Fluoride glasses: multiphonon relaxations

One of the main spectroscopic properties that differentiate fluoride glasses from silica-based glasses is the low multiphonon emission rate. These non-radiative relaxations that may strongly compete with radiative processes in rare-earth ions are nearly three orders of magnitude lower in ZBLAN glass than in silicate, as shown in Fig. 2. This property is directly related to the fundamental vibration modes of the host and, therefore, varies basically in the same manner as the infrared absorption edge. 

The (3H4 - 3F4 3H6 - 3F4) cross-relaxation is an efficient energy transfer mechanism for Tm3+ concentration greater than 0.1 mol% in indium-based fluoride glasses [124], Therefore, the intermediate 3F4 level can be populated not only by successive multiphonon relaxations from 3F3, but also through this cross-relaxation process [125],... 

Fig. 3. Dependence of multiphonon relaxation on the number of phonons in the glass former needed to bridge the energy gap between the luminescent /-level and the closest lower-lying level. The curves refer to glasses of the following types (1) phosphate (2) borate (3) silicate (4) tellurite (5) fluoroberyllate (6) germanate (7) zirconium fluoride (ZBLA) (8) aluminum lanthanide sulfide (ALS) and gallium lanthanide sulfide (GLS) (9) the crystalline hosts YjAljOj CYAG), and (10)...

From the point of view of luminescent behavior, the cascading down between eight excited /-levels of holmium(III) in ZBLA glass is one of the most spectacular effects in fluoride glasses (fig. 8). The multiphonon relaxation being... 

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