Lithium Niobate Substrate Research Articles

Surface acoustic wave quasi-Bessel beams generated by symmetrically tilted interdigital transducers

Formation of surface acoustic wave (SAW) quasi-Bessel beams on a piezoelectric substrate through superposition of plane waves generated by interdigital transducers tilted symmetrically about the propagation axis is numerically and experimentally demonstrated. Acting as an axicon, the tilted transducers provide a facile way for quasi-Bessel beam generation. Finite-element method simulations reveal that non-diffracting Bessel beams, whose length and width are 193 and 1.38 wavelengths, respectively, can be obtained on a YX-128∘ lithium niobate substrate for an axicon angle of 15 degrees. The corresponding values for 20 degrees are 146 and 1.05 wavelengths, respectively. For a wavelength of approximately 300 micrometers, transmission spectra show that Bessel beam formation can be achieved at frequencies around 13.3 MHz. Bessel beam is visualized through a thin liquid film of methanol on the substrate. SAW Bessel beams can be utilized in acoustophoresis in microfluidic systems and sensing applications.

Opto-Microfluidic Integration of the Bradford Protein Assay in Lithium Niobate Lab-on-a-Chip

This paper deals with the quantification of proteins by implementing the Bradford protein assay method in a portable opto-microfluidic platform for protein concentrations lower than 1.4 mg/mL. Absorbance is measured by way of optical waveguides integrated to a cross-junction microfluidic circuit on a single lithium niobate substrate. A new protocol is proposed to perform the protein quantification based on the high correlation of the light absorbance at 595 nm, as commonly used in the Bradford method, with the one achieved at 633 nm with a cheap commercially available diode laser. This protocol demonstrates the possibility to quantify proteins by using nL volumes, 1000 times less than the standard technique such as paper-analytical devices. Moreover, it shows a limit of quantification of at least 0.12 mg/mL, which is four times lower than the last literature, as well as a better accuracy (98%). The protein quantification is obtained either by using one single microfluidic droplet as well by performing statistical analysis over ensembles of several thousands of droplets in less than 1 min. The proposed methodology presents the further advantage that the protein solutions can be reused for other investigations and the same pertains to the opto-microfluidic platform.

Band structure of love wave in a one-dimensional piezoelectric layered phononic crystal with imperfect interface

The propagation of Love wave in a phononic crystal consisting of superlattice lithium niobate substrate carrying a piezoelectric waveguide layer is analyzed. The interface between the waveguide layer and the substrate is imperfectly bonded, which is described by means of a shear spring model. Numerical results show that the bandgap shifts to a lower frequency range and the bandwidth decreases as the interface imperfection increases. Moreover, the transmission coefficient calculations using finite element simulation revealed that a perfect interface results in more effective bandgap attenuation.

Quantum noise source based on shot noise of a balanced photodetector with a tunable integrated optical beam splitter

A broadband quantum noise source based on detection of shot noise of a balanced photodetector is demonstrated. Precise electro-optical tuning of the balanced photodetector circuit was carried out by an integrated optical beam splitter constructed in the form of the dual output Mach-Zehnder interferometer on a lithium niobate substrate. The classical component of the detected noise related to the relative intensity noise of a laser diode was suppressed by more than 15 dB. At the maximum laser power of 100 mW, the power spectral density of detected shot noise was 12 dB higher than the level of technical noise of the measuring system in the frequency band above 3 GHz. Keywords: quantum random number generator, vacuum fluctuations, shot noise measurements, integrated optics, lithium niobate

Planarization of Lithium Niobate Surface Using a Thin Film Catalyst in Pure Water

A catalytically assisted etching method, named Catalyst-Referred Etching (CARE) was applied to the planarization of Lithium Niobate (LN) surface, which is widely used for optical waveguides, optical modulators, piezoelectric applications. The study demonstrates that an atomically smooth surface with less than 0.1 nm root-mean-square roughness could be achieved on a LN substrate using a thin metal film and pure water as the catalyst and etching solution, respectively. All residual stress and surface damage could be removed completely thanks to the removal mechanism of CARE.

Superconducting NbN thin films on various (X/Y/Z-cut) lithium niobate substrates

Lithium niobate (LN) exhibits outstanding properties in various application of photonics, electronics, and optoelectronics, showing potentials in integration. Due to the directional dependence of LN tensor properties, optical elements made up by LN favor the type of LN substrate. To introduce high-performance superconducting nanowire single-photon detectors (SNSPDs) to LN-integrated photonics chips, superconducting NbN thin films with thicknesses from 3 to 50 nm were deposited on X-cut, Y-cut, and Z-cut LN substrates using magnetron sputtering at room temperature. The different thickness dependencies of T c, δT c, and residual resistance ratios are observed in NbN thin films on different LN substrates. NbN thin films on X-cut and Y-cut LN substrates are polycrystalline with a transition temperature (T c) of ∼6 K for a 6 nm thick film. While NbN thin films are epitaxially textured on Z-cut LN substrates with T c of 11.5 K for a 6 nm thick film. NbN-SNSPD on X-cut LN substrates shows a weak saturation trend of its system detection efficiency; however, the performance of NbN-SNSPD on Z-cut LN substrates is limited. We evaluated the selection of cuts and concluded that X-cut and Y-cut LN are more suitable to be a platform of integrated LN photonic chips from the aspect of NbN-SNSPD. This study helps fabricate high-performance SNSPDs on fully integrated photonics chips on LN substrates.

Optimizing the Design of Surface-Acoustic-Wave Ring Resonator by Changing the Interdigitated Transducer Topology

Introduction. Previous works considered the frequency characteristics and methods for fixing sensitive elements in the form of a wave ring resonator on surface acoustic waves in a housing made of various materials, as well as the influence of external factors on sensitive elements. It was found that the passband in such a case is sufficiently wide, which can affect adversely signal detection when measuring acceleration using the sensitive element under development. Therefore, it has become relevant to reduce the sensitive element’s bandwidth by changing the design of the interdigitated transducer (IDT).Aim. To demonstrate an optimal topology for an IDT with a low bandwidth, leading to improved signal detection when acceleration affects the sensitive element.Materials and methods. The finite element method and mathematical processing in AutoCAD and in COMSOL Multiphysics.Results. Nine topologies of IDT are proposed. All these types were investigated using the COMSOL Multiphysics software on lithium niobate substrates, which material acts as a sensitive element. The frequency characteristics are presented. The data obtained allowed an optimal design of the ring resonator to be proposed: an IDT with rectangular pins without selective withdrawal.Conclusion. Self-generation in a ring resonator can be performed by withdrawing no more than one pair of IDTs for 10 or more periods. In this case, the withdrawal of IDTs should be uniform. With an increase in the number of IDT withdrawals, the geometry of the ring resonator is violated, and the wave leaves the structure. The presence of a shared bus keeps the surface acoustic wave inside the IDT structure, and the narrowing of the periods towards the inner part of the structure makes it possible to improve the frequency characteristics of the ring resonator on surface acoustic waves.

Design and Manufacture of Traveling-wave Electro-optic Modulator for X-band LFM Signal Generation

In this paper, a photonic-based microwave system technology is described, and a traveling-wave electro-optic modulator is designed and manufactured as a key component. The fabricated modulator is composed of a metal diffusion waveguide for optical transmission and a planar waveguide electrode on lithium niobate substrate for microwave transmission. The electro-optic response bandwidth of I and Q channels in a fabricated dual parallel Mach-Zehnder modulator were measured for 27.67 and 28.11 GHz, respectively. Photonic four times up-converted X-band frequency and linear frequency modulated signal were confirmed using the fabricated electro-optic modulator by S-band input signal. The confirmed broadband signal can be applied to a microwave system for surveillance and high-resolution ISAR imaging.

Surface Acoustic Wave Mitigation of Precipitate Deposition on a Solid Surface─An Active Self-Cleaning Strategy.

We demonstrate the application of a 20 MHz frequency surface acoustic wave (SAW) in a solid substrate to render its surface “self-cleaning”, redirecting the deposition of precipitating mass onto a nearby inert substrate. In our experiment, we confine a solution of poly(methyl methacrylate) polymer and a volatile toluene solvent between two substrates, lithium niobate and glass, at close proximity. We render the glass surface low energy by employing hydrophobic coating. In the absence of SAW excitation, we observe that the evaporation of the solvent yields polymer coating on the higher energy lithium niobate surface, while the glass surface is mostly devoid of polymer deposits. The application of a propagating SAW in the lithium niobate substrate mitigates the deposition of the polymer on its surface. As a response, we observe an increase in the deposition of the polymer precipitates on glass. Above a SAW power threshold, the polymer appears to deposit solely on glass, leaving the surface of the lithium niobate substrate devoid of polymer mass.

Design of Hybrid Waveguide Structures for High-Efficiency Integrated Optical Superconducting Single Photon Detectors On Ti:LiNbO3Waveguides

A configuration of integrated optical superconducting single photon detectors on lithium niobate substrates based on conventional titanium in-diffused waveguides with hybrid waveguide structures for enhancing the detector efficiency is proposed and analyzed. The sensing element in the form of a meander-like superconducting nanowire from niobium nitride is covered with an additional hybrid waveguide from a dielectric material with a higher refractive index than the LiNbO3 substrate to increase the light absorption. A special mode converter in the region free from a superconducting nanostructure is used to excite a localized mode with the maximum absorption coefficient that strongly interacts with the superconductive nanostructure. Silicon and titanium dioxide are considered and compared as suitable materials for the hybrid waveguide structures. Silicon based structures give a higher light concentration and, hence, a higher absorption coefficient. However, they are very demanding to the technological accuracy. Titanium dioxide structures potentially can be produced by the standard thin-film deposition technique and contact photolithography. The estimated absorption coefficient of 1.31 dB/μm is four orders of magnitude higher than that for the detectors with a standard titanium in-diffused waveguide, and a high transformation mode efficiency opens a way for fabrication of integrated optical superconducting single photon detectors with the efficiency comparable with other competing material platforms.

Spontaneous Polarization Reversal Induced by Proton Exchange in Z-Cut Lithium Niobate α-Phase Channel Waveguides.

The α-phase waveguides directly produced in one fabrication step only are well known for preserving both the excellent nonlinear properties and the ferroelectric domains orientation of lithium niobate substrates. However, by using the piezoresponse force microscopy (PFM), we present a coherent study on ferroelectric dipoles switching induced by the fabrication process of α-phase waveguides on Z-cut congruent lithium niobate (CLN) substrates. The obtained results show that the proton exchange process induces a spontaneous polarization reversal and a reduction in the piezoelectric coefficient d33. The quantitative assessments of the impact of proton exchange on the piezoelectric coefficient d33 have been quantified for different fabrication parameters. By coupling systematic PFM investigation and optical characterizations of α-phase protonated regions and virgin CLN on ±Z surfaces of the samples, we find a very good agreement between index contrast (optical investigation) and d33 reduction (PFM investigations). We clearly show that the increase in the in-diffused proton concentration (increase in index contrast) in protonated zones decreases the piezoelectric coefficient d33 values. Furthermore, having a high interest in nonlinear performances of photonics devices based on PPLN substrates, we have also investigated how deep the spontaneous polarization reversal induced by proton exchange takes place inside the α-phase channel waveguides.

Optical characterization of a neodymium-doped lithium-niobate-on-insulator (LNOI)

A neodymium (Nd) doped lithium-niobate-on-insulator (LNOI) was fabricated from a lithium niobate (LN) substrate diffusion-doped with Nd prior to the LNOI fabrication process. Diffusion doping enables the fabrication of differently doped regions on a single LNOI substrate, a key advantage for future applications in highly integrated photonics. Fluorescence spectra, emission cross-section, fluorescence lifetime, and small-signal gain of Nd:LNOI were determined for the first time to our knowledge and compared with data published for diffusion-doped ridge or channel waveguides in LN substrates. For an 11 mm-long sample, we achieve a small-signal gain for π (σ) polarization of about 14 (10) dB when pumping with a Ti:Sapphire laser, which is promising for future active components in Nd:LNOI like amplifiers and lasers.

Finite element simulation and characterization of one-port heterostructured surface acoustic wave resonator

The research work presents the study of a heterostructured surface acoustic wave (SAW) resonator consisting of an interdigital transducer (IDT), lithium niobate (LN), and silicon (Si) substrate. In the proposed structure, Silicon is the base substrate, above it layer of LN is placed then IDT is designed on it. The presented analysis includes the variation of the quality factor and electromechanical coupling coefficient with the change in the width of the piezoelectric layer. The electromechanical coupling coefficient (K 2), quality factor (Q-factor), velocity, resonance, admittance, and anti-resonance frequency have been studied for the thickness of the different layers of LN film. The higher quality factor of one port SAW devices reduces the energy loss and the K 2 of the devices indicates faster energy conversion. The Q-factor and K 2 are found to be depending on the thickness of LN, resonance, and anti-resonance frequency.

Utilization of n-dodecane as coupling layer for reusable acoustofluidic microchips

In this study, n-dodecane used as the coupling layer for reusable acoustofluidic microchips was investigated. n-dodecane has the similar viscosity to that of water. However, it possesses much lower surface tension and higher boiling point compared to water. When dispensing a droplet of n-dodecane on lithium niobate (LiNO3) substrate with interdigital transducers and placing the polydimethylsiloxane microchannel on top of it, n-dodecane can easily wick through and completely occupy the interstitial space. Moreover, it can be readily removed from the substrate without leaving residue. The experimental results showed that the reusable acoustofluidic microchips can be operated at higher input voltages and longer duration when applying n-dodecane as the coupling layer. Attenuation of the acoustic radiant force was observed through decrease of the particle velocity, which is in agreement with the literature results. Decreasing the thickness of the coupling layer can alleviate the attenuation effect and a linear relationship between particle velocity and thickness on a semi-log plot was obtained.

Tailorable Brillouin Light Scattering in a Lithium Niobate Waveguide

Stimulated Brillouin scattering (SBS) lasers based on silicon waveguides with large SBS gain have been widely used in frequency tunable laser emissions, mode-locked pulse lasers, low-noise oscillators, optical gyroscopes and other fields. However, among SBS lasers, the realization of Brillouin laser output often requires a longer waveguide length, which not only increases waveguide loss but also increase the size of the device. As a new medium, lithium niobate has been fabricated into a new type of hybrid structure. Meanwhile, the width of a suspended waveguide is adjusted to tune the phonon frequency of an SBS laser based on lithium niobate substrate. Simulation results show that the tunable forward SBS effect is realized in a lithium niobate-suspended optical waveguide, showing a larger forward stimulated Brillouin scattering gain of 0.31 W−1m−1. The tunable phonon frequency ranges from 1 to 15 GHz. Therefore, utilizing the photon–phonon conversion effect, the waveguide system with LiNbO3 will pave a new way forward with better integration.

Submicron Iron-Garnet Films on Lithium-Niobate Substrates Obtained by Ion-Beam Deposition

Submicron Iron-Garnet Films on Lithium-Niobate Substrates Obtained by Ion-Beam Deposition

Micro motion of a piezoelectric linear actuator driven by liquid interacting with Rayleigh surface acoustic wave

The research work presents a newly developed mechanism using liquid for surface acoustic wave (SAW) actuator making linear motion resulting in a no-load lightweight design. The proposed actuator consists of Interdigital Transducers (IDTs) fabricated on two ends of lithium niobate (LiNbO3) substrate and a cylindrical slider placed over the surface of the substrate in contact with liquid. The actuator uses Rayleigh wave traveling on the surface of the piezoelectric stators that transmit into a liquid medium which drives a cylindrical slider in translational motion. As the slider is not placed directly in contact with the stator, so the damage of piezo substrate or slider due to direct frictional contact is avoided. Due to this advantage, the longevity of the device will be enhanced and the driving load becomes easier. The movement of the slider on the liquid surface can be made to move in reverse direction by energizing the IDTs fabricated on other side of the substrate. The finite element simulation of the proposed SAW actuator and the experimental results demonstrates the slider reaches a steady-state velocity of 37.5 mm/s for application of wave excitation.

Sub- and supersonic elastic waves in an annular hole phononic metamaterial

Surface acoustic wave (SAW) devices are used in a wide range of applications including sensing and microfluidics, and are now being developed for applications such as quantum computing. As with photonics, and other electromagnetic radiation, metamaterials offer an exciting route to control and manipulate SAW propagation, which could lead to new device concepts and paradigms. In this work we demonstrate that a phononic metamaterial comprising an array of annular hole resonators can be used to realise frequency control of SAW velocity. We show, using simulations and experiment, that metamaterial patterning on a lithium niobate substrate allows control of SAW phase velocities to values slower and faster than the velocity in an unpatterned substrate; namely, to ~85% and ~130% of the unpatterned SAW velocity, respectively. This approach could lead to novel designs for SAW devices, such as delay lines and chirp filters, but could also be applied to other elastic waves.

Acoustofluidic Separation of Proteins Using Aptamer-Functionalized Microparticles.

We propose an acoustofluidic method for the triseparation of proteins conjugated with aptamer-coated microparticles inside a microchannel. Traveling surface acoustic waves (TSAWs) produced from a slanted-finger interdigital transducer (SFIT) are used to separate the protein-loaded microparticles of different sizes via the TSAW-driven acoustic radiation force (ARF). The acoustofluidic device consists of an SFIT deposited onto a piezoelectric lithium niobate substrate and a polydimethylsiloxane (PDMS) microfluidic channel on top of the substrate. The TSAWs propagating on the substrate penetrate into the sample fluid flow, where the human protein-conjugated microparticles are suspended, inside the PDMS microchannel. The microparticles are subjected to the TSAW-driven ARF with varying magnitude depending on their size and thus flow along different streamlines, leading to triseparation of the proteins. In this work, we used two different-sized streptavidin-functionalized polystyrene (PS) microparticles to capture two kinds of aptamers (apt15 and aptD17.4), which were labeled with a respective biotin molecule at one end. The biotin ends of the aptamers were attached to the microparticles through streptavidin-biotin linkage, whereas the free ends of the aptamers were used to capture their target proteins of thrombin (th) and immunoglobulin E (IgE). The resultant PS-apt15-th and PS-aptD17.4-IgE complexes, as well as mCardinal2, were used for experimental demonstration of acoustofluidic triseparation of the human proteins. We achieved simultaneous separation of proteins of three kinds (th, IgE, and mCardinal2) for the first time via the TSAW-driven ARF in the proposed acoustofluidic device.

Application of surface acoustic wave to enhance the photocatalytic degradation efficiency of methylene blue

The TiO2 thin film was grown on a [Formula: see text] Y-axis cut, X-axis propagation lithium niobate substrate by radio frequency magnetron sputtering to fabricate a surface acoustic wave device. The sputtering ratios of argon to oxygen at 20% and post-annealing temperature at [Formula: see text]C for 3 h were controlled to improve the photocatalytic activity of TiO2 thin film. The effects of work frequency and input voltage of the surface acoustic wave device on the methylene blue photocatalytic degradation efficiency of the TiO2 thin film were investigated. The TiO2 thin film was vibrated at radio frequency by surface acoustic wave device to increase the scattering and reactive frequency for methylene blue in liquid. The surface acoustic wave device operated at 28.65 MHz work frequency and 0.95 V input voltage revealed the highest photocatalytic degradation efficiency of methylene blue. Compared to the TiO2 thin film combined with and without surface acoustic wave device, the surface acoustic wave device enhanced the TiO2 thin film photocatalytic degradation efficiency to 46%.