We show theoretically with the simplest possible model that the intensity of an upconversion luminescence that is excited by the sequential absorption of n photons has a dependence on absorbed pump power P, which may range from the limit of P n down to the limit of P 1 for the upper state and less than P 1 for the intermediate states. The two limits are identified as the cases of infinitely small and infinitely large upconversion rates, respectively. In the latter case, the dependence of luminescence intensities from intermediate excited states on pump power changes with the underlying upconversion and decay mechanisms. In certain situations, energytransfer upconversion and excited-state absorption can be distinguished by the measured slopes. The competition between linear decay and upconversion in the individual excitation steps of sequential upconversion can be analyzed. The influence of nonuniform distributions of absorbed pump power or of a subset of ions participating in energy-transfer upconversion is investigated. These results are of importance for the interpretation of excitation mechanisms of luminescent and laser materials. We verify our theoretical results by experimental examples of multiphoton-excited luminescence in Cs 3 Lu 2 Cl 9 :Er 3ϩ , Ba 2 YCl 7 :Er 3ϩ , LiYF 4 :Nd 3ϩ , and Cs 2 ZrCl 6 :Re 4ϩ .

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We describe a novel approach to directly measure the energy of the narrow, low-lying isomeric state in 229Th. Since nuclear transitions are far less sensitive to environmental conditions than atomic transitions, we argue that the 229Th optical nuclear transition may be driven inside a host crystal with a high transition Q. This technique might also allow for the construction of a solid-state optical frequency reference that surpasses the short-term stability of current optical clocks, as well as improved limits on the variability of fundamental constants. Based on analysis of the crystal lattice environment, we argue that a precision (short-term stability) of 3×10(-17)<Δf/f<1×10(-15) after 1 s of photon collection may be achieved with a systematic-limited accuracy (long-term stability) of Δf/f∼2×10(-16). Improvement by 10(2)-10(3) of the constraints on the variability of several important fundamental constants also appears possible.

Hexagonal Sodium Yttrium Fluoride Based Green and Blue Emitting Upconversion Phosphors. -Pure hexagonal Na 3x Ln 2-x F 6 (Ln: Y, Nd, Er, Tm, Yb; x = 0.45) phosphor powder samples are synthesized from Ln2O3, Na2CO3, HBr, and HF in H2O (550-540°C, 40 h). As revealed by powder XRD the samples crystallize in the space group P63/m with Z = 1. The samples doped with 18% Yb + 2% Er and 25% Yb + 0.3% Tm show the highest upconversion efficiencies for green and blue emission, respectively. The obtained phosphor materials show no degradation under high-power IR laser excitation. -(KRAEMER*, K. W.; BINER, D.; FREI, G.; GUEDEL, H. U.; HEHLEN, M. P.; LUETHI, S. R.; Chem. Mater. 16 (2004) 7, 1244-1251; Inst. Chem. Biochem., Univ. Bern, CH-3012 Bern, Switz.; Eng.) -W. Pewestorf 26-017

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Since the first demonstration of net cooling twenty years ago, optical refrigeration of solids has progressed to outperform all other solid-state cooling processes. It has become the first and only solid-state refrigerator capable of reaching cryogenic temperatures, and now the first solid-state cooling below 100 K. Such substantial progress required a multi-disciplinary approach of pump laser absorption enhancement, material characterization and purification, and thermal management. Here we present the culmination of two decades of progress, the record cooling to ≈ 91 K from room temperature.

A spectroscopic investigation of an extensive series of Er 3ϩ -doped and Er 3ϩ ,Yb 3ϩ -codoped soda-limesilicate ͑SL͒ and aluminosilicate ͑AS͒ glasses is presented. Compared to SL glasses, 4 f transitions in AS glasses show higher oscillator strengths, larger inhomogeneous broadening, and smaller crystal-field splittings of the respective excited-state multiplets. The Er 3ϩ excited-state relaxation dynamics is adequately described by a combination of the Judd-Ofelt model and the energy-gap law. With the exception of 4 I 13/2 , multiphonon relaxation is dominant for all excited states, making it possible to efficiently pump the 1.55 m 4 I 13/2 → 4 I 15/2 emission by excitation of 4 I 11/2 at around 980 nm. The absolute 4 I 13/2 luminescence quantum yield, for low 980-nm excitation density (ϳ5 W/cm 2 ), , is ϳ0.9 at 0.4 mol % Er 2 O 3 and drops to about 0.65 upon increasing Er 2 O 3 to 1.2 mol %, indicating the onset of energy-transfer processes. Samples with high OH Ϫ impurity concentration suffer from significantly higher quenching of 4 I 13/2 luminescence at higher Er 3ϩ concentrations. Energy migration to the minority of Er 3ϩ ions coordinated to OH Ϫ , followed by efficient multiphonon relaxation accounts for this effect. At low excitation densities, the strong near-infrared absorption of Yb 3ϩ in combination with efficient Yb→Er energy transfer increases the 4 I 13/2 population density in Yb 3ϩ ,Er 3ϩ -codoped samples by up to 2 orders of magnitude compared to equivalent samples without Yb 3ϩ .The dependence of on Yb 3ϩ codotation of 0.4 mol % Er 2 O 3 -doped samples predicts that a minimum of ϳ0.8 mol % Yb 2 O 3 is required to achieve efficient sensitization of Er 3ϩ by Yb 3ϩ . The relative intensities of upconversion luminescence from 4 S 3/2 and 2 H 11/2 are used to analyze internal sample heating in detail. Due to the high absorption cross section of Yb 3ϩ , increasing the Yb 3ϩ concentration in Yb 3ϩ ,Er 3ϩ -codoped samples of given length increases the absorbed power and subsequently the total density of multiphonon emission, leading to internal temperatures of up to 572 K in 0.4 mol % Er 2 O 3 samples codoped with 4 mol % Yb 2 O 3 and excited with 51 kW/cm 2 . Multiphonon relaxation from 4 I 13/2 is shown to be inefficient even at these high internal sample temperatures. From upconversion luminescence spectra of a series of glasses, the thermal conductivity is estimated to be between 3.5ϫ10 Ϫ2 and 7.7ϫ10 Ϫ2 W m Ϫ1 K Ϫ1 , in good agreement with the known value of 4.8ϫ10 Ϫ2 W m Ϫ1 K Ϫ1 for soda-lime-silicate glass. ͓S0163-1829͑97͒03436-X͔