Quantum dot-based thermal spectroscopy and imaging of optically trapped microspheres and single cells

Laser-induced thermal effects in optically trapped microspheres and single cells are investigated by quantum dot luminescence thermometry. Thermal spectroscopy has revealed a non-localized temperature distribution around the trap that extends over tens of micrometers, in agreement with previous theoretical models besides identifying water absorption as the most important heating source. The experimental results of thermal loading at a variety of wavelengths reveal that an optimum trapping wavelength exists for biological applications close to 820 nm. This is corroborated by a simultaneous analysis of the spectral dependence of cellular heating and damage in human lymphocytes during optical trapping. This quantum dot luminescence thermometry demonstrates that optical trapping with 820 nm laser radiation produces minimum intracellular heating, well below the cytotoxic level (43 °C), thus, avoiding cell damage.

Analysis of vacuum ultraviolet electronic spectra of Ce3+ and Pr3+ ions in Ca9Lu(PO4)7: crystal-field calculations and simulation of optical spectra

The 4f-5d excitation and emission spectra of Ce(3+) and Pr(3+) ions in Ca9Lu(PO4)7 as recently reported (2012 J. Phys.: Condens. Matter 24 385502) were further analyzed and simulated by employing the effective Hamiltonian model for the 4f(N) and 4f(N-1)5d electronic configurations of impurity lanthanide ions and the exchange charge model of crystal-field theory. The multi-site effect on the 4f-5d transition spectra was explicitly discussed from the points of view of the local structure and site occupation ratios of lanthanide ions in Ca9Lu(PO4)7. An excellent agreement between the predicted and measured spectra confirms the validity of the performed calculations. Based on these energy level and intensity calculation results, the radiative lifetimes of the 5d-4f emissions of Ce(3+) and Pr(3+) ions have been modeled to show nearly independent temperature trends. Comparison with the measured lifetimes suggests the nonradiative relaxation process in this host is probably related to the intrinsic defect states. In addition to the studies of the 4f-5d transitions, a general theoretical scheme to calculate the lowest 4f-6s transition energy of the Ce(3+) ion was proposed for the first time on the basis of the ligand polarization model. The predicted 6s energy position of the Ce(3+) ion in Ca9Lu(PO4)7 is solid evidence corroborating our previous spectroscopic assignment.

High pressure luminescence spectra of CaMoO4:Ln3+ (Ln = Pr, Tb)

Photoluminescence spectra and luminescence kinetics of pure CaMoO(4) and CaMoO(4) doped with Ln(3+) (Ln = Pr or Tb) are presented. The spectra were obtained at high hydrostatic pressure up to 240 kbar applied in a diamond anvil cell. At ambient pressure undoped and doped samples exhibit a broad band emission extending between 380 and 700 nm with a maximum at 520 nm attributed to the MoO(4)(2-) luminescence. CaMoO(4) doped with Pr(3+) or Tb(3+) additionally yields narrow emission lines related to f-f transitions. The undoped CaMoO(4) crystal was characterized by a strong MoO(4)(2-) emission up to 240 kbar. In the cases of CaMoO(4):Pr(3+) and CaMoO(4):Tb(3+), high hydrostatic pressure caused quenching of Pr(3+) and Tb(3+) emission, and this effect was accompanied by a strong shortening of the luminescence lifetime. In doped samples, CaMoO(4):Pr(3+) and CaMoO(4):Tb(3+), quenching of the emission band attributed to MoO(4)(2-) was also observed, and at pressure above 130 kbar this luminescence was totally quenched. The effects mentioned above were related to the influence of the praseodymium (terbium) trapped exciton PTE (ITE-impurity trapped exciton) on the efficiency of the Pr(3+) (Tb(3+)) and MoO(4)(2-) emissions.

PEG-capped, lanthanide doped GdF3 nanoparticles: luminescent and T2 contrast agents for optical and MRI multimodal imaging

A facile method for the synthesis of water dispersible Er(3+)/Yb(3+) and Tm(3+)/Yb(3+) doped upconverting GdF(3) nanoparticles is reported. Strong upconversion emissions are observed in the red (for Er/Yb doped) and near-infrared (for Tm/Yb doped) regions upon laser excitation at 980 nm. The PEG coating ensures a good dispersion of the system in water and reduces the radiationless de-excitation of the excited states of the Er(3+) and Tm(3+) ions by water molecules. The r(2) relaxivity values are quite high with respect to the common T(2)-relaxing agents (22.6 ± 3.4 mM(-1) s(-1) and 15.8 ± 3.4 mM(-1) s(-1) for the Tm/Yb and Er/Yb doped samples, respectively), suggesting that the present NPs can be interesting as T(2) weighted contrast agents for proton MRI purpose. Preliminary experiments conducted in vitro, in stem cell cultures, and in vivo, after subcutaneous injection of the lanthanide-doped GdF(3) NPs, indicate scarce toxic effects. After an intravenous injection in mice, the GdF(3) NPs localize mainly in the liver. The present results indicate that the present Er(3+)/Yb(3+) and Tm(3+)/Yb(3+) doped GdF(3) NPs are suitable candidates to be efficiently used as bimodal probes for both in vitro and in vivo optical and magnetic resonance imaging.

Interconfigurational 5d → 4f luminescence of Ce3+ and Pr3+ in Ca9Lu(PO4)7

Ca(9)Lu(PO(4))(7):Ce (3+) and Ca (9)Lu (PO (4))(7):Pr (3+) polycrystalline materials were synthesized by solid state reaction at high temperature. The materials were characterized by powder x-ray diffraction (XRPD). The luminescence spectroscopy and the excited state dynamics of these compounds were investigated upon excitation with UV/VUV synchrotron radiation. Both materials showed efficient and fast 5d-4f emission upon direct VUV excitation into the 5d levels but only Ca(9)Lu(PO(4))(7):Ce (3+) revealed luminescence upon excitation across the bandgap. The decay kinetics of the 5d-4f emission upon VUV intra-center excitation is characterized by a decay time of 29 ns for Ce (3+) and 17 ns for Pr (3+) with no significant build-up after the excitation pulse. For the both compounds, no significant temperature dependence of the 5d-4f emission lifetime was observed within the range 8-300 K.

High pressure luminescence spectra of CaMoO4:Pr3+

Steady state and time resolved luminescence measurements of CaMoO(4) doped with Pr(3+) as a function of hydrostatic pressure in the 1-175 kbar range are presented. It has been observed that with increasing pressure the spectral features shift towards lower energies, the decay times of both (3)P(0) and (1)D(2) emitting levels become shorter and the intensity of the (3)P(0) emission decreases to complete quenching at about 110 kbar, whereas that of the (1)D(2) emission increases in the 0-100 kbar range and then rapidly decreases when the pressure exceeds 127 kbar. A variation of the structure of the spectral manifolds indicates that a pressure induced phase transition of the host lattice occurs in the 80-100 kbar range. The quenching of the luminescence and the shortening of the decay times have been accounted for by means of a model that takes into account the role played by a praseodymium trapped exciton in the excited state dynamics of the investigated material.

Hyperfine structure of Ho3+ levels and electron-phonon coupling in YPO4 single crystals

High resolution spectroscopy (the finest being 0.01 cm(-1)) was applied in the 75-25,000 cm(-1) and 9-300 K ranges to a 1 mol% holmium doped Y PO(4) single crystal with two purposes: (1) to study the hyperfine splitting of Ho(3+) energy levels of interest for possible quantum manipulation media and (2) to analyze the electron-phonon interaction. The hyperfine structure was clearly revealed for a high number of lines in a wide wavenumber range (up to ~21,500 cm(-1)) and for a large number of multiplets. Several hyperfine patterns were monitored, differing in the number of components (a maximum of 16 could be easily distinguished in a single beautiful pattern), in their separation, and in their relative statistical weight. These features were all understood in terms of a crystal-field model, whose results are in good agreement with experiments and account for the involved level symmetry, the type of transitions (electric and magnetic dipole allowed), and the contribution of a second-order (pseudoquadrupolar) hyperfine coupling between close levels. The electron-phonon interaction, investigated through the thermally induced line shift, was critically discussed in the framework of single phonon coupling and of two phonon Raman scattering models.

First measurement of chiral dynamics in π- γ → π- π- π+

The COMPASS Collaboration at CERN has investigated the π- γ → π- π- π+ reaction at center-of-momentum energy below five pion masses, sqrt[s]<5m(π), embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, t'<0.001  GeV2/c2. Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3π-invariant mass m(3π)=sqrt[s] were extracted. The absolute cross section was determined in seven bins of sqrt[s] with an overall precision of 20%. At leading order, the result is found to be in good agreement with the prediction of chiral perturbation theory over the whole energy range investigated.

NIR-to-NIR two-photon excited CaF2:Tm3+,Yb3+ nanoparticles: multifunctional nanoprobes for highly penetrating fluorescence bio-imaging

In this study, we report on the remarkable two-photon excited fluorescence efficiency in the "biological window" of CaF(2):Tm(3+),Yb(3+) nanoparticles. On the basis of the strong Tm(3+) ion emission (at around 800 nm), tissue penetration depths as large as 2 mm have been demonstrated, which are more than 4 times those achievable based on the visible emissions in comparable CaF(2):Er(3+),Yb(3+) nanoparticles. The outstanding penetration depth, together with the fluorescence thermal sensitivity demonstrated here, makes CaF(2):Tm(3+),Yb(3+) nanoparticles ideal candidates as multifunctional nanoprobes for high contrast and highly penetrating in vivo fluorescence imaging applications.

Random lasing in Nd:LuVO4 crystal powder

Room temperature random lasing action is demonstrated for the first time in a low concentrated neodymium doped vanadate crystal powder. Laser threshold and emission efficiency are comparable to the ones obtained in stoichiometric borate crystal powders. The present investigation provides a complete picture of the random lasing characteristics of Nd-doped vanadate powder both in the spectral and temporal domain, together with a simplified model which accounts for the most relevant features of the random laser.

Temperature and pressure dependence of the optical properties of Cr3+-doped Gd3Ga5O12 nanoparticles

Since the crystal-field strength at the Cr(3+) site is very close to the excited-state crossover (ESCO), this work investigates the optical properties of Cr(3+)-doped Gd(3)Ga(5)O(12) (GGG) nanoparticles as a function of temperature and pressure in order to establish the effect of the ESCO on the optical behaviour of nanocrystalline GGG. Luminescence, time-resolved emission and lifetime measurements have been performed on GGG:0.5% Cr(3+) nanoparticles in the 25-300 K temperature range, as well as under hydrostatic pressure up to 20 GPa. We show how low temperature and high pressure progressively transforms Cr(3+)(4)T(2) --> (4)A(2) broadband emission into a ruby-like (2)E --> (4)A(2) luminescence. This behaviour together with the lifetime dependence on pressure and temperature are explained on the basis of the spin-orbit interaction between the (4)T(2) and (2)E states of Cr(3+).

Lanthanide doped upconverting colloidal CaF2 nanoparticles prepared by a single-step hydrothermal method: toward efficient materials with near infrared-to-near infrared upconversion emission

Colloidal Er(3+)/Yb(3+), Tm(3+)/Yb(3+) and Ho(3+)/Yb(3+) doped CaF(2) nanoparticles have been prepared by a one-pot hydrothermal procedure and their upconversion properties have been investigated.

Eu3+-Doped Y2O3-SiO2 Nanocomposite Obtained by a Sol-Gel Method

A Y2O3-SiO2 nanocomposite doped with Eu3+ was obtained by a sol-gel method and characterized by X-ray diffraction, IR, 29Si NMR and laser-excited luminescence spectroscopy. It was found that small (2-3 nm) yttria nanoparticles are homogeneously dispersed in, and interacting with, the amorphous silica matrix. Luminescence spectroscopy indicates that the Eu3+ ion is preferentially located inside or at the surface of highly disordered Y2O3 nanoparticles. These luminescent nanocomposites form a class of materials which could find applications in the field of phosphors.

Synthesis and Characterization of Luminescent ZnO Powders Produced by Thermally-Induced Doping

A low cost and efficient method of doping pure zinc oxide powders for luminescence applications is presented. The powders have been obtained from pure, non-luminescent ZnO by thermally induced doping under a nitrogen flux and in the presence of ZnS powders. Structural characterization has been carried out by X-ray diffraction analysis (XRD), while luminescence properties have been studied with both a spectrofluorimeter and microraman spectrometer. Emission spectra have shown an increase of the luminescence with treatment temperature. The XRD patterns of all samples showed, along with reflections of the hexagonal phase of ZnO, other reflections ascribed to the hexagonal phase of ZnS. Upon further annealing in air at 800°C, the ZnS reflections disappear even though the luminescence still persists with a comparable intensity. Luminescence is ascribed to ZnO reduction due to the interaction of S with ZnO. It has also been observed that excitation with a He-Ne laser beam (λ=633 nm) with a high specific power (5×10−3 W·µm−2) yielded blue-green luminescence in spite of excitation below the band-to-band absorption threshold.

Luminescence and decay properties of the 1D2 level of Pr3+-doped YPO4

This investigation reports on the spectroscopic properties of trivalent Pr in YPO4. In particular, we show how the luminescence from the 1D2 level depends on the polarization of the emission and on the temperature of the sample. We also report on the decay kinetics of the 1D2 level, which show the lifetime decreasing as temperature is increased. Our observed lifetime of 440 µs is at low temperature and is significantly longer than that reported by other workers. We explain the temperature dependence of the 1D2 emission by considering the population redistribution in the 1D2 level, without invoking the existence of multiphonon relaxation. Finally, we estimate the radiative decay rates of individual crystal field levels within the 1D2 manifold.

Observation of a J(PC)=1-+ exotic resonance in diffractive dissociation of 190 GeV/c π- into π- π- π+

The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the π- π- π+ final state using a 190  GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420,000 events taken at values of the squared 4-momentum transfer t' between 0.1 and 1  GeV2/c2. The well-known resonances a1(1260), a2(1320), and π2(1670) are clearly observed. In addition, the data show a significant natural-parity exchange production of a resonance with spin-exotic quantum numbers J(PC)=1-+ at 1.66  GeV/c2 decaying to ρπ. The resonant nature of this wave is evident from the mass-dependent phase differences to the J(PC)=2-+ and 1++ waves. From a mass-dependent fit a resonance mass of (1660±10(-64)(+0))  MeV/c2 and a width of (269±21(-64)(+42))  MeV/c2 are deduced, with an intensity of (1.7±0.2)% of the total intensity.

Spectroscopy of f-f radiative transitions of Y b(³+) ions in ytterbium doped orthophosphates at ambient and high hydrostatic pressures

Studies of absorption and luminescence spectra, and luminescence decay times of the intrashell f-f transitions of Y b(³+) ions in bulk orthophosphates at different temperatures and at high hydrostatic pressures are presented. The spectroscopic properties of Y₀.₉₉Y b₀.₀₁PO₄ crystal with a tetragonal zircon structure and Gd₀.₉₉Y b₀.₀₁PO₄ and La₀.₉₅Y b₀.₀₅PO₄ crystals with a monoclinic monazite structure are compared. Experiments at ambient pressure were performed at temperatures from 10 to 295 K whereas measurements at high pressure were performed in a diamond-anvil cell at 10 K with a pressure up to 160 kbar. The results reveal a high thermal and pressure stability of the optical properties of the examined crystals as well as the dependence of Y b(³+) emission properties on the local symmetry of the site occupied by the ytterbium dopant.

Optical transition probabilities in Er3+- and Tm3+-doped LiLa9(SiO4)6O2 crystals

In this work, Er(3+) and Tm(3+)-doped LiLa(9)(SiO(4))(6)O(2) crystals have been grown from an Li(2)MoO(4) flux in the 1360-940 °C temperature range. Optical absorption spectra have been measured to obtain the experimental oscillator strengths of the transitions from the ground state to the excited levels. Judd-Ofelt calculations have been performed to estimate the Ω(2), Ω(4) and Ω(6) intensity parameters. The dynamics of selected Er(3+) and Tm(3+) manifolds have been investigated under selective pulsed excitation in order to determine the energy gap law by comparing the observed decay rates with the Judd-Ofelt predictions.

Nanocrystalline lanthanide-doped Lu3Ga5O12 garnets: interesting materials for light-emitting devices

Nanocrystalline Lu(3)Ga(5)O(12), with average particle sizes of 40 nm, doped with a wide variety of luminescent trivalent lanthanide ions have been prepared using a sol-gel technique. The structural and morphological properties of the powders have been investigated by x-ray powder diffraction, high resolution transmission electron microscopy and Raman spectroscopy. Structural data have been refined and are presented for Pr(3+), Eu(3+), Gd(3+), Ho(3+), Er(3+) and Tm(3+) dopants, while room temperature excited luminescence spectra and emission decay curves of Eu(3+)-, Tm(3+)- and Ho(3+)-doped Lu(3)Ga(5)O(12) nanocrystals have been measured and are discussed. The Eu(3+) emission spectrum shows typical bands due to 5D(0)-->7F(J) (J = 0, 1, 2, 3, 4) transitions and the broadening of these emission bands with the non-exponential behaviour of the decay curves indicates the presence of structural disorder around the lanthanide ions. Lanthanide-doped nanocrystalline Lu(3)Ga(5)O(12) materials show better luminescence intensities compared to Y(2)O(3), Gd(3)Ga(5)O(12) and Y(3)Al(5)O(12) nanocrystalline hosts. Moreover, the upconversion emission intensity in the blue-green region for the Tm(3+)- and Ho(3+)-doped samples shows a significant increase upon 647.5 nm excitation with respect to other common oxide hosts doped with the same lanthanide ions.