Chapter XXV Quantum Generators

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• Imagine an atom with excess energy. Without external action, the atom would, after a period of time, spontaneously emit light, whose properties (e.g. its direction) are random — only the energy is fixed. • This situation changes when the atom is shone with a light wave of the corresponding energy. There is then a higher probability that the atom will also simply copy the remaining properties of the incident wave: It then emits light that is in step with the original and propagates in the same direction....

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Nội dung Text: Chapter XXV Quantum Generators

GENERAL PHYSICS III Optics & Quantum Physics
Chapter XXV Quantum Generators §1. Principle of light amplification §2. Properties of laser beams
Quantum generators are light sources that work by a special principle of light amplification Light Amplification by Stimulated Emission of Radiation
§1.Principle of light amplification: 1.1 Radiative processes: E2 1.1.1 Absorption: • Atoms absorb light (photon) and transist from a lower to higher (exited) energy levels E1 • Absorption rate:  1 dN    12 N 1 W  dt  abs   12 FN 1 • Note: Typical value of N1 ~ 1020/cm3  ~ 10-19 ÷ 10-18 cm2 12
1.1.2 Spontaneous emission: E2 • The emission of light that takes place completely randomly. • When, and in which direction, the light will be emitted? It cannot be determined before it actually happens. E1 • We cannot manage this emission.  2 dN   AN 2  dt sp N 2 / sp The process is spontaneous by nature. This gives the effect its name: spontaneous emission.
1.1.3 Stimulated emission: • Imagine an atom with excess energy. Without external action, the atom would, after a period of time, spontaneously emit light, whose properties (e.g. its direction) are random — only the energy is fixed. • This situation changes when the E1 atom is shone with a light wave of the corresponding energy. There is then a higher probability that the atom will also simply copy the remaining properties of the incident wave: It then emits light that is E2 in step with the original and propagates in the same direction. • In contrast to spontaneous emission, this effect is known as stimulated emission. This is the effect that causes the light in the laser to be amplified, and also gives the process its name: Light Amplification by Stimulated Emission of Radiation.
• The rate: dN   2   21 N 2 W  dt st 21 FN 2 • Einstein showed: W 12 W 21 and 12 21 • For degenerate levels: g 1W 12  g 2W 21 g 112  g 221
• The LASER idea:  2  1  dN dN Fout Fin        dz   dt   dt   st abs  g 2 N1  dF 21 FN 2  12 FN 1 21 F  2   N dz  g1  • Thermodynamical equilibrium: E  E N2 g2  2 1 N 2 g1 g 2 N1  e kBT    N2  1 0 N1 g1 N1g 2 g1 Absorption
• It means that the thermodynamical equiblirium situation corresponds to the absorption. • The possibility of emission requires non-equilibrium (unnatural) situation: It must be g 2 N1 N2  0 It is called “population inversion” g1 • Innumerable copies are produced from a few light waves. However, to achieve this effect the light waves must be reflected back and past the atoms again and again. This is done using mirrors. The mirrors are used to capture the light waves — in such a way that the individual waves are exactly superimposed and oscillate in step. The result is known as a standing wave.
• Light amplification: If we supply continually with new energy to the atoms, for example by using a bright light  there are more atoms with additional energy than without  a population inversion is created (this effect is called “the pump” )  Laser start to work. 1.2 Pumping schemes: 1.2.1 Two-level laser: Suppose that we try to increase N 2 with string light hto create a population inversion  This won’t work !
1.2.2 Three-level laser: • By fast decay processes from level 3 down to level 2 we can create population inversion • Ruby lasers is based on this scheme 1.2.3 Four-lavel laser: • It is a better way to create population inversion • Example: Nd:YAG laser ( Neodimium-doped ytrium aluminium garnet Y3Al5O12 )
1.2.4 Quasi-three (or quasi-four) level laser: • The lower lasing level is partially occupied in thermal equilibrium •Example: Yb: YAG (Ytterbium)
§2. Properties of laser beams: • Laser rays have many valuable properties that cannot be found in light form other sources • Laser rays have many applications in science, technology, medicine … The distinguished properties of laser rays are Monochromaticity Temporal and spacial coherence Directionality 2.1 Monochromaticity: • Lasers amplify light waves that have definite frequency • But there is still a narrow spread for  because of finite upper state and interaction with surounding environment.
• Example: For Laser Nd:YAG  = 1.064   2.8 x 1014 Hz,  m; =   spead is  ~ 3 kHz the      10-11 very small !   ~   A laser beam is almost monochromatic light beam. 2.2 Temporal and spacial coherence: It means that different points in the laser beam have definite relation in phase. We can define wave fronts for a laser beam.  Laser light waves are almost sinusoidal in time and in space.
2.3 Directionality: • The property of coherence and the result of the laser cavity make Laser beams ot be high directional.  Laser beams are almost parallel light beams ! • Because of high directionality we can creat very bright beams and concentrate light energy in an exact locations  a rich supply of applications !
Laser types Gas (HeNe, CO2), Liquid (dye), Solid-State (Nd:YAG, Yb:YAG, Ruby, Ti:Sapphire, Fiber, Semiconductor), Chemical (HF), X-ray,… A scheme of laser