Crystalline Lithium Niobate Research Articles

Low loss ridge waveguides on the hybrid single crystalline silicon and lithium niobate thin films

The hybridization of silicon (Si) thin film and lithium niobate (LN) thin film with remarkable properties, including the excellent optical properties of LN, excellent electrical properties and mature processing advantages of Si, can provide an important material platform for integrated photonic applications. In this work, a 4-inch single crystalline Si thin film wafer was prepared using wafer bonding, grinding and polishing techniques on LN thin film substrate. The films were characterized by using high-resolution transmission electron microscopy (HRTEM). The interplanar spacings and strain of Si film were calculated by analysis of high-resolution X-ray diffraction (HRXRD). Si ridge straight waveguides were fabricated on the hybrid thin films, and the transmission loss was 1.2 dB/cm. The near-field mode distribution of the waveguide was investigated by combining FDTD (Finite-Difference Time-Domain) simulation and end coupling measurement.

Observation of Anapole Resonances in Lithium Niobate Metasurfaces with Significantly Enhanced Second Harmonic Generation

AbstractBenefiting from their large second‐order nonlinear coefficients and high integration capabilities, crystalline lithium niobate (LN) films have shown great application prospects in the field of nonlinear metasurfaces. It is necessary to endow LN metasurfaces with optical resonances to further boost nonlinear optical responses. Among these optical resonances, anapole resonances carried by LN metasurfaces have been predicted to efficiently enhance second harmonic generations (SHG) but have never been experimentally realized. Anapole resonance requires ideal nanostructures to have steep sidewalls and a high filling ratio, but the intrinsic hardness and inert chemical properties of LN materials pose great challenges for the fabrication of LN nanostructures. Here, a multi‐gas component dry‐etching technique is proposed to prepare various LN nanostructures, achieving typical LN nanopillar arrays with a sidewall angle of ≈85° at a depth of 300 nm and a filling ratio of 52%. Importantly, nanostructured LN metasurfaces are designed and fabricated to experimentally realize anapole resonances at a fundamental wavelength of ≈800 nm, demonstrating a ≈30‐fold enhancement in second harmonic generation compared with bare LN films. The work provides promising strategies for the versatile fabrication of LN nanostructures and their applications in nonlinear meta‐optics.

Hydrothermal preparation of crystalline lithium niobate photocatalysts for effective degradation of dye-containing contaminated wastewater by ultraviolet to visible light irradiation

Hydrothermal preparation of crystalline lithium niobate photocatalysts for effective degradation of dye-containing contaminated wastewater by ultraviolet to visible light irradiation

Strong-Acid-Catalyzed Formation of Crystalline LiNbO3 at Low Ambient Temperatures.

A crystalline LiNbO3 material was synthesized at 80 °C by an optimized sol-gel method using a double alkoxide alcoholic solution in the presence of hydrogen peroxide (H2O2) and strong acids. The same reaction in the presence of water or acetic acid resulted in amorphous powders with fewer impurities but which crystallized only at 450 °C and higher temperatures. The purity of the crystalline material obtained at 80 °C is strongly dependent on the Li/Nb molar ratio used for the reaction. It appears that the combination of strong acids with H2O2 in air generates perfect conditions for the synthesis of low-temperature crystalline lithium niobate oxide derivatives and can be extended to various other metal oxides. The developed synthetic method opens the potential for the coating of low-melting-point conductive polymer materials with LiNbO3 crystalline films.

Effect of crystal orientation on the hardness and strength of piezoelectric LiNbO3 substrates for microelectronic applications

Piezoelectric single crystalline materials are paramount for high-speed data transfer in 5G technologies. The functionality of the end-devices demands temperature independent frequency filtering and high surface acoustic wave velocities, which are associated with the orientation dependent thermo-physical properties of the piezoelectric substrate material. Single crystalline Lithium Niobate (LiNbO3), cut in particular directions, has proven to have outstanding functional properties, yet its brittle character along with the highly anisotropic mechanical properties may limit its use in demanding applications.In this study, the effect of crystal orientation on hardness and on mechanical strength is demonstrated by comparing nanoindentation results and finite element analysis supported biaxial strength experiments for two LiNbO3 samples with different orientation. It is demonstrated that the crystal anisotropy leads to differences in hardness up to ∼ 20% between both orientations, with the characteristic strength being double in the harder direction. The observed correlation is rationalized based on the effect of surface finish and distinct sub-surface damage in the corresponding crystal orientations. Additional strength measurements on nano-scratched samples revealed a significantly higher remaining strength for the harder orientation due to less (sub-) surface damage. These findings can be exploited in future design of single crystalline substrate materials with higher reliability.

Epitaxy of LiNbO3: Historical Challenges and Recent Success

High-quality epitaxial growth of thin film lithium niobate (LiNbO3) is highly desirable for optical and acoustic device applications. Despite decades of research, current state-of-the-art epitaxial techniques are limited by either the material quality or growth rates needed for practical devices. In this paper, we provide a short summary of the primary challenges of lithium niobate epitaxy followed by a brief historical review of lithium niobate epitaxy for prevalent epitaxial techniques. Available figures of merit for crystalline quality and optical transmission losses are given for each growth method. The highest crystalline quality lithium niobate thin film was recently grown by halide-based molecular beam epitaxy and is comparable to bulk lithium niobate crystals. However, these high-quality crystals are grown at slow rates that limit many practical applications. Given the many challenges that lithium niobate epitaxy imposes and the wide variety of methods that have unsuccessfully attempted to surmount these barriers, new approaches to lithium niobate epitaxy are required to meet the need for simultaneously high crystalline quality and sufficient thickness for devices not currently practical by existing techniques.

Characterisation of hydrogen ion implantation damage in quartz, lithium niobate and tellurium dioxide by Raman spectroscopy

We report observations of the Raman spectra of 10 keV hydrogen ion-implanted crystalline quartz, lithium niobate and tellurium dioxide over a wide range of ion fluences (– protons/cm). We observe the emergence and growth of a new fluorescence feature in the Raman spectra (peaked at 1535 cm) as a function of fluence in all three materials. When measured, the position and shape of this feature is nearly identical in each material. However, the appearance of this feature in tellurium dioxide has a slightly varied behaviour. Based on our observations, we suggest that this signal is due to the 557.7 nm photoluminescence of interstitial atomic O formed radiolytically. While interstitial molecular O2 has previously been observed in irradiated quartz, the presence of this atomic oxygen signature in proton-irradiated lithium niobate and tellurium dioxide has not been previously reported. Our results are important for the use of these materials, and other crystalline oxides, in charged particle (proton) radiation rich environments, such as those encountered by spaceborne instrumentation.

Elucidating the role of precursors in synthesizing single crystalline lithium niobate nanomaterials: a study of effects of lithium precursors on nanoparticle quality.

A number of solution-based procedures have been realized for the synthesis of lithium niobate (LiNbO3) nanoparticles (NPs). Relatively little is, however, known about the influences of the selection of lithium (Li) precursors on the resulting dimensions, shapes, crystallinity, and purity of the products. A comparative study is provided herein on the role of different Li precursors during the synthesis of LiNbO3 NPs. To the best of our knowledge, this study provides the first systematic comparison of the effects of various Li reagents on the preparation of LiNbO3 NPs through solvothermal processes. This solution-phase approach was tuned by the inclusion of Li precursors that either lacked carbon based anions (e.g., F-, Cl-, Br-, I-, OH-, NO3-, or SO42-) or contained carbon-based anions (e.g., C2H5O-, C2H3OO-, C5H7OO-, or CO32-). All other variables were held constant during the synthesis, such as reaction temperature, solvent, niobium precursor, and surfactants. The results of these studies suggest that the type of Li precursor selected plays an important role in nanoparticle formation, such as through controlling the uniformity, crystallinity, and aggregation of LiNbO3 NPs. The average diameter of the resulting NPs can also vary from ∼30 to ∼830 nm as a function of the Li reagent used in the synthesis. The selection of Li precursors also influences the phase purity of the products. This comparative study on the preparation of crystalline LiNbO3 NPs represents a critical step forward to understand the influences and roles of precursors in the design of synthetic processes for the preparation of a variety of alkali metal niobates (e.g., including NaNbO3 and KNbO3) and crystalline metal oxide-based NPs containing other transition metals (e.g., titanium, tantalum).

Advances in on-chip photonic devices based on lithium niobate on insulator

Crystalline lithium niobate (LN) is an important optical material because of its broad transmission window that spans from ultraviolet to mid-infrared and its large nonlinear and electro-optic coefficients. Furthermore, the recent development and commercialization of LN-on-insulator (LNOI) technology has opened an avenue for the realization of integrated on-chip photonic devices with unprecedented performances in terms of propagation loss, optical nonlinearity, and electro-optic tunability. This review begins with a brief introduction of the history and current status of LNOI photonics. We then discuss the fabrication techniques of LNOI-based photonic structures and devices. The recent revolution in the LN photonic industry has been sparked and is still being powered by innovations of the nanofabrication technology of LNOI, which enables the production of building block structures, such as optical microresonators and waveguides of unprecedented optical qualities. The following sections present various on-chip LNOI devices categorized into nonlinear photonic and electro-optic tunable devices and photonic-integrated circuits. Some conclusions and future perspectives are provided.

Fabrication of single crystalline LiNbO3 thin films for wideband bulk acoustic wave filters

Single crystalline lithium niobate (LN) thin films with a Y43-cut are fabricated by crystal-ion-slicing technique, and the B-staged benzocyclobutene (BCB) polymer is used as the bonding medium for the transferring of the LN thin film on to a homogeneous LN substrate. The thickness of the LN film is about 910 nm, and low energy ion irradiation is used to treat the surface of the film, which reduces the roughness from 12.4 nm down to 3.6 nm. Bulk acoustic wave (BAW) resonator is fabricated based on the thin LN layer, and the electromechanical coefficient (k2t) of the LN film reaches 16.3%. A 3-stage BAW filter is obtained, and the 3 dB bandwidth of the filter is 8.5%, demonstrating the single crystalline LN thin film with large k2t is promising for wide band filter.

Electrically Micro‐Polarized Amorphous Sodo‐Niobate Film Competing with Crystalline Lithium Niobate Second‐Order Optical Response

AbstractThe design of active optical devices integrating second‐order nonlinear (SONL) optical responses typically relies on the use of dielectric crystalline materials such as lithium niobate (LN) or semiconductors such as GaAs. Despite high SONL susceptibilities, these materials present important geometry constrains inherent to their crystalline nature limiting the complexity of the designed photonic systems. Conversely, amorphous materials are versatile optical media compatible with broad platform designs possessing a wide range of optical properties attributable to their composition flexibility. Demonstrated here for the first time in an amorphous inorganic material, a magnitude of SONL optical susceptibility (χ(2) = 29 pm V−1 at 1.06 µm) comparable to that of LN single crystal is reported. By using a thermo‐electrical imprinting process, fine control of the induced uniaxial anisotropy is demonstrated at the micrometer scale. This work paves the way for the future design of integrated nonlinear photonic circuits based on amorphous inorganic materials enabled by the spatially selective and high SONL optical susceptibility of these promising and novel optical materials.

Lithium Niobate Metasurfaces

AbstractLithium niobate is a multi‐functional material, which has been regarded as one of the most promising platforms for the multipurpose optical components and photonic circuits. Targeting miniature optical components and systems, lithium niobate microstructures with feature sizes of several to hundreds of micrometers are demonstrated, such as waveguides, photonic crystals, micro cavities, and modulators, and so on. In order to demonstrate the possibilities toward further shrinking the footprint of the lithium niobate devices with new optical functionalities, here, subwavelength nanograting metasurfaces are fabricated based on a crystalline lithium niobate film. Due to the collective lattice interactions between isolated ridge resonances, distinct transmission spectral resonances are observed, which could be tunable by varying the structural parameters. Furthermore, the metasurfaces are capable of showing high‐efficiency transmission structural colors as a result of structural resonances and intrinsic high transparency of lithium niobate in visible spectral range. The results pave the way for new types of ultracompact photonic devices based on lithium niobate.

Sol-Gel and Electrospinning Synthesis of Lithium Niobate-Silica Nanofibers

Lithium niobate-silica fibers were produced by the combination of the sol-gel method and the electrospinning technique. Two sol-gel solutions starting from niobium-lithium ethoxide and tetraethyl orthosilicate were prepared and then mixed with polyvinylpyrrolidone; the solutions were electrospun in a coaxial setup. The obtained lithium niobate-silica polymeric fibers were approximately 760 nm in diameter. Raman spectroscopy confirmed the composite composition by showing signals corresponding to lithium niobate and silica. Scanning electron microscopy showed coaxial fibers with a diameter of around 330 nm arranged as a fibrillar membrane at 800 °C. At 1000 °C the continuous shape of fibers was preserved; the structure is composed of silica and lithium niobate nanoparticles within the fibers. The formation of crystalline lithium niobate and amorphous SiO2 phase was also confirmed by XRD peaks.

Lithium niobate micro-disk resonators of quality factors above 107.

We report on the fabrication of crystalline lithium niobate microresonators with quality factors above 107, as measured around 770nm wavelength. Our technique relies on femtosecond laser micromachining for patterning a mask coated on the lithium niobate on insulate (LNOI) into a microdisk, followed by a chemo-mechanical polishing process for transferring the disk-shaped pattern to the LNOI. Nonlinear processes including second-harmonic generation and Raman scattering have been demonstrated in the fabricated microdisk.

Resistor Network Modeling of Conductive Domain Walls in Lithium Niobate

AbstractHere the concept of a 2D resistor network (2D RN) is applied in order to model the electrical conductivity along sheet‐like domain walls (DWs) in single crystalline lithium niobate (sc‐LNO). The only input to the RN modeling approach is the DW inclination angle distribution, as measured previously with respect to the polar c‐axis. The simulations then show that a 2D network of Ohmic resistors not only adequately accounts for the different boundary conditions envisaged in experiments, but equally well provides a direct link between the local domain wall conductivity (DWC) and the DW inclination angle α. Moreover, the RN simulations can be directly compared to local‐scale transport measurements, as obtained by scanning probe techniques. The conceptual simplicity and the low computational effort make the present RN modeling approach a useful tool for both the advanced interpretation and evaluation of potential DWC ferroelectrics.

The Effects of Admixtures on Resistance to Radiation of Lithium Niobate Crystals

The studies of optical absorption and transmission of crystalline lithium niobate compounds modified by rare-earth and alkali-earth elements: LiNbO3: Y (0.46 wt %), LiNbO3: Y (0.32 wt %), Mg (0.24 wt %), LiNbO3: Mg (0.27 wt %), LiNbO3: Gd (0.004, 0.04, 0.26, and 0.43 wt %), and ostensibly pure LiNbO3 is reported. The features of absorption and transmission are examined with respect to the dosage of γ-irradiation, the annealing temperature, and the type and concentration of modifying admixtures. The features revealed in different ways of bleaching γ-irradiated and annealed in vacuum ostensibly pure lithium niobate crystals are used to refine the mechanisms of developing electron and point defects and to propose a model of processes taking place at γ-irradiation.

Formation of bidomain structure in lithium niobate plates by the stationary external heating method

The method of development of the bidomain structure in single crystalline lithium niobate plates based on the creation of a given temperature gradient distribution through a sample thickness by stationary heating is considered. Heating the LiNbO3 plate, which is placed between two silicon plates, is implemented by light energy emitted by lamps of the photonic annealing setup, which is absorbed by silicon. The scheme of the technological cell provides the formation and control over heat fluxes penetrating a ferroelectric plate and forming temperature gradients required for the controlled formation of two domains with the opposite polarization vectors (a head-to-head domain structure). The efficiency of light absorption for the formation of heat sources, which can be used for symmetric and asymmetric heating, which determines the position of the conditional surface with a zero temperature gradient and, consequently, a domain boundary position, is confirmed experimentally. In the LiNbO3 plate with a thickness of 1.6 mm and length 60 mm, a symmetric bidomain structure with oppositely directed polarization vectors is formed. The dependence of the bending strain of a console-clipped sample on electric voltage is studied in the temperature range −300 to +300 V; the strain amplitude is more than 35 μm. The high linearity and repeatability of the electric voltage-bending strain characteristic is shown.

Utilizing dynamic annealing during ion implantation: synthesis of silver nanoparticles in crystalline lithium niobate

Silver nanoparticles (NPs) embedded in lithium niobate were fabricated via ion beam synthesis and are suitable for various plasmonic applications, e.g. enhancement of optical nonlinear effects. After room temperature silver implantation, annealing in the temperature range of 400–600 °C was performed in order to recrystallize the damaged lithium niobate surface layer. The shape of the silver NPs, their optical properties as well as the structural properties of their surrounding matrix have been analyzed for various annealing steps. TEM investigations show that annealing at 400 °C does not lead to recrystallization of the damaged lithium niobate. A recrystallization occurs upon increasing the annealing temperature to 500 or 600 °C, but simultaneously a second phase consisting of lithium triniobate forms. This is additionally supported by XRD measurements. By utilizing dynamic annealing, i.e. implanting silver at elevated temperatures of 400 °C, it is shown that the LiNbO3 matrix stays single crystalline during ion implantation and no LiNb3O8 is formed. This is additionally verified by comparing the positions of the surface plasmon resonances with calculations based on Mie’s scattering theory.

Primary ion implantation and recoil implantation effects in Cs depth profiling of thin metallic layers on LiNbO3

Depth profiles obtained under Cs bombardment of samples of crystalline lithium niobate (LiNbO3) covered with a thin layer of metal display some artifacts related to metal/LiNbO3 interface effects. The altered yields of Li and Nb secondary ions in the first 50 nm after erosion of the metal layer can be interpreted in terms of different stopping powers between metal and LiNbO3. In this study, two typical samples are investigated and the artifacts are qualitatively explained by comparing experimental profiles and simulations performed with the transport of ions in matter code. The similarity between experiment and simulation suggests that the ballistic interpretation of the artifacts is appropriate. Copyright © 2012 John Wiley & Sons, Ltd.

A SIMS Study on Li Diffusion in Single Crystalline and Amorphous LiNbO<sub>3</sub>

We investigated lithium self-diffusion in amorphous and single crystalline lithium niobate at low temperatures of 323, 423 and 623 K. The diffusivity was studied by secondary ion mass spectrometry (SIMS), using ion beam sputtered 6LiNbO3 as a tracer source. Our intention was to get information how structural disorder influences ionic diffusivity, while chemical composition remains unchanged. The results indicate an increase of the Li diffusivity by about eight orders of magnitude in the amorphous state.