Cut Lithium Niobate Research Articles

Y-Z cut lithium niobate longitudinal piezoelectric resonant photoelastic modulator.

The capability to modulate the intensity of an optical beam has scientific and practical significance. In this work, we demonstrate Y-Z cut lithium niobate acousto-optic modulators with record-high modulation efficiency, requiring only 1.5 W/cm2 for 100% modulation at 7 MHz. These modulators use a simple fabrication process; coating the top and bottom surfaces of a thin lithium niobate wafer with transparent electrodes. The fundamental shear acoustic mode of the wafer is excited through the transparent electrodes by applying voltage with frequency corresponding to the resonant frequency of this mode, confining an acoustic standing wave to the electrode region. Polarization of light propagating through this region is modulated at the applied frequency. Polarization modulation is converted to intensity modulation by placing the modulator between polarizers. To showcase an important application space for this modulator, we integrate it with a standard image sensor and demonstrate 4 megapixel time-of-flight imaging.

Area Dependence of Effective Electromechanical Coupling Coefficient Induced by On-Chip Inductance in LiNbO3-Based BAW Resonators

To solve the problem of filter bandwidth in 5G communication, it is urgent to develop an acoustic resonator with a large effective electromechanical coupling coefficient (Keff2). In this paper, the dependence between the resonance area and the performance of the bulk acoustic wave (BAW) resonator is studied. The solidly mounted resonators (SMRs) based on 43° Y cut lithium niobate (LN) were fabricated by the wafer transfer technique. The on-chip inductor was integrated with the BAW resonator through a pad electrode. Resonators with different resonant areas were fabricated and tested. Finite element modeling (FEM) simulation of acoustic resonators and electromagnetic (EM) simulation of layout were carried out, respectively. The Modified Butterworth Van Dyke (MBVD) model was used to analyze the results, and simulation of the Mason model was adopted. The results show that the dependency relationship between the resonant area and the effective electromechanical coupling coefficient can be induced by on-chip inductance. In the resonant area range of 20 × 20 μm2~160 × 160 μm2, the Keff2 increases from 11.97% to 43.28%.

Discrete switching in the ion sliced lithium niobate thin films with thick dielectric layer

Domain growth and shape evolution during local switching under the action of biased conductive tip of scanning probe microscope were studied in Z + cut lithium niobate on insulator (LNOI) with 1 µm-thick SiO2 layer between LN film and grounded electrode. Piezoresponse force microscopy and confocal Raman microscopy were used for domain imaging. The domain shape evolution during switching was qualitatively reconstructed. The crucial role of discrete switching on the kinetics of domain growth was revealed. The stability of the switched domains was confirmed. The obtained knowledge will be used for creation of the periodical domain structures by local switching.

Large regular arrays with submicron domains written by low-voltage e-beam on –Z cut of lithium niobate

Contact-free formation of large arrays with a uniform distribution of submicron size domains on the LiNbO3 surface is an urgent and not fully solved problem. Such arrays are widely used, including in optical quasi-phase matching devices for nonlinear frequency conversion. In this work large arrays of submicron domains were first written using low-voltage electron-beam irradiation on a –Z cut of a bulk lithium niobate sample with a grounded conductive coating. Very uniform periodic structures of submicron domains with periods of 1 μm or 1.5 μm were obtained at 5 kV and 10 kV. Surface etching was used to identify the domains within days of electron beam writing. It was found that the stability of the submicron domains became worse as the area of the irradiated squares used in the writing schemes decreased. The spatial intervals between the squares also affect the regularity of the arrays formed. When interpreting the experimental data obtained, the results of electric field distribution simulation at the corresponding writing schemes were used. Based on these results, it has been suggested that two effects competitively influence the success of large regular arrays of submicron domains formation. The electrostatic repulsion between the closest submicron domains and the convergence of electron-beam charges up to the formation of sufficiently large merging zones in which the electric field has sufficient strength. The conclusions obtained could be useful for understanding the mechanism of submicron domain formation in the injected charge zones and could contribute to the further development of electron-beam domain engineering.

Domain-Wall p - n Junction in Lithium Niobate Thin Film on an Insulator

Domain-wall nanoelectronics has attracted extensive attention since the discovery of conductive domain walls, where domain-wall $p$-$n$ junctions are indispensable for device applications. Here, we report on the fabrication of a domain-wall $p$-$n$ junction based on $n$-type and $p$-type conductive domain walls in an $x$-cut lithium niobate thin film on an insulator using a lateral-electric-field poling technique. The electron and hole mobilities in the $n$-type and $p$-type domain walls are measured to be 337 and $93\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.1em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively, more than 400 times higher than those in the parent bulk crystal. The successful fabrication of a domain-wall $p$-$n$ junction with large electron and hole mobilities in the domain walls makes lithium niobate on an insulator a promising platform for domain-wall nanoelectronics and optoelectronics.

Simulation of Wide Band BAW Filter for N78 Application

Based on the requirements of large bandwidth filters for 5G communications, finite element models for material analysis and filter design are established. Characteristics of lithium niobate under different cut angle are simulated, and 36° Y cut lithium niobate is used in resonator with SiO2/Mo acoustic reflector. The influence of top electrode is evaluated and the area of the electrode is designed. A method of co-simulation using finite element model and modified Butterworth van Dyke model is introduced. A wide band filter with relative bandwidth of 10.1% is designed while the insertion loss is less than 2 dB, which can satisfy the requirement of N78 band.

Fabrication of high frequency SAW devices using tri-layer lift-off photolithography

A tri-layer lift-off photolithography technique is presented to enable the fabrication of surface acoustic wave (SAW) devices with near GHz fundamental operating frequencies. SAW devices require high-quality micron-scale features for high frequency operation, a requirement that challenges traditional UV photolithography techniques. The presented process is a modified version of a previously reported tri-layer photolithography process intended for Si and SiO2 substrates which allows for compatibility with materials that are piezoelectric and pyroelectric, often used as the substrate in SAW devices. The process uses a lithographic tri-layer consisting of layers of lift-off resist (LOR) on the bottom, back anti-reflection coating (BARC) in the middle, and photoresist (PR) on top. The addition of the BARC layer prevents back reflection of exposure light, improves the structural integrity of the lithographic stack, and decouples the PR and LOR development, improving resolution by a factor of two over traditional lift-off photolithography techniques. We demonstrate the fabrication of a SAW device with an interdigital transducer (IDT) pitch of 4 μm (minimum feature size of 1 μm) on 128° Y-X cut lithium niobate. The device produces a dip in the measured S11 spectrum at 994.5 MHz, corresponding to the fundamental operating frequency of the device. The fundamental operating frequency of the SAW device is also determined theoretically via numerical modelling and found to be 995.5 MHz. The process thus enables the reliable fabrication of high frequency SAW devices that exhibit excellent agreement with numerical results.

Microliter ultrafast centrifuge platform for size-based particle and cell separation and extraction using novel omnidirectional spiral surface acoustic waves.

Asymmetric surface acoustic waves have been shown useful in separating particles and cells in many microfluidics designs, mostly notably sessile microdroplets. However, no one has successfully extracted target particles or cells for later use from such samples. We present a novel omnidirectional spiral surface acoustic wave (OSSAW) design that exploits a new cut of lithium niobate, 152 Y-rotated, to rapidly rotate a microliter sessile drop to ∼10 g, producing efficient multi-size particle separation. We further extract the separated particles for the first time, demonstrating the ability to target specific particles, for example, platelets from mouse blood for further integrated point-of-care diagnostics. Within ∼5 s of surface acoustic wave actuation, particles with diameter of 5 μm and 1 μm can be separated into two portions with a purity of 83% and 97%, respectively. Red blood cells and platelets within mouse blood are further demonstrated to be separated with a purity of 93% and 84%, respectively. These advancements potentially provide an effective platform for whole blood separation and point-of-care diagnostics without need for micro or nanoscale fluidic enclosures.

Domain patterning of non-polar cut lithium niobate by focused ion beam

The domain switching by ion beam irradiation of single-domain Y-cut plates of congruent lithium niobate doped by 5 mol% MgO was studied. Dot irradiation resulted in the formation of wedge-like domains grown along Z- polar direction. The nonlinear dependences of the domain sizes on irradiation conditions were obtained. The model of domain growth on non-polar cut was proposed in frame of kinetic approach. The absence of the domain merging along Z- polar direction was attributed to the deceleration of the forward growth by electrostatic interaction of the positive charges at the charged domain walls and injected positive ions.

High coupling efficiency grating couplers on lithium niobate on insulator.

We demonstrate monolithically defined grating couplers in Z-cut lithium niobate on insulator for efficient vertical coupling between an optical fiber and a single mode waveguide. The grating couplers exhibit ∼ 44.6%/coupler and ∼ 19.4%/coupler coupling efficiency for TE and TM polarized light respectively. Taperless grating couplers are investigated to realize a more compact design.

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.

Charging and domain switching in ferroelectrics LiNbO3 by electron beam

Comprehensive experimental results on the kinetic characteristics of LiNbO3 charging process are reported. Secondary electron emission (SEE) yield, surface potential, and the sum of leakage and displacement currents were experimentally obtained during electron beam irradiation in the energy range from 200 eV to 10 keV for both sides of the Z-cut. We found that at the initial stage of charging, the SEE current that were collected by a hemispherical electrode changes its sign. It was explained by the emergence of a large displacement current on the collector. The direction of this current is opposite to the directions of SEE and backscattered electron currents. If a large positive charge arises on the surface of ferroelectrics, the positive displacement current dominates the secondary emission current; this fact was confirmed experimentally. Also, a high value of the negative surface potential in the electron energy range 200–700 eV for −Z cut of lithium niobate was found, though in this energy region, positive charging is observed for nonpolar dielectrics. The influence of domain repolarization currents on the measured emitted electrons and displacement current signals is shown.

ВЛИЯНИЕ ОБРАТНОГО ПРОТОННОГО ОБМЕНА НА СТРУКТУРУ И СВОЙСТВА ПРОТОНООБМЕННЫХ СЛОЕВ НА X-СРЕЗЕ КРИСТАЛЛА НИОБАТА ЛИТИЯ

The changes in the structure and optical properties of proton-exchange layers during reverse proton exchange were investigated on X cut lithium niobate. It was shown that the phase transformations in proton-exchange layers at reverse proton exchange proceed faster than during annealing at the same temperature;  1 phase particles are not formed at reverse proton exchange.

Deformation Anisotropy of Y + 128°-Cut Single Crystalline Bidomain Wafers of Lithium Niobate

Bidomain single crystals of lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) are promising materials for use as actuators, mechanoelectrical transducers, and sensors capable of working in a wide temperature range. One need to take into account the anisotropy of the properties of the crystalline material when such devices are designed. In this study we investigated deformations of bidomain round shaped Y + 128°-cut wafers of lithium niobate in an external electric field. The dependences of the piezoelectric coefficients on the rotation angles were calculated for lithium niobate and lithium tantalate and plotted for the crystal cuts which are used for the formation of a bidomain ferroelectric structure. In the experiment, we utilized an external heating method and long-time annealing with the lithium out-diffusion method in order to create round bidomain lithium niobate wafers. Optical microscopy was used to obtain the dependences of the bidomain crystals’ movements on the rotation angle with central fastening and the application of an external electric field. We also modelled the shape of the deformed bidomain wafer with the suggestion that the edge movement depends on the radial distance to the fastening point quadratically. In conclusion, we revealed that the bidomain Y + 128°-cut lithium niobate wafer exhibits a saddle-like deformation when a DC electric field is applied.

Determining the sign of a polar surface of lithium niobate crystal by UV reflectance spectroscopy

We propose to reveal the +Z and–Z surfaces of a polar cut lithium niobate crystal by measuring its UV reflection spectrum. By the example of a congruent lithium niobate, it is shown that the intensities of light reflection from polar crystal surfaces of different signs in the region of 190—260 nm differ by up to several percent. The depth of short-wave radiation penetration into surface layers of the crystal in the spectral range of intrinsic absorption is estimated. It is shown that the proposed method can be used for determining the surface signs of polar crystal layers with thicknesses from several dozen to several hundred microns.

Deformation anisotropy of Y + 128° –cut single crystalline bidomain wafers of lithium niobate

Bidomain single crystals of lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) are promising material for usage as actuators, mechanoelectrical transducers and sensors working in a wide temperature range. It is necessary to take into account anisotropy of properties of crystalline material when such devices are designed. Inthis study we investigated deformations of bidomain round shapedY+ 128°-cut wafers of lithium niobate in an external electric field. Dependencies of piezoelectric coefficients on rotation angles were calculated for lithium niobate and lithium tantalate and plotted for the crystal cuts which are used for bidomain ferroelectric structure formation. In experiment, we utilized external heating method and long-time annealing with lithium out-diffusion method in order to create round bidomain lithium niobate wafers. In order to obtain dependencies of the bidomain crystals’ movements on the rotation angle with central fastening and external electric field application optical microscopy was used. We also modeled a shape of the deformed bidomain wafer with a suggestion that the edge movement depends on the radial distance to the fastening point quadratically. In conclusion, bidomainY+ 128°-cut lithium niobate wafer exhibits saddle-like deformation when DC electric field is applied.

Interdomain region in single-crystal lithium niobate bimorph actuators produced by light annealing

The interdomain region of a bidomain strucrture formed in 127°-cut lithium niobate single crystals using light annealing has been studied by optical and scanning probe microscopies. A periodic subdomain structure on the 180° macrodomain wall is visualized by piezoresponse force microscopy. The piezoresponse signal (polarization) is shown to be a power-law function of the domain width with an exponent n = 0.53.

Comparative study of 0° X-cut and Y + 36°-cut lithium niobate high-voltage sensing.

A comparison study between Y + 36° and 0° X-cut lithium niobate (LiNbO3) was performed to evaluate the influence of crystal cut on the acoustic propagation to realize a piezoelectric high-voltage sensor. The acoustic time-of-flight for each crystal cut was measured when applying direct current (DC), alternating current (AC), and pulsed voltages. Results show that the voltage-induced shift in the acoustic wave propagation time scaled quadratically with voltage for DC and AC voltages applied to X-cut crystals. For the Y + 36° crystal, the voltage-induced shift scales linearly with DC voltages and quadratically with AC voltages. When applying 5 μs voltage pulses to both crystals, the voltage-induced shift scaled linearly with voltage. For the Y + 36° cut, the voltage-induced shift from applying DC voltages ranged from 10 to 54 ps and 35 to 778 ps for AC voltages at 640 V over the frequency range of 100 Hz-100 kHz. Using the same conditions as the Y + 36° cut, the 0° X-cut crystal sensed a shift of 10-273 ps for DC voltages and 189-813 ps for AC voltage application. For 5 μs voltage pulses, the 0° X-cut crystal sensed a voltage induced shift of 0.250-2 ns and the Y + 36°-cut crystal sensed a time shift of 0.115-1.6 ns. This suggests a frequency sensitive response to voltage where the influence of the crystal cut was not a significant contributor under DC, AC, or pulsed voltage conditions. The measured DC data were compared to a 1-D impedance matrix model where the predicted incremental length changed as a function of voltage. When the voltage source error was eliminated through physical modeling from the uncertainty budget, the combined uncertainty of the sensor (within a 95% confidence interval) decreased to 0.0033% using a Y + 36°-cut crystal and 0.0032% using an X-cut crystal for all the voltage conditions used in this experiment.

Investigation of high-temperature ultrasonic transducer design using lithium niobate piezocomposite

Design and fabrication of lithium niobate piezocomposite transducers is aimed at achieving high temperature ultrasonic NDT measurements at 400°C. We investigated three aspects: exploring design parameters at room temperature by comparison of experimental and modelled results for lithium niobate piezocomposites with epoxy matrix; high temperature electromechanical testing of lithium niobate piezocomposites with cement matrix; and fabrication and testing of a high temperature transducer operating at 400°C for defect detection. The piezocomposites were made with a 1-3 structure using y/36°-cut lithium niobate single crystal material. Typical piezocomposite thickness was 1 mm, pillar width 0.4-0.8 mm, kerf width 0.5 mm, volume fraction of lithium niobate 30-40%, and pillar aspect ratio (pillar height to pillar width) range from 1 to 6. Operating frequency of the samples was between 1 and 4 MHz. Results on lithium niobate piezocomposite with an epoxy matrix with pillar aspect ratio 3, 4, and 6 indicated that high coupling coefficient kt should be achievable even with low pillar aspect ratio. Piezocomposite material was heated in air to 400-500°C over several thermal cycles and showed good stability of the electromechanical impedance spectrum. A 13 x 13 mm 2 transducer with operating frequency 3 MHz was used to demonstrate detection of an artificial defect in a steel block using high temperature couplant on a hot plate at 400°C.

Investigation of the Orientational Dependence the Lateral Piesoresponse in Y–cut Periodically Poled Lithium Niobate Single Crystals

The angular dependence of the lateral component piesoresponse in Y−cut lithium niobate single crystals with a regular domain structure was investigated in piezoresponse force microscopy mode. It is shown that visualization of the domain structure is determined by the orientation of the X direction of the crystal relative to the scan direction: the greatest contrast between domains with opposite orientation of the polarization vectors takes place at locations along an X−direction line and the direction of scanning.