Lithium Tantalate Crystals Research Articles

Effects of interlayer SiO2 or Au on the performances of LNOS-SAW

Abstract Surface acoustic wave (SAW) resonators and filters based on lithium niobate (LN) or lithium tantalate crystals have been widely used in modern wireless communication systems. However, acoustic energy in these kinds of devices is lost in the substrate due to longitudinal attenuation. The enhancements of quality factor and coupling coefficient are still in demand for further applications. This work proposes a SAW based on LN thin film on a sapphire substrate (LNOS). Besides, we explored the effects of using different bonding materials on the LNOS-SAW performances. The presence of the interlayer not only plays a role in bonding but also has an impact on the performance of the resonator. Considering the compatibility of the process in this method, 2 μm-thick silicon dioxide (SiO2) or 1 μm-thick gold (Au) is designed as the interlayer. As the displacement diagrams show, the acoustic energy of the SAW with interlayer is reflected at the interface, and the energy leakage is suppressed. The simulation results show that the presence of SiO2 or Au causes a decrease in resonant frequencies of the resonators, and the effect is more pronounced with Au. Significantly, the presence of SiO2 enhances the coupling efficiency of the resonator while the Au interlayer brings a more obvious improvement in the quality factor.

Pyroelectric Properties and Applications of Lithium Tantalate Crystals

Lithium tantalate crystals, as a type of pyroelectric material, stand out from many other pyroelectric materials due to the advantages of high Curie temperature, large pyroelectric coefficient, high figure of merits, and environmental friendliness. Due to the pyroelectric effect caused by their spontaneous polarization, lithium tantalate crystals have broad application prospects in wide spectral bandwidth and uncooled pyroelectric detectors. This article reviews the pyroelectric properties of lithium tantalate crystals and evaluates methods for pyroelectric properties, methods for modulating pyroelectric properties, and pyroelectric detectors and their applications. The prospects of lithium tantalate thin films, doped lithium tantalate crystals, and near stoichiometric lithium tantalate crystals as response components for pyroelectric detectors are also discussed.

Microcavity Mach-Zehnder refractive index sensing in LiTaO3 crystal waveguides by a combination of femtosecond laser inscription and diamond blade dicing

Based on planar waveguides, refractive index sensors have certain advantages over conventional optical fiber sensors, including better mechanical stability and lower cross-responses to factors such as vibrations. In this study, we report for the first time a microcavity Mach-Zehnder interferometric refractive index sensor fabricated within a planar waveguide. The sensor consists of double-line waveguides made of lithium tantalate crystals written by femtosecond laser and microcavities formed by diamond blade dicing. The refractive index sensitivity of the sensor reaches approximately 1856.1 nm/RIU, with extremely low temperature cross-responses. This work provides a new alternative for developing biosensors based on femtosecond laser-written waveguides. In the future, by selecting appropriate materials and optimizing the structure, the sensing performance of this sensor is expected to be further enhanced.

Performance of LiTaO3 Crystals and Thin Films and Their Application

Lithium tantalate (LiTaO3, or LT) crystal is widely used in optical applications, infrared detection, and acoustic surface wave devices because of its excellent piezoelectric, pyroelectric, and nonlinear optical properties. In this paper, we discuss the defect structure of LT; the preparation method for LT; the influence of doping on LT; and LT’s application in optical, acoustic, and electrical devices. We mainly analyzed the structure and physical properties of LT crystal, the preparation of LT crystal and LT thin films, the periodic polarization of LT crystal, the reduction of LT wafers, and the application potential of LT crystals in lasers and acoustic surface filters according to the most recent research. We also provide an overview of future research directions for LT in the fields of acoustics, optics, and other fields. The applications of LT in 5G, 6G, SAW filters, nonlinear optical devices, and waveguides are expected to provide additional breakthroughs.

Preparation, Properties, and Applications of Near Stoichiometric Lithium Tantalate Crystals

Lithium tantalate crystal is widely used in optical devices, infrared detectors and surface acoustic wave devices because of its excellent piezoelectric, acousto-optic and nonlinear optical properties. The Li content of near stoichiometric lithium tantalate (NSLT) crystal is higher than that of congruent lithium tantalate (CLT) crystal. Therefore, the performance of NSLT crystal is better than that of CLT crystal in some aspects. This article reviews the physical properties, preparation methods and current research status in acoustics and optics of NSLT crystals. It also looks forward to the improvement of NSLT crystal preparation methods and their applications in surface acoustic wave (SAW) filters and optics. With the increase of Li content, the acoustic performance of NSLT crystals is expected to be comprehensively improved, achieving the application of SAW filters in 5G communication.

Li self-diffusion and ion conductivity in congruent LiNbO3 and LiTaO3 single crystals

Lithium niobate and lithium tantalate crystals are technologically important metal oxides with exceptional combinations of ferroelectric, piezoelectric, acoustic, optical, and electrical properties. The self-diffusion of both, the ionic constituents and the underlying point defects, is especially important for the overall electrical conductivity. To get insight into their dynamics, we investigate in this work Li self-diffusion in congruent ${\mathrm{LiNbO}}_{3}$ and ${\mathrm{LiTaO}}_{3}$ single crystals from different suppliers up to a temperature of $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, using isotope-enriched $^{6}\mathrm{LiNbO}_{3}$ and $^{6}\mathrm{LiTaO}_{3}$ tracer layers in combination with secondary ion mass spectrometry depth-profile analysis. The diffusivities of the two isostructural materials are identical within error limits and can be described by the Arrhenius law with an activation energy of 1.35 eV in the range from ${150}^{\ensuremath{\circ}}$ to $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Furthermore, the electrical conductivity is determined between $400{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and can described by an activation energy of about 1.34 eV. This is in excellent agreement with the energy barrier for the diffusion of a single Li vacancy as determined by nudged elastic band calculations based on density-functional theory. The Li-ion conductivities calculated from the diffusivities in ${\mathrm{LiNbO}}_{3}$ and ${\mathrm{LiTaO}}_{3}$ are identical within the error margins with the overall conductivities obtained from impedance spectroscopy measurements. This indicates that the migration of ${\mathrm{Li}}^{+}$ is able to explain the overall electrical conductivity below $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ down to $180{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$.

Enhanced surface blistering efficiency of H+ implanted lithium tantalate by chemical reduction modification

The crystal ion slicing (CIS) fabricated heterogeneously integrated lithium tantalate (LiTaO3, LT) thin film is prospective in optical modulators, infrared sensors, and acoustic filters, therefore improving the transferring efficiency of the LT thin film will definitely promote its development and application. In the present work, the chemical reduction method was found effective to enhance the forming ability of H+ implantation-induced defects and surface blistering efficiency of lithium tantalate crystal during the CIS process. According to the investigations by XPS, XRD, RBS, Micro-Raman, and TEM on the reduction-induced defects, H+ implantation-induced defects, and the surface blistering behavior, the treated black lithium tantalate (BLT) present a higher blistering efficiency, confirming the contribution of reduction-induced defects in implantation and blister formation process than the referenced congruent lithium tantalate (CLT). In detail, the improvement is in ascribing to the existing intrinsic oxygen vacancy defects in BLT. These defects increase the forming efficiency of Ta defects in the implantation process and enhance the diffusion of H+ in the blistering formation process, thus decreasing the activation energy for the blistering process and increasing the film exfoliation area. These results highlight an avenue for enhancing ion slicing efficiency of piezoelectric and pyroelectric single crystal oxide.

Charged domain walls in lithium niobate and lithium tantalate crystals with composition gradients

The formation of charged domain walls (CDWs) in the bulk was studied in lithium tantalate LiTaO3 (LT) and lithium niobate LiNbO3 (LT) crystals with various composition gradients along the polar axis produced by partial vapor transport equilibration (VTE). Under the action of the pyroelectric field upon cooling after the phase transition, the shape of a CDW in LT changed significantly, in contrast to LN: the ledges appeared at CDW and grew toward the polar surfaces. The creation of tail-to-tail and head-to-head CDWs with controlled shape is important for various applications.

Light-induced ordering of nanodomains in lithium tantalate as a result of multiple scanning by IR laser irradiation

The ordering of a light-induced structure of isolated circular ferroelectric nanodomains was discovered in lithium tantalate crystals under multiple scanning by infrared laser irradiation. The effect was considered as domain arising and growth under the action of alternating in sign pyroelectric field arising during sample heating and subsequent cooling. The circular domains appeared due to 1D to 2D shape transformation after the second scan and grew during subsequent scanning by merging with arising domains accompanied by shape restoration. The ordering of the domain pattern during multiple scanning characterized by an increase in the peak of the autocorrelation function was attributed to domain interaction. This mechanism was confirmed by computer simulation using the kinetic approach based on the analogy between the growth of domains and crystals. It was demonstrated that the quasi-regular pattern of one-size circular domains could be created by scanning with shift. The discovered domain ordering effect is similar to that in magnetic materials.

The impact of the simultaneous presence of Li and Nb(Ta) vacancies in the defect structure of pure LiBO3 (B=Nb, Ta) on the curie temperature

The lithium niobate (LiNbO3, "LN") and lithium tantalate (LiTaO3, "LT") crystals are piezoelectric materials that exhibit intrinsic or extrinsic defect structures, which immediately impact their optical and electrical properties. In this study, we examine the influence of point defects on the Curie temperature (Tc) for different samples of pure LN and LT, using three vacancy models: the tantalum (niobium) vacancy model, the lithium vacancy model and the mixed lithium and tantalum (niobium) vacancy model. Based on the Safaryan approach, it was found that the calculated values for the tantalum (niobium) vacancy model did not agree with the experimental data. However, the lithium vacancy model and the mixed vacancy model showed good agreement between experimental and theoretical results. In conclusion, the proposed mixed vacancy model provides the best description of the defect structure in LN and LT materials, and the Tc, is influenced by the structural defects present in the LN and LT compounds.

Raman Scattering in a Double-Doped Single Crystal LiTaO3:Cr(0.2):Nd(0.45 wt%)

The Raman spectra of a lithium tantalate crystal doubly doped with chromium and neodymium LiTaO3:Cr(0.2):Nd(0.45 wt%) have been studied in this paper. Raman spectra of the first and second orders have been found to be located against the background of a luminescent halo with a maximum at ≈1250 cm−1. Several Raman bands have been detected in the frequency range of 900–2000 cm−1. Their frequencies were 940, 1034, 1113, 1171, 1250, 1343, 1428, 1491, 1582, 1735, 1838, and 1925 cm−1. These bands correspond to overtone processes. We have determined that the frequencies of 1838 and 1925 cm−1 bands are significantly higher than the exact value of the overtone frequency corresponding to the fundamental mode 4A1(z)LO (864 cm−1).

Observation of X-rays during heating a pyroelectric crystal by an infrared laser

A pyroelectric X-ray source is proposed, in which a lithium tantalate crystal is heated by an infrared laser with a wavelength of 10.6 μm. X-ray spectra measured during irradiation of the crystal with infrared radiation and during natural cooling of the crystal include characteristic X-ray radiation of atoms contained in the structural parts of the source, as well as bremsstrahlung of electrons with energies above 50 keV. An 8 mm sodium chloride window was used to inject 64 W infrared radiation into a vacuum chamber with the pyroelectric crystal installed.

Preparation and defect structure analysis of near-stoichiometric lithium tantalate wafers

A vapour transfer equilibrium (VTE) method has been used to prepare near-stoichiometric lithium tantalate (NSLT) crystals with different Li contents. The NSLT crystals were tested and analyzed by differential thermal analysis (DTA) and X-ray photoelectron spectroscopy (XPS) to investigate the effect of Li content on the Curie temperature and internal defects of NSLT crystals. This study found that when the Li content increased in the NSLT wafer, the binding energy corresponding to the peak of the Ta4f electron layer in the XPS spectrum first decreased and then increased, indicating that the proportion of Ta valence states was different in wafers with different Li contents. From XPS energy spectrum analysis, it can be seen that the lithium tantalate crystal contains Ta5+, Ta4+, Ta3+ and lower-valence Ta. As the Li content increases in the NSLT wafer, Ta4+ disappears and the proportion of Ta5+ decreases initially, follows by a later increase and then subsequent further decrease. However, the change in proportion of Ta3+ and lower-price Ta is completely opposite to that of Ta5+, showing a trend of first rising, then falling and then finally rising again. Moreover, when the Li content is 49.751% in the NSLT wafer, the proportion of Ta5+ reaches a maximum, showing that at this Li concentration the NSLT crystal has a more perfect lattice structure. In this study, we propose a mixed defect model in which polarons coexist with Li vacancies and Ta inversion, explaining the change in Ta valence state in lithium tantalate crystals. This model is more in line with the observed results in this work. The new hybrid defect model and the variation law of Ta valence state with Li concentration proposed in this paper provide a new direction and experimental proof for the defect study of NSLT crystals, and also provide a theoretical basis to explore the Li content at the best physical properties of NSLT crystals.

Q-switched mode-locked laser generation by Au nanoparticles embedded in LiTaO3 crystals

Au nanoparticles (NPs) in lithium tantalate (LiTaO3) crystal were prepared by ion implantation technique. The microstructure of the formed Au NPs was observed by transmission electron microscope (TEM). The linear and non-linear optical response of the samples was investigated and Z-scan measured that the Au NPs embedded LiTaO3 has saturable absorption properties. Based on this, the sample was used as a saturable absorber (SA) embedded in a waveguide laser system to achieve a 1 μm Q-switched mode-locked laser with a pulse width of 90 ps and a repetition frequency of 6.54 GHz.

Recoverable and rewritable waveguide beam splitters fabricated by tailored femtosecond laser writing of lithium tantalate crystal

We report on an integrated 1 × 5 beam splitter based on optical waveguides with type I and type II modifications of femtosecond laser (fs-laser) writing in lithium tantalate (LiTaO3) crystal. The cladding waveguides consisting of type-II modified tracks are used for optical signal transmission, photon crosstalk reduction, and mode field regulation. The single-line waveguides with type I modification are utilized for light beam splitting. Type-I single-line waveguides are with relatively weak thermal stability, which are utilized to produce a recoverable and rewritable optical beam splitter, and the structure still possesses good transmission properties after the reconstruction. Especially, the type-I modified waveguides can be rewritten in very short time (1–2 min). The beam splitter shows good performance in outputting programmable optical signals, which provides a possible strategy for the development of erasable photonic data processors.

Polar modes and overtone states in Raman spectra of lithium tantalate crystals

The conditions for the excitation of transverse and longitudinal polar modes in the processes of Raman scattering of light in mono- and polydomain lithium tantalate single crystals are established using a backscattering geometry. The relative intensity of the overtone Raman spectrum on the longitudinal and transverse fundamental polar excitations is much higher in the polydomain sample. Unfortunately, the exact reason for the difference in the Raman spectra of the mono- and polydomain lithium tantalate samples is unknown. The frequencies of the two overtone bands exceeded the exact value of the overtone frequency corresponding to the fundamental longitudinal polar mode $4{A}_{1}(\text{LO})$, which clearly indicates the existence of bound phonon pairs (biphonons). This is consistent with theory and has been shown for both mono- and polydomain samples. On the basis of the general theory of bound states of quasiparticles, the possibility of the existence of bound states of quadrupole excitations of the antipolar type in the vibrational spectrum of a polydomain crystal of lithium tantalate is predicted.

Properties of high-temperature poling ferroelectric crystals congruent solid solution LiNb0.5Ta0.5O3

Lithium niobate and lithium tantalate are among the most important and most widely used materials in acousto-optics and acoustoelectronics. These materials have high values of piezoelectric constants, which makes it possible to use these materials as actuators; however, their use is limited by the thermal instability of a lithium niobate crystal and the low Curie temperature (TC) of a lithium tantalate crystal. LiNb(1-x)TaxO3 crystals have to overcome the aforementioned limitations of individual compounds.Crystals LiNb0.5Ta0.5O3 were grown by the Czochralski method, of good quality. Comparative studies of the features of high-temperature single domainization of LiNb0.5Ta0.5O3 crystals have been carried out. The main differences in the technological regimes for single-domainization of congruent LiNb0.5Ta0.5O3 crystals from congruent LiNbO3 crystals are demonstrated. The parameters of high-temperature electrodiffusion processing LiNb0.5Ta0.5O3 crystals are presented, which make it possible to obtain single-domain crystals for further study of their physical properties.

Transformation of initial domain structure by ac electric field in lithium tantalate crystals with composition gradient

A transformation of the initial three-layer domain structure in lithium tantalate with composition gradient and formation of charged domain wall (CDW) under application of ac electric field have been studied on the surface and in the bulk. We have demonstrated the abilities to produce several types of domain structure by ac field treatment: (1) a layer of the maze structure in the bulk with lower thickness and domain wall concentration, (2) a CDW situated just in the center of the plate, and (3) a decay of the initial domain structure leading to formation of large through domains.

Raman scattering of light on overtone states of transverse and longitudinal polar optical modes in a single crystal of lithium tantalate

Raman scattering spectra in single crystals of lithium tantalate at 180-degree scattering geometry in a wide spectral frequency range (50 ÷ 2800 cm−1) were studied. Raman peaks were found in 1300 ÷ 2000 cm−1 region with frequencies exceeding doubled frequencies of LiTaO3 crystal lattice fundamental vibrations of A1 and E-type. The conditions for the manifestation of bound polar phonons in lithium tantalate Raman spectra were analyzed. The spectral intensity of Raman scattering in the overtone region of the spectrum of lithium tantalate was calculated taking into account the presence of a biphonon, and a satisfactory agreement between theory and experiment was obtained.

High-temperature poling treatment of congruent ferroelectric LiNb0.5Ta0.5O3 solid solution single crystals

Lithium niobate and tantalate are among the most important and widely used materials of acoustooptics and acoustoelectronics. They have high piezoelectric constants enabling their use as actuators. Their use is however restricted by the thermal instability of lithium niobate crystals and the low Curie temperature TC of lithium tantalate crystals. Overcoming these drawbacks typical of some compounds is possible by growing LiNb1-xTaxO3 single crystals. Good quality LiNb1-xTaxO3 single crystals have been grown using the Czochralsky technique. High-temperature poling process of LiNb1-xTaxO3 single crystals has been studied. The main differences between the process modes required for poling of congruent LiNb1-xTaxO3 single crystals and congruent LiNbO3 single crystals have been demonstrated. Parameters of high-temperature electric diffusion processing of LiNb1-xTaxO3 single crystals that provide for singledomain crystals for further study of physical properties have been reported.