Lect21 - Lecture 21 Lasers Atoms Molecules Solids etc...

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Lecture 21, p 1 Lecture 21: Lasers, Atoms, Molecules, Solids, etc. Review and Examples + e r + e ψ even
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Lecture 20, p 2 Lasers Photons are emitted when the electrons in atoms go from a higher state to a lower state Conversely, photons are absorbed when the electrons in atoms go from a lower state to a higher state photon Emission photon Absorption Fermions and bosons: Electrons, protons, and neutrons are fermions. Two identical fermions cannot occupy the same quantum state. (exclusion principle) Photons (and many atoms) are bosons. Unlike fermions, bosons actually “prefer” (to be explained soon) to be in the same quantum state. This is the physical principle on which lasers are based.
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Lecture 20, p 3 Suppose we have an atom in an excited state. Eventually (at some random time) it will emit a photon and fall to the lower state. The emitted photon will go in a random direction. This is called “ spontaneous emission ”. Suppose, however, that before the spontaneous emission occurs, another photon of the same energy (emitted by another atom) comes by. Its presence will stimulate the atom to emit its photon. Two identical photons! Photon emitted by some other atom We now have two photons in the same quantum state: the same frequency, the same direction, and the same polarization. As they travel, they will stimulate even more emission. Lasers photon Emission Stimulated emission
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Lecture 20, p 4 Lasers Light Amplification by Stimulated Emission of Radiation Tube of gas Mirrors at both ends 99.99% 100% Laser operation (one kind): Tube of gas with mirrored ends. Excite the atoms to the upper state (mechanism not shown). Spontaneous emission generates some photons. Photons that travel along the axis are reflected. Other directions leak out the sides. Because the amplification process is exponential, the axial beam quickly dominates the intensity. One mirror allows a small fraction of the beam out.
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Lasers LASER: “Light Amplification by Stimulated Emission of Radiation” What if you don’t have an atom that emits the color you want? “Make” one (e.g., by stressing the crystal lattice). Did you know: Semiconductors don’t naturally emit in the red. Charles Henry (UIUC ’65) discovered how to combine layers of different semiconductors to make a ‘quantum well’ (essentially a 1-D ‘box’ for electrons). By adjusting the materials, one could shift the emission wavelengths into the visible. Nick Holonyak (UIUC ECE Prof) used this to create the first visible LEDs & visible laser diodes.
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Lecture 21, p 6 Act 1 The Pauli exclusion principle applies to all fermions in all situations
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This note was uploaded on 04/04/2011 for the course PHYSICS 214 taught by Professor Mestre during the Spring '11 term at University of Illinois at Urbana–Champaign.

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Lect21 - Lecture 21 Lasers Atoms Molecules Solids etc...

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