Metastable energy levels

The optical pumping efficiency and output power of many rare earth lasers can be increased by codoping the medium with other ions which absorb pump radiation and effectively transfer the excitation to the upper laser level. This transfer may be either radiative or nonradiative. In general, sensitization schemes used for phosphors and other luminescence phenomena are also applicable to lasers (Van Uitert, 1966). Requirements for the sensitizer ion include (a) no ground- or excited-state absorption at the laser wavelength, (b) absorption bands which complement rather than compete with absorption bands of the laser ion, since the fluorescence conversion efficiency usually is less for the former, (c) one or more metastable energy levels above the upper laser level,... 

Figure 6.2 Energy level diagrams for (a) ruby, (b) Nd YAG and (c) excimer lasers, (a) Lasing occurs between the metastable energy levels and the ground level (b) lasing occurs between the metastable levels (c) lasing occurs between the upper bound state and the lower free electronic state of a molecule.

Figure 9.24 Energy-level diagram for a luminescent species, in which a metastable state slows the rate of emission. The metastable state is also termed an ion trap...

In many real lasers, the upper laser level, 2 in Figure 2.6(a), is a metastable level that is, it has a long lifetime compared to the lower laser level (t > ti). If we can pump efficiently into such a longer-lived upper level, and provided that there is a lower energy level, E in Figure 2.6(a), with a short lifetime, then a population inversion is very likely to be established. 

Figure 5.61 summarizes the temperature behavior of decay time r and quantum efficiency xj of the blue luminescence from benitoite in the forms ln(r) and ln(q) as a function of reciprocal temperature 1/T. Figure 5.62.a demonstrates a suitable energy levels scheme. After excitation the metastable level 1 is populated due to nonradiative fast transition from excited level. Between levels 1 and 2 the equilibrium population is established due to nonradiative transition. The relative quantum yield of the blue emission may be described by simple Arrhenius equation ... 

Figure 6.97 Schematic illustration of energy levels in mby that are used to create a populated metastable electronic state, which can then be stimulated to emit monochromatic, coherent radiation for a laser. Reprinted, by permission, from J. F. Shackelford, Introduction to Materials Science for Engineers, 5th ed., p. 607. Copyright 2000 by Prentice-HaU, Inc.

Example Problem Determine the temperature at which kBT is equal to the energy level splitting for the metastable state of 123Te at 247.6 keV in an external magnetic field of 4.0 tesla (T). 

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