Luminescence laser materials

In the first topic, we will briefly describe a semi-empirical method that is commonly nsed to estimate the radiative transition probabilities from energy levels of (RE) + ions in crystals. This is certainly very nsefnl in order to determine the efficiency of a (RE) + based system as a luminescent or laser material. In the previous chapter (Section 5.7), we have described a method for determining the qnantnm efficiency of a Inminescent system. However, the application of that method is limited to certain... 

Chromium activated ruby was the first laser material and its luminescence properties are carefully studied. It is a classical example of Cr + in octahedral crystal field. Here Cr + substitutes the AP ions, while such a possibihty can be rationalized by an excellent chemical fit of Cr in place of Al. Ruby is a high crystal field material and thus the T2g state Hes above the E2g level. Pumping is accomplished by a spin-allowed transition into the state, while emission occurs from the level without vibrational broadening and almost all excited... 

Pressure effects on the energy transfer between f elements of the same kind were studied by Merkle et al. (1981) for the case of Nd3+-Nd3+ pairs in Ndx Y xP50i4 (x = 1,0.1). This material was studied in detail because of its potential use as a stoichiometric laser material. An outstanding property is a very weak concentration quenching of the luminescence. The total luminescence decay rate of the 4F3/2 multiplet in Ndx Y xP50i4 (x = 1,0.1) underpressure is shown in fig. 17. Obviously the stoichiometric compound shows a much larger increase of the decay rate than the doped compound. 

The quantum yields of fluorescence of the different systems have also been determined relative to a single crystal of neodymium-doped YAG for which a quantum yield of unity has been assumed (Heller, 1968a). The quantum yields obtained, even if they are accurate only within a factor of two, follow the same trend as for the lifetimes, with the highest values for the acidic solutions 0.70 and >0.75 in presence of S11CI4 and SbCls, respectively. Neutral and basic solutions are less luminescent and have quantum yields of 0.5 and 0.4, respectively. Identical measurements performed on a sodium-compensated neodymium-doped calcium tungstate crystal lead to a value of 0.5. The high quantum efficiency and the low threshold (between 2 and 40 J) of these Nd3+ SeOCl2 systems clearly demonstrate that liquids are not inherently inferior to solids as laser materials. 

Many minerals can be made to luminesce under various excitation sources, usually UV light, but in relatively few cases is the mechanism understood in detail. Best understood is luminescence due to transitions between localized states in the unfilled d-orbitals of transition metal ions and localized states in the unfilled f-orbitals of rare earth ions. Rare earth ions, important in the development of synthetic phosphor and laser materials, are uncommon among naturally occurring minerals. 

Some applications other than laser materials are the following luminescent materials for lighting, for display in cathode-ray tubes, and for X-ray radiography scintillator materials electroluminescent thin films glasses for solar concentrators colored materials for all types of applications (e.g., pigments). The greater part of these applications were reviewed in refs. 2 and 3. Optical centers can in many cases also be used as probes of the surroundings. 

The unique luminescent properties of rare earth metal clathrochelates have been used in the development of luminescent materials (luminophores and laser materials). The luminescence of these clathrochelates in solution makes their application as biological probes and concentrators of the luminescence (i.e., the antenna effect ) promising. These complexes can also serve as efficient molecular devices to convert UV light absorbed by the ligand to lanthanide ion luminescence in the visible region. Even in very dilute (10-5 mol l-i) solutions, the conversion of irradiated photons to luminescent ones has been observed to occur at a rate of approximately 1%. For rare earth metal aqua ions at the same concentration, the efficiency of conversion is equal to 4 x IQ- % [212, 390-392]. 

Neodymium. Neodymium can be present in relatively high concentrations in fluorapatites. Gaft et al. (2001a) lists Nd analyses for several natural apatites that are higher than any other REE except Ce, and at a concentration level of about 40% of the Ce value. Nd emission is well into the IR, and it is not sensitized by most of the other REE. Hence, Nd emission is expected to be relatively independent of other impurities, and will not contribute to visible luminescence. However, Nd -doped synthetic apatites are excellent laser materials, due to several physical attributes of the Nd electronic structure in the host lattice. Detailed evaluation of the optical properties of Nd in Ba fluorapatite... 

Finally that intriguing application yielding the laser. In luminescence the radiative decay of the excited state to the ground state occurs by spontaneous emission, i.e. the emission processes on different activator ions are not correlated. If, by some means, the majority of the luminescent ions are in the excited state (this situation is called population inversion), a single spontaneously emitted photon (quantum of radiation) may stimulate other excited ions to emit. This process is called stimulated emission. It is monochromatic, coherent and non-divergent. Laser action depends on emission by a stimulated process. Actually the word la.ser is an acronym for light amplification by stimulated emission of radiation. This book does not deal with lasers or laser physics. However, we will deal with the material where the stimulated emission is generated if this is useful for our purpose. F.very laser material is after all also a luminescent material. 

A laser material should satisfy several requirements. One of these is that it shows efficient luminescence. Further, laser materials should be characterized by careful spectroscopic measurements. 

The excited ions of the pumped laser materials in a laser resonator can be de-excited by various radiative (either laser or luminescence) and nonradiative (electron-phonon interaction or energy transfer) processes. Also, the amount of ions participating in laser emission is dependent on the laser emission efficiency ( /i). The laser emission is produced by the excited ions inside the laser mode volume and pumped above the threshold. The excited ions inside the pumped volume but outside the laser mode volume and those that form the inversion of population at the laser threshold can be de-excited by luminescence and nonradiative processes. 

The luminescence of lanthanide ions in solids has been the subject of several books or chapters [1-12] and has an immense scope. Since 1 was asked to write this review within a short time, I have failed to give a comprehensive survey but have given a qualitative overview of several areas with some references to more quantitative treatments. Topics such as lanthanide luminescence of laser materials [4,8,13,14], sensors [15], hybrid materials [16], organolanthanide, and coordination compounds [17] are missing. So what is here in October 2009 - basically, a description of the luminescence spectra of lanthanide ions in the solid state and some applications of phosphor materials. 

Luminescence of lanthanides in crystalline form is maybe the best-known application for the common people. The lighting and display industry has utilized the unique long lifetime luminescence as phosphors and laser industry has exploited lanthanide crystals since population inversion is easy to achieve with Iruig lifetime laser materials. Beyond the luminescence use, lanthanides are used in magnets, glass production, as colorants, contrast agents, in computer memories - the... 

Mohler R, White W (1995) Influence of structural order on the luminescence of oxide spinels Ci " activated spinels. J Electrochem Soc 142 3923-3927 Moncorge R, Cormier G, Simkon D, Capobianco J (1991) Fluorescence analysis of chromium doped forsterite (Mg2Si04). IEEE J Quantum Electron 27 114-120 Moncorge R, Manaa H, Boulon G (1994) Cr and Mn active centers for new solid state laser materials. Opt Mater 4 139-144... 

Volume 7 summarizes new trends on liquid crystals, display, and laser materials. The topics include liquid crystals for electro-optic applications, switchable holographic polymer-dispersed liquid crystals, electrochromism and electrochromic materials for displays, materials for solid-state dye lasers, photophysical properties of laser orientational relaxation processes in luminescence, and lasing of dyes and photosensitive materials for holographic recording. 

The luminescence spectrum of Nd +-activated NaGdGe04 crystals has been studied by Kaminskii et al. (1980). The compound is a promising laser material. The... 

Sol-gel derived materials with function of generating lights are listed under the heading of (A) Generation of light in Table 1-3. Actually, these materials convert a light to a new useful light by the use of luminescence, laser and non-linear optical effects. 

Our main purpose is to compare absorption and luminescence spectra of R(IH) in glasses with the behavior in crystalline compounds and in aqueous solution. In one way, it is surprising how close many of the similarities are, and also how technologically attractive glasses can be as laser materials compared to crystals. It may be noted that the largest-scale terawatt lasers, such as the SHIVA and NOVA systems in Livermore, CA, are neodymium(III) glasses. We summarize most of the major results described in this chapter in seven conclusions ... 

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