Reflection from white spaces Removes charge Creates map of original print Pass through toner Ink sticks to charge on plate Print Press against paper to transfer ink Faithful copy of original Color copying Passes for the 3 primary colors

26 Light as a Particle: Photons Transparency and color Photodetectors Photoconductors Photoelectronics Photocopiers Photoemitters Phosphors Light-emitting diodes (LED) Lasers

27 Photoemitters: Phosphors A phosphor is an ionic emitter Incident radiation (ћω i =E i ) excites ion Excited ion relaxes in lattice, changing energy Excited state returns to ground state, emitting photon with E e = ћω Since ω ω I, photon is emitted from the material Phosphors used in monitors, etc. Multiple phosphors used for color images

28 Photoemitters: Light Emitting Diodes (LED) E - e e e e e e e Ó G + E conduction band EG E conduction band Ed EG valence band valence band x k k To generate light from a p-n junction: Use a direct-gap semiconductor in forward bias Charge recombinations generate photons Color set by band gap Long search for blue LED solved by GaN

29 Lasers: Light Amplification by Stimulated Emission of Radiation 2 phonon 3 mirror half-silvered mirror Ó 12 Ó 13 stimulated emission 1 Three-level laser (ruby): Excite with incident radiation Transition to level with difficult transition to ground state (inverted population) Transitions stimulate further transitions, create beam of photons in phase Light emission from laser Mirrors used for multiple reflections to amplify in phase beam Mechanism such as half-silvered mirror to emit amplified light

30 Lasers: Four-Level Lasers Three-level laser: Problem ћω 13 can stimulate transition 1 3 One photon lost for each transition - loss of efficiency Four-level laser: Solution Lasing transition to transient state 4 Immediate transition 4 1 empties level 4 High efficiency since ћω 14 has nothing to excite

31 Semiconductor Lasers - + e e e e e e e ћω G Heterojunction GaAs Laser GaAs (n) GaAs (p) GaAlAs (p) Use direct-gap semiconductor (GaAs) Note GaAs gap such that light is infrared Create well where electrons are trapped Pump high density of carriers exceed recombination rate Recombination enhanced by stimulated emission laser

PHYSICS nd TERM Outline Notes (continued)

PHYSICS nd TERM Outline Notes (continued) PHYSICS 2800 2 nd TERM Outline Notes (continued) Section 6. Optical Properties (see also textbook, chapter 15) This section will be concerned with how electromagnetic radiation (visible light, in particular)