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Thin film lithium niobate on insulator (LNOI), as an emerging photonic material, provides a promising platform to realize compact, high-speed, and high-efficient electro-optic (EO) modulators. However, due to the large coupling loss caused by the large mode field mismatch between standard single mode fibers and the LNOI waveguides, edge or grating couplers are indispensable to LNOI modulators. In this paper, we propose a novel edge coupler employing staircase structure. The proposed coupler is feasible with i-line lithography and avoids high precise alignment. Moreover, such coupler can be applied to wafer-scale manufacture with low cost. The simulation results show that a low input/output coupling loss of 1.14 dB/1.62 dB can be achieved at 1.55 m wavelength for transverse electric (TE) polarized light signals. Specifically, benefited from the novel staircase design, the proposed edge coupler shows a unique property of polarization extinction ratio larger than 10 dB, which will further improve the modulation efficiency and depth of the EO modulators based on X-cut LNOI.

Chip-scale electro-optic (EO) frequency combs are expected to play an essential role in future high-capacity optical communications systems and next-generation mobile communications. The application requires integrated EO frequency comb generators featuring good spectral flatness, high modulation bandwidth, low driving voltage and low insertion loss simultaneously. Here, we demonstrate a flat-top optical flat comb (OFC) generator based on high-performance lithium niobate on insulator modulators. The OFC generator shows a low on-chip loss (2.1 dB), a low driving voltage over a broad frequency range, and a large 3-dB EO bandwidth. Moreover, with consuming power of less than 2W, the presented device produces 13 lines with a power variation of less than 1.2 dB and a line spacing of 31 GHz which can further be extended to 67 GHz.

The upconversion detection is a promising method to detect infrared radiation. It is proved that the focused beam makes a contribution to improving the upconversion efficiency. A near-infrared detector at room-temperature based on sum frequency generation(SFG) with focused beams is demonstrated. To enhance the upconversion efficiency, the signal and pump beams are tightly focused by the same focal lens. To reduce that the dispersion of the focal lens has a negative effect on upconversion efficiency in a tightly focusing system, the relation between the upconversion efficiency and the focused beams in our system is investigated. In the setup, the signal and pump beams are combined by a polarization maintaining wavelength division multiplexer (PMWDM) that is connected on a collimator. The mixed beam is focused into a periodically poled lithium niobate (PPLN) bulk by a lens of 35mm focal length. The signal pulse at 1550nm is converted to 863nm with a 172mW continuous-wave pump beam at 1950nm. The converted signal is measured by a photomultiplier tube(PMT) and results shows the maximum upconversion efficiency is 5.23×10-4 while the pump power is 172mW.

A fully-functional photonic integrated circuit (PIC) platform with supporting active and passive components in the green part of the visible spectral regime is of significant research interest for next-generation optical systems. Here we design highly-integrated ∼3.5 mm long PICs at green wavelengths, which consist of on-chip GaAs-based near-infrared pump lasers, SiN-LiNbO3 hybrid waveguides and ring resonators for nonlinear frequency conversion. The waveguides in the PICs are designed to eliminate etching of LiNbO3, reducing fabrication challenges. Efficient wavelength of 1062 nm pump to 531 nm coherent light is achieved by employing modal phase matching. Unlike a quasi-phase matching technique, modal phase matching enables poling-free operation and further eases device fabrication with comparable performance in terms of second harmonic generation efficiency. The effective nonlinear mode-overlap factor between 1062-nm-TE00 and 531-nm-TE01 modes in the hybrid waveguide is calculated to be 26%. For robust on-chip light coupling between pump laser and waveguide, a calculated maximum coupling efficiency of −2.3 dB is achieved. The theoretical work presented is an initial step towards demonstrating complex non-telecom PICs which could offer a comprehensive range of photonic functionalities.

This article presents a thermal control design method for a surface acoustic wave (SAW) device. We designed a heat-dissipation structure and packaging scheme to solve three key issues observed in SAW devices using anisotropic crystals as piezoelectric substrates in acoustofluidics (e.g., lithium niobate): SAW chip cracking caused by thermal stress, SAW chip cracking caused by mismatched thermal expansion coefficients of the packaging materials, and enhancement of the structural strength and stability of the SAW chip. This study establishes the physical model of the designed structure and the relationship between the steady-state working temperature and the physical properties of the material. By comparing these physical properties and numerical calculations, we identified nanosilver adhesive as the most effective bonding material between the SAW chip and the heat sink. In addition to designing and fabricating, we also evaluated our SAW devices experimentally. The results not only confirmed that the abovementioned three key problems were solved but also demonstrated the significant enhancement of the stability of the SAW device.

We propose and experimentally demonstrate a high-performance polarization splitter-rotator based on the lithium niobate-on-insulator platform, consisting of an adiabatic taper, an asymmetric directional coupler, and a multimode interference mode splitter. Two injecting modes with orthogonal polarization states were separated, and the transverse magnetic (TM) polarization state was rotated to the transverse electric (TE) polarization state simultaneously. The designed device is fabricated in a single-step etching process, providing an extinction ratio of 20 dB over 80 nm wavelength range and insertion loss of less than 0.65 dB for both TE0 and TM0 modes launched. Lithium niobate-on-insulator, polarization splitter-rotator, extinction ratio.

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.

We propose and demonstrate a Mach-Zehnder modulator in Z-cut lithium niobate thin film (LNTF) with a vertical electric field structure. By placing the metal electrodes on top and bottom of the waveguide rather than the usual lateral configuration, the electric field is fully overlapping the optical field. Such a configuration reduces the critical requirement of electrode alignment as needed in X/Y-cut LN based devices. Both the simulation and process details to realize the proposed device are demonstrated. The measured static performance accords well with the simulations. Additionally, we developed a new method to accurately characterize the dynamic performance of the Z-cut LNTF modulator, and the measured tuning efficiency is around 8.84 pm/V. Our proposed device validates the feasibility of integrated Z-cut LNTF based modulators, and will likely extend the research area of integrated lithium niobate photonics.

This letter reports on the passive voltage amplification provided by X-cut Lithium Niobate (LN) S0 mode resonators when implemented as Matching Networks (MN) in the front-end of ultra-low-power, Internet of Things (IoT) receivers. In the first section, a simple gain analytical model is described. In the second part, devices design rules and geometries are disclosed. In the third and last section, measurements methodology and results are presented. Measured gains of 38.9 dB and 32.5 dB are reported at 110 and 570 MHz, respectively, for a capacitive load of ~100 fF. These performance represent, to the knowledge of the authors, the largest passive voltage gains ever recorded at the given frequencies for MEMS-based matching networks.

Lithium niobate LiNbO3 (LN) on insulator wafers (LNOI) representing a submicron-thick LN film bonded to a SiO2 layer deposited on a thick LN substrate have been manufactured recently by ion slicing. Today, LNOI is one of the most promising materials for fabrication of various integrated optical devices, including waveguides, electro-optical modulators, and wavelength convertors, due to their high drop in refractive index at the interface between the film and the SiO2 layer. Creation of the stable domain structures with submicron periods in LNOI will improve the properties of nonlinear optical devices. Periodically poled LN with periods below 300 nm can be used to implement an optical parametric oscillator in the backward configuration. We have studied experimentally the ways to create the stripe domains and periodical domain structures in LNOI with a dielectric layer under the film by local switching. The identified scenarios of the domain evolution were attributed to the ineffective screening of depolarization field. We have shown that stripe domains and periodic domain structures can be produced by scanning with a biased tip only at temperatures above 80 °C. Periodical stable domain structures with periods down to 300 nm have been created.

In this paper, a systematic study of the relationship between nonlinear crystal length and intracavity second-harmonic generation (SHG) using MgO-doped periodically-poled lithium niobate (MgO:PPLN) is presented. The experimental results demonstrate a relationship between the maximum SHG power generated and the full-width at half maximum (FWHM) of the crystal’s temperature tuning curve to the length of the nonlinear optical crystal. It was shown that maximum SHG power increases rapidly with the increase of MgO:PPLN length, reaching a saturation length (~2 mm), which is much shorter than that predicted by the single-pass SHG theory. This saturation length of the MgO:PPLN crystal is almost independent on 808 nm pump power for typical powers used in continuous wave intracavity SHG lasers. In addition to this saturation effect, a broadening effect was also observed, the FWHM of the temperature tuning curve was shown to have a larger FWHM than that predicted by the single-pass SHG theory for MgO:PPLN shorter than the saturation length. This work has the benefit of allowing engineers to optimize nonlinear crystal length when developing intracavity SHG based diode-pumped solid state (DPSS) lasers.

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...

Optical isolators today are exclusively built on magneto-optic principles but are not readily implemented within photonic integrated circuits. So far, no magnetless alternative1–22 has managed to simultaneously combine linearity (that is, no frequency shift), linear response (that is, input–output scaling), ultralow insertion loss and large directional contrast on-chip. Here we demonstrate an electrically driven optical isolator design that leverages the unbeatable transparency of a short, high-quality dielectric waveguide, with the strong attenuation from a critically coupled absorber. Our concept is implemented using a lithium niobate racetrack resonator in which phonon-mediated13 photonic Autler–Townes splitting10,16,23,24 breaks the chiral symmetry of the resonant modes. We demonstrate isolators at wavelengths one octave apart near 1,550 nm and 780 nm, fabricated from the same lithium-niobate-on-insulator wafer. Linear isolation is demonstrated with simultaneously 39 dB contrast and 10 dB bandwidth up to ~200 MHz. Non-magnetic optical isolators are demonstrated using phonon-mediated photonic Autler–Townes splitting. The on-chip lithium niobate devices simultaneously achieve ultralow insertion loss and high contrast.

In this letter, we experimentally demonstrate the first slow-light Mach-Zehnder modulator (MZM) on a hybrid ithin-film Lithium Niobate platform. The Bragg grating waveguides are served as phase shifters, which enhance the modulation efficiency with the slow-light effect. Compared with large-signal modulation efficiency ( $\text{V}_\pi \cdot \text {L} =0.67\,\,\text{V}\cdot $ cm) and an ultra-compact footprint (0.3 mm $\times1.2$ mm). Data transmission up to 60 Gbps and an on-chip insertion loss of 1.9 dB are also achieved. This modulator provides a potential solution for high-intensity integration, low-power transmission, and optical biosensing.

Lithium niobate integrated photonics has recently received significant attention because it exploits the attractive properties of lithium niobate on an integrated platform which provides strong optical confinement as well as high photonic integration density. Although many optical functionalities of lithium niobate have been demonstrated on a chip in the telecom band, the visible and near-visible regimes are less explored. This is mainly because devices with a relatively smaller feature size are required which increases fabrication difficulty. Here, we explored the acousto-optic effect of lithium niobate on a chip at near-visible wavelengths (765–781 nm) and demonstrated acousto-optic modulation with the modulation frequency up to 2.44 GHz. We adopted an etchless process for the device fabrication and applied the principle of bound states in the continuum to optimize the device performance. By demonstrating functionality at near-visible wavelengths, our devices will enable many on-chip applications ranging from frequency metrology to quantum information processing.

Lithium niobate (LN), as a nonlinear material with a large nonlinear susceptibility, has been widely employed in second harmonic generation (SHG) up to ultraviolet (UV) frequency range due to its broad low-absorption window. In nanophotonics, it is possible to harness the Mie resonances associated with the single dielectric particles to boost the nonlinear light–matter interactions. Here, we fabricate single Mie-resonant LN nanospheres on a SiO2 substrate via the femtosecond (fs) laser ablation technique. By exploiting the magnetic dipole (MD) Mie resonance, UV SHG from the LN nanosphere is significantly enhanced with a measured conversion efficiency of 4.45 × 10−8 under the excitation of an fs laser at 750 nm. The single LN nanospheres achieved in this work could serve as Mie resonators for building nonlinear nanophotonic devices such as frequency converters and quantum light sources, etc.

We demonstrate that terahertz radiation can be generated from titanium-diffused magnesium oxide-doped lithium niobate optical strip waveguides when pumped by a single laser source with a wavelength of 1.55 μm. Titanium-in-diffusion was performed on a magnesium oxide-doped lithium niobate substrate by the deposition of titanium in a 40-µm wide strip pattern and then annealing in a vacuum furnace. Prism-coupled Cherenkov-phase matching was utilized to extract the terahertz emission from the waveguide. The contrast between the emission from the waveguide against the bulk crystal when the sample was moved along its crystal facet was also recorded, where an improvement of ~ 18.0% in the time-domain peak signal of the waveguide was observed. The waveguide’s performance was also measured against that of a commercial terahertz emitter, a photoconductive antenna, where it was found that the time-domain signals of the two were comparable. The terahertz power spectra also revealed that the lithium niobate-based waveguides have a broader bandwidth by more than 1 THz, with a similar signal-to-noise ratio. As an effect, the waveguides gave a better signal at higher frequencies and, at times, reaching as high a difference of 20 dB. These results indicate the possibility of using titanium-in-diffusion to generate intense terahertz emission from titanium-doped lithium niobate waveguides and that these strip waveguides are viable alternatives to commercial terahertz emitters, especially at high frequencies. It is expected that much better output can be obtained from an optimized waveguide design and with more appropriate optics.

This work reports an experimental study of acoustic loss in thin-film lithium niobate (LiNbO3) using acoustic delay lines (ADLs). Unlike prior resonator-based quality factor ( $Q$ ) studies, this approach directly extracts the damping in thin-film LiNbO3, avoiding the influence of other intricate loss mechanisms, e.g., anchor loss and electrode-induced loss. Acoustic attenuation of fundamental symmetric (S0) and shear horizontal (SH0) waves are studied in suspended LiNbO3 thin films of different thicknesses. The attenuation is significantly higher in thinner LiNbO3 films, suggesting the LiNbO3 crystal degradation during the microfabrication as the primary loss origin. Nevertheless, the extracted equivalent $Q$ in thin-film LiNbO3 is still higher than reported values, suggesting that anchor design and electrode quality remain the bottlenecks for higher $Q$ . The proposed loss extraction framework is readily extendable to other acoustic thin-film structures. [2021-0107]

In this article, near-zero drift and high electromechanical coupling acoustic resonators have been designed and demonstrated. The acoustic resonator is based on Lamb acoustic waves in a bimorph composed of lithium niobate on silicon dioxide. Our approach breaks through a performance boundary in conventional Lamb-wave resonators by introducing the bimorph while operating at higher order resonant modes. This enables the resonator to achieve frequency scalability, a low-temperature coefficient of frequency, and high electromechanical coupling altogether. The electromechanical coupling and temperature coefficient of the resonator were analytically optimized for the A3 mode through adjusting the thicknesses of different materials in the bimorph. Resonators with different dimensions and stack thickness were fabricated and measured, resulting in a temperature coefficient of frequency ranging from −17.6 to −1.1 ppm/°C, high electromechanical coupling ranging from 13.4% to 18%, and quality factors up to 800 at 3.5 GHz. The achieved specifications are adequate for fifth-generation (5G) sub-6-GHz frequency bands n77 and n78.

Lithium niobate, dueto its strong electro-optic effect, is an excellent material for high-performance optical modulators. Hybrid integration of thin film lithium niobate and silicon photonic circuits makes it possible to fully exploit potentials of the two material systems. In this paper, we introduce a detailed design procedure for silicon and lithium niobate hybrid integrated modulator using coplanar line electrodes based on Mach-Zehnder interferometer push-pull configuration. A multiphysics model for the crossing section of the modulation section is proposed and analyzed. The results show that optimizing solely the $V_{\pi } L$ product would not lead to the best 3-dB bandwidth for a certain half-wave voltage due to the increased microwave losses. There exists an optimal ground-signal electrode gap value, which is about 8–9${\,\mu m}$ for the present modulator structure. For these optimized structures, 3-dB bandwidths can reach 45 GHz and 137 GHz with half-wave voltages of 2 V and 4 V, respectively, for a lithium niobate waveguide total thickness of 600 nm and a ridge height of 200 nm.

This article develops a novel theoretical and modeling approach for a surface acoustic wave (SAW) acoustoelectric amplifier applicable to thin-film overlay resistive or semiconductor material that is within or outside the acoustic path. The theory is developed for a direct-coupled SAW and a thin-film resistive interaction layer. The analysis uses the combination of coupling of modes and charge control analysis to develop a complete model describing the small-signal operation and the large-signal saturation effect. The model is developed for the case of direct coupling of thin films in the propagation path or coupling electrodes either inside or outside the propagation path. The approach describes the phenomenological physics and amplifier's key parameters. Predictions are compared to previously published experimental results on lithium niobate using a monolayer graphene film that yielded continuous-wave operation and a net terminal gain.

In this work, we present the first group of gigahertz low-loss, wideband, and high power handling delay lines (ADLs) using a thin-film lithium niobate (LiNbO3)-on-sapphire platform. The ADLs leverage a single-phase unidirectional transducer (SPUDT) to efficiently excite the shear horizontal surface acoustic wave (SH-SAW) in the film stack. The fabricated miniature SH-SAW ADL at 1.1 GHz shows a low insertion loss (IL) of 2.8 dB, a wide fractional bandwidth (FBW) of 6.14%, and a fast phase velocity of 5127 m/s. The device also features a high 1-dB compression point (P1dB) of 30.4 dBm. The temperature coefficient of frequency is −45 ppm/K. ADLs with delays between 12 and 172 ns have been implemented, with IL between 2.8 and 8.3 dB. SH-SAW propagation characteristics are extracted, showing a group velocity of 4747 m/s and a propagation loss of 6.73 dB/mm or 31.9 dB/ $\mu \text{s}$ . The simultaneous low-loss and high power handling illustrate the great potential of LiNbO3-on-sapphire for RF and cross domain applications at gigahertz.

This letter presents a new class of High-overtone Bulk Acoustic Resonators (HBARs) with only top electrodes but excellent performance using lithium niobate (LN) thin films on conductive silicon carbide (SiC). The 4-inch single-crystalline lithium niobate thin film on conductive SiC (LNCSiC) substrate was prepared by ion-cutting process. Since the conductive SiC serves as both the resonant cavity and the floating electrode, the HBARs based on LNCSiC with only top electrodes were designed and fabricated. The devices show a wide frequency response span exceeding 2.0 GHz, a frequency spacing of 10 MHz, an excellent f $\cdot $ Q $_{\textit {LAKIN}}$ product of ${9.6}\times {10} ^{{13}}$ Hz ( f $\cdot $ Bode-Q of ${1.1}\times {10} ^{{14}}$ Hz), and a low temperature coefficient of frequency (TCF) of −18.9 ppm/°C. The frequency response shift of the proposed HBAR can be achieved by just adjusting the in-plane orientation of the top electrodes. Additionally, a multi-band filter was numerically synthesized based on the proposed HBARs, showing a spectral spacing of 10 MHz and a passband span of more than 2.0 GHz. Upon further optimizations, the proposed HBARs with a wide frequency response, a high- f $\cdot $ Q , and a low-TCF will be very promising for radio-frequency applications.

For the first time, yttrium-iron-garnet films with a thickness of 0.5 µm on nonoriented single-crystal lithium niobate LiNbO3 substrates are obtained by the ion-beam sputtering–deposition method. The surface profile and composition of the samples obtained are studied by atomic force microscopy, scanning electron microscopy, and X-ray phase analysis. Based on the data of optical and magnetic measurements, as well as the results of studies of spin-wave propagation in ferrite-garnet films on LiNbO3 substrates, we conclude that the reproducible formation of high-quality structures by nonepitaxial methods is possible for obtaining optically transparent straintronic materials, as well as the development of microwave electronic materials.

This paper reports a process for the growth of zinc–erbium codoped lithium niobate single crystals using a growth charge synthesized from a Nb2O5:Zn:Er precursor and Li2CO3. Characterization of the LiNbO3:Zn:Er crystals by high-speed evaluation of the number of scattering centers, piezoacoustic measurements, photoinduced light scattering, and laser conoscopy has demonstrated that they have high optical quality. The proposed technological approaches are aimed at designing functional materials with fundamentally new characteristics for electronics, acoustoelectronics, and integrated, quantum, and laser optics.

We propose and demonstrate a type of highly efficient tunable Mach-Zehnder interferometer (MZI) in Z-cut lithium niobate thin film (TFLN) based on the thermo-optical effect. By placing the electrode directly above the TFLN waveguide, a more uniform and higher heat distribution was obtained, compared with the conventional horizontal structure. Both the principle and design strategies of the proposed scheme are described. The process flow of the device fabrication is demonstrated in Z-cut TFLN. The measured static results accord well with the simulations. About 0.121 nm/V tuning efficiency is observed, which is nearly double the tuning efficiency based on the electro-optic effect, and may prove useful when high switching speed is not the primary consideration. The presented thermo-optic tuning scheme can be extended to other scenarios easily and may satisfy many different application requirements in integrated lithium niobate photonics (LNPs).

High requirements on the electrical parameters of the filters are made in wireless radio communication systems. A number of tasks require bandpass filters with relative bandwidths of 8-12 %. In this case, the filter must have insertion losses of not more than 10 dB, have a rejection is not worse than 40 dB, unevenness in the bandwidth of not more than 1 dB. In addition, the amplitude frequency responses of the filter must have steep slopes due to the closely spaced frequency bands of neighboring communication systems. Due to its small size and other advantages, ladder resonator filters on surface acoustic waves are widely used in communication systems. To realize the high requirements of this type of filters, it is necessary not only to select all the topology parameters of the individual resonators included in the filter very accurately, but also to have a good computational theory and the necessary material parameters for the selected model at the design stage. Purpose: to show on the example of comparison of calculated and experimental frequency responses of ladder filters the validity of the method of extraction of the necessary parameters obtained for an infinite periodic structure by the finite element method for calculating of finite structures of real inter digital transducer and reflective gratings. Results: the method of extraction of parameters necessary for calculation by the method of connected modes on the basis of P-matrices is offered. The technique is based on the analysis of infinite periodic electrodes by the finite element method. Bandpass filters with a relative bandwidth 8-12 % on a piezosubstrate 49° YX LiNbO3 were designed and manufactured based on the proposed theory. It is shown that for this material, the calculation must take into account the direct radiation of bulk acoustic waves, since the design of ladder type filters, the radiation falls into the projected bandwidth of the filters. These calculations are confirmed by the results of experiments.

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. Surface acoustic waves are important in a wide range of applications such as telecommunication filters, sensors, and microfluidic devices. Here, patterning of a phononic metamaterial formed of annular hole resonators enables frequency control of the surface wave phase velocities.

A piezoelectric disk gyroscope fabricated with a single-crystal lithium niobate (LN) wafer has been developed for inertial navigation systems for localization during automated driving. LN has a high electromechanical coupling coefficient and a resonator made of a low-cost single-crystal wafer shows a high quality factor, which is suitable for high-performance vibratory gyroscopes. In this study, its crystallographic anisotropies in both in elastic and piezoelectric properties were considered to realize mode matching in the two-wine-glass-mode vibration of a disk resonator. We chose a 155°-Y-cut LN wafer because we found that its elastic compliance is uniform in any in-plane direction. Its anisotropic piezoelectric properties were considered to optimize the electrode design to oscillate the wine-glass modes efficiently. A 25.8-mm-diameter, 330- $\mu \text{m}$ -thick disk resonator with 16 electrodes on each side was designed and fabricated by photolithography and grinding. The as-fabricated LN disk resonator was successfully oscillated in two wine-glass modes with a small frequency split of 0.7% at 95 kHz resonant frequency. Gyroscope operation was successful and the scale factor was $0.35~\mu \text{V}$ /°/s without any preamplifier. [2021-0022]

Optical metasurfaces have been extensively investigated, demonstrating diverse and multiple functionalities with complete control over the transmitted and reflected fields. Most optical metasurfaces are however static, with only a few configurations offering (rather limited) electrical control, thereby jeopardizing their application prospects in emerging flat optics technologies. Here, we suggest an approach to realize electrically tunable optical metasurfaces, demonstrating dynamic Fresnel lens focusing. The active Fresnel lens (AFL) exploits the electro-optic Pockels effect in a 300-nm-thick lithium niobate layer sandwiched between a continuous thick and nanostructured gold film serving as electrodes. We fabricate and characterize the AFL, focusing 800-900 nm radiation at the distance of 40 $\mathrm{\mu}$m with the focusing efficiency of 15 % and demonstrating the modulation depth of 1.5 % with the driving voltage of $\pm 10$ V within the bandwidth of $\sim\! 4$ MHz. We believe that the electro-optic metasurface concept introduced is useful for designing dynamic flat optics components., Comment: 9 pages, 4 figures

We propose and demonstrate a type of highly efficient thermally tuned interferometer in lithium niobate on insulator (LNOI) thin film using an air bridge structure. By removing the oxide below the lithium niobate (LN) waveguide, the heat transferred to the substrate is greatly reduced. Thus, more heat is confined inside the LN waveguide and a higher tuning efficiency is realized. Additionally, by directly placing the thermal electrode above the LN waveguide, a more uniform and stronger thermal distribution is realized compared with a horizontal structure. Both the design and process details of the proposed device are demonstrated. The improvement of the proposed new structure is about 4.9-fold compared with a typical structure without the air bridge modification according to our measured results. The proposed structure can be extended to other application scenarios easily and will likely boost the development of integrated lithium niobate photonics.

This work presents the development of a novel process flow for the fabrication of Lithium niobate Lamb wave resonator that allows full control of the shape and size of the released area (undercut) necessary for stabilizing quality factor. We then investigate the influence of the inactive regions of the resonator (i.e. undercut, anchor, bus and gap) on $Q$ ; and determine the optimum dimensions for those regions in order to limit the flow of energy escaping from the resonator’s body resulting in overall improvement in $Q$ . We report devices with electromechanical coupling ( $k_{t}^{2}$ ) of 41% and quality factor ( $Q$ ) of 1900 at around 290 MHz resulting in the highest-ever achieved Figure-of-Merit (FoM) of 780 for SH0 mode resonators in X-cut LN. [2020-0363]

The creation of the stripe domain structures with submicron periods by local switching using a biased tip of a scanning probe microscope was studied on Z-cut of lithium niobate on insulator (LNOI)....

All-dielectric optical metasurfaces are a workhorse in nano-optics, because of both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair production is enhanced up to 2 orders of magnitude, compared to an unpatterned film of the same thickness and material. These results enable flat-optics sources of entangled photons - a new promising platform for quantum optics experiments.

In the present work, the contribution of Helium implantation fluence on the Crystal-Ion-Slicing fabrication (CIS) of Y-cut LiNbO3 film were systematically investigated. Y-cut Lithium niobate (LN) thin films with a submicron thickness on the Y-cut LN substrates were successfully fabricated by CIS technique by employing He implantation. By varying the implantation fluence, the structure evolution of the LN exfoliation layer was systematically investigated by Rutherford backscattering/channeling (RBS/C), X-ray Diffraction (XRD), Raman spectroscopy, as well as Atomic Force Microscopy (AFM). Upon gradually increasing the implantation fluence from 1.0 x 1016 to 4.0 x 1016 cm-2, the lattice damage together with the tensile strain in the as-implanted LN becomes intensive according to the Raman spectroscopy investigations. In addition to the crystalline quality, the root mean square roughness becomes 1.4 nm upon increasing fluence till 4.0 x 1016 cm-2. The blistering threshold fluence for Y-cut LN is found to be between 1.0 x 1016 and 2.0 x 1016 cm-2, and the Arrhenius plot of the blistering time as a function of reciprocal temperature shows that the activation of blistering is 2.14 eV, 1.77 eV and 1.44 eV for samples under fluence of 2.0 x 1016, 3.0x 1016 and 4.0 x 1016 cm-2, respectively. Our experimental results unambiguously contribute to exploring the avenue of achieving high-quality layer, particularly highly anisotropic ones, by CIS techniques.

This work presents an improved design that exploits dispersion matching to suppress the spurious modes in the lithium niobate first-order antisymmetric (A1) Lamb wave mode resonators. The dispersion matching in this work is achieved by micro-machining the lithium niobate thin film to balance the electrical and mechanical loadings of electrodes. In this article, the dispersion matchings of the A1 mode in lithium niobate based on different metals are analytically modeled and validated with finite-element analysis. The fabricated devices exhibit spurious-free responses with a quality factor of 692 and an electromechanical coupling coefficient of 28%. The demonstrated method herein could overcome a significant hurdle that is currently impeding the commercialization of A1 devices.

Electro-optic modulators are utilized ubiquitously ranging from applications in data communication to photonic neural networks. While tremendous progress has been made over the years, efficient phase-shifting modulators are challenged with fundamental tradeoffs, such as voltage-length, index change-losses or energy-bandwidth, and no single solution available checks all boxes. While voltage-driven phase modulators, such as based on lithium niobate, offer low loss and high speed operation, their footprint of 10’s of cm-scale is prohibitively large, especially for density-critical applications, for example in photonic neural networks. Ignoring modulators for quantum applications, where insertion loss is critical, in this work we distinguish between current versus voltage-driven modulators. We focus on the former, since current-based schemes of emerging thin electro-optical materials have shown unity-strong index modulation suitable for heterogeneous integration into foundry waveguides. Here, we provide an in-depth ab-initio analysis of obtainable modulator performance based on heterogeneously integrating low-dimensional materials, i.e., graphene, thin films of indium tin oxide, and transition metal dichalcogenide monolayers into a plurality of optical waveguide designs atop silicon photonics. Using the fundamental modulator tradeoff of energy-bandwidth-product as a design-quality quantifier, we show that a small modal cross section, such as given by plasmonic modes, enables high-performance operation, physically realized by arguments on charge-distribution and low electrical resistance. An in-depth design understanding of phase-modulator performance, beyond doped-junctions in silicon, offers opportunities for micrometer-compact yet energy-bandwidth-ratio constrained modulators with timely opportunities to hardware-accelerate applications beyond data communication towards photonic machine intelligence, for instance; where both performance and integration-density are critical.

Hybrid Lithium Niobate (LN) and Silicon photonic (SiPh) integration platform has emerged as a promising candidate to combine the scalability of silicon photonics with the excellent modulation performance of LN. Mach-Zehnder modulators (MZMs) based on this platform exhibit outstanding performance with low insertion loss, low drive voltage, and large bandwidth. In this paper, we discuss the technologies for realizing hybrid LN/SiPh platform. The configuration and key metrics of LN/SiPh MZM are analyzed in detail. Furthermore, various functional devices derived from the Mach-Zehnder interferometer (MZI) configuration are also reviewed.

The addition of active, nonlinear, and nonreciprocal functionalities to passive piezoelectric acoustic wave technologies could enable all-acoustic and therefore ultra-compact radiofrequency signal processors. Toward this goal, we present a heterogeneously integrated acoustoelectric material platform consisting of a 50 nm indium gallium arsenide epitaxial semiconductor film in direct contact with a 41° YX lithium niobate piezoelectric substrate. We then demonstrate three of the main components of an all-acoustic radiofrequency signal processor: passive delay line filters, amplifiers, and circulators. Heterogeneous integration allows for simultaneous, independent optimization of the piezoelectric-acoustic and electronic properties, leading to the highest performing surface acoustic wave amplifiers ever developed in terms of gain per unit length and DC power dissipation, as well as the first-ever demonstrated acoustoelectric circulator with an isolation of 46 dB with a pulsed DC bias. Finally, we describe how the remaining components of an all-acoustic radiofrequency signal processor are an extension of this work. Radio frequency signal processing (RFSP) currently involves a mix of components with differing operation principles, which hinders miniaturisation. Here, Hackett et al. succeed in creating acoustic non-reciprocal circulators, amplifiers, and passive filters, paving the way for all acoustic single-chip RFSP.

We recently introduced piezoelectric lithium niobate (LN) based transparent ultrasound transducers (TUT) as a new platform for developing multimodal optical, ultrasound, and photoacoustic imaging (PAI) systems. However, LN based TUT is limited in its signal-to-noise ratio due to material's low piezoelectricity ( $d_{33}$ ). In this letter, we report, for the first time, a 0.2-mm-thick transparent lead magnesium niobate-lead titanate (PMN-PT) based TUT (PMN-PT-TUT) for ultrasound and photoacoustic applications and compared its performance with a 0.25-mm-thick transparent LN based TUT (LN-TUT). To improve the ultrasound energy transmission efficiency, TUTs were fabricated with a two-matching-layer design. This resulted in a dual frequency response with center frequencies of 7.8 MHz/13.2 MHz and corresponding bandwidths of 28.2%/66.67% for PMN-PT-TUT, as well as center frequencies of 7.2 MHz/11.8 MHz and bandwidths of 36.1%/62.7% for LN-TUT. The optical transmission rate of PMN-PT-TUTs and LN-TUTs are measured as $\sim$ 73 and $\sim$ 91%, respectively, at 532 nm optical wavelength. The PMN-PT-TUT exhibited higher sensitivity compared to LN-TUT with a nearly threefold higher pulse echo amplitude and more than twofold higher photoacoustic amplitude. Furthermore, optical resolution photoacoustic microscopy experiments on phantom targets demonstrated lateral resolutions of 7 and 5.1 $\mu$ m and axial resolutions of 285.6 and 375.9 $\mu$ m for PMN-PT-TUT and LN-TUT, respectively. These results indicated that PMN-PT is a viable alternative to LN for developing TUT based multimodal ultrasound and PAI systems.

An electro-optic Pockels cell can show very good optical switching behavior for its very good first-order non-linear effect. The response time or the switching time of this type of cell is also very fast. When an electric field is applied to it in a proper way, then it changes the propagation of a polarized light beam passing through it, for which there is found a change in refractive index occurring in it. The change in refractive index is linearly proportional to the applied electric field. This causes a change in optical path inside the material for the propagating light signal, which results in a phase modulation of the light beam. Electro-optic crystals were seen for massive uses in phase and intensity modulation–based all-optical long-haul communication systems. In integrated optics, this effect also has strong uses for processing of optical data. In this paper, the authors propose a new scheme where a single biasing electrical field is parallelly applied in two orthogonal directions at a time on a specific light signal, instead of one biasing signal on the light beam. This parallel application of biasing signal enhances the depth of modulation.

The pyroelectric response has been studied for electro-optic modulators utilizing X-cut LiNbO3 integrated-optical chips. Since this response induces the modulator drift that appears only at fast ch...

We have studied the domain forward growth (growth in polar direction) on the non-polar cut of lithium niobate crystal as a result of focused ion beam irradiation. The nonlinear dose dependencies of...

In this paper, we present a novel transmission line architecture in thin-film lithium niobate (TFLN) platforms to improve the velocity match between the microwave and the optical wave. Compared to conventional coplanar waveguide (CPW), the microwave index ($n_m$) of the proposed slotted electrodes can be optimized from 2.1 to 3 while maintaining the high modulation efficiency and 50-$\Omega$ impedance match. Equivalent-circuit model analysis and finite-element simulation are performed. The simulated half-wave voltage (V$_\pi$) of 1.2 V and E-O modulation bandwidth greater than 80 GHz is obtained for a 2-cm-long modulator. By utilizing the slotted slow-wave electrode, TFLN Mach-Zehnder modulators with CMOS-compatible operating voltage and 3-dB modulation bandwidth greater than 100 GHz are potentialized.

Photonic integrated circuits have found a remarkable range of applications in both fundamental science and commercial products. The development of photonic integrated circuits that use highly efficiency nonlinear optical materials as optical waveguides opens opportunities to achieve low power, on chip optical signal processing. In this article, we discussed the latest developments of two state-of-the-art second order nonlinear optical waveguide platforms, namely lithium niobate on insulator and aluminum gallium arsenide on insulator. We also presented some new progress we made in device performance and system integration. The demonstrated ultra-efficient conversion in those platforms paves the way for high performance optical signal processing, bringing an essential functionality from bench-top level into photonic integrated circuits.