Lithium Niobate Single Crystals Research Articles

State-of-the-Art and Practical Guide to Ultrasonic Transducers for Harsh Environments Including Temperatures above 2120 °F (1000 °C) and Neutron Flux above 1013 n/cm2.

In field applications currently used for health monitoring and nondestructive testing, ultrasonic transducers primarily employ PZT5-H as the piezoelectric element for ultrasound transmission and detection. This material has a Curie–Weiss temperature that limits its use to about 210 °C. Some industrial applications require much higher temperatures, i.e., 1000–1200 °C and possible nuclear radiation up to 1020 n/cm2 when performance is required in a reactor environment. The goal of this paper is the survey and review of piezoelectric elements for use in harsh environments for the ultimate purpose for structural health monitoring (SHM), non-destructive evaluation (NDE) and material characterization (NDMC). The survey comprises the following categories: 1. High-temperature applications with single crystals, thick-film ceramics, and composite ceramics, 2. Radiation-tolerant materials, and 3. Spray-on transducers for harsh-environment applications. In each category the known characteristics are listed, and examples are given of performance in harsh environments. Highlighting some examples, the performance of single-crystal lithium niobate wafers is demonstrated up to 1100 °C. The wafers with the C-direction normal to the wafer plane were mounted on steel cylinders with high-temperature Sauereisen and silver paste wire mountings and tested in air. In another example, the practical use in harsh radiation environments aluminum nitride (AlN) was found to be a good candidate operating well in two different nuclear reactors. The radiation hardness of AlN was evident from the unaltered piezoelectric coefficient after a fast and thermal neutron exposure in a nuclear reactor core (thermal flux = 2.12 × 1013 ncm−2; fast flux 2 (>1.0 MeV) = 4.05 × 1013 ncm−2; gamma dose rate: 1 × 109 r/h; temperature: 400–500 °C). Additionally, some of the high-temperature transducers are shown to be capable of mounting without requiring coupling material. Pulse-echo signal amplitudes (peak-to-peak) for the first two reflections as a function of the temperature for lithium niobate thick-film, spray-on transducers were observed to temperatures of about 900 °C. Guided-wave send-and-receive operation in the 2–4 MHz range was demonstrated on 2–3 mm thick Aluminum (6061) structures for possible field deployable applications where standard ultrasonic coupling media do not survive because of the harsh environment. This approach would benefit steam generators and steam pipes where temperatures are above 210 °C. In summary, there are several promising approaches to ultrasonic transducers for harsh environments and this paper presents a survey based on literature searches and in-house laboratory observations.

“Shutdown” of the Proton Exchange Channel Waveguide in the Phase Modulator under the Influence of the Pyroelectric Effect

It is shown that the termination of the channeling of the fundamental radiation mode in the waveguide can be observed upon heating of an optical integrated circuit based on proton exchange channel waveguides formed in a lithium niobate single crystal. This process is reversible, but restoration of waveguide performance takes tens of minutes. The effect of the waveguide disappearance is observed upon rapid heating (5 K/min) from a low temperature (minus 40 °C). This effect can lead to a temporary failure of navigation systems using fiber optic gyroscopes with modulators based on a lithium niobate crystal.

Raman Scattering in Non-Stoichiometric Lithium Niobate Crystals with a Low Photorefractive Effect

Raman spectra of lithium niobate single crystals strongly doped by zinc and magnesium, it has been established, contain low-intense bands with frequencies 209, 230, 298, 694, and 880 cm−1. Ab ignition calculations fail to attribute these bands to fundamental vibrations of A2 symmetry type unambiguously. Such vibrations are prohibited by the selection rules in the space group C3V6 (R3c). Ab initio calculations also proved that low-intense “extra” bands with frequencies 104 and 119 cm−1 definitely do not correspond to vibrations of A2 symmetry type. We have paid special attention to these extra bands that appear in LiNbO3 single crystals Raman spectra despite the fact that they are prohibited by the selection rules. In order to do so, we have studied a number of lithium niobate single crystals, both nominally pure and doped, by Raman spectroscopy. We have assumed that some “extra” bands correspond to two-particle states of acoustic phonons with a total wave vector equal to zero. We have also detected a Zn concentration area (0.05–0.94 mol.% ZnO in a crystal) where doped crystal structure is more ordered: The order of alternation of the main, doping cations, and vacancies along the polar axis is increased, and oxygen octahedra are less distorted.

Magnetoelectric Effect in the Bidomain Lithium Niobate/Nickel/Metglas Gradient Structure

The paper describes the magnetoelectric (ME) effect in a gradient laminate structure consisting of a LiNbO3/Ni/metglas multilayer based on a bidomain lithium niobate single crystal with a Y + 128° cut. The main purpose of the work is to obtain a self‐biased structure. A theoretical and experimental study is conducted to determine the optimal thickness of the Ni film for creating such a structure. The application of self‐biased structures based on gradient bidomain crystals in ME devices can significantly reduce their weight and dimensions as well as improve their electrical characteristics.

Real-Time 3D Imaging of Nanoscale Ferroelectric Domain Wall Dynamics in Lithium Niobate Single Crystals under Electric Stimuli: Implications for Domain-Wall-Based Nanoelectronic Devices

Challenges in the fields of renewable energy harvesting, data storage, and nanoelectronics have resparked interest in ferroelectric domain walls (DWs) as tunable, nanoscale elements. However, the study of such structures has mostly relied on 2-dimensional, rather slow imaging techniques such as scanning probe microscopy. Therefore, Cherenkov second harmonic generation (CSHG) microscopy has been established as a technique suitable for the nondestructive imaging of ferroelectric DWs and their 3-dimensional (3D) evolution. Here, we report on the real-time and in situ 3D DW kinetics when inspecting electrical-field-biased 200 μm thick lithium niobate (LNO) single crystals. A linear electric field increase up to +4.0 kV/mm (antiparallel to the direction of spontaneous polarization PS) resulted in the collapse of laser-poled hexagonal domains into cone-like structures. The average inclination was measured to rise up to 2.5°. Head-to-head (h2h) domain walls dominated. Simultaneously, the domain wall current (DWC) was recorded in situ. It increased by 4 orders of magnitude to 1 μA. The DW mobility increased dramatically as a function of depth. Moreover, a significant asymmetry was found, as DW mobility was much higher along crystallographic (Y+) directions. Subsequently, the electric field was reversed and swept to −3.6 kV/mm. While the hexagonal domain shapes were restored for moderate electric fields, the domains separated into many nucleating spike domains when exceeding a critical threshold of −3.5 kV/mm. DWC increased dramatically through this process, reaching magnitudes of up to 1 mA. The understanding of DW dynamics upon electric stimulation was used to realize a two-port DW-based nanoswitch. Alternating positive and negative electric fields were applied to a neutral hexagonal domain contacted purely via solid electrodes. The field strengths were kept well below the critical threshold of spike domain formation. The electrical conductivity of such a device could be tuned over 4 orders of magnitude, i.e., deliberately switched on and off. Our findings support the development of future DW-based nanoelectronic devices.

Determination of the Chemical Composition of Lithium Niobate Powders

Existent methods for determining the composition of lithium niobate single crystals are mainly based on their variations due to changes in their electronic structure, which accounts for the fact that most of these methods rely on experimental techniques using light as the probe. Nevertheless, these methods used for single crystals fail in accurately predicting the chemical composition of lithium niobate powders due to strong scattering effects and randomness. In this work, an innovative method for determining the chemical composition of lithium niobate powders, based mainly on the probing of secondary thermodynamic phases by X-ray diffraction analysis and structure refinement, is employed. Its validation is supported by the characterization of several samples synthesized by the standard and inexpensive method of mechanosynthesis. Furthermore, new linear equations are proposed to accurately describe and determine the chemical composition of this type of powdered material. The composition can now be determined by using any of four standard characterization techniques: X-Ray Diffraction (XRD), Raman Spectroscopy (RS), UV-vis Diffuse Reflectance (DR), and Differential Thermal Analysis (DTA). In the case of the existence of a previous equivalent description for single crystals, a brief analysis of the literature is made.

Determination of optical parameters of lithium niobate films by srectrophotometry

Lithium niobate films on silicon substrates were synthesized by high−frequency magnetron sputtering of a target. The resultant film was a layer of polycrystalline lithium niobate. By the method of spectrophotometry we obtained the spectral dependences of the reflectance in the wavelength range 300—700 nm at small angles of incidence. The angular dependence of p− and s− polarized light were measured for a discrete set of wavelengths from 300 to 700 nm increments of wavelength 50 nm and increments for angles of 1°. The values of the refractive indicies, film thickness and extinction coefficients were determined using a numerical method for solving inverse problems. As the film is absorbing we accepted the simulation optical system as an isotropic monolayer absorbing film on a semi-infinite absorbing substrate with a sharp interface. Initial approximation for the solution of inverse problems were defined by the methods based on the estimation of the interference extrema position in the reflection-angular spectra. Values of the refractive indicies of the film differ from the values typical for LiNbO3 single crystals obtained both from the reference literature, and by refractive indices direct goniometric method measurements of a certified standard enterprise sample (SES) made from a lithium niobate single crystal. We additionally studied the specimens with X−ray diffraction and scanning probe microscopy. These deviations are attributed to the film inhomogeneity, the presence of the second phase, and disordering of the structure. Inclusions of the second phase in the form of crystallites with a predominant orientation along the Z axis are observed.

Pulse-induced transient blue absorption related with long-lived excitonic states in iron-doped lithium niobate

Transient absorption is studied in Fe-doped lithium niobate single crystals with the goal to control and probe a blue absorption feature related with excitonic states bound to Fe$_\textrm {Li}$Li defect centers. The exciton absorption is deduced from the comparison of ns-pump, supercontinuum-probe spectra obtained in crystals with different Fe-concentration and Fe$^{2+/3+}_\textrm {Li}$Li2+/3+-ratio, at different pulse peak and photon energies as well as by signal separation taking well-known small polaron absorption bands into account. As a result, a broad-band absorption feature is deduced being characterized by an absorption cross-section of up to $\sigma ^\textrm {max}(2.85$σmax(2.85 eV$) = (4\pm 2)\cdot 10^{-22}$)=(4±2)⋅10−22 m$^{2}$2. The band peaks at about 2.85 eV and can be reconstructed by the sum of two Gaussians centered at 2.2 eV (width $\approx 0.5$≈0.5 eV) and 2.9 eV (width $\approx 0.4$≈0.4 eV), respectively. The appropriate build-up and decay properties strongly depend on the crystals’ composition as well as the incident pulse parameters. All findings are comprehensively analyzed and discussed within the model of $\textrm {Fe}^{2+}_{\textrm {Li}}-\textrm {O}^{-}-\textrm {V}_{\textrm {Li}}$FeLi2+−O−−VLi excitonic states.

Superfast domain wall motion in lithium niobate single crystals. Analogy with crystal growth

In this paper, we studied the superfast domain walls that appear after domain merging during polarization reversal in congruent lithium niobate single crystals by in situ optical visualization with a high temporal resolution. The superfast domain walls were observed during the fast transformation of the domain shape from concave polygonal that appears after merging to the convex polygonal one with slow domain walls only. Orientations and velocities of superfast domain walls were measured during transformation of an artificial polygonal X-shaped domain (with all concave angles appearing after merging) to a hexagonal one. The “convex” growth of isolated hexagonal domains was governed by the slowest domain walls, while the “concave” growth after domain merging—by the superfast walls with three orders of magnitude higher velocity. We reconstructed experimentally the v-plot (kinetic Wulff plot) for domain wall motion by analysis of both convex and concave domain growth. The slowest walls (Y-walls) correspond to the v-plot deep cusps having six-fold symmetry, whereas the superfast walls correspond to the v-plot maxima having three-fold symmetry with a difference between Y+ and Y− crystallographic directions. The field dependences of the velocities of the superfast domain walls followed the activation law, taking into account the existence of the bias field.

Raman Scattering in Lithium Niobate and Lithium Tantalate Single Crystals and Ceramics

We have found conditions for high-speed measurements of Raman spectra of lithium niobate and lithium tantalate single crystals and ceramics. To measure Raman spectra, we used 180° scattering geometry and a minispectrometer equipped with an array detector. The Raman spectra obtained at two distinct orientations of the single crystals relative to the excitation beam direction have been shown to have different frequencies of transverse and longitudinal optical modes. The Raman spectra of the ceramics are mainly contributed by transverse optical modes.

Fabrication of Piezoelectric Platforms for Studying Strain in Electrocatalysis

The fabrication of piezoelectric supports for straining thin film electrodes using standard photolithography, sputter coating, and atomic layer deposition is detailed. An interdigitated array on the back of single crystal lithium niobate piezoelectric substrates allows large electric fields to be applied to strain thin films on the front of the device. A thin metal oxide insulating layer prevents electronic communication between the electrode of interest and the underlying piezoelectric. Platinum thin films (<10 nm) can be sputtered and annealed to produce quality electrodes. Microelectrode arrays can be patterned on the electrode via another oxide layer or insulating resist. The various challenges of fabricating and testing these devices as well as future prospects are discussed.

Highly sensitive magnetic field sensor based on a metglas/bidomain lithium niobate composite shaped in form of a tuning fork

This study reports the creation of a highly sensitive, low-frequency magnetic field sensor based on a composite multiferroic consisting of a bidomain lithium niobate/metglas laminate shaped in form of a tuning fork. An efficient suppression of acoustic and thermal noises in the measurements of AC magnetic fields has been achieved. As a piezoelectric component we used a y + 128°-cut lithium niobate single crystal. A metglas foil (serving as a magnetostrictive component) was antisymmetrically bonded to each tine of the tuning fork. The sensor demonstrated a 6.7 times increase of the sensitivity to magnetic fields as compared to a single-plate magnetoelectric (ME) sensor: the magnetic field detection limit was enhanced from 20 pT to 3 pT at a frequency of ca. 318 Hz, without any additional shielding from external noises. The advantages of the ME sensors based on bidomain lithium niobate over those based on PZT or PMN-PT are a much higher thermal stability, anhysteretic piezoelectric effect, large resistance to creep, lead-free nature and simple and cheap fabrication process. Ultimately, the tuning-fork ME sensors based on bidomain lithium niobate single crystals might be used in low frequency, ultra-sensitive, cheap and high-temperature magnetic field sensors for biomedical or space applications.

Annealing stability of the domain structure in periodically poled MgO doped lithium niobate single crystals

The stability of periodical domain structure during high-temperature annealing was studied in MgO doped lithium niobate single crystals by analysis of efficiency and homogeneity of the second harmonic generation (SHG) signal and optical visualization of the domain pattern. Annealing at temperatures up to 600 °C with up to 50 heating/cooling cycles resulted in weak changes of SHG performance and domain structure, while annealing at 1000 °C resulted in eight times decrease in the average SHG power and two times decrease in the effective periodical domain structure depth. Backswitching was attributed to the action of electric field induced by the crystal composition gradient caused by Li out-diffusion.

Tilt control of the charged domain walls in lithium niobate

The utilization of charged domain walls (CDWs) as nanoelectronic devices requires three technological aspects to be developed: (1) controllable CDW creation, (2) tuning of CDW geometry, and (3) CDW removal. We have studied CDW formation in lithium niobate single crystals during polarization reversal using liquid and solid-state electrodes, as well as their combination. It was shown that CDW can be formed by two alternative procedures: (1) by forward switching for the liquid electrode at the Z+ polar surface and solid at the Z− one, (2) by backward switching for solid at the Z+ polar surface and liquid at the Z− one. The created CDW can be transformed to an almost neutral domain wall by the application of the field pulse of the reverse polarity for both procedures. As a result, the tilt of the formed CDW can be tuned reversibly in the range from 0.2° to 1.2° resulting in a change between isolated and highly conductive states. We have demonstrated that the created CDW can be used as a nanoelectronic channel for local electrolysis opening the additional possibilities for ferroelectric lithography.

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.

Low-Frequency Vibration Sensor with a Sub-nm Sensitivity Using a Bidomain Lithium Niobate Crystal

We present a low-frequency sensor for the detection of vibrations, with a sub-nm amplitude, based on a cantilever made of a single-crystalline lithium niobate (LiNbO3) plate, with a bidomain ferroelectric structure. The sensitivity of the sensor-to-sinusoidal vibrational excitations was measured in terms of displacement as well as of acceleration amplitude. We show a linear behavior of the response, with the vibrational displacement amplitude in the entire studied frequency range up to 150 Hz. The sensitivity of the developed sensor varies from minimum values of 20 μV/nm and 7 V/g (where g = 9.81 m/s2 is the gravitational acceleration), at a frequency of 23 Hz, to peak values of 92.5 mV/nm and 2443 V/g, at the mechanical resonance of the cantilever at 97.25 Hz. The smallest detectable vibration depended on the excitation frequency and varied from 100 nm, at 7 Hz, to 0.1 nm, at frequencies above 38 Hz. Sensors using bidomain lithium niobate single crystals, as sensitive elements, are promising for the detection of ultra-weak low-frequency vibrations in a wide temperature range and in harsh environments.

Shear Piezoelectric and Dielectric Properties of $${\hbox {LiNbO}}_{3}$$ LiNbO 3 , PMN-PT and PZT-5A at Low Temperatures

We have measured the low-temperature shear piezoelectric and dielectric constants of single-crystal lithium niobate ( $$\hbox {LiNbO}_{3}$$ ) and lead magnesium niobate–lead titanate (PMN-PT), and of ceramic lead zirconium titanate (PZT-5A) transducers between room temperature and 78 mK. The piezoelectric and dielectric coefficients $$d_{15}$$ and $$K^{\sigma }_{15}$$ all decrease with temperature, although the total change in $$d_{15}$$ is only about 7% for $$\hbox {LiNbO}_3$$ . The values of $$d_{15}$$ for PZT-5A and PMN-PT are much larger at room temperature but decrease much more rapidly, by factors of 4 for PZT-5A and 10 for PMN-PT. For $$\hbox {LiNbO}_3$$ , $$d_{15}$$ is constant below 50 K, but in both PZT-5A and PMN-PT $$d_{15}$$ continues to decrease nearly linearly to the lowest temperatures. The behavior of the dielectric constant of each material mirrors that of $$d_{15}$$ , reflecting their common ferroelectric origins. The piezoelectric voltage constants $$g_{15}$$ are similar in the three materials and are only weakly temperature dependent. For actuator applications where large displacements are needed, PMN-PT and PZT-5A have much larger $$d_{15}$$ values than $$\hbox {LiNbO}_3$$ , but this advantage essentially disappears at low temperatures and $$\hbox {LiNbO}_3$$ is a better choice in many applications. For sensor applications where $$g_{15}$$ determines a transducer’s output voltage, the three materials have similar sensitivity for high-frequency applications like ultrasonics. At low frequencies, however, they are less sensitive as voltage sensors and the use of charge or current amplifiers is preferable.

Temperature-Dependent and Threshold Behavior of Sm3+ Ions on Fluorescence Properties of Lithium Niobate Single Crystals.

Temperature-dependent and threshold behavior of Sm3+ ions on fluorescence properties of lithium niobate (LiNbO3, LN) single crystals were systematically investigated. The test materials, congruent LiNbO3 single crystals (Sm:LN), with various concentrations of doped Sm3+ ions from 0.2 to 2.0 mol.%, were grown using the Czochralski technique. Absorption spectra were obtained at room temperature, and photoluminescence spectra were measured at various temperatures in the range from 73 K to 423 K. Judd–Ofelt theory was applied to calculate the intensity parameters Ωt (t = 2, 4, 6) for 1.0 mol.% Sm3+-doped LiNbO3, as well as the radiative transition rate, Ar, branching ratio, β, and radiative lifetime, τr, of the fluorescent 4G5/2 level. Under 409 nm laser excitation, the photoluminescence spectra of the visible fluorescence of Sm3+ mainly contains 568, 610, and 651 nm emission spectra, corresponding to the energy level transitions of 4G5/2→6H5/2, 4G5/2→6H7/2, and 4G5/2→6H9/2, respectively. The concentration of Sm3+ ions has great impact on the fluorescence intensity. The luminescence intensity of Sm (1.0 mol.%):LN is about ten times as against Sm (0.2 mol.%):LN at 610 nm. The intensity of the fluorescence spectra were found to be highly depend on temperature, as well as the concentration of Sm3+ ions in LiNbO3 single crystals, as predicted; however, the lifetime changed little with the temperature, indicating that the temperature has little effect on it, in Sm:LN single crystals. Sm:LN single crystals, with orange-red emission spectra, can be used as the active material in new light sources, fluorescent display devices, UV-sensors, and visible lasers.

Optical Properties of Copper-Doped Lithium Niobate Crystals

We have studied the optical properties of copper-doped lithium niobate crystals using semiconductor light-emitting diodes and lasers emitting in the visible and ultraviolet spectral regions as excitation sources. The results demonstrate that, at an excitation frequency approaching the frequency of electronic absorption in copper ions, there is discrete resonance Raman scattering in the form of a frequency comb. The observed Raman satellites of discrete light scattering are due to polar longitudinal A1 optical modes of the lithium niobate single crystal. Under excitation by a light-emitting diode with a wavelength of 520 nm, we observe a sharp increase in discrete light scattering intensity in comparison with the photoluminescence excited by shorter wavelength excitation sources.

Self-Organized Formation of Quasi-Regular Ferroelectric Nanodomain Structure on the Nonpolar Cuts by Grounded SPM Tip.

The understanding of self-organization processes at the micro- and nanoscale is of fundamental interest and is important to meet the great challenges in further miniaturization of electronic devices to the nanoscale. Here, we report self-organized quasi-regular nanodomain structure formation on the nonpolar cut of a ferroelectric lithium niobate single crystal. These structures were formed along the trajectory of grounded scanning probe microscope tip approaching or moving away from the freshly switched region. Detailed analysis of the formed structures revealed internal organization by the length of the needle-like domains, which ranged from uniform to quasi-periodic and even chaotic modes as a function of distance from the switched region. Comprehensive investigations and numerical simulations allowed to attribute explored phenomena to charge injection during the field application and further switching under the action of electric field induced by injected charges near the tip. Self-organization and quasi-periodicity were explained by the effective screening and long-range electrostatic interaction between the individual needle-like domains.