Multiple phosphors used for color images JW Morris Jr University of California

Multiple phosphors used for color images jw morris jr

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Multiple phosphors used for color images
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Photoemitters: Light Emitting Diodes (LED) 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
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Lasers: Light Amplification by Stimulated Emission of Radiation 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 mirror half-silvered mirror stimulated emission
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 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 34 has nothing to excite
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Semiconductor Lasers 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
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Magnetic Materials Sources of magnetism Unpaired electrons Inner core: transition metals and rare earth elements Electron bands (secondary) Electron orbit (secondary) Types of magnetism – Diamagnetism : Electron orbit changed in magnetic field – Paramagnetism : Disordered, unpaired spins align in magnetic field Magnetic order: Unpaired atomic moments spontaneously order at low T Adjacent moments parallel: ferromagnets Adjacent moments antiparallel: antiferromagnets ( ferrimagnets )
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Sources of Magnetic Fields Circulating current creates magnetic moment For a closed current loop of area A: Magnetic moment : m = IA n For solenoid of N turns per meter: Magnetic field : H = NIA n Spinning electron creates magnetic moment m = m B ( Bohr magneton ) I m = IA n A M = m B (Bohr magneton) H = NI n
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Basic Relations for the Magnetic Field Magnetic field : H Magnetic flux : B Magnetization in material : M Magnetic susceptibility : χ B = μμ 0 H B = μ 0 H + M ( ) M = χ H μ = 1 + χ B 2 B 1 H 1 H 2 M 2 Boundary conditions: Normal: B = µ 0(H+M) is continuous Transverse: H = (B/ µ 0)-M is continuous µ 0 (=1 in free space) H 1 H 2 M 2 H 1
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J.W. Morris, Jr.
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  • Fall '08
  • Staff
  • Magnetism, Magnetic Field, University of California, J.W. Morris

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