László Kovács

Wigner Research Centre for Physics · Crystal Physics

The results of a series of pump–probe spectral hole-burning experiments are presented on Yb3+- or Er3+-doped Li6Y(BO3)3 (LYB) single crystals in the temperature range of 2–14 K and 9–28 K, respectively. The spectral hole has a complex structure for Yb3+ with superposed narrow and broad bands, while a single absorption hole has been observed for Er3...

Electro-optical control of on-chip photonic devices is an essential tool for efficient integrated photonics. Lithium niobate on insulator (LNOI) is an emerging platform for on-chip photonics due to its large electro-optic coefficient and high nonlinearity. Integrating quantum emitters into LNOI would extend their versatility in classic photonics to...

Congruent lithium niobate (LiNbO3) prepared by sintering was ground under wet conditions in a planetary mill in order to produce nanocrystals. By using gradually lower sizes of the balls in the mill, the final particle size of the crystals could be reduced to about 12–15 nm. The particle size achieved as well as the lithium oxide (Li2O) loss of the...

The recent Special Issue on lithium niobate (LiNbO3) is dedicated to Prof. Schirmer and his topics and contains nineteen papers, out of which seven review various aspects of intrinsic and extrinsic defects in single crystals, thin films, and powdered phases; six present brand-new results of basic research, including two papers on Li(Nb,Ta)O3 mixed...

The present review is intended to interest a broader audience interested in the resolution of the several decades-long controversy on the possible role of oxygen-vacancy defects in LiNbO3. Confronting ideas of a selected series of papers from classical experiments to brand new large-scale calculations, a unified interpretation of the defect generat...

The energy levels of Dy3+ ions have been determined in lithium yttrium borate (Li6Y(BO3)3) single crystals in a wide spectral range between 3000 and 40,000 cm−1 using optical absorption and luminescence spectroscopy, which also allow for an analysis of the ground state. The crystal field splittings of the 6H15/2 ground state and all excited states...

Electro-optical control of on-chip photonic devices is an essential tool for efficient integrated photonics. Lithium niobate on insulator (LNOI) is an emerging platform for on-chip photonics due to its large electro-optic coefficient and high nonlinearity [1]. Integrating quantum emitters into LNOI would extend their versatile use in classic photon...

Cooperative luminescence (CL) of Yb³⁺ pairs was investigated in Yb-doped Li6Y(BO3)3 crystals. It was reliably distinguished from other types of up-conversion luminescence related to the energy transfer from Yb³⁺ ions to other rare-earth ions unintentionally introduced into the crystal. The CL was observed in the region of 19,000–21,000 cm⁻¹ (526-47...

Rare-earth (RE = Er 3+ , Nd 3+ , or Yb 3+) ion-doped stoichiometric LiNbO 3 crystals were grown by the Czochralski and the high-temperature top-seeded solution growth methods. For the 0.22-0.87 mol% concentration range of the RE oxides in the melt/solution, in addition to the well-known hydroxyl (OH −) vibrational band in undoped stoichiometric LiN...

Polycrystalline, cerium-doped yttrium oxyorthosilicate (Y2SiO5:Ce, YSO:Ce) and lutetium yttrium oxyorthosilicate (Lu2-xYxSiO5:Ce, LYSO:Ce) scintillator materials have been synthesized and characterized. Samples have been prepared at 1400 °C with the aid of LiF flux. The initial concentration of the flux and the cerium content has strongly affected...

Terahertz time-domain spectroscopy measurements were performed on 0.7 mol% Mg-doped stoichiometric lithium niobate crystal with ordinary and extraordinary polarization in the 4-460 K temperature range. The absorption coefficient and refractive index spectra were recorded in the terahertz frequency range from 0.5 to 1.8 THz. The data extracted from...

The absorption coefficient and refractive index spectra of 0.7 mol% Mg-doped stoichiometric lithium niobate were determined in the terahertz frequencies. The phase-matching parameters extracted from time-domain measurements were given in a wide temperature range.

Transition metal ion (TM = Fe3 +, Cr3+, Ti4+) doped stoichiometric LiNbO3 crystals have been grown by the high temperature top-seeded solution growth and Czochralski methods. Vibrational bands of hydroxyl ions (O H −) have been observed for dopants above a threshold concentration at wavenumbers of 3502 cm−1 for Fe3+ and Cr3+, and 3486 cm−1 for Ti4+...

Lithium niobate (LiNbO3, LN) nanocrystals were prepared by ball-milling of the crucible residue of a Czochralski grown congruent single crystal, using a Spex 8000 Mixer Mill with different types of vials (stainless steel, alumina, tungsten carbide) and various milling parameters. Dynamic light scattering and powder X-ray diffraction were used to de...

Many dopants are incorporated into LiNbO3 (LNO) to modify the physical properties, particularly many optical properties. To understand these changes, the substitution site for the dopant must be known. Here we provide a detailed EXAFS study of the local environment about the metal (M) ion dopants indium, erbium, and hafnium in LNO, using crystals g...

In this review article, we discuss photoluminescence phenomena mediated by polarons in lithium niobate (LNO). At first we present the fundamentals on polaron states in LNO and their energy levels, i.e., on free and bound electron polarons, on hole polarons as well as on bipolarons. We discuss the absorption measurements on reduced as well as on dop...

The energy levels of Er³⁺ ions have been determined in lithium yttrium borate (Li6Y(BO3)3) single crystals in a wide spectral range between 6000 and 40000 cm⁻¹ together with an analysis of the ground state, using optical and EPR spectroscopy. The crystal field splittings of the ⁴I15/2 ground state and those of nearly all excited states up to the ⁴D...

Dielectric material parameters of lithium tantalate (LT) in the terahertz region have been investigated using terahertz time-domain spectroscopy (THz-TDS). Undoped congruent, undoped stoichiometric, and Mg-doped stoichiometric LT crystals were measured. The Mg content was 0.5 and 1.0 mol% for the stoichiometric composition. Index of refraction and...

Index of refraction and absorption coefficient spectra of pure paratellurite (α-TeO2) crystal as a potential material for terahertz (THz) applications were determined in the 0.25–2 THz frequency range at room temperature by THz time domain spectroscopy (THz-TDS). The investigation was performed with beam polarization both parallel (extraordinary po...

We compare extended x-ray absorption fine structure (EXAFS) data at the Zn K edge for a low concentration of Zn (0.7 mol%) in a stoichiometric crystal with that for higher Zn concentrations (nominally 5 and 9 mol%) in congruent LiNbO3 (LNO). Note that stoichiometric and congruent LNO have significantly different optical properties. We find no signi...

The existence of a recently described monoclinic phase (C2/c, Z = 8) (Beregi et al., 2012) in addition to the well-known Huntite type rhombohedral (R32) polytypic modification of the GdAl3(BO3)4 (GAB) crystal at room temperature provides a unique possibility to investigate the incorporation of rare earth dopants into slightly modified crystal latti...

A pump-probe-type saturation spectroscopic experiment has been performed at 980 nm to measure the homogeneous linewidth of the \(^4\)I\(_{11/2}\)–\(^4\)I\(_{15/2}\) transition and the lifetime of the \(^4\)I\(_{11/2}\) state of erbium embedded into lithium niobate single crystals. There are several non-equivalent Er\(^{3+}_{\mathrm{Li}^+}\)–V\(_{\m...

Owing to the extraordinary richness of its physical properties, congruent lithium niobate has attracted multidecade-long interest both for fundamental science and applications. The combination of ferro-, pyro-, and piezoelectric properties with large electro-optic, acousto-optic, and photoelastic coefficients as well as the strong photorefractive a...

Solid sampling high resolution continuum source graphite furnace atomic absorption spectrometry (SS-HR-CS-GFAAS) methods were developed and studied for the fast and sensitive quantitation of Li and Na additives in microsamples of cerium-doped yttrium oxyorthosilicate (Y2SiO5:Ce) scintillator materials. The methods were optimized for solid samples b...

Polycrystalline, cerium-doped yttrium oxyorthosilicate (Y1.99Ce0.01SiO5, YSO:Ce) scintillator materials have been synthesized and characterized. The samples have been prepared using a mixed-powder route with LiF and NaF additives. The final reaction products have been analyzed for phase purity by X-ray powder diffraction (XRD) and Raman spectroscop...

We report the result of a series of pump–probe spectral hole burning measurements on Yb3+ doped stoichiometric and congruent lithium niobate single crystal. The measurements were performed with a stabilized diode laser tuned to the nm transition of the dopant ions. The crystal samples were placed in a closed cycle helium cryostat, the experiments h...

Several optical methods including ultraviolet absorption, infrared absorption of the hydroxyl ions, Raman spectroscopy, and the Z-scan method have been used to determine the damage resistance threshold in 0-0.72 mol. % Zr-containing, flux-grown, nearly stoichiometric LiNbO₃ single crystals. All spectroscopical methods used indicate that...

Ferroelectric K3Li2Nb5O15 (KLN-1) crystals have been grown by the top-seeded solution growth method from pure, Na+, Rb+ and Cs+ doped melt. The impact of alkali additives was assessed all over the entire pulling range by investigating the variation of the structural and physical properties by using spectroscopic and dielectric methods. The incorpor...

Li6Y(BO3)3 and Li6Gd (BO3)3 powders were synthesized using wet chemical method. Lithium-nitrate, yttrium-/gadolinium-nitrate and boric acid solutions were mixed in stoichiometric composition, and a colloidal precipitate was obtained by adjusting the pH of the mixture. Li6Y(BO3)3/Li6Gd (BO3)3 phase formation was achieved by annealing the precursors...

Solid sampling (SS) graphite furnace atomic absorption spectrometry (GFAAS) and solution-based (SB) methods of GFAAS, flame atomic absorption spectrometry (FAAS), inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) were elaborated and/or optimized for the determination of Cr,...

Raman spectra have been measured in both pure and alkali doped potassium lithium niobate (KLN) crystals. One of the A(1)(TO) Raman modes at about approximate to 640 cm(-1) measured in x(zz)x backscattering geometry is sensitive to the changes of Nb concentration. Other phonon modes at approximate to 306 cm(-1) and approximate to 350 cm(-1) correspo...

Hexagonal rare-earth manganites (RMnO3 with R = Er, Tm, Yb) have been studied by high resolution absorption spectroscopy in the infrared spectral range around the Néel-temperature (TN ≈ 80 K). Series of transitions have been assigned for the R rare-earth ions in trigonal crystal field. Some of the Stark components of the rare-earth multiplets split...

The optical absorption spectra of hexagonal rare-earth manganites (RMnO3 with R=Er, Tm, Yb) have been measured in the 5000–13,000cm−1 infrared spectral range as a function of temperature between 8 and 300K. A series of transitions have been assigned for the trivalent rare-earth ions in trigonal crystal field. Some of the Stark components of the rar...

A first principles quantum mechanical calculation of the vibrational energy levels and transition frequencies associated with protons in stoichiometric LiNbO(3) single crystal has been carried out. The hydrogen contaminated crystal has been approximated by a model one obtains by translating a supercell, i.e., a cluster of LiNbO(3) unit cells contai...

A new absorption band has been detected at 4009 cm−1 in stoichiometric LiNbO3 single crystals. Based on present and earlier experiments and recent calculations, this band has been assigned as a combination transition involving the OH stretching and an OH librational mode. The librational mode participating in this combination is of lower fundamenta...

We work out a simple, pulsed pump-probe measurement scheme to measure the homogeneous linewidth of an atomic transition in an inhomogeneously broadened spectral line in a solid state environment. We apply the theory to the I_11/2 -- I_15/2 optical transition of erbium in LiNbO_3:Er^3+ crystal. Beside obtaining the homogeneous linewidth, we have est...

The geometry of the most stable configuration of OH− defects in nearly stoichiometric, hydrogen-contaminated, LiNbO3 has been determined from first principles theoretical calculations. In the most stable configuration the proton is located near a bisector of an oxygen triangle and is tilted by 4.3 degrees out of the oxygen plane towards a lithium v...

The Europhysical Conference on Defects in Insulating Materials, organized in the period 12-16 July 2010 in Pécs, Hungary by the Research Institute for Solid State Physics and Optics, Budapest and the Institute of Physics of the University of Pécs, was the 11th European conference in the alternate series of EURODIM and ICDIM. The first meeting in Ar...

OH stretching vibrations in LiNbO3 (LN) crystals having various Li/Nb ratios and doped with Mg above the photorefractive threshold concentration were investigated in the 20–500 °C temperature range. Beside the 3534 cm−1 above-threshold band another peak at about 3465 cm−1, known from below-threshold stoichiometric LN, was found to appear at a tempe...

Several weak absorption bands have been observed in the optical absorption spectra of pure and rare-earth-doped YAl3(BO3)4 single crystals in the 3350– 3650 cm−1 wave number region. Two of them, peaking at about 3377 cm−1 and 3580 cm−1 in the 8 K spectra, appear in most of the samples. They are tentatively attributed to the stretching mode of OH− i...

Formation of periodic domain structure in Mg, Y and Mg+Y co-doped congruent LiNbO3 single crystals grown by off-centered Czochralski method in the X and 148°Y pulling directions has been investigated. The developed domain structures were explained as functions of the thermal gradient during growth and the amount of Y and Mg dopants, the latter dopa...

The OH stretch mode spectra have been measured in both ferroelectric (KLN-1) and non-ferroelectric (KLN-2) potassium lithium niobate crystals grown from different melt compositions. O─H bond directions of about 15, 60, and 15 degrees with respect to the c-axis have been determined for the three main OH bands in KLN-1 peaking at about 3440, 3500, an...

The room temperature stability region of the tetragonal tungsten bronze-type potassium lithium niobate (K3Li2Nb5O15, KLN) has been determined by X-ray phase analysis on ceramic samples synthesized by solid state reaction in the composition range of [K2O]=28–33mol%, [Li2O]=12.5–20.5mol% and [Nb2O5]=50.5–55.5mol%. Lattice parameters and temperature d...

Fundamental and multi-mode transitions of the tetrahedral unit modes (A1 and F2) in Bi12SiO20 (BSO), Bi12GeO20 (BGO), and Bi12TiO20 (BTO) sillenite crystals have been studied by measuring Raman scattering and FTIR absorption spectra. The ν1 and ν3 frequencies related to the A1 symmetric and F2 asymmetric stretching modes, respectively, were obtaine...

A simple and accurate spectroscopic method is proposed to determine the composition of LiNbO3 crystals close to the stoichiometric composition. The method is based on the evaluation of the vibrational spectrum of hydroxyl ions that are always present in air-grown crystals. The intensity ratio R=I3480/I3465 of two band components peaking at about 34...

LiNbO3:Mg crystals doped with 0–8 mol. % Mg with stoichiometric, intermediate and congruent compositions were systematically investigated by Raman spectroscopy in backscattering y(zx)y, y(zz)y and z(xx)z geometries. The damping was found to be a very sensitive parameter for the characterization of the crystal composition. The half-widths of E(TO3)–...

Beside the ferroelectric Curie-temperature, the ultraviolet and infrared spectra of potassium lithium niobate (K3Li2Nb5O15, KLN) were shown to be very sensitive to the compositional variations observed in the crystals along the growth axis. The absorption edge shifts to the UV with decreasing Nb content, independently of the K/Li ratio of the cryst...

The presence of hydroxil ions has been detected for the first time in as-grown scheelite type crystals. A number of stretching vibrational bands were observed in PbMoO4 and BaWO4 crystals at 8 K, while only very weak OH– absorption was found in CaMoO4, SrMoO4 and SrWO4 in the 3200–3600 cm–1 wavenumber range. The anharmonicity of the stretching mode...

LiNbO3 crystals grown from Na2O, Rb2O, and Cs2O containing fluxes were characterized by infrared and Raman spectroscopic methods. Pure, nearly stoichiometric crystals with 49.7–50.0 mol% Li2O content were obtained from solvents containing Rb2O and Cs2O, while Na2O entered the crystal as a dopant. The intensity ratio of two OH– absorption peaks at 3...

Lithium niobate is a material with many important technological applications due to its diverse physical properties. Using a recently derived interatomic potential, intrinsic defect energies have been calculated leading to conclusions about the defect properties of the material that are compared with experimental conclusions. The incorporation of d...

LiNbO3:Mg crystals doped with 0-8 mol% Mg with stoichiometric, intermediate and congruent compositions were systematically investigated by Raman spectroscopy in backscattering y(zx)y and y(zz)y geometries. The halfwidths of E(TO3) – E(TO9) and A1(TO1) – A1(TO4) bands, which have a significant composition dependence for undoped LiNbO3 crystals, show...

Lithium niobate is a material with many important technological applications as a result of its diverse physical properties. Using a recently derived interatomic potential, intrinsic defect energies have been calculated leading to conclusions about the defect properties of the material that are compared with experimental conclusions. The incorporat...

In order to understand the photochromic effect in terbium gallium garnet doubly doped with calcium and cerium, the spectral characteristics of absorption changes induced by heating and irradiation are studied. Based on the findings we conclude that the photochromic band at $420\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ originates from defects involvin...

We report on the synthesis of a new relaxor compound with chemical formula Pb0.78Ba0.22Sc0.5Ta0.5O3 (PBST). Single crystals were obtained by the high-temperature solution growth method. The structure of the new compound is of double-perovskite type with face-centred cubic symmetry at room temperature. The frequency dependence of the dielectric cons...

The photosensitive properties of a centrosymmetric gadolinium gallium garnet crystal doped with calcium are investigated at room temperature. Elementary holograms can be recorded over a wide range of wavelengths in the visible spectral range. The photosensitive properties are studied experimentally using beam coupling and angular response experimen...

X2O (X=Na, K, Rb, Cs) solvents were studied for the flux growth of stoichiometric LiNbO3. All alkali metal oxide solvents decrease the crystallization temperature of LiNbO3 as compared to that using the conventional melt growth technique. The properties of crystals grown from K2O, Rb2O and Cs2O fluxes were found to be similar, but constant stoichio...

MgO-doped near-stoichiometric LiNbO3 crystals have been grown by the high-temperature top-seeded solution growth method from the K2O–Li2O–Nb2O5 flux. The critical threshold concentration of MgO required to minimize photorefractive damage in the crystals has been determined on the basis of the liquid–solid phase relations. The lowest MgO concentrati...

This paper describes the derivation of a new interatomic potential for lithium niobate, and its use in the calculation of lattice and intrinsic defect properties of the material. As an example of extrinsic defects, magnesium doping is considered. The results are compared with relevant experimental data.

The effect of neutron irradiation on photorefractive gratings in Li Nb O ₃: Fe single crystals is studied experimentally. The observed phenomena result from the large effective cross section of ⁶ Li for thermal and cold neutrons and from the large number of the electrons excited to the conduction band by the high kinetic energ...

The presence of hydroxyl ions has been detected for the first time in as-grown Pb5(GeO4)(VO4)2 (PGV) and Bi2(MoO4)3 (BMO) single crystals by the Fourier Transform Infrared (FTIR) absorption technique. A single, but relatively broad absorption band ( 24 cm-1) related to the stretching vibration of OH- ions has been observed in PGV at 3561.5 cm-1 wav...

The UV absorption edge and IR spectrum of the OH- ion vibration have systematically been measured in LiNbO3 samples for a wide composition and Mg concentration range. The Mg content (0-9 mol %) of the crystals with congruent, stoichiometric and an intermediate composition has been determined by atomic absorption spectroscopy. At a Mg threshold conc...

Spectroscopic properties of the Dy3+-doped YAl3(BO3)4 crystal were studied from the vacuum-ultraviolet to near-infrared spectral range. Optical results were interpreted in terms of the 4fn, 4fn−15d and charge transfer transitions. It was found that the VUV electronic excitation is transferred to the optically active ions giving emission from the 4F...

The stretching mode of the hydroxyl ions in SrxBa1−xNb2O6 crystals with x=0.38 to 0.79 has been measured by the Fourier transform infrared spectroscopy technique. The main absorption band at about 3493 cm−1 is accompanied by a shoulder on the low energy side. The shape of the shoulder depends strongly on the Sr/Ba ratio of the crystal which allows...

The Ti dopant occupying Li or Nb sites and the charge transfer processes induced by thermochemical reduction and optical bleaching treatments have been investigated in LiNbO3 systems using optical absorption and EPR. The Ti3+ centers, built preferentially at Nb sites in heavily Mg-codoped crystals, are shown to have absorption bands at 1.620.08eV a...

The infrared absorption spectrum of the OH-stretch-mode in lithium tantalate is measured for crystals of different compositions. The spectrum exhibits two prominent lines which are representative for ideal and for Li-deficient sites in the crystal, respectively. Position, shape, and intensity of the two lines thus depend on crystal composition in a...

Polarized absorption spectra of CsLiB6O10 crystals have been measured between −186 and 25 °C in the 1500–7500 cm−1 wavenumber range. A series of absorption bands have been observed with intensities depending on the thermal history of the crystals. Three of them have been assigned as the fundamental H–O–H bending mode (δ=1650 cm−1), and symmetric an...

Instead of unstable solutions for conventional four-wave mixing equation set, we obtain the stationary soliton solution for the dynamic grating amplitude and stable auto-oscillations of the dynamic grating. The dynamics of grating recording is described by the sine-Gordon equations in transmission four-wave mixing with non-local grating. The method...

The absorption, emission spectra and lifetime of the 4F9/2 fluorescent level of Dy3+ in YAl3(BO3)4 crystal were investigated. Energy level structure was constructed from absorption and emission spectra recorded at 5 K. The measured oscillator strengths of the transitions between the J manifolds at 300 K were compared with those derived from the Jud...

The wave-mixing problem is considered with respect to the amplitude of the dynamic grating forming in photorefractive crystals. For non-local response and transmission geometry the grating amplitude dynamics is described by a sine-Gordon equation. Hence the grating amplitude has a soliton form. For an experimental verification the spatial distribut...

The bottom seeded fluxothermal (BSFT) high-temperature solution growth technique was used to grow Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PZN) relaxor crystals. During the process, the seed was held in a cold spot of a Pt crucible having a relatively high temperature gradient. The method can separate the perovskite and pyrochlore phases, decrease the s...

For Ce³⁺, Er³⁺ and Yb³⁺ ions, electron paramagnetic resonance (EPR) spectra typical for S' = 1/2 ions are measured for YAl3(BO3)4 (YAB) single crystal. The spectra show axial symmetry indicating that all three dopants replace Y³⁺ at the given dopant concentration. Corresponding - and hyperfine Ã-tensors are