Stoichiometric Lithium Niobate Research Articles

Three-photon and four-photon absorption in lithium niobate measured by the Z-scan technique

Open-aperture Z-scan measurements have been carried out to investigate the three-photon (3 PA) and four-photon absorption (4 PA) coefficients at 800 nm and 1030 nm wavelengths, respectively in congruent and stoichiometric lithium niobate (cLN, sLN) with different concentrations of Mg doping. The laser pulse duration at the two wavelengths were 40 and 190 fs. The peak intensity inside the crystals varied between approximately 110 and 550 GW/cm2. The 3 PA and 4 PA coefficients were evaluated using a theoretical model and the results suggest that their minima are at or around the Mg doping level corresponding to the threshold for suppressing photo-refraction for both cLN and sLN. This result can be attributed to the contribution of crystal defects to the 3 PA and 4 PA processes. Furthermore, the 4 PA at 1030 nm exhibited greater nonlinear absorption than the 3 PA at 800 nm under the same intensity level. Possible reasons for this unexpected behavior are discussed. Overall, comparing the 3 PA and 4 PA values of these crystals will enable for selection of the optimum composition of LN crystal for efficient THz generation and for other nonlinear optical processes requiring high pump intensities.

Investigations on SHG properties of LiNbO3/PMMA nanocomposites

An organic–inorganic nanocomposite (LN/PMMA) was prepared using nearly stoichiometric lithium niobate (LiNbO3; LN) nanoparticles dispersed homogeneously into a polymer matrix, poly(methyl methacrylate) (PMMA). This composite is having anisotropic LN nanocrystals, oriented randomly in an isotropic host matrix of PMMA. So far, to the best of our knowledge, different nonlinear optical (NLO) properties have been studied for organic–inorganic nanocomposites in thin film and waveguide form, the thickness of which was in micron and submicron order. In this article, for the first time, we have presented SHG properties of thick LN/PMMA nanocomposites of thickness, 0.1 mm and above. LN/PMMA nanocomposite samples of different thickness and different concentrations of LN nanoparticles were fabricated. The attenuation coefficients of these samples, measured at laser wavelengths of 1064 nm and 532 nm, show a deviation from Beer-Lambert law. In the LN/PMMA nanocomposite, the effect of the randomly oriented LN nanoparticles on second harmonic generation (SHG) has been studied as a function of (a) input pulse energy of the fundamental laser (1064 nm), (b) LN nanoparticle concentration and (c) the sample thickness. The net effect was found to be the result of two competitive phenomena; harmonic generation and different losses from LN nanoparticles. Thermal effects at high input pulse energy also play important role in second harmonic generation. Through these studies, one can get an idea about the collective behavior of LN nanoparticles, particularly effect on their linear and nonlinear optical properties, and how they can be dispersed in isotropic host matrices (to different concentration levels) to achieve the desired NLO effect.

Photoluminescence of Nominally Pure Lithium Niobate Single Crystals Produced by Various Technologies

Volume and surface photoluminescence of congruent and stoichiometric lithium niobate crystals obtained using various technologies was studied. The luminescence intensity of the stoichiometric crystal was less than that of the congruent one. Volume luminescence of the crystals was mainly caused by NbLi defects while luminescence quenching of the near-surface layer was observed in the long-wavelength spectral region (>500 nm) because of energy scattering in crystal-lattice vibrations and increased luminescence intensity of the $$ {\mathrm{Nb}}_{\mathrm{Nb}}^{4+}-{\mathrm{O}}^{-} $$ pair. Luminescence bands with maxima at 426 and 446 nm were caused by complex defects in the form of electron–hole pairs $$ {\mathrm{Nb}}_{\mathrm{Nb}}^{4+}-{\mathrm{O}}^{-} $$ in which the Nb atom was bound to oxygen atoms by covalent and ionic bonds. An increase of the Li/Nb ratio led to a shift of the luminescence bands to shorter wavelengths and a change of the fundamental absorption edge of the studied crystals.

Temperature-dependent terahertz time-domain spectroscopy study of Mg-doped stoichiometric lithium niobate

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 measurements are given in simple and concise form in order to provide easily usable practical information for those who would like to use this material in the terahertz range. Through a practical example it was also pointed out, that the effect of the temperature must not be neglected during the design and adjustment of terahertz sources if the goal is to maximize the optical-to-THz conversion efficiency.

Stoichiometric Lithium Niobate Crystals: Towards Identifiable Wireless Surface Acoustic Wave Sensors Operable up to 600 °C

Wireless surface acoustic wave (SAW) sensors constitute a promising solution to some unsolved industrial sensing issues taking place at high temperatures. Currently, this technology enables wireless measurements up to 600-700$^\circ$C at best. However, the applicability of such sensors remains incomplete since they do not allow identification above 400$^\circ$C. The latter would require the use of a piezoelectric substrate providing a large electromechanical coupling coefficient K 2 , while being stable at high temperature. In this letter, we investigate the potentiality of stoichiometric lithium niobate (sLN) crystals for such purpose. Raman spectroscopy and X-ray diffraction attest that sLN crystals withstand high temperatures up to 800$^\circ$C, at least for several days. In situ measurements of sLN-based SAW resonators conducted up to 600$^\circ$C show that the K 2 of these crystals remains high and stable throughout the whole experiment, which is very promising for the future achievement of identifiable wireless high-temperature SAW sensors.

Defect Complexes in Stoichiometric Lithium Niobate Crystals Prepared by Different Processes

Using IR absorption spectroscopy in the stretching region of OH groups and Raman spectroscopy in the region of two-particle states of acoustic phonons, we have demonstrated that high-temperature top-seeded solution growth from a melt of congruent composition containing 6.0 wt % K2O flux in Kristall-2 commercially available growth units makes it possible to obtain nearly stoichiometric LiNbO3 crystals having approximately the same perfection of the proton sublattice as in stoichiometric crystals grown from a melt containing 58.6 mol % Li2O, but considerably surpassing them in general optical and structural homogeneity.

Photoelectric fields in doped lithium niobate crystals

Photoinduced light scattering (PILS) in nominally pure stoichiometric and congruent lithium niobate single crystals (LiNbO3), and ones doped with B3+, Cu2+, Zn2+, Mg2+, Gd3+, Y3+, Er3+ cations was studied. All crystals have a relatively low effect of photorefraction and are promising materials for frequency conversion, electro-optical modulators and shutters. It was found that the photovoltaic and diffusion fields for some crystals have a maximum at a wavelength of 514.5 nm. All the crystals studied are characterized by a maximum of the integral intensity of the speckle structure of the PILS at a wavelength of 514.5 nm.

Processing and characterization of improved congruent lithium niobate

LiNbO3 exists in two forms: Most commercial devices are inexpensively fabricated from congruent melt that is ca. 1.5% off-stoichiometry. This produces devices of high optical quality for nonlinear optics but also high loss. Stoichiometric lithium niobate also exists, but in smaller more expensive specimens. Here we report the preparation and characterization of low-loss congruent samples. These are shown to be relatively free from extended defects.

Temperature-dependent ferroelectric properties of near stoichiometric lithium niobate single crystal

The temperature dependent ferroelectric hysteresis properties of stoichiometric lithium niobate single crystals have been studied in this article. It is found that remnant polarization (Pr) and the coercive field (Ec) increase monotonically with increasing electric field at 293 K, and the Pr at 76 K is smaller than that at 293 K due to less switchable domains can be activated at low temperature, whereas the Ec at 76 K is larger than that at 293 K due to the trapped oxygen vacancy cannot be detrapped easily at low temperature. However, both the Pr and Ec increase when the temperature decreases from 293 to 247 K, then decrease with further decreasing temperature to 76 K. The pinning of the switched domains induces the increase of Pr and the deepening and widening of the domain wall-defect interaction potential well makes the activated domains difficult to be back-switched leads to the increase of Ec when the temperature decreases from 293 to 247 K, while less domains will be activated in the domain switching process induces the decrease of Pr and the frozen of domains with higher activation energy is the cause of Ec decreasing from 247 to 76 K.

Physicochemical and Optical Characteristics of LiNbO3 Single-Crystals Doped with Boron

The chemical interaction in the Li2CO3–Nb2O5 and Li2O–B2O3–Nb2O5 systems and crystallization features of LiNbO3 crystals from the melts containing boron impurity are considered. Raman spectra of LiNbO3:B crystals grown from a congruent melt containing ~0.55–1.24 mol % of B2O3 are studied. There are noticeable changes in the entire Raman spectrum upon doping with boron, which indicates a change in alternation of the main cations and vacancies along the polar axis of the LiNbO3:B crystal and “perturbation” of oxygen octahedra. Moreover, expansion of oxygen octahedra is anisotropic with an increase in boron concentration in the melt. Boron, hardly entering the structure of the lithium niobate crystal, significantly changes the structure of the melt and thus has a significant effect on structure and physical characteristics of the LiNbO3:B crystals. The results obtained for the LiNbO3:B crystals are compared with those obtained for nominally pure stoichiometric (LiNbO3stoich) and congruent (LiNbO3cong) lithium niobate crystals.

Photoelectric Fields and Band Gap in Doped Lithium Niobate Crystals

Photoinduced light scattering and optical spectroscopy have been used to study the photorefractive effect and determine the band gap in nominally undoped congruent and stoichiometric lithium niobate crystals, as well as in a series of congruent LiNbO3 crystals doped with Mg, Zn, B, Gd, Y, and Er cations and LiNbO3 single crystals codoped with Mg:Gd, Mg:Fe, Mg:Y, and Mg:Ta.

Domain Shape Appeared in Stoichiometric Lithium Niobate as a Result of Ion Beam Irradiation

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Photoelectric Fields and Band Gap in Doped Lithium Niobate Crystals

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Optical properties of lithium niobate crystals

According to optical absorption spectra the width of the forbidden band of nominally pure congruent and stoichiometric lithium niobate crystals, as well as a series of congruent crystals doped with cations of Mg2+(0.35wt.%), Zn2+(2.05), B3+(0.12), Gd3+(0.26, 0.44, 0.51), Y3+(0.46), Gd3+(0.23):Mg2+(0.75), Mg2+(0.86):Fe3+(0.0036), Ta5+(1.13):Mg2+(0.01), Y3+(0.24):Mg2+(0.63), Er3+(3.1) is determined. The photorefractive effect in crystals, their structural and optical homogeneity was studied by photo induced and Raman scattering methods, laser conoscopy.

Optical homogeneity and photorefractive properties of stoichiometric and congruent lithium niobate crystals grown using charges of different origins

A stoichiometric lithium niobate crystal (LiNbO3 st) and congruent lithium niobate crystals grown from a charge prepared using cyclohexanone as an extractant (LiNbO3 cong(CHN)) and a charge prepared using cyclohexanone and carboxylic acid dimethylamides as extractants (LiNbO3 cong(CHN + A)) have been characterized by photoinduced light scattering and laser conoscopy. The results demonstrate that the LiNbO3 cong(CHN + A) crystals are rather homogeneous along their growth direction and possess good optical properties, similar to those of the LiNbO3 cong(CHN) crystal. At the same time, the LiNbO3 cong(CHN + A) crystal offers significantly better electro-optical properties (r e = 29.3 pm/V).

Lattice Site of Rare-Earth Ions in Stoichiometric Lithium Niobate Probed by OH− Vibrational Spectroscopy

Rare-earth (RE = Er3+, Nd3+, or Yb3+) ion-doped stoichiometric LiNbO3 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 LiNbO3, a new infrared absorption band was observed at about 3500 cm−1, similar to the case of the trivalent optical damage resistant (ODR) dopants In3+ and Sc3+. By comparing the frequencies and polarization dependences of the bands to those detected for ODR ion containing crystals, they are attributed to the stretching vibration of OH− ions in RE3+Nb-OH− complexes. Consequently, above a given concentration threshold, some of the rare-earth ions are assumed to occupy niobium sites in the LiNbO3 lattice. The same model is also suggested for RE-doped congruent LiNbO3 crystals containing over-threshold (>5 mol %) amounts of the Mg-co-dopant.

Manifestation of structural features of LiNbO3:Zn and LiNbO3:Mg crystals in their IR absorption spectra in the stretching region of OH– groups

Analysis of IR absorption spectra of LiNbO3:Zn (0.04–4.46 mol % ZnO) and LiNbO3:Mg (0.19–5.91 mol % MgO) single crystals in the stretching region of OH– groups has been used to gain insight into composition-dependent structural changes in the crystals. The results demonstrate that, characteristically, the OH– groups occupy different sites in the doped and congruent LiNbO3 crystals and have different quasielastic O–H bond constants in their structure. In stoichiometric lithium niobate crystals, all of the OH– sites and quasi-elastic O–H bond constants are identical. At threshold Zn and Mg dopant concentrations, the frequencies, widths, and intensities of the observed lines change sharply. The linewidths in the spectra of the LiNbO3:Zn crystals near their first concentration threshold (≈2.0 mol % ZnO) and the LiNbO3:Mg crystals near their first and second concentration thresholds (≈3.0 and 5.5 mol % MgO) decrease markedly, indicating ordering of the position of the OH– groups in the structure of the crystals.

Optical amplifier based on an Er:MgO-doped near stoichiometric lithium niobate waveguide

We report on an optical signal amplifier based on an Er:MgO-doped near stoichiometric lithium niobate (Er:MgO:SLN) waveguide. The Er:MgO:SLN waveguide was fabricated using swift carbon ion irradiation combined with precision diamond blade dicing. Under 980 nm laser pumping, the waveguide provides a 2.13 dB/cm gain at 1536 nm, 1.49 dB/cm gain at 1552 nm, and 1.37 dB/cm gain at 1565 nm, with the pumping power of 99.5 mW. This work demonstrates the potential application of swift ion irradiated Er:MgO:SLN waveguides for the optical amplifiers in the C communication band.

Substitution mechanisms and location of Co2+ ions in congruent and stoichiometric lithium niobate crystals derived from electron paramagnetic resonance data

Electron paramagnetic resonance (EPR) spectra and their angular dependencies were measured for Co2+ trace impurities in stoichiometric samples of lithium niobate doped with rhodium. It was found that Co2+ substitutes for Li+ in the dominant axial center (CoLi) and that the principal substitution mechanism in stoichiometric lithium niobate is 4Co2+ ↔ 3Li+ + Nb5+. The four Co2+ ions can occupy the nearest possible cation sites by occupying a Nb site and its three nearest-neighbor Li sites, creating a trigonal pyramid with C3 symmetry, as well as non-neighboring sites (e.g. a CoNb–CoLi pair at the nearest sites on the C3 axis with two nearby isolated single Co2+ ions substituted for Li+). In congruent crystals and samples with Li content enriched by vapor transport equilibrium treatment the excess charge of the Co2+ centers is compensated by lithium vacancies located rather far from the Co2+ ions for the dominant axial center or in the nearest neighborhood for low-symmetry satellite centers (the Co2+ ↔ 2Li+ substitution mechanism). The use of exact numerical diagonalization of the spin-Hamiltonian matrices explains all the details of the EPR spectra and gives a value for hyperfine interaction A|| that is several times smaller than that obtained using perturbation formulae. The refined values of A and g-tensor components can be used as reliable cornerstones for ab initio and cluster calculations.

Investigation of the cluster formation in lithium niobate crystals by computer modeling method

The processes occurring upon the formation of energetically equilibrium oxygen-octahedral clusters in the ferroelectric phase of a stoichiometric lithium niobate (LiNbO3) crystal have been investigated by the computer modeling method within the semiclassical atomistic model. An energetically favorable cluster size (at which a structure similar to that of a congruent crystal is organized) is shown to exist. A stoichiometric cluster cannot exist because of the electroneutrality loss. The most energetically favorable cluster is that with a Li/Nb ratio of about 0.945, a value close to the lithium-to-niobium ratio for a congruent crystal.