Rare earth fluoride glasses

Extensive research including the study of radiative and non-radiative properties of rare-earth ions has been carried out. Especially, the Judd-Ofelt theory has been applied to most rare-earth — fluoride-glass combinations. Typical Judd-Ofelt parameters are reported in Table 3 for ZBLAN glass [31-34], An exhaustive list of such parameters for glasses and crystals can be found in Ref. [35]. 

Rare-earth fluorides are good fluorine-ion conductors (see sect 5.2). Addition of aliovalent cations significantly increases the fluorine-ion conductivity even fiulher. For example, single crystals of Lap3 doped with Eup2 are widely used in commercial apphcations of ion-selective electrodes (ISE) as specific electrodes (Frant and Ross 1966). In the field of ISE, only the pH-sensing glass electrode is more widely used. Fluoride-ion detection is important in sea water, water minerals, rocks, fossils and minerals, biomedical applications, potable water and plant and animal metabolism. 

Tada M., Fujihara S., Kimura T. Sol-gel processing and characterization of alkaline earth and rare-earth fluoride thin films. J. Mater. Res. 1999 14 1610-1616 Takahashi M., Izuki M., Kanno R., Kawamoto Y. Up-conversion characteristics of Er in transparent oxyfiuoride glass-ceramics. J. Appl. Phys. 1998 83 3920-3922 Takashima M., Kano G. Preparation and electrical condnctivity of binary rare earth metal fluoride oxides. Solid State Ionics 1987 23 99-106... 

There are a great number of potential glasses and some can be extremely complex. All contain silica and alumina and an alkaline earth or rare earth oxide or fluoride. The two essential glass types are SiOj-AljOg-CaO and SiOj-AljOg-CaFj, from which all others are derived. 

Samarium salts are used in optical glass, capacitors, thermoionic generating devices, and in sensitizers of phosphors. The metal is doped with calcium fluoride crystals for use in lasers. It also is used along with other rare earths for carbon-arc lighting. Its alloys are used in permanent magnets. 

Active optical properties of rare-earth-doped fluoride glasses... 

Fluoride glasses are hosts of prime interest for rare-earth spectroscopy. They show low phonon energies, which induce high quantum efficiencies of rare-earth... 

Judd-Ofelt parameters Q, (r = 2, 4, 6) of rare-earth ions in ZBLAN fluoride glasses... 

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. 

Fig. 2. Multiphonon emission rates of rare-earth ions in silicate, fluoride and chlorofluoride glasses. The position of three levels or Er3+ ions are indicated, with respect to the energy gap to the next-lower level (reproduced with permission from Eur. J. Solid State Inorg. Chem., 31 (1994) 337 [44]).

Little basic research has been devoted to Yb3+ ion itself which possesses only one excited state [151]. However, as shown previously, this ion is of special interest for a number of energy transfer and up-conversion processes, which take advantage of the strong 2F7/2 — 2F5/2 absorption band of Yb3+. In addition, efficient lasing around 1.02 can be obtained with this ion and laser cooling effects have been observed. These points are developed in the next section devoted to applications of rare-earth-doped fluoride glasses. 

Characteristics of rare-earth-doped ZBLAN fluoride glass fiber lasers, (i) incident, (1) launched, and (a) absorbed pump powers, (uc) up-conversion pumping... 

Because of the wide transmission range and low phonon energies of fluoride glasses, the observation of numerous rare-earth laser lines is possible at wavelengths beyond 2 fim, where the transmission of silica fibers is extremely poor. Laser sources around 2 /jm are of special interest because they belong, not only to the eye-safe spectral domain, but also to an optical transparency window of the atmosphere. Two fluoride glass fiber lasers have been demonstrated in that region. First, a Ho3+ laser with the 5I7 -> 5I8 transition at 2.024 /on which delivers 250 mW with 60%... 

Rare-earth-doped fluoride glasses are suitable for some other specific applications that require laser transitions at wavelengths greater than 2 pm, efficient up-conversion mechanisms or ultra-high quantum efficiencies. 

From a material point of view, amplification at 1.3 jum and up-conversion processes will benefit from the use of glasses with lower phonon energies than fluorides. As far as rare-earth spectroscopy is concerned, sulfide glasses are for the moment the most promising materials. However, significant improvements are still to be made with sulfides in terms of glass chemical purity and fiber drawing processes. 

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