The absorption and emission cross sections of the transition between the ground spin-orbit multiplet and the lowest excited multiplet were measured for Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/ ions in a variety of crystalline hosts. The materials that were investigated include LiYF/sub 4/, BaY/sub 2/F/sub 8/, Y/sub 3/Al/sub 5/O/sub 12/, LaF/sub 3/, KCaF/sub 3/, YAlO/sub 3/, and La/sub 2/Be/sub 2/O/sub 5/. The absolute magnitudes of the emission cross sections were determined from the absorption spectra, with the aid of the principle of reciprocity. The calculated radiative emission lifetimes derived from these measured cross sections agree well with the measured emission decay times for most materials. The potential use of these rare-earth-doped materials in pulsed laser applications requires that the ground state exhibit adequate splitting to minimize the detrimental effects of the ground state thermal population, and also that the emission cross section be sufficiently large to permit efficient extraction energy. The systems based on Ho/sup 3+/ in the eightfold coordinated sites of LiYF/sub 4/, BaY/sub 2/F/sub 8/, and Y/sub 3/Al/sub 5/O/sub 12/ appear to be the most promising. >

Infrared cross-section measurements for crystals doped with Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/

Photoluminescent NaYF(4):Yb(3+)/Tm(3+) nanocrystals are ideally suited for in vitro and in vivo photoluminescence (PL) bioimaging due to their virtue of near-infrared to near-infrared (NIR-to-NIR) upconversion (UC); they display PL with a peak at approximately 800 nm if excited at approximately 980 nm. Here, we report the synthesis of monodisperse NaYF(4):Yb(3+)/Tm(3+) nanocrystals of ultrasmall size (7-10 nm) with high UC efficiency. The intensity of their NIR UC emission was demonstrated to increase by up to 43 times along with an increase in the relative content of Yb(3+) ions from 20 to 100%, with a corresponding decrease in the Y(3+) content from 80 to 0%. The achieved ultrasmall NaYbF(4):2% Tm(3+) nanocrystals manifest NIR PL emission, which is 3.6 times more intense than that from 25-30 nm sized NaYF(4):20% Yb(3+)/2% Tm(3+) nanocrystals, previously synthesized and used for in vitro and in vivo bioimaging. An optimization of both size and UC PL efficiency of NIR-to-NIR nanocrystals provides us with highly efficient optical imaging probes for bioapplications.

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Ultrasmall monodisperse NaYF4:Yb3+/Tm3+ nanocrystals with enhanced near-infrared to near-infrared upconversion photoluminescence

Publisher Summary This chapter describes the symmetry aspects of the crystal field and the parametrization of the energy level scheme. It provides an overview of the experimental data of trivalent lanthanide ions doped into crystalline host matrices. The selection rules for induced electric dipole (ED) and magnetic dipole (MD) transitions for systems with even or odd numbers of f electrons in different site symmetries are presented. Methods for the assignment of crystal-field levels are discussed and the determination of phenomenological crystal-field parameters is described. Such a set of crystal-field parameters in combination with a suitable set of free-ion parameters allows calculating crystal-field energy levels and reconstructing the energy diagram of the 4fn configuration. In this way, it is not only possible to check the validity of the crystal-field model but also to get information about energy levels, which cannot be detected experimentally.

Chapter 155 Rationalization of crystal-field parametrization

We describe fundamental measurements of the properties of thulium (Tm)-doped silica and power scaling studies of fiber lasers based on the material. Data on the high-lying Tm:silica energy levels, the first taken to our knowledge, indicate that pumping at 790 nm is unlikely to lead to fiber darkening via multiphoton excitation. Measurement of the cross-relaxation dynamics produces an estimate that, at the doping levels used, as much as 80% of the decay of the Tm level pumped is due to cross relaxation. Using a fiber having a 25-mum-diameter, 0.08 numerical aperture (NA) core, we observed fiber laser efficiencies as high as 64.5% and output powers of 300 W (around 2040 nm) for 500 W of launched pump power, with a nearly diffraction-limited beam. At these efficiencies, the cross-relaxation process was producing 1.8 laser photons per pump photon. We generated 885 W from a multimode laser using a 35-mum, 0.2-NA core fiber and set a new record for Tm-doped fiber laser continuous-wave power.

Tm-Doped Fiber Lasers: Fundamentals and Power Scaling

The theoretically determined slope efficiency and threshold pump power for continuous wave (CW) operation of a Tm-doped silica fiber laser are presented. The associated rate equations are solved using standard techniques and, in conjunction with the published and our measured spectroscopic parameters as input, the model was used to examine the fiber laser output for a variety of fiber and pump configurations. After good agreement was achieved between the model calculations and the published experimental measurements, the model was used to examine the relative performance of the fiber laser when the pump wavelength was varied over the /sup 3/F/sub 4/, /sup 3/H/sub 5/, and /sup 3/H/sub 4/ absorption bands of Tm/sup 3+/. The calculated maximum slope efficiencies were determined to be /spl sim/40, /spl sim/57, and /spl sim/84%, respectively, for each of the /sup 3/F/sub 4/, /sup 3/H/sub 5/, and /sup 3/H/sub 4/ absorption band pump schemes and the threshold pump power over the range of pump schemes was determined to vary by only 28%. The model was further used to analyze the fiber laser output when the fiber length, Tm/sup 3+/ concentration and /sup 3/H/sub 4/ energy level lifetime were varied and the consequences on the operation of the fiber laser with these variations are discussed.

Theoretical modeling of Tm-doped silica fiber lasers

Etude des spectres d'absorption du Tm 3+ :Y 3 Al 5 O 12 entre 19 et 026 μm aux temperatures 15 et 90K; et entre 08 et 035 μm a 16 K L'emission excitee par laser obtenue a 90 K est aussi etudiee, des etats multiples Tm 3+ : 1 D 2 , 1 G 4 , 3 H 4 et 3 F 4 vers l'etat fondammental 3 H 6  L'emission a partir de l'etat 1 D 2 inclue des transitions vers les niveaux Stark des etats 3 F 4 , 3 F 3 , et 3 F 2  L'analyse des spectres d'emission confirme l'eclatement experimental du champ cristallin deduit d'une analyse de donnees d'absorption de la bande chaude

Spectra and energy levels of Tm3+:Y3Al5O12.

The crystal-field energy-level structures of three different Er 3+ -doped garnet systems are analyzed and compared in this study The garnet hosts are Y 3 Al 5 O 12 (YAG), Y 3 Sc 2 Al 3 O 12 (YSAG) doped with Tm 3+ as a sensitizer ion, and Y 3 Sc 2 Ga 3 O 12 (YSGG) doped with Cr 3+ as a sensitizer ion The focus is on energy levels assigned to Er 3+ ions substituted for Y 3+ at dodecahedral (D 2 symmetry) sites in the cubic garnet lattices

Energy levels and correlation crystal-field effects in Er3+-doped garnets.

nets Y3A1)012 (YAG), Y3Sc~A1,0„(YSAG), Gd3Sc~Ga30l2 (GSGG), and LagLu2Ga30]2 (LLGG). The majority of Nd'+ ions substitute for Y'+, Gd'+, or La'+ ions in sites of D2 point-group symmetry in the lattice. Individual Stark levels within each manifold have been identified experimentally up to 40000 cm ' and compared with results obtained from a theoretical calculation. The model Hamiltonian includes Coulombic, spin-orbit, and interconfiguration interaction terms for the 4f atomic configuration of Nd'+ and crystal-field terms in D2 symmetry, including spin-correlated crystal-field (SCCF) contributions. The Hamiltonian was diagonalized within the complete 4f SLJMJ basis set which includes 364 states. Comparison between 148 (Nd: YAG), 140 (Nd: YSAG), 143 (Nd:GSGG), and 107 (Nd:LLGG) observed and calculated Stark levels (without SCCF) yields rms deviations of 18, 19, 17, and 16 cm, respectively. Results indicate that SCCF contributions yield modest improvement in the calculated value of the overall crystal-field splitting of the energy levels.

Comparative analysis of Nd3+(4f3) energy levels in four garnet hosts.

Absorption, fluorescence, and site-selective excitation spectra of Pr 3+ : Y 3 Al 5 O 12 are reported between 0.4 and 6.7 μm at several temperatures between 1.6 and 90 K. The complexity of the spectra indicates that Pr 3+ ions occupy several different sites. The most intense spectra, representing the majority of the Pr 3+ ions in dodecahedral lattice sites, are analyzed on the basis of electric-dipole selection rules for D 2 site symmetry. Weak spectra are reported but not analyzed due to the difficulty in assigning levels to a particular site. Analyses of intense spectra establish the symmetry of 17 Γ 1 , 12 Γ 2 , 10 Γ 3 , and 12 Γ 4 Stark levels. These 51 levels are compared with the results of a crystal-field splitting calculation. A Hamiltonian consisting of Coulombic, spin-orbit, and crystal-field (D 2 symmetry) terms was diagonalized for all manifolds of the Pr 3+ (4f 2 ) configuration. The rms deviation between calculated and experimental levels is 11 cm −1 .

Symmetry, selection rules, and energy levels of Pr3+:Y3Al5O12

Absorption spectra of Ho3+ ions in yttrium aluminum garnet (Ho3+:YAG) are reported between 2.16 and 0.23 μm at various temperatures between 4 K and room temperature. Laser‐excited excitation spectra and emission spectra from the 5F4 and 5S2 multiplet manifolds to the ground state manifold 5I8 were obtained at 4, 20, and 55 K. The majority of Ho3+ ions substitute for Y3+ ions in sites of D2 point‐group symmetry in the lattice. Over 1000 temperature‐dependent transitions (hot bands) establish 280 experimental Stark levels of the 4 f 10(2S+1LJ) multiplet manifolds. Symmetry labels Γ1, Γ2, Γ3, or Γ4, appropriate to D2 symmetry, have been assigned to Stark levels up to 43 000 cm−1. Experimental levels are compared with results obtained from a theoretical calculation. The model Hamiltonian includes Coulombic, spin‐orbit, and interconfiguration interaction terms for the 4 f 10 atomic configuration of Ho3+ and crystal‐field terms in D2 symmetry. The Hamiltonian was diagonalized within the 50 lowest 2S+1LJ manifol...

Marian E. Hills

Papers

Spectra and energy levels of Tm3+:Y3Al5O12.

Energy levels and correlation crystal-field effects in Er3+-doped garnets.

Comparative analysis of Nd3+(4f3) energy levels in four garnet hosts.

Symmetry, selection rules, and energy levels of Pr3+:Y3Al5O12

Energy levels and crystal quantum states of trivalent holmium in yttrium aluminum garnet