Laser ions trivalent rare earths

Finally, in the last section of this chapter (Section 6.6), we will treat two aspects that are of great relevance in the optical spectroscopy of solids. First, we will introduce a semi-empirical method (due to Judd, 1962 Ofelt,1962) that analyzes the absorption spectra of trivalent rare earth ions in crystals to search for new efficient phosphors and solid state lasers. Secondly, we will treat a relatively new topic related to optical centers in solids the optically induced cooling of trivalent ytterbium doped solids. 

Chapter 6 is devoted to discussing the main optical properties of transition metal ions (3d" outer electronic configuration), trivalent rare earth ions (4f 5s 5p outer electronic configuration), and color centers, based on the concepts introduced in Chapter 5. These are the usual centers in solid state lasers and in various phosphors. In addition, these centers are very interesting from a didactic viewpoint. We introduce the Tanabe-Sugano and Dieke diagrams and their application to the interpretation of the main spectral features of transition metal ion and trivalent rare earth ion spectra, respectively. Color centers are also introduced in this chapter, special attention being devoted to the spectra of the simplest F centers in alkali halides. 

Figure 2.7 Energy levels of trivalent rare earth ions. (A.P.B. Sinha— Fluorescence and Laser Action in Rare Earth Chelates in Spectroscopy in Inorganic Chemistry Ed. CNR Rao and JR Ferraro.)...

Energy migration in concentrated systems has been an issue of research in the last decade. Especially since lasers became easily available, the progress has been great. In Section VIII we will first consider the case that S is an ion to which the weak-coupling scheme applies. In practice this case consists of the trivalent rare earth ions. Subsequently we will deal with the case where S is an ion to which the intermediate-or strong-coupling scheme applies. 

Fig. 35.1 . Energy levels and laser transitions of trivalent rare earth ions. Wavelengths of transitions are in (um.

Trivalent rare-earth ions present a unique instance for which a priori calculations can be made from a small number of parameters, which are calculated theoretically and/or derived from simple experiments on small samples. Sueh predictions are of value in devising materials based on transparent media doped by rare-earth ions. The cross sections and performance of glass lasers can be predicted with quite good accuracy based on such calculations (Reisfeld and Jprgensen, 1977 Reisfeld, 1984a,b). 

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

This review will be concerned with fluorescent-lifetime studies upon condensed systems (that is, glasses, liquids, or crystals) containing rare-earth ions, and will to a large extent deal only with trivalent ions pumped by optical means. Laser phenomena themselves will not be considered, because a number of very excellent review articles (5-7) and books (8, 9) already exist on this subject. 

Figure 1.6 The energy level diagram for trivalent lanthanide ions [7]. (With kind permission from Springer Science+Business Media Lasers and Excited States of Rare Earths, 1977, p. 93, R. Reisfeld, and C.K. Jorgensen, figure 2, Springer-Verlag, Berlin.)...

The term upconversion describes an effect [1] related to the emission of anti-Stokes fluorescence in the visible spectral range following excitation of certain (doped) luminophores in the near infrared (NIR). It mainly occurs with rare-earth doped solids, but also with doped transition-metal systems and combinations of both [2, 3], and relies on the sequential absorption of two or more NIR photons by the dopants. Following its discovery [1] it has been extensively studied for bulk materials both theoretically and in context with uses in solid-state lasers, infrared quantum counters, lighting or displays, and physical sensors, for example [4, 5]. Substantial efforts also have been made to prepare nanoscale materials that show more efficient upconversion emission. Meanwhile, numerous protocols are available for making nanoparticles, nanorods, nanoplates, and nanotubes. These include thermal decomposition, co-precipitation, solvothermal synthesis, combustion, and sol-gel processes [6], synthesis in liquid-solid-solutions [7, 8], and ionothermal synthesis [9]. Nanocrystal materials include oxides of zirconium and titanium, the fluorides, oxides, phosphates, oxysulfates, and oxyfluoiides of the trivalent lanthanides (Ln ), and similar compounds that may additionally contain alkaline earth ions. Wang and Liu [6] have recently reviewed the theory of upconversion and the common materials and methods used. 

To illustrate the spectroscopic features of rare earth ions, in the following and throughout this section we shall use trivalent neodymium as a paradigm. This is appropriate since (a) most of the physical phenomena relevant to rare-earth lasers are present in Nd and (b) Nd is the most extensively used rare earth for lasers and has been lased in all forms of condensed matter - crystals, glasses. 

In the subsections to follow, trivalent and divalent rare earth ions and transitions used for crystal lasers are reviewed. The literature on rare earth crystal lasers is too voluminous to attempt a comprehensive listing of all ion-crystal combinations. Instead, the following references and tables therein should be consulted Kaminskii and Osiko (1970), Weber (1971b), and Kaminskii... 

In conclusion, the field of rare earth lasers is mature, but it is not exhausted. Additional laser schemes and materials will undoubtedly be exploited. For example, there are 1639 free-ion energy levels associated with the 4f" configurations of the thirteen trivalent lanthanide ions. Yet, of the 192 177 possible transitions between pairs of levels, only 34 have been used for lasers. It is certain that, given suitable pump sources and materials, stimulated emission involving many more transitions can be obtained. This is particularly true with the increasing availability of lasers at new wavelengths for pump sources and of tunable lasers for selective excitation of levels. 

In the P" phase, trivalent cations like Eu + can also be substituted for Na+. For instance, when a slab of this crystal is heated in EuClj powder for 24 h, almost all the Na is replaced by Eu. This vapor-phase ion exchange utihzing fast ion transport is a typical example of materials synthesis by soft chemistry. Unfortimately, rare earth P"-aluminas are not good ionic conductors, but they are promising candidates as crystals for solid state lasers. ... 

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