Tài liệu Chapter XXV Quantum Generators pptx

GENERAL PHYSICS III GENERAL PHYSICS III Optics & Quantum Physics Chapter XXV Chapter XXV Quantum Generators Quantum Generators §1. Principle of light amplification §2. Properties of laser beams Light Amplification by Stimulated Emission of Radiation Quantum generators are light sources that work by a special principle of light amplification §1.Principle of light amplification: 1.1 Radiative processes: 1.1.1 Absorption: • Atoms absorb light (photon) and transist from a lower to higher (exited) energy levels • Absorption rate: 112 112 1 FN NW dt dN abs          E 1 E 2 • Note: Typical value of N1 ~ 10 20 /cm 3  12 ~ 10 -19 ÷ 10 -18 cm 2 1.1.2 Spontaneous emission: • 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. • We cannot manage this emission. sp sp N AN dt dN  / 2 2 2         E 2 E 1 The process is spontaneous by nature. This gives the effect its name: spontaneous emission. 1.1.3 Stimulated emission: E 1 E 2 • 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. • 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 . 221 221 2 FN NW dt dN st          • Einstein showed: • The rate: 2112 WW  2112  and • For degenerate levels: 212121 WgWg  212121  gg  • The LASER idea: dz dt dN dt dN FF absst inout                     12 dz g Ng NFFNFNdF        1 12 221112221  • Thermodynamical equilibrium: 01 1 12 2 21 12 1 2 1 2 12    g Ng N gN gN e g g N N Tk EE B Absorption • It means that the thermodynamical equiblirium situation corresponds to the absorption. • The possibility of emission requires non-equilibrium (unnatural) situation: It must be 0 1 12 2  g Ng N It is called “population inversion” • 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 ! . PHYSICS III GENERAL PHYSICS III Optics & Quantum Physics Chapter XXV Chapter XXV Quantum Generators Quantum Generators §1. Principle of light amplification. of laser beams Light Amplification by Stimulated Emission of Radiation Quantum generators are light sources that work by a special principle of light amplification