Lithium Niobate Substrate Research Articles

Polarization separation in titanium-diffused waveguides on lithium niobate substrates

The influence of technological parameters of the process of thermal diffusion of titanium in lithium niobate substrates on the polarization-dependent losses of obtained optical waveguide has been theoretically studied. It is established that the anisotropy of refractive index variation leads to different conditions of polarization eigenmode cutoff that can be used for separating extraordinary and quenching ordinary polarization modes. Experiments with titanium-diffused waveguides showed the possibility of extraordinary polarization mode separation above 40 dB in the C-range (1530–1565 nm) of telecommunication wavelengths.

Visualization of Surface Acoustic Waves in Thin Liquid Films.

We demonstrate that the propagation path of a surface acoustic wave (SAW), excited with an interdigitated transducer (IDT), can be visualized using a thin liquid film dispensed onto a lithium niobate (LiNbO3) substrate. The practical advantages of this visualization method are its rapid and simple implementation, with many potential applications including in characterising acoustic pumping within microfluidic channels. It also enables low-cost characterisation of IDT designs thereby allowing the determination of anisotropy and orientation of the piezoelectric substrate without the requirement for sophisticated and expensive equipment. Here, we show that the optical visibility of the sound path critically depends on the physical properties of the liquid film and identify heptane and methanol as most contrast rich solvents for visualization of SAW. We also provide a detailed theoretical description of this effect.

Development and experimental verification of a finite element method for accurate analysis of a surface acoustic wave device

Accurate analysis of surface acoustic wave (SAW) devices is highly important due to their use in ever-growing applications in electronics, telecommunication and chemical sensing. In this study, a novel approach for analyzing the SAW devices was developed based on a series of two-dimensional finite element method (FEM) simulations, which has been experimentally verified. It was found that the frequency response of the two SAW device structures, each having slightly different bandwidth and center lobe characteristics, can be successfully obtained utilizing the current density of the electrodes via FEM simulations. The two SAW structures were based on XY Lithium Niobate (LiNbO3) substrates and had two and four electrode finger pairs in both of their interdigital transducers, respectively. Later, SAW devices were fabricated in accordance with the simulated models and their measured frequency responses were found to correlate well with the obtained simulations results. The results indicated that better match between calculated and measured frequency response can be obtained when one of the input electrode finger pairs was set at zero volts and all the current density components were taken into account when calculating the frequency response of the simulated SAW device structures.

Low cost batch fabrication of microdevices using ultraviolet light-emitting diode photolithography technique

Solid-state technology has enabled the use of light-emitting diodes (LEDs) in lithography systems due to their low cost, low power requirement, and higher efficiency relative to the traditional mercury lamp. Uniform irradiance distribution is essential for photolithography to ensure the critical dimension (CD) of the feature fabricated. However, light illuminated from arrays of LEDs can have nonuniform irradiance distribution, which can be a problem when using LED arrays as a source to batch-fabricate multiple devices on a large wafer piece. In this study, the irradiance distribution of an UV LED array was analyzed, and the separation distance between light source and mask optimized to obtain maximum irradiance uniformity without the use of a complex lens. Further, employing a diffuser glass enhanced the fabrication process and the CD loss was minimized to an average of 300 nm. To assess the performance of the proposed technology, batch fabrication of surface acoustic wave devices on lithium niobate substrate was carried out, and all the devices exhibited identical insertion loss of −18 dB at a resonance frequency of 39.33 MHz. The proposed low-cost UV lithography setup can be adapted in academic laboratories for research and teaching on microdevices.

Spreading dynamics of a partially wetting water film atop a MHz substrate vibration

A MHz vibration, or an acoustic wave, propagating in a solid substrate may support the convective spreading of a liquid film. Previous studies uncovered this ability for fully wetting silicon oil films under the excitation of a MHz Rayleigh surface acoustic wave (SAW), propagating in a lithium niobate substrate. Partially wetting de-ionized water films, however, appeared immune to this spreading mechanism. Here, we use both theory and experiment to reconsider this situation and show partially wetting water films may spread under the influence of a propagating MHz vibration. We demonstrate distinct capillary and convective (vibrational/acoustic) spreading regimes that are governed by a balance between convective and capillary mechanisms, manifested in the non-dimensional number θ3/We, where θ is the three phase contact angle of the liquid with the solid substrate and We ≡ ρU2H/γ; ρ, γ, H, and U are the liquid density, liquid/vapour surface tension, characteristic film thickness, and the characteristic velocity amplitude of the propagating vibration on the solid surface, respectively. Our main finding is that the vibration will support a continuous spreading motion of the liquid film out of a large reservoir if the convective mechanism prevails (θ3/We < 1); otherwise (θ3/We > 1), the dynamics of the film is governed by the capillary mechanism.

Lithium Niobate Polarization Splitter Fabricated Using One-Step Diffusion

Polarization splitters with zinc–nickel and gallium indiffusion optical waveguides fabricated on Z-cut lithium niobate substrates are presented. The device fabrication process needs only one diffusion step and, therefore, is greatly simplified. Experimental results show that, at 1.55- $\mu \text{m}$ wavelength, the extinction ratios for the TE and TM modes are 27.8 and 24.5 dB, respectively.

Generation of higher-dimensional modal entanglement using a three-waveguide directional coupler

In this paper, we propose a method for the generation of higher-dimensional modal entanglement through a type II spontaneous parametric down-conversion process using a three-waveguide directional coupler in a periodically poled lithium niobate substrate. We show that by a proper design, it is possible to achieve an output state of two photons occupying three different spatial modes. The advantage of using such waveguide structure is its flexibility and the design space availability to achieve desired characteristics of the photon pairs generated in the down-conversion process.

Alignment nature of ZnO nanowires grown on polished and nanoscale etched lithium niobate surface through self-seeding thermal evaporation method

High aspect ratio catalyst-free ZnO nanowires were directly synthesized on lithium niobate substrate for the first time through thermal evaporation method without the use of a buffer layer or the conventional pre-deposited ZnO seed layer. As-grown ZnO nanowires exhibited a crisscross aligned growth pattern due to step bunching of the polished lithium niobate surface during the nanowire growth process. On the contrary, scratches on the surface and edges of the substrate produced well-aligned ZnO nanowires in these defect regions due to high surface roughness. Thus, the crisscross aligned nature of high aspect ratio nanowire growth on the lithium niobate surface can be changed to well-aligned growth through controlled etching of the surface, which is further verified through reactive-ion etching of lithium niobate. The investigations and discussion in the present work will provide novel pathway for self-seeded patterned growth of well-aligned ZnO nanowires on lithium niobate based micro devices.

A Novel Coupled Resonator Photonic Crystal Design in Lithium Niobate for Electrooptic Applications

High-aspect-ratio photonic crystal air-hole fabrication on bulk Lithium Niobate (LN) substrates is extremely difficult due to its inherent resistance to etching, resulting in conical structures and high insertion losses. Here, we propose a novel coupled resonator photonic crystal (CRPC) design, combining a coupled resonator approach with that of Bragg gratings. CRPC design parameters were optimized by analytical calculations and FDTD simulations. CRPC structures with optimized parameters were fabricated and electrooptically tested on bulk LN annealed proton exchange waveguides. Low insertion loss and large electrooptic effect were observed with the fabricated devices, making the CRPC design a promising structure for electrooptic device applications.

An optimisation of IDTs for surface acoustic wave sensor

Acoustic wave devices have been applied for detecting chemical and physical phenomena in gas and liquid applications. For the surface acoustic wave sensors (SAW sensors), the mechanical waves propagating through a piezoelectric crystal include Rayleigh and shear modes. They are influenced by the shape and size of the metal electrodes interdigital transducers (IDTs). Traditionally, IDTs were fabricated in the form of long straight parallel fingers on the piezoelectric substrate. This paper presents a piezoelectric device with focused IDTs (FIDTs) placed on a Y–Z lithium niobate (LiNbO3) substrate to achieve Rayleigh wave energy convergence. The energy conservation principle, called the Rayleigh principle, is used to prove the effective performance of IDTs with concentric shape. Amplitude fields of the conventional and concentric structures are shown. In addition, the energy loss and the output signal changes are compared between the two cases.

Numerical study of acoustophoretic motion of particles in a PDMS microchannel driven by surface acoustic waves.

We present a numerical study of the acoustophoretic motion of particles suspended in a liquid-filled PDMS microchannel on a lithium niobate substrate acoustically driven by surface acoustic waves. We employ a perturbation approach where the flow variables are divided into first- and second-order fields. We use impedance boundary conditions to model the PDMS microchannel walls and we model the acoustic actuation by a displacement function from the literature based on a numerical study of piezoelectric actuation. Consistent with the type of actuation, the obtained first-order field is a horizontal standing wave that travels vertically from the actuated wall towards the upper PDMS wall. This is in contrast to what is observed in bulk acoustic wave devices. The first-order fields drive the acoustic streaming, as well as the time-averaged acoustic radiation force acting on suspended particles. We analyze the motion of suspended particles driven by the acoustic streaming drag and the radiation force. We examine a range of particle diameters to demonstrate the transition from streaming-drag-dominated acoustophoresis to radiation-force-dominated acoustophoresis. Finally, as an application of our numerical model, we demonstrate the capability to tune the position of the vertical pressure node along the channel width by tuning the phase difference between two incoming surface acoustic waves.

Mid-infrared integrated waveguide modulators based on silicon-on-lithium-niobate photonics

Heterogeneous integration techniques, such as direct bonding, have enabled solutions to many problems facing integrated photonics. In particular, the relatively new field of mid-infrared (mid-IR) integrated photonics has been hindered by the availability of functional, transparent substrates in this wavelength range. The key to achieving compact, high-performance optical modulation and frequency conversion is the monolithic integration of silicon photonics with a material with high second-order nonlinear susceptibility. By transferring large areas of thin, monocrystalline silicon to bulk lithium niobate (LiNbO3) substrates, the first silicon-based platform to exploit the Pockels or linear electro-optic effect in the mid-IR range is achieved. Integrated Mach–Zehnder interferometer modulators with an extinction ratio of ∼8 dB, a half-wave voltage-length product of 26 V·cm, and an on-chip insertion loss of 3.3 dB are demonstrated at a wavelength of 3.39 μm. Ultrathin optical waveguides fabricated and characterized on this platform exhibit a low transverse electric mode linear propagation loss of 2.5 dB/cm. Future capabilities such as wideband difference frequency generation for integrated mid-IR sources are envisioned for the demonstrated silicon-on-lithium-niobate platform.

Modeling of Process Mechanisms in Pulsed Laser Micro Machining on Lithium Niobate Substrates

The mechanism behind the laser ablation of LN is investigated using near infrared pico-second-pulsed laser. A model of the mechanism is developed, deriving the mechanical, thermal, and photonic properties of LN in addition to doing preliminary experiments on laser ablation with controlled laser fluence. β is material removal using the nonthermal process via multiphoton ionization, γ is nonthermal material removal with chipping or cracking produced by generated heat (but at temperatures below the melting point), and δ is material removal using the thermal process with temperatures above the melting point, resulting in resolidification at the surface and the adhesion of oncemolten burrs around the processed area. In a process modes map constructed through exhaustive experiments on laser ablation under various irradiation conditions (at specific energy ρ and with number of pulse shotsN’), different contributions of ρ andN’ in the machining process are found. In terms of machining quality, desirable conditions in the control of laser irradiations are the use of weaker ρ and increasedN’ to keep thermal damage to a minimum and to raise the removal rate.

Impact of humidity on surface acoustic wave propagation in vanadium pentoxide xerogel–lithium niobate structure

The impact of ambient humidity on surface acoustic wave (SAW) propagation in the structure consisting of vanadium pentoxide xerogel (V2O5∙nH2O) layer deposited on a piezoelectric lithium niobate (LiNbO3) substrate has been studied. Thin V2O5∙nH2O layers were synthesized using sol–gel method and employed as humidity sensing materials. Freshly prepared layer drastically reduced the transmitted SAW signal, which later increased with time reaching saturation after more than 120 h at the level well below the free-surface value. The experimental observations of SAW behaviour were discussed and found related to the variations in resistance and dielectric permittivity of the V2O5∙nH2O.

Graphene and carbon black nano-composite polymer absorbers for a pyro-electric solar energy harvesting device based on LiNbO3 crystals

A novel scheme for solar energy harvesting based on the pyro-electric effect has been demonstrated. The proposed harvester is based on an optical system focusing solar radiation onto a ferroelectric crystal (i.e. lithium niobate). The face exposed to the heating source is coated with a nanocomposite material (i.e. carbon black and graphene particles) that greatly improves the adsorption of solar radiation. The solar energy focused onto the crystal through a simple optical system allows one to induce a thermal gradient able to generate electric charges. Experiments have been carried out indoor as well as outdoor (in Pozzuoli, Naples, Italy, on December). Results show that two configurations appear to be preferable: (a) pyro-electric element with carbon black-based coating and a Fresnel lens (surface of about 100cm2); (b) pyro-electric element with graphene-based coating and a Fresnel lens (surface of about 600cm2). In both experimental arrangements the maximum temperature variation reached locally onto the lithium niobate substrate is relatively high with peaks greater than 250°C. The maximum electrical power peak is of about 90μW and about 50μW for (a) and (b) respectively. The results of this first investigation are encouraging for further development of more efficient harvesting devices.

Poloidal flow and toroidal particle ring formation in a sessile drop driven by megahertz order vibration.

Poloidal flow is curiously formed in a microliter sessile water drop over 157-225 MHz because of acoustic streaming from three-dimensional standing Lamb waves in a lithium niobate substrate. The flow possesses radial symmetry with downwelling at the center and upwelling around the periphery of the drop. Outside this frequency range, the attenuation occurs over a length scale incompatible with the drop size and the poloidal flow vanishes. Remarkably, shear-induced migration was found to drive toroidal particle ring formation with diameters inversely proportional to the frequency of the acoustic irradiation.

Spectral broadening in femtosecond pulse written filamentary waveguides in periodically poled lithium niobate.

The authors report the filamentary waveguide formation and the significant spectral broadening based on periodically poled lithium niobate substrate. The modified morphology contributes to the combined effects of optical diffraction and self-focusing with the dependence on pulse intensity. Up to 4 times broadening of the FF wave and about 47 nm spanning of the SH wave with the pump power of 19.5 mW are achievable under 1550 nm excitation. Spectral evolution by cubic nonlinearity inside the waveguide has been obtained numerically, and provides a reasonable agreement with the experimental results.

An acoustically enhanced gold film Raman sensor on a lithium niobate substrate

The surface enhanced Raman scattering effect has shown immense potential for detecting trace amounts of explosive vapor molecules. To date, efforts to produce a commercially available, reliable SERS sensor have been impeded by an inability to separate the electromagnetic enhancement produced by the metallic nanostructure from other signal enhancing effects. Here, we show a new Raman sensor that uses surface acoustic waves (SAWs) to produce controllable surface structures on gold films deposited on LiNbO3 substrates that modulate the Raman signal of a target compound (thiophenol) adsorbed on the films. We demonstrate that this sensor can dynamically control the Raman signal simply by changing the SAW's amplitude, allowing the Raman signal enhancement factor to be directly measured with no variation in the concentration of the target compound. The physically adsorbed molecules can be removed from the sensor without physical cleaning or damage, making it possible to reuse it for real‐time Raman detection. Copyright © 2014 John Wiley & Sons, Ltd.

Terahertz wave generation by plasmonic-enhanced difference-frequency generation

We propose an efficient and compact plasmonic surface-enhanced terahertz generation scheme based on nonlinear difference-frequency generation inside a metal–insulator–metal structure. Gold nanowire arrays are planted on top of the surface of a lithium niobate (LN) substrate with second-order nonlinearity to enhance both the nonlinear wavelength conversion and waveguide terahertz waves at the same time. Our numerical simulations show that our structures are capable of generating both tunable continuous and ultrafast-pulsed terahertz sources. We also discuss further improvements on the conversion efficiency by combining with Ti-diffusing LN waveguides.

Scattering of surface acoustic waves by a system of topographical irregularities comparable to a wavelength

Scattering of surface acoustic waves (SAWs) in a reflective delay line, which is a Y+128°-cut lithium niobate substrate with aluminum interdigital transducers and reflecting structures located on its surface, is investigated by the finite-element method. The effects of scattering of SAW RF pulses by various surface irregularities are investigated in order to determine the topological parameters that minimize the loss of the SAW energy caused by the bulk-mode radiation into the substrate. The dependences of the energy coefficients of reflection and absorption of SAW RF pulses on the height and width of the reflecting irregularities whose dimensions are comparable to a wavelength have been determined.