Emission cross sections and spectroscopy of Ho3+ laser channels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho3+ laser channels in KGd(WO4)2 crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho3+ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω2=15.35×10-20 cm2, Ω 4=3.79×10-20 cm2, Ω6 =1.69×10-20 cm2. The 7-300-K lifetimes obtained in diluted (8·1018 cm-3) KGW:0.1% Ho samples are: τ(5F3)=0.9 μs, τ( 5S2)=19-3.6 μs, and τ(5F5 )=1.1 μs. For Ho concentrations below 1.5×1020 cm-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τph -1(0)=βexp(-αΔE), where β=1.4×10-7 s-1, and α=1.4×103 cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho3+ emission cross section σEM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σEM values achieved for transitions to the 5I8 level are ≈2×10-20 cm2 in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the 5I7→5I8 channel.This work was supported by Comisión Interministerial de Ciencia y Tecnología under Projects MAT99-1077 and 2FD97-0912.Peer reviewe

New strategies invonving upconverting nanoparticles for determining moderate temperatures by luminescence thermometry

Producción CientíficaHere we analyze alternative luminescence thermometry techniques to FIR, such as intensity ratio luminescence thermometry between the emission arising from two electronic levels that are not necessarily thermally coupled, but that show different evolutions with temperature, and lifetime luminescence nanothermometry in (Ho,Tm,Yb):KLu(WO4)2 and (Er,Yb):NaY2F5O nanoparticles. (Ho,Tm,Yb):KLu(WO4)2 nanoparticles exhibited a maximum relative sensitivity of 0.61% K−1, similar to that achievable in Er-doped systems, which are the upconverting systems presenting the highest sensitivity. From another side, (Er,Yb):NaY2F5O nanocrystals show great potentiality as thermal sensors at the nanoscale for moderate temperatures due to the incorporation of additional non-radiative relaxation mechanisms that shorten the emission lifetime generated by the oxygen present in the structure when compared to (Er,Yb):NaYF4 nanoparticles exhibiting the highest upconversion efficiency. We used those nanoparticles for ex-vivo temperature determination by laser induced heating in chicken breast using lifetime-based thermometry. The results obtained indicate that these techniques might constitute alternatives to FIR with potential applications for the determination of moderate temperatures, with sensitivities comparable to those that can be achieved by FIR or even higher.Ministerio de Economía, Industria y Competitividad (Projects MAT2013-47395-C4-1-4-R, and TEC2014-55948-R)Catalan Government (Projects no. 2014SGR1358 and 2013FI_B 01032

Green up-conversion of Er3+ in KGd(WO4)2 crystals. Effects of sample orientation and erbium concentration

Pumping with infrared light resonant to the energy position of 4 I 11/2 and 4 I 9/2 multiplets respectively has excited green up-conversion of Er3+ in KGd(WO4)2 single crystal. At room temperature the maximum green-emission intensity is achieved by pumping with light polarized parallel to the C2 symmetry axis of the crystal (//p) at 981 and 801.5 nm, while pumping with light parallel to the principal m axis (//m) has maximum up-conversion at 978.2, 806 and 800 nm. The emission is weakly polarized. The maximum of the emission peaks at 547.8 nm if the light is analyzed parallel to the C2 axis or at 552.4 nm for light perpendicular to it. The largest emission intensity was achieved with an erbium concentration about 3×1020 cm-3. A schematic model of the up-conversion process is suggested.This work is supported by CICyT under projects 2FD97-0912 and MAT99-1077.Peer reviewe

Thulium doped monoclinic KLu(WO4)2 single crystals: growth and spectroscopy

This paper presents the crystal growth and optical characterization of thulium-doped KLu(WO4)2 (KLuW). Thulium-doped KLuW macrodefect-free monoclinic single crystals (a*×b×c≈10×7×15 mm3) were grown by the top seeded solution growth slow cooling method with dopant concentrations of 0.5%, 1%, 3% and 5% atomic in solution. The evolution of unit cell parameters in relation with thulium doping was studied by X-ray powder patterns. Thulium energy levels in the KLuW host were determined by 6 K polarized optical absorption. The Judd–Ofelt parameters determined were Ω2=9.01×10-20 cm2, Ω4=1.36×10-20 cm2 and Ω6=1.43×10-20 cm2. The maximum emission cross section for the 1.9 μm emission, calculated by Füchtbauer–Ladenburg method, is 1.75×10-20 cm2, at 1845 nm with E//Nm. The intensity decay time from the emitting levels 1G4 and 3H4 levels in relation to the concentration were studied. For the lowest thulium concentration, the measured decay times from 1G4 and 3H4 emitting levels are 140 μs and 230 μs, respectively.We gratefully acknowledge financial support from the Education and Science Ministry of Spain under Project MAT-2004-20471-E, MAT 2005-06354-C03-02 and CIT-020400-2005-14 and from the Generalitat de Catalunya under project 2005SGR658. This paper was also funded by the EU-Commission Project DT-CRYS (STRP-NMP3-CT-2003-505580). M.C. Pujol and M. Rico are supported by the Education and Science Ministry of Spain under the Ram´on y Cajal programme.Peer reviewe

Crystalline structure and optical spectroscopy of Er3+-doped KGd(WO4)2 single crystals

KGd(WO4)2 single crystals doped with Er3+ have been grown by the flux top-seeded-solution growth method. The crystallographic structure of the lattice has been refined, being the lattice constants a=10.652(4), b=10.374(6), c=7.582(2) Å, β=130.80(2)°. The refractive index dispersion of the host has been measured in the 350–1500 nm range. The optical absorption and photoluminescence properties of Er3+ have been characterised in the 5–300 K temperature range. At 5 K, the absorption and emission bands show the (2J+1)/2 multiplet splittings expected for the C2 symmetry site of Er in the Gd site. The energy positions and halfwidths of the 72 sublevels observed have been tabulated as well as the cross sections of the different multiplets. Six emission band sets have been observed under excitation of the 4F7/2 multiplet. The Judd–Ofelt (JO) parameters of Er3+ in KGW have been calculated: Ω2=8.90×10-20 cm2, Ω4=0.96×10-20 cm2, Ω6=0.82×10-20 cm2. Lifetimes of the 4S3/2, 4F9/2, and 4I11/2 multiplets have been measured in the 5–300 K range of temperature and compared with those calculated from the JO theory. A reduction of the 4S3/2 and 4I11/2 measured lifetimes with increasing erbium concentration has been observed, moreover the presence of multiphonon non-radiative processes is inferred from the temperature dependence of the lifetimes.This work has been supported by CICyT under project number TIC96-1039.Peer reviewe

Measurement and crystal field analysis of energy levels of Ho3+ and Er3+ in KGd(WO4)2 single crystal

Optical absorption and photoluminescence spectra performed on Ho3+ (4f(10)) and Er3+ (4f(11)) doped KGd(WO4)(2) single crystals were measured at selected temperatures between 5 and 80 K. In this monoclinic matrix. space group C2/c (No. 15) the lanthanide ions replacing Gd3+ occupy a single crystallographic site with symmetry C-2. The interpretation of these spectra and the simulation of the splitting of L-2S+1(J) levels were carried out in the frame of the phenomenological single-electron crystal field theory, through the simultaneous diagonalization of the energy matrices of the free-ion (fi) and crystal field (cf) interactions. A total of 20 fi and 14 cf adjustable parameters were considered for the reproduction of experimental energy levels and their associated wavefunctions. The simulated energy level series are in excellent accordance with the experimental data. i.e. rms deviations, sigma =14.5 and 15.6 cm(-1) for Ho3+ and Er3+, respectively. The comparison with similar results for Pr3+. (4f(2)) and Nd3+ (4f(3)) allowed establishment of the variation trends of the cf parameters with the number of 4f electrons.This work is supported by CICyT under grant numbers 2FD97-0912 and MAT1999-1077.Peer reviewe

Yb sensitising of Er3+ up-conversion emission in KGd(WO4)2:Er:Yb single crystals

KGd(1−x−y)ErxYby(WO4)2 (x/y=0.024/0, 0.02/0.022 and 0.019/0.066) single crystals have been grown by the top seeded solution growth slow cooling method using K2W2O7 as solvent. Erbium concentration was selected at [Er]=1.2–1.5×1020 cm−3 in order to minimise Er–Er non-radiative losses and the ytterbium concentration was varied in the range [Yb]=0–4.2×1020 cm−3. The optical absorption in the 850–1100 nm spectral range is characterised by the overlap between ytterbium 2F5/2 and erbium 4I11/2 manifolds. At room temperature the green 4S3/2 Er3+ emission has been excited by energy transfer from the 2F5/2 (934.2, 953.2 and 980.8 nm) ytterbium multiplet. The up-converted emission is weakly polarised and within the concentration range studied its intensity increases with the Yb3+ concentration.This work has been supported by CICyT under projects 2FD97-0912 and MAT99-1077.Peer reviewe

Optical spectroscopy of Pr3+ in KGd(WO4)2 single crystals

The polarized optical absorption (OA) and photoluminescence of Pr3+ doped KGd(WO4)(2) (KGW) single crystals have been measured at selected temperatures between 7 and 300 K. For the studied Pr concentrations, [Pr] = 0.03 x 10(20)-1.9 x 10(20) cm(-3) a unique site is observed. 74 energy levels were experimentally determined for this site and labelled with the appropriate A or B irreducible representations corresponding to the C-2 symmetry of the Gd point site in KGW. The set was fitted to a Hamiltonian of adjustable parameters including free-ion as well as real B-q(k) and complex S-q(k) crystal-field parameters. A good simulation of the experimental crystal field energies was achieved with a root mean square deviation sigma = 15.3 cm(-1). Distortions in the oxygen bonds to Pr3+ are found to contribute to the broadening of some OA bands, particularly those related to the D-1(2) multiplet. The OA edge is determined by the interconfigurational 4f--> 4f(1)5d(1) Pr3+ transition peaking at 34 200 cm(-1). From the average 300 K OA cross sections the radiative lifetimes of the Pr3+ multiplets have been calculated considering the standard and modified Judd-Ofelt (JO) theories. A better agreement with the experimental results is obtained by the standard theory: the average JO parameters obtained at (Omega)over-bar(2) = 12.0 x 10(-20) cm(2), (Omega)over-bar(4) = 8.15 x 10(-20) cm(2) and (Omega)over-bar(6) = 2.64 x 10(-20) cm(2). Electrons excited to the P-3(0) multiplet decay very efficiently to the D-1(2) multiplet even at 15 K. In samples with [Pr] greater than or equal to 0.3 x 10(20) cm(-3) the excitation of the D-1(2) multiplet decays non-radiatively by an electric dipole-dipole transfer between Pr neighbours.This work is supported by CICyT under projects TIC96-1039 and MAT99-1077 and by MONOCROM SL (Spain).Peer reviewe

White light upconversion in Yb-sensitized (Tm, Ho)-doped KLu(WO4)2 nanocrystals: the effect of Eu incorporation

Monoclinic Yb-sensitized (Tm, Ho)-doped KLu(WO4)2 nanocrystals of ∼100 nm size have been synthesized by the modified Pechini sol-gel method. Their diode laser near-infrared (∼980 nm) excited upconversion emission properties have been characterized at power densities in the range 30-355 W cm-2. Bright white light composed of blue ∼475 nm, green ∼540 nm, and red ∼650 nm emissions, corresponding to Tm 3+ 1G4 → 3H6, Ho 3+ 5S2, 5F4 → 5I8, and Ho3+ 5F5 → 5I8 electronic transitions, respectively, was generated by adjusting the Yb, Tm, and Ho contents in KLu(WO4)2 nanocrystalline samples. Chromaticity coordinates of the emitted white light can be tuned by modifying the excitation power density. The effect of Tm and Ho on the luminescence dynamics has been described by analyzing the upconverted emission intensity dependence on the excitation power, as well as from Stokes and decay time measurements. The effect on upconversion properties of further codoping with Eu in these (Tm, Ho, Yb)-doped KLu(WO4)2 nanocrystals has also been studied.This work is supported by the Spanish Government under projects, MAT2011-29255-C02-01-02, TEC2010-21574-C02-02 and PI09/90527; and the Catalan Government under project 2009SGR235. EW Barrera is supported by Catalan Government for the fund provided through the fellowship 2012FI_B200136. The support of the COST Action CM1006 by the STSM-CM1006-050512-017263 grant is gratefully acknowledged.Peer Reviewe