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Energy transfer between neighboring ions or molecules in a solid medium is a well-established phenomenon. The following section will outline briefly the main features of this phenomenon, which has some important implications for solid state laser performance. Energy transfer is a process by which one ion, excited to a certain energy ħω exe , transfers part of its excitation energy to a second neighboring fluorescent ion that lacks or has a weak absorption band in the relevant pumping band. The fluorescence of the second neighboring ion will be enhanced as a result of this energy transfer, sometimes by several orders of magnitude. In another extreme case, the fluorescence of the excited ion is quenched as a result of energy transfer. In this case, a fluorescent ion is pumped into a metastable level E 1. A neighboring ion that has no absorption in the pumping bandwidth, as well as no fluorescence (or a small emission quantum yield) from a level lying close to E 1, will accept part or all of the excitation energy deposited on E 1. As a result, a reduction in the overall emission occurs. Energy transfer is one way to increase the lasing efficiency of rare-earth ions by codoping the solid medium with ions that have broad, allowed absorption spectra in the UV or visible range and are capable of transferring the excitation energy efficiently to the lasing ion. The process of electronic energy transfer is used to increase the population of the lasing level by several orders of magnitude. The increase in population density of the lasing level has obvious advantages in improving the efficiency of the laser system and obtaining a low-threshold laser with low thermal load on the rod. Energy transfer can be radiative or nonradiative. The radiative process is a trivial case and will be discussed here briefly.
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