Lect20 - Lecture 20: ons Consequences of Quantum Mechanics:...

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Lecture 20: Consequences of Quantum Mechanics: ons qu nc s of Quantum M chan cs Effects on our everyday lives Lecture 20, p 1
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Today Why do some solids conduct – others do not – others are intermediate Metals, Insulators and Semiconductors Understood in terms of Energy Bands and the Exclusion Principle Solid-state semiconductor devices The electronic states in semiconductors Transistors, . . . Lasers Bosons and Fermions Stimulated emission of photons. Superconductivity py Electrical conduction with zero resistance! All the electrons in a metal cooperate to form a single quantum state Lecture 20, p 2
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Bands and Band Gaps Occur for all Types of Waves in Periodic Systems Electron in a crystal – a periodic array of atoms yp s of Wa s n r o c Syst ms Energy Bands Band Gap: no states Light propagating through a periodic set of layers with different index of refraction – an interference filter lternating index n nd n Photon Transmitted? Alternating index n 1 and n 2 Lecture 20, p 3 Reflected?
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Lecture 20, p 4
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Interference Filter Light propagating through a periodic set of layers with alternating index of refraction. lternating index n nd n Photon Transmitted? Alternating index n 1 and n 2 Reflected? The behavior light waves in this material is the same as that of electron waves in a string of square wells. For certain wavelengths there is complete reflection, because that wavelength cannot exist inside the material. This is a stop band ”. Lecture 20, p 5 For other wavelengths there is complete transmission. This is a pass band ”.
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Conventional (dye) filters absorb unwanted colors. Interference Filter Interference filters do not absorb light (do not get hot) and are used in many applications: TV, photography, astronomy, … Examples: Red , Green , Blue (RGB) Yellow , Cyan, Magenta What is magenta ? Answer: Magenta = Red + Blue (no Green ) magenta filter must have a stop band in the green but pass red and blue: A magenta filter must have a stop band in the green, but pass red and blue: Demonstration with a ommercial interference filter Lecture 20, p 6 commercial interference filter
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Why Some Solids Conduct Current nd Others Don’t and Others Don t Conductors, semiconductors, and insulators: Their description is provided in terms of: Energy Bands for electrons in solids The Pauli exclusion principle In order for a material to conduct electricity, it must be possible to get the electrons moving ( i.e. , give them some energy).
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Lect20 - Lecture 20: ons Consequences of Quantum Mechanics:...

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