Legfrissebb eredmények

A csoport legfrissebb eredményei (angolul)

( 2016)

Crystal growth of niobates. — A series of Fe+Ti double-doped stoichiometric LiNbO3 (LN) crystals has been grown to optimize/balance photorefractive and wave-guiding properties. Rare-earth (Er, Nd, Yb) and transition-metal (Fe, Ti) doped over-threshold stoichiometric LN crystals have been grown for IR spectroscopic studies showing that these dopants (M) may also incorporate at Nb sites forming MNb ‒ OH -type complexes like optical-damage-resistant (Mg, Zn, In, Sc, Hf, Zr, Sn) ions.

Spectroscopy of lithium yttrium orthoborate (Li6Y(BO3)3, LYB) single crystals. — Czochralski grown LYB:Dy crystals suitable for scintillator and laser communication systems (Fig. 1) have been characterized by temperature and polarization dependent FTIR absorption and luminescence measurements. Up to the 4I15/2 multiplet, all Stark levels split by low-symmetry crystal field could be determined. The most intense luminescence was detected at 577 nm (4F9/26H13/2 transition).

Figure 1. High optical quality LYB crystal doped with Dy.

Coherent Yb-pair emission in Li6Y1-xYbx(BO3)3 crystals (x=0.01, 0.05, 0.20) was successfully generated using laser excitation at »972 nm. At 6 K, the emission showed a detailed structure containing at least 13 band components in the 470-540 nm range. The laser intensity dependence of the integrated area of the spectra was explained by an energy transfer mechanism via Yb3+ chains in the crystal lattice.

Spectroscopy of the tissue-equivalent dosimeter material lithium tetraborate (LTB). —Luminescence studies of Mn doped LTB single crystals complemented by EPR analysis in the 33 GHz Q-band revealed rechargeable Mn2+ centres incorporated in the lithium sublattice, compensated by mobile lithium vacancies. Both electrons and holes created by ionising radiation are trapped by Mn2+ centres, their recombination accounting for the thermoluminescent readout of the radiation dose.

Spectroscopy of LiNbO3:Yb nano-powders. LN nano-powders have been prepared by high-energy ball milling from bulk crystals, using alumina, steel or tungsten carbide vials. The type of mills, number of balls (1 or 2), milling time and the ball-to-powder ratio has been varied to produce the smallest grain size. After resonant irradiation at λ=980 nm, a fluorescent signal with a lifetime of 0.5 ms at »40 K, but surviving also at RT was found in a LiNbO3:Yb nano-powder with a characteristic grain size of 390 nm. Using a saturation spectroscopic method in KBr-LiNbO3:Yb pellets with characteristic size of 390 nm, the population relaxation kinetics and magnetic field splitting of the spectral hole was determined for the 2F7/22F5/2 transition of Yb3+. The same processes were studied for 70 nm pellets as well, but only a single strong spectral hole component with a lifetime longer than 10 h was found instead, that could only be erased by a 30 min annealing of the sample above 90 K.

Transient absorption of small polarons and excitons in LiNbO3:Mg. — Picosecond absorption in the 0.3-1.05 eV mid-infrared region induced by 100 fs pumping pulses with 2.5 eV photon energy was attributed to free electrons originating from hot electron-hole pairs undergoing phonon-assisted dissociation and cooling, as well as their subsequent trapping as Nb4+ polarons at regular Nb sites. The concentrations of hot, cold, and polaronic charge carriers could be derived based on a kinetic model. Long-lived absorption in the near infrared and visible region could be interpreted as due to the survival of trapped polarons and pinned excitons near lattice defects, respectively (Fig. 2).

Figure 2. Left: subpicosecond pulse-induced two-photon absorption in LiNbO3:Mg (green) and the resulting number densities of hot, cold and polaronic carriers (orange, blue and yellow lines, respectively). Right: early recombination (1), excitonic stabilisation (2) and dissociation (3) of hot electron-hole pairs located on neighboring matrix ions of the LiNbO3 lattice.

Dielectric parameters of lithium tantalate (LiTaO3) in the terahertz range. The refractive indices and absorption coefficients of congruent, stoichiometric and 0.5 or 1.0 mol% Mg-doped LiTaO3 crystals have been investigated using polarization dependent terahertz time-domain spectroscopy in the 0.3-2.0 THz range. The data were successfully fitted by a three-term Lorentz oscillator model. Stoichiometry was shown to have a significant influence, while Mg had a much smaller effect. LiTaO3 can be a promising THz generator material due to the lack of the three-photon absorption at 800 nm.

Analytical methods for environmental and advanced materials. — The distributions of mass, water-soluble inorganic salts and mineral elements of size-segregated aerosols, precursor gaseous pollutants, black carbon, and nanoparticles over the Southern North Sea have been studied. The nano-aerosol count, originating from ocean-going ships, peaked at lower diameters (»28 nm) than those observed for smaller (e.g. fishing) boats (45-50 nm) and depended also on weather conditions.

Solid phase extraction with high-resolution continuum-source graphite-furnace atomic absorption spectrometry has been developed for speciation of inorganic As in geothermal, well and pretreated water samples from four Hungarian waterworks. Total As in well waters varied between 40-120 μg/L. Occurrence of As(III) in well waters exceeded 80% of the total As, while As(V) was predominant (≈90%) in pretreated waters, but below the health limit value of the 98/83/EC Council Directive (10 μg/L).

Eredmények 2016-ban

Crystal growth of niobates and borates. – Stoichiometric LiNbO3 (sLN) crystals doped with 1-2 at% rare-earth ions (Sm3+, Nd3+, Yb3+) have been grown by the Czochralski and the high-temperature top-seeded solution growth (HTTSSG) methods for IR spectroscopic experiments. An extra large prism (Fig. 1) has been prepared for tilted-front pumping ultra-short THz pulse generation using an sLN:Mg crystal grown by a semi-continuous feeding HTTSSG technique with a Mg-dopant concentration slightly above the photorefractive threshold. Li6Y0.99Pr0.01(BO3)3 (LYB:Pr) and β-barium borate (BBO) crystals have also been grown for spectroscopic and nonlinear optical applications.

Figure 1. Mg-doped stoichiometric LiNbO3 crystal prism prepared for high-intensity THz pulse generation.

Characterization of polaronic distortion in LiNbO3. — Polarization around trapped charges in oxides is assumed to be strongly confined but up to now experimental data on the distortion were lacking. We used a molecular probe (OH ions) to determine the displacements of nearest-neighbour ions around O small polarons in LiNbO3 crystals created by ps pump pulses in the blue region. Some of the holes were temporarily trapped at oxygens in the direct vicinity of impurity OH ions which could be detected by the absorption of probe pulses in the infrared as a transient 3 cm-1 redshift of the stretcing vibration frequency of part of the OH ions. The vibration was described by a Morse potential, the adjacent polaron by point-charge potential changes caused by the trapped charge and its shifted neighbours, both modifying the Morse potential. Assuming a  ̴12% increase of the nearest Nb5+ neighbour distance from the trapping site (Fig. 2), agreement with the experimental redshift and earlier expectations could be achieved. On the other hand, assuming only a trapped hole without polaronic halo was found to result in a blueshift of the OH frequency, contradicting experiment.

 

Figure 2. Electrostatic potential changes due to hole polaron trapping (in the origin), enlarged arrows indicating the polaronic displacements of neighbouring lattice ions.

Spectroscopy of lithium yttrium orthoborate (Li6Y(BO3)3, LYB) single crystals. – The electronic dipole orientation of the transition of Yb3+ ion pairs in 20 mol% Yb-doped LYB was shown to be parallel to the chain of rare-earth sites in the crystal lattice using high-resolution infrared absorption spectroscopy. Luminescence emission generated by coherent pair excitation has been successfully demonstrated by applying a high-intensity laser source. It was shown by SIESTA quantum-chemical calculations that the incorporation of Yb3+ ion pairs at nearest neighbour Y sites is energetically more favourable than for larger Yb-Yb distances. The spin-Hamiltonian tensors of isolated Yb3+ centres in LYB have been derived from angular dependent EPR spectra measured for 1 mol% Yb.

Using saturation spectroscopy in LYB:Er at 8 K the homogeneous linewidth, the population relaxation kinetics, and the magnetic-field-induced splitting of the spectral holes for the 4I11/24I13/24I15/2 transitions of Er3+ ions have been determined. The sensitivity of the spectral hole distances to the external magnetic field intensity is   ̴20% lower in LYB:Er than found earlier in LiNbO3:Er. The same splitting and sensitivity was also confirmed by using high resolution FTIR absorption spectroscopy for a magnetic induction of 120 mT.

Analytical methods for environmental and advanced materials. – Airborne particulate matter (PM) over two rural roads of Piên (South Brazil), paved with ultramafic rocks, was analysed with EPMA, XRF and XRD to characterize elemental composition and crystallinity. Single-particle compositions indicated a few percentages of serpentine and amphibole in PM. The deposition efficiency of chrysotile in two principal segments of the human respiratory system was estimated using a lung deposition model. Almost half of the inhaled particles deposited in the respiratory system. Asbestos depositions were significant (~25 %) in lower airways, even under rest situation and nose breathing.

Interaction between bovine serum albumin (BSA) and silicon carbide (SiC), investigated in nanocrystals by fluorescence spectroscopic and simulation methods, was found to be both dynamic and static. A new model was applied to describe the dynamic quenching effect with huge bimolecular quenching constants. The average binding distance between BSA and SiC was derived from the fitted model parameters and Förster’s theory. Computer simulation of molecular docking identified a possible docking site of SiC in the surrounding of the tryptophan-134 residues of BSA.

High-resolution continuum-source graphite furnace atomic absorption spectrometry (HR-CS-GFAAS) methods were developed for the determination of the Al contaminant and the main component Si in SiC nanocrystals with a size-distribution of 1–8 nm dispersed in non-aqueous solution. Similar procedures starting from solid samples were optimized for the vaporization/atomization processes of Mg, evaporating lithium niobate optical single crystals doped with various amounts of Mg.