Color center production by femtosecond-pulse laser irradiation in fluoride crystals

Color centers are lattice vacancy defects trapping electrons or holes. They are easily created in single crystals by irradiation with ionizing radiation. We report the production of color centers in LiF and LiYF₄ single crystals by ultrashort high-intensity laser pulses (60 fs, 12.5 GW). An intensity threshold for color center creation of 1.9 and 2 TW/cm² was determined in YLF and LiF, respectively, which is slightly smaller than the continuum generation threshold. Due to the high energy density of the coherent radiation of the focused laser beam, we were able to identify a large amount of F centers, which gave rise to aggregates such as F₂, F ₂ ⁺ , and F ₃ ⁺ . The proposed mechanism of formation is based on multiphoton excitation, which also produces short-lived F ₂ ⁺ centers. It is also shown that it is possible to write tracks in the LiF crystals with dimensional control.     

Compact,diode-side-pumped and Q-switched Nd:YLiF<Subscript>4</Subscript> laser cavity operating at 1053 nm with diffraction limited beam quality

This work reports a diode-side-pumped and passively Q-switched Nd:YLiF₄ (YLF) laser operating at 1053 nm using a new laser resonator concept. Very stable pulses of 1 mJ energy with less than 10 ns pulse duration are obtained at 1 kHz repetition rate in a very simple, compact, and robust cavity that uses a double bounce configuration to achieve TEM₀₀ operation.     

Imaging cytoplasmic cAMP in mouse brainstem neurons

Background  

cAMP is an ubiquitous second messenger mediating various neuronal functions, often as a consequence of increased intracellular Ca²⁺ levels. While imaging of calcium is commonly used in neuroscience applications, probing for cAMP levels has not yet been performed in living vertebrate neuronal tissue before.     

Stable isotopic studies of earthworm feeding ecology in tropical ecosystems of Puerto Rico

Feeding strategies of earthworms and their influence on soil processes are often inferred from morphological, behavioral and physiological traits. We used (13)C and (15)N natural abundance in earthworms, soils and plants to explore patterns of resource utilization by different species of earthworms in three tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus and Estherella sp. showed less (15)N enrichment ((15)N = 3-6 per thousand) than exotic Pontoscolex corethrurus ((15)N =7-9 per thousand) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in (15)N enrichment ((15)N = 6-9 per thousand), suggesting the potential for interspecific competition for N resources. A tabonuco forest converted to pasture contained only P. corethrurus which were less enriched in (15)N than those in the forest sites, but more highly enriched in (13)C suggesting assimilation of C from the predominant C(4) grass. These results support the utility of stable isotopes to delineate resource partitioning and potential competitive interactions among earthworm species. Copyright 1999 John Wiley & Sons, Ltd.     

Study of color centers produced in thulium doped YLF crystals irradiated by electron beam and femtosecond laser pulses

In this work, we report the influence of the presence of photochromic and color centers in the photobleaching of thulium ions blue emission in YLF (YLiF₄) crystals doped with 1 mol% Tm (3+). The samples were irradiated at room temperature both with electron beam and high intensity ultrashort pulses from a Ti:Sapphire CPA laser system. In both irradiations the production of photochromic and color centers was observed via the absorption bands in the UV and visible ranges. Pure LiF and pure and oxygen doped YLF crystals were used to identify the color centers produced and their optical properties. From a phenomenological model it was possible to study the interaction between color centers and thulium ions, and their effect in photobleaching and photodarkening behaviors. Finally, the blue up laser level population was computed using a rate equation analysis.     

Lasing of a single crystal Nd fluoride fiber

We present laser action in a single crystal fluoride fiber obtained by the micro-pulling-down technique. The 700 μm diameter and 1 cm long Nd:LiYF₄ fiber was pumped by a beam shaped diode bar emitting at 806 nm. A peak power of 300 mW was achieved with a slope efficiency of 12%. When considering the pump power fraction absorbed by the laser mode, a slope efficiency of 37% was achieved. To the best of our knowledge, this is the first report of a Nd: LiYF₄ fiber laser.     

Growth and optical characteristics of Ce-doped and Ce : Na-codoped BaLiF3 single crystals

Ce-doped and Ce : Na-codoped BaLiF₃ single crystals were grown by the Czochralski technique under reactive atmosphere. Na⁺-ions, used as a charge-compensating impurity, compete with Ce³⁺-ions for Ba²⁺ sites in the host, reducing the Ce³⁺ incorporation and degrading the mechanical integrity of crystals. Ce-doped BaLiF₃ showed potential for UV tunable lasers.     

Phase equilibria and prospects of crystal growth in the system LiF–GdF3–LuF3

Scheelite type LiGdF₄, LiLuF₄, and mixtures of both end members were prepared by a hydrofluorination route from the rare earth oxides and commercial LiF. The samples were purified by melting in HF/Ar mixtures, and were investigated by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and energy dispersive X‐ray spectroscopy (EDX) techniques. Both end members show unlimited miscibility in the solid phase. Mixed crystals containing at least 65 mol‐% LiLuF₄ melt under direct formation of the liquid phase. The gap between solidus and liquidus is narrow. LiGdF₄ and mixed crystals with less then 65 mol‐% LiLuF₄ decompose peritectically under formation of (Gd,Lu)F₃. Crystal growth is expected to be possible either from Lu‐rich melts with the appropriate scheelite composition or from Gd‐rich melts containing an excess of LiF. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)