Stimulated Emission in Organic Materials

Stimulated emission is quantified by the (wavelength dependent) excited state cross-section agg. In the case that no other processes are important, stimulated emission leads directly to amplification of light. In a material with a volume density Aexc of excited Si-states this amplification can be described by 

Here (Tabs,i is the cross-section for an absorption from the Si -level to higher lying states, Nt is the density of triplet excitations and (Tt is the corresponding cross-section for absorption into higher lying triplet levels. Since the first two terms on the right hand side both depend linearly on the density N xc of excited Si-states an effective stimulated emission cross-section iVsc,cn = sc- ahs,i can be defined. A quantitative treatment of the triplet absorption is more complicated since the density of molecules in the 7 -stale has to be known. 

Generally, photooxidation has an even stronger negative effect on lasing and stimulated emission in conjugated polymers than it has on the EL-performance. It not only reduces the number of excited Si states but additionally creates charged absorbing species that partly compensate the stimulated emission due to die neutral excited states. 

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Rare earth laser action has been obtained for two groups of liquids metallo-organic and inorganic aprotic liquids. The first group are chelate lasers and are reviewed by Lempicki and Samelson (1966) research on aprotic materials and systems for high-power, pulsed liquid lasers are reviewed by Samelson and Kocher (1974). Stimulated emission in both liquids occurs between 4f states of trivalent rare earths. Optical pumping is via xenon-filled flashlamps in optical cavities and resonators similar to those used in solid-state lasers. Rare earth liquid lasers have only been operated pulsed. 

In 1962, it was pointed out that europium complexes dissolved in organic solvents or in a polymer matrix have optical properties that make them potential candidates for laser materials. At the same time. Whan and Crosby and Filipescu and coworkers have also suggested the potential application of rare-earth chelates in lasers. Lempicki and Samelson in 1963 were the first researchers to obtain stimulated emission at... 

Ordered conjugated organic materials exhibit low trap densities and high charge carrier mobilities, for both electrons and holes. As an example, in single crystals of anthracene [87], the hole and electron mobilities are approximately 1 cm V s and the crystal photoluminescence quantum yield at room temperature is almost unity. Both electroluminescence [87] and stimulated emission... 

Lacquer—A type of organic coating in which rapid drying is effected by evaporation of solvents. Laser—A term for light amplification by stimulated emission of radiation laser is a cavity with mirrors at the ends, filled with material such as crystal, glass, liquid, gas, or dye. It produces an intense beam of light with the unique properties of coherency, coUimation, and monochromaticity. 

Here we recall a few important physical concepts and technological issues related to the properties of stimulated emission and lasing action shown by a few classes of light-emitting polymer nanofibers. These fibers are better presented in Seetion 5.5. A complete, definitive description of the operation principles of laser materials can be found in the book by O. Svelto. Furthermore, for readers interested in more details on organic lasers, excellent specialized reviews exist in the literature. ... 

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