Near-stoichiometric Lithium Niobate Crystals Research Articles

Near Stochiometric LiNbO3 Crystal: the Piezoelectric Features and the Shear Horizontal Guided Wave Transducer for Structural Health Monitoring up to 650 °C.

The application of shear horizontal (SH) guided wave transducers in high-temperature structural health monitoring (SHM) is a topic of significant interest across various industrial engineering sectors. In this study, we utilized the novelty piezoelectric crystal of near stoichiometric lithium niobate (NSLN), which exhibited a robust piezoelectric response (d15 = 77.6 pC/N@room temperature). Next, the pure thickness shear vibration mode d15' through size optimization was designed. It was demonstrated that the NSLN-based ultrasonic guided wave transducers utilizing the optimum d15' mode were proficient in transmitting and receiving pure fundamental SH wave (SH0 wave) along two orthogonal main directions (0° and 90°) over a wide frequency range (100-350 kHz), exhibiting strong response to the SH0 wave. Under the driving voltage of 100 V, the signal voltages of the NSLN-based transducer were found to be on the order of 200.3 and 11.8 mV at room temperature and high temperature of 650 °C, respectively. Moreover, the NSLN-based SH0 transducer showcased its better defect localization ability, and the signal-to-noise ratio (SNR) sensitivity of NSLN-based transducer was evaluated to be 16.1 dB at high temperature of 650 °C. To sum up, the ultrasonic wave transducer based on NSLN crystal demonstrated higher potential applications for in situ SHM under elevated temperatures.

Defect structure of near-stoichiometric Mg-doped LiNbO3 crystals prepared by different method

Mg-doped (0.5 mol% and 1 mol%) near-stoichiometric lithium niobate (nSLN) crystals were prepared by Li-rich growth method and VTE technique, respectively. Their defect structures were characterized by high resolution X-ray diffraction (HRXRD), OH– vibrational spectra, Raman spectra and ultraviolet-absorption spectra. The crystalline perfection analysis by HRXRD measurements reveals that diffraction curves of all Mg-doped nSLN crystals contain a single diffraction peak, signifying the absence of internal sub-grain boundary. In comparison to SLN-R crystals (grew by Li-rich growth method), the crystalline quality of SLN-V crystals (prepared by VTE technique) is better. The results indicate that MgNb3+ defect complexes exist in SLN-R crystals, even if Mg content is under the threshold, and that most of Mg2+ occupy Li site, extra low Mg2+ and the unavoidable trivalence impurities occupy Nb site in SLN-V crystals. The cation sublattice disorder of SLN-R crystals increases relative to SLN-V crystals. A remarkably weak OH– absorption band indicates that H+ content is extremely low in SLN-V crystals.

Growth, structure, physical and chemical characteristics in a series of LiNbO3:Er crystals of different composition grown in one technological cycle

A series of LiNbO3:Er ([Er] = 0.08–2.71 wt%) crystals were grown due to one technology in a single technological cycle. A near-stoichiometric lithium niobate (NSLN) was grown as a control. Concentration dependences of LiNbO3:Er and NSLN crystals structure characteristics were studied. A full-profile analysis of polycrystals X-ray patterns was used to analyze models of LiNbO3:Er atom structure. Structure characteristics of LiNbO3:Er crystals are compares with that of NSLN crystal. Optical and atomic-force microscopy were used to study growth irregular and regular domain microstructures and periodic nano-structures in as-grown LiNbO3:Er crystals.

Photoluminescence and Particular Features of the Defect Structure of Congruent and Near-Stoichiometric Lithium Niobate Crystals Obtained Using Different Technologies

Bulk photoluminescence of congruent and near-stoichiometric lithium niobate crystals that were obtained by different technologies is studied. It is found that the luminescence intensity in crystals whose composition is close to stoichiometric is lower than that in congruent crystals. It is shown that not only luminescence centers of the main types, which are caused by the occurrence of NbLi, VLi, etc., point defects, but also complex defects that are caused by the occurrence of OH groups in the structure can contribute to luminescence.

Фотолюминесценция и особенности дефектной структуры конгруэнтных и близких к стехиометрическому составу кристаллов ниобата лития, полученных по разным технологиям

Photoluminescence was studied in the volume of congruent and near-stoichiometric lithium niobate crystals obtained by different technologies. Luminescence intensity in near-stoichiometric crystals was found to be lower than that in a congruent crystal. We have shown that not only basic centers (caused by point defects NbLi, VLi) contribute to luminescence, but also complex defects caused by existence of OH-groups.

The effect of an optical pump on the absorption coefficient of magnesium-doped near-stoichiometric lithium niobate in terahertz range

The absorption coefficient of magnesium-doped near-stoichiometric lithium niobate crystal is measured by terahertz time-domain spectroscopy in a frequency range of 0.2 THz–0.9 THz at room temperature. The absorption coefficient is modulated by external optical pump fields. Experimental results show that the absorption coefficient of near-SLN:Mg crystal is approximately in a range of 22 cm−1–35 cm−1 in a frequency range of 0.2 THz–0.9 THz and tunable up to nearly 15%. Further theoretical analysis reveals that the variation of absorption coefficient is related to the number of light-induced carriers, domain reversal process, and OH− absorption in this crystal.

Influence of In2O3 codoping on the upconversion emission properties of Er3+-doped near-stoichiometric LiNbO3 crystals

Two near-stoichiometric lithium niobate (NSLN) crystals codoped with Er3+(1mol%) and In3+ (0 and 0.3mol%) were grown from Li-rich melt ([Li]/[Nb]=1.38, molar ratio) by the conventional Czochralski method. The Er, In contents and [Li]/[Nb] ratios in the crystals were measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES). The OH− absorption spectra were investigated to characterize the defect structure of the crystals. The optical damage resistance ability was characterized by the transmission facula distortion method. The results indicate that the optical damage resistance ability is enhanced by one order of magnitude for Er:NSLN codoped with 0.15mol% In2O3. The electronic transitions of Er3+ ions were analyzed based on ultraviolet–visible absorption spectra. The upconversion fluorescence spectra excited by an 800nm femtosecond laser were measured at room temperature. The results show that three emission peaks centered at 535, 551 and 673nm exist in both crystals and the intensities at these three peaks of In:Er:NSLN are larger than those of Er:NSLN, but an additional 400nm peak appears in In:Er:NSLN. The 400nm emission peak is verified experimentally to be the second harmonic generation of 800nm light.

Waveguide structures for the visible and near-infrared wavelength regions in near-stoichiometric lithium niobate formed by swift argon-ion irradiation

We report the fabrication of a waveguide structure in a near-stoichiometric lithium niobate crystal using 200-MeV argon-ion irradiation at a fluence of 2 × 10(12) ions/cm2. Guided modes were detected in the visible and near-infrared wavelength regions, suggesting that the waveguide can be used at fiber communications wavelengths. The refractive index profiles of the waveguide were reconstructed from the effective index functions. Micro-Raman spectra recorded in the waveguide layer and the substrate showed that the Li/Nb ratio was preserved in the waveguide layer after swift argon-ion irradiation.

The impact of MgO-doped near-stoichiometric lithium niobate crystals on the THz wave output characteristics

The control experimental study on the THz wave parametric oscillator (TPO) output characteristics based on the congruent LiNbO3 crystal (CLN) and stoichiometric MgO-doped lithium niobate (SLN) crystal is performed. As a nonlinear medium in the aspect of the THz wave output experiments show that the congruent LiNbO3 crystal is more stable than the SLN crystal. Compared with the CLN crystal SLN showed significant photorefractive effect which adversely the stability of the THz wave output. Experiments indicated that different molar concentration of MgO doped can significantly change the photorefractive properties of SLN crystal. The results showed that with the increase of MgO doping concentration the photorefractive of SLN gradually become weaker and THz wave output stability has the significantly increase. The output stability of mol 5.0% MgO droped SLN crystal has not significantly different with the CLN. In the contrast experiment of TPO with the 160mm cavity length and 65mm crystal length the pump laser threshold of the 5% mol MgO: SLN crystal decreased by 23% than the CLN crystal while the peak THz energy output increased 28%.

An excellent crystal for high resistance against optical damage in visible-UV range: near-stoichiometric zirconium-doped lithium niobate

Near-stoichiometric zirconium-doped lithium niobate crystals were fabricated and their optical damage resistance was investigated. It was found that these crystals can withstand a light intensity of 20 MW/cm2 at 514.5 nm cw laser, 80 GW/cm2 at 532 nm pulse laser, and 120 kW/cm2 at 351 nm cw laser. The minimum switching field is only 1.00 kV/mm for 0.5 mol% zirconium-doped lithium niobate crystal. These properties suggest that the near-stoichiometric zirconium-doped lithium niobate crystals will be an excellent candidate for quasi-phase matching technique.

Two-wave mixing of ion-implanted photorefractive waveguides in near-stoichiometric Fe:LiNbO 3 crystals

We report on the two-wave mixing of light in photorefractive waveguides in H ion-implanted Fe-doped near-stoichiometric lithium niobate crystals. For pump light of 632.8 nm wavelength a gain coefficient as high as 15 cm −1 is found. A response time of the order of a few seconds is achieved for micro-watt input powers.

Drive frequency dependent phase imaging in piezoresponse force microscopy

The drive frequency dependent piezoresponse (PR) phase signal in near-stoichiometric lithium niobate crystals is studied by piezoresponse force microscopy. It is clearly shown that the local and nonlocal electrostatic forces have a great contribution to the PR phase signal. The significant PR phase difference of the antiparallel domains are observed at the contact resonances, which is related to the electrostatic dominated electromechanical interactions of the cantilever and tip-sample system. Moreover, the modulation voltage induced frequency shift at higher eigenmodes could be attributed to the change of indention force depending on the modulation amplitude with a piezoelectric origin. The PR phase of the silicon wafer is also measured for comparison. It is certificated that the electrostatic interactions are universal in voltage modulated scanning probe microscopy and could be extended to other phase imaging techniques.

Influence of MgO codoping on the optical properties of Er3+-doped near-stoichiometric LiNbO3

Four near-stoichiometric lithium niobate (NSLN) crystals codoped with Er3+ (1 mol%) and MgO (0, 0.5, 1.0, and 2.0 mol%) were grown from K2O-based flux in air using top seeded solution growth technique. The [Li]/[Nb] ratio, estimated from the blueshift of ultraviolet absorption edge, is 97.2% in NSLN:Er. MgO; codoping can increase the segregation coefficient of Er3+ in NSLN:Er:MgO crystal. The photorefractive damage threshold is enhanced by three orders of magnitude for NSLN:Er codoped with 1 mol% MgO, it coincides with the peak shift of OH− absorption spectrum from 3481 to 3535 cm−1. Judd–Ofelt theory based on absorption spectra is used to analyze the influence of MgO concentration on the Judd–Ofelt intensity parameter, transition strength, fluorescence branching ratio, and stimulated emission cross section. From the time-resolved emission spectra and the comparison among emission spectra, two Er3+ crystal-field sites are ascertained in NSLN:Er codoped with 2 mol% MgO, this coincides with the bimodal structure in X-ray photoelectron spectrometry spectra. The upconversion processes under pulse excitation is proposed based on the pump energy dependence and decay kinetics. The distribution of Er3+-clustered sites in NSLN:Er:MgO series is discussed based on the nonexponential decay curves monitored at 550 nm under two-photon excitation.

Ultraviolet light assisted periodic poling of near-stoichiometric, magnesium-doped lithium niobate crystals

Ferroelectric domain reversal by application of electric-fields to near-stoichiometric lithium niobate crystals containing 1.3 mol % magnesium is investigated. The +z-face of the crystal is coated with a patterned photoresist layer that yields both a structured ultraviolet light absorber and a structured electric insulator. Periodic ferroelectric domain inversion in a z-cut crystal is then achieved by combination of illumination with light of the wavelength of 334 nm that reduces the coercive field strength inside the crystal by up to 50% and application of an external electrical field. Domain patterns with a period length of 10 μm are fabricated in a 500 μm thick crystal.

近化学计量比铌酸锂晶体在非相干紫外光敏化下的双色全息存储

Nonvolatile two-color holographic recording gated by incoherent ultraviolet (UV) light centered at 365 nm is investigated in near-stoichiometric lithium niobate crystals. The influence of thermal treatment on the two-color recording is studied. The results show that thermal reduction tends to improve the two-color recording performance, whereas thermal oxidation degrades the two-color recording. With an incoherent 0.2-W/cm2 UV gating light and a 0.25-W/cm2 semiconductor recording laser at 780 nm, a two-color recording sensitivity of 4\times10^{-3} cm/J and a recording dynamic range characterized by M/# of 0.12 are achieved in a 2.2-mm thermally reduced near-stoichiometric lithium niobate crystal. We attribute the improvement to the prolonged lifetime of small polarons and the increased absorption at the gating wavelength due to thermal reduction.

Single crystal growth of MgO-doped near-stoichiometric lithium niobate crystals and fabrication of Ti:PPLN devices

Quasi-phase matching (QPM) frequency conversion devices have been studied for applications of high-speed all-optical signal processing due to their ultrafast nonlinear optical response and wide conversion bandwidth. Among several ferroelectric crystals used, MgO-doped near-stoichiometric LiNbO 3 (MgSLN) crystals have advantages including low dopant content, low coercive field and high damage threshold. In this work, 2 in boules of MgSLN crystals were grown from Li-rich melts by the melt supplying technique. Then Ti-diffusion waveguides were fabricated on Z-cut plate of as-grown crystals. Finally, periodically poled optical gratings with period of 18 μm were made on the plate by high-voltage electric-field poling process. The compositional uniformity of as-grown crystals was detected by optical spectroscopy and the quality of Ti:PPLN devices was observed by optical microscopy, respectively. The effects of several key parameters including domain structure, compositional uniformity and polish process on the quality of Ti:PPLN devices were discussed. The way to improve the quality of Ti:PPLN devices was proposed.

Influence of Nd:Zn codoping in near-stoichiometric lithium niobate

Near-stoichiometric lithium niobate (SLN) crystals doped with up to 1.6 mol % Zn and codoped with various Nd concentrations in the melt (0.2, 0.5, 0.9, and 1.5 mol %) (Nd:Zn:SLN) are grown from 58.6 mol % Li(2)O using conventional Czochralski technique. Crystals are pulled at the rate of 0.35 mmh with seed rotation at 9 rpm. Concentrations of Zn and Nd in the crystal are varied by adding appropriate amounts of ZnO and Nd(2)O(3) to the starting composition. Unit cell parameters of the grown crystals are calculated by Rietveld refinement method using FULLPROFF software. Domain structure studies are carried out by chemical etching followed by microscopic examination. Dielectric studies reveal the existence of piezoelectric resonance at high frequencies. Enhancement in dielectric constant and tan delta in Nd doped samples has been attributed to the space charge polarization. Nd doped samples exhibit reduction in the relative permittivity after oxygen annealing. Transmission spectra of Nd:Zn:SLN crystals in the UV region exhibit blueshift in the cutoff wavelength. In Mid Infrared (MIR) region crystals doped with 1.6 mol % Zn have shift in the OH absorption peak from 2873 to 2833 nm. Judd-Ofelt analysis carried out on the absorption spectra of codoped crystal yields the lifetime of 104 mus for the metastable state (4)F(32). The branching ratio for the electronic transition from (4)F(32) to (4)I(112) is high compared to that for (4)F(32) to (4)I(132), indicating a higher emission cross section for the former transition. Laser damage threshold evaluated using 532 nm, 5 ns pulsed neodymium doped yttrium aluminum garnet laser, shows an increase by two orders of magnitude for crystals doped with 1.6 mol % Zn. Photorefractive damage threshold for these crystals shows an enhancement of four orders of magnitude due to increase in the photoconductivity.

The two photorefractive centres in iron doped nearly stoichiometric lithium niobate crystals

This paper investigates the photorefractive properties of iron doped lithium niobate with different [Li]/[Nb] ratios. The experimental results show two photorefractive centres for iron doped near-stoichiometric lithium niobate crystal. Besides Fe2+ and Fe3+ ions, small polarons and bipolarons are considered as another photoactive centre.

Properties of Ru-doped near-stoichiometric lithium niobate crystals produced by vapor transport equilibration

Ruthenium (Ru)-doped near-stoichiometric lithium niobate crystals (Ru:SLN) were prepared by the vapor transport equilibration (VTE) technique from Ru-doped congruent lithium niobate (CLN) grown using the Czochralski method. Increasing the duration time of the VTE treatment caused the ratio of [Li]/[Nb] in the crystal to increase, which in turn caused the absorption edges to shift to violet. The absorption coefficient in the UV/VIS absorption spectra at 530 nm decreased because of the change in the valence of the Ru ions cause by the VTE treatment. The OH − absorption spectra showed that the broad band located at 3482 cm −1 gradually decreased while another band located at 3466 cm −1 gradually increased and the shape became sharper as the VTE treatment time increased. When the VTE treatment time reached 200 h, Ru-doped near-stoichiometric lithium niobate crystals were obtained. Two-beam coupling examination with a 532 nm laser showed that the Ru-doped near-stoichiometric lithium niobate crystals had a larger exponential gain coefficient, lower diffraction efficiency, faster response time, and higher sensitivity than did the Ru-doped CLN crystals.

Bond-Energy Study of Photorefractive Properties of Doped Lithium Niobate Crystals

A bond-energy method is proposed to study the photorefractive behaviors of doped lithium niobate (LN) crystals on the basis of the bond valence model. The dopant occupancy in the LN matrix can be determined by comparing the deviation of its bond energy in different lattice locations at both Li+ and Nb5+ sites. Furthermore, the optical damage resistant (ODR) ions can be easily distinguished from those dopants by the increased bond energy when they are introduced into the Nb5+ sites, which gives us a bond-energy criterion when searching for new ODR ions. The suppression of the photorefractive effect of doped ODR ions more than their threshold concentration can be attributed to the increase of photoconductivity; it has been shown that such a phenomenon results from the decreased concentration and mobility of the charge carriers with increasing bond energy. According to our bond-energy viewpoint, the threshold concentration of ODR ions in both congruent and near-stoichiometric LN crystals can be determined quantitatively via the assumption that all doped LN crystals should endure a same variation of bond energy.