Recent results

Results in 2018

LiNbO₃ single and nanocrystals. – A cost-effective, crucible-rest-refilling technology has been developed for the serial high-temperature top-seeded solid solution growth of high-quality Mg-doped stoichiometric LiNbO₃ crystals, resistant to optical damage. Nanocrystals have been prepared by shaker and planetary milling from LiNbO₃ crystals of congruent composition. While dry shaker milling resulted in particle sizes of a few hundred nm with gray coloration due to partial reduction, wet planetary milling with 0.1 mm zirconia balls in zirconia vial produced the desired 10-20 nm size.

Using EXAFS spectroscopy, the incorporation of Er, In and Hf dopants was determined in LiNbO₃ single crystals. Er mostly substitutes at the Li site with a small fraction at Nb site, with little change between stoichiometric and congruent LiNbO₃. In ions also primarily substitute at the Li site in congruent LiNbO₃; Hf appears to be nearly evenly distributed between Li and Nb sites. While charge-compensating defects are required, self-compensating schemes are not consistent with the data.

Using transient absorption spectroscopy in the mid/near infrared region in Mg-doped LiNbO₃ crystals, the dissociation and thermalization of hot excitons produced by 100 fs laser pulses at ~2.5 eV with power up to 2 PW/m² was shown to occur on a ~200 fs timescale leading to localized Nb⁴⁺ small polarons via an interim state attributed to electrons without local lattice relaxation. Exciton hopping and pinning on dipolar crystal defects have been demonstrated to play decisive roles in the luminescence components observed under pulsed excitation.

Spectroscopy of lithium yttrium orthoborate (Li₆Y(BO₃)₃, LYB) single crystals. – Undoped LYB single crystals grown by the Czochralski method were investigated for their suitability for optical applications. Two bands near the UV absorption edge, at about 220 and 240 nm, have been found with no and a small difference in their amplitudes between the top and bottom parts of the crystals for stoichiometric and off-stoichiometric compositions, respectively. XRF and luminescence measurements revealed no contamination of the crystals, so the UV bands can be attributed to intrinsic defects distributed more homogeneously in stoichiometric crystals.

The amplitude of the lowest energy absorption band of Yb³⁺ ions in LYB single crystals was observed to increase upon cooling from 300 to 9 K by more than two orders of magnitude, while the halfwidth decreased to 0.16 cm^-1. This strong temperature dependence explains the anomalous behavior of the time and intensity dependence of the cooperative pair emission previously investigated in similar crystals. Cooperative pair absorption was successfully demonstrated in a Li₆Yb(BO₃)₃ crystal (where Yb is a matrix ion) in the same wavenumber range where pair emission in LYB:Yb was observed. Spectral hole burning in LYB doped with monoisotopic ¹⁶⁶Er³⁺ ions has been investigated in magnetic fields. For coaxial field orientation, two pairs of spectral side holes were detected with a magnetic sensitivity of about ±75 and ±177 MHz/mT, while these holes split to more components in the case of a general direction of the magnetic field.

Figure 1. UV spectra of slices cut from the top and bottom parts of stoichiometric and off-stoichiometric LYB crystals, the latter showing changed amplitudes.

Designing and building a confocal microscope. – A new confocal microscope has been designed from the ground up to detect photons from a single-photon source. The samples mounted in a cryostat can be moved by synchronized motors. Optimal optical components were chosen for laser excitation and the detection of the emitted photons. A setup comprising a monochromator and a photon counter has also been developed for testing and optimizing solid state samples and suspensions.

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Figure 2. Design (a) and realization (b) of the confocal microscope stand.

Analytical methods for environmental and advanced materials. – An open-air ELCAD-OES system fitted with a low-resolution mini UV spectrometer has been built for on-line monitoring of river sediments. By correcting for structured molecular background emissions, the signal-to-noise ratios for Cd, Cu, Mg, Mn, Ni, Zn, etc. can be significantly improved. Acidic pressure digestion and flame atomic absorption methods were developed for major and minor constituents (Al, Ca, Fe, K, Mg, Mn, Na, Si, Sr, Ti) of alginite, which is an environmentally and agriculturally strategic mineral in Hungary.

 

Results in 2017

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 M_Nb ‒ 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 ⁴I_15/2 multiplet, all Stark levels split by low-symmetry crystal field could be determined. The most intense luminescence was detected at 577 nm (⁴F_9/2 → ⁶H_13/2 transition).

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

Coherent Yb-pair emission in Li₆Y_1-xYb_x(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 ²F_7/2 – ²F_5/2 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).

 

Results in 2016

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. Li₆Y_0.99Pr_0.01(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 ⁴I_11/2 – ⁴I_13/2 – ⁴I_15/2 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.