ls2_unit_5

ls2_unit_5 - THE INTERACTION OF RADIATION AND MATTER:...

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THE INTERACTION OF RADIATION AND MATTER: SEMICLASSICAL THEORY PAGE 47 R. Victor Jones, March 14, 2000 V. ELEMENTARY LASER MODELS -- RATE EQUATION APPROACH: P OPULATION I NVERSION AND “L ASERING IN A T HREE -L EVEL S YSTEM Obviously, the trick in building masers and lasers has been to find means to "invert" the equilibrium population of states. 22 First conceived by Charles Townes in 1951, Gordon, Zeiger and Townes in 1954 demonstrated the first ammonia beam maser. Degenerate quantum states of the ammonia molecule (NH 3 ) are split by the tunneling of the nitrogen atom across the plane formed by the three hydrogen atoms. In the Columbia maser, thermal populations of molecules in the split ground state (23,800 MHz) are spatially separated by an electrostatic field to achieve population inversion in the maser resonant cavity. While this two-level system was clearly seminal, it was the multi-level paradigm of THE CONFIGURATION OF THEODORE MAIMAN’S RUBY LASER (1960) 22 Recently the subject of lasing has become of considerable interest. See, for Thomas Mossberg’s very interesting Web Seminar on the subject (http://decryptor.uoregon.edu/~mosswww/moss-seminar.html).
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THE INTERACTION OF RADIATION AND MATTER: SEMICLASSICAL THEORY PAGE 48 R. Victor Jones, March 14, 2000 Bloembergen’s three-level ruby maser which galvanized the search for possible laser systems. To quote from A Laser Tutorial (http://members.aol.com/WSRNet/tut/t1.htm) “Finding substances in which a population inversion can be set up is central to the development of new kinds of lasers. The first material used was synthetic ruby. Ruby is crystalline alumina (Al 2 O 3 ) in which a small fraction of the Al 3+ ions have been replaced by chromium ions, Cr 3+ . It is the chromium ions that give rise to the characteristic pink or red colour of ruby and it is in these ions that a population inversion is set up in a ruby laser. In a ruby laser, a rod of ruby is irradiated with the intense flash of light from xenon-filled flashtubes. Light in the green and blue regions of the spectrum is absorbed by chromium ions, raising the energy of electrons of the ions from the ground state level to one of the broad F bands of levels. Electrons in the F bands rapidly undergo non-radiative transitions to the two metastable E levels. A non-radiative transition does not result in the emission of light; the energy released in the transition is dissipated as heat in the ruby crystal. The metastable levels are unusual in that they have a relatively long lifetime of about 4 milliseconds (4 x 10 -3 s), the major decay process being a transition from the lower level to the ground state. This long lifetime allows a high proportion (more than a half) of the chromium ions to build up in the metastable levels so that a population inversion is set up between these levels and the ground state level. This population inversion is the condition required for stimulated emission to overcome absorption and so give rise to the amplification of light. In an assembly of
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This note was uploaded on 02/29/2012 for the course PHYSICS 216 taught by Professor Staff during the Fall '11 term at BU.

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ls2_unit_5 - THE INTERACTION OF RADIATION AND MATTER:...

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