Lithium Niobate Layer Research Articles

High Q lithium niobate metasurfaces with transparent electrodes for efficient amplitude and phase modulation

Lithium niobate (LN)-based metasurfaces have demonstrated remarkable potential in integrated electro-optically adjustable metadevices with the maturation of thin film LN on insulator (LNOI) technology. Here, we proposed a type of high Q factor tunable metasurface with etchless LN, which is electrically driven in the vertical direction by using transparent conductive film. A transmission amplitude modulation of over 60 dB at a voltage of 20 V is realized through guided mode resonances created at the LN layer with a Q factor of 1320. Meanwhile, phase modulation is also realized with a reflective design by adding a gold layer at the bottom of the metasurface. With a gate voltage of 80 V, about 1.75π phase modulation is achieved while keeping reflection over 92%. Our proposed device achieves effective modulation of optical amplitude and phase in the near-infrared band, which lays a good foundation for the development of high performance LN-based active nanophotonic devices.

Hybrid silica and thin-film lithium niobate waveguides and Y-junctions

Silica (SiO2) strip-loaded waveguides and Y-junctions on lithium niobate (LN) thin film were designed, fabricated and experimentally investigated. We first studied the optical confinement factor distribution in strip-loaded waveguides. The Simulations showed that most of the power (>82%) was confined in the LN layer, and the dimensions of the SiO2 loading strip had a relatively small influence on the optical confinement factor distribution. The SiO2 film was deposited with the magnetron sputtering method. The deposited SiO2 layer approached stoichiometric SiO2, and a smooth surface was obtained. A group of waveguides with widths of 2–5 μm were fabricated using lift-off technology. The 2 μm-wide waveguide displayed low propagation losses of 0.2 dB/cm for the quasi-TE (q-TE) mode and 0.8 dB/cm for the quasi-TM (q-TM) mode at the wavelength of 1550 nm. Y-junctions with small sizes were designed and fabricated based on the low-loss waveguides. The transmission could reach 70–80%, and the splitting ratio maintained at a stable level near 1:1 for the Y-junctions S1 and S4. The low-loss strip-loaded waveguides and Y-junctions offer the possibility of combining SiO2 and LN materials for integrated optics. Additional high-performance photonic devices and circuits using this method are expected in the future.

Numerical analysis of a strip-loaded silicon rich nitride-thin film lithium niobate hybrid waveguides

Hybrid integration of silicon rich nitride and lithium niobate on insulator (SRN-LNOI) is an emerging material platform for photonic integrated circuits (PIC). In this paper, we present a systematic numerical investigation of the mode properties of a strip-loaded SRN-LNOI hybrid waveguide at 1550 nm wavelength using the full-vectorial finite difference method. Considering the anisotropic nature of the lithium niobate (LN) crystal, the effective refractive indices of the transverse electric and transverse magnetic modes of strip-loaded SRN-LN hybrid waveguides were analyzed. The single-mode condition, zero-birefringence, effective mode area, and power distribution in terms of the geometrical parameters of the strip-loaded SRN-LN hybrid waveguide are discussed in detail. Furthermore, the optical power transmission in both straight and bent waveguides, as well as the different characteristics of the optical power confinement of the fundamental modes in the SRN and LN layers were analyzed. This study provides useful information for designing high-performance photonic devices on a hybrid SRN-LNOI platform for future PIC applications.

Giant Second Harmonic Generation from Membrane Metasurfaces.

Metasurfaces have emerged as a fascinating framework for nonlinear optics, which have advantages of a compact footprint and unprecedented flexibility in manipulating light. But their nonlinear responses are generally limited by the short interaction lengths with light. Therefore, further enhancement is highly desired for building high-efficiency nonlinear devices. Here, we experimentally demonstrate a record high second harmonic generation (SHG) efficiency of 2.0 × 10-4 using lithium niobate (LN) membrane metasurfaces. Benefiting from the large refractive index contrast in the vertical direction and high fabrication quality, distinct spectral resonances and tight field confinements in the LN layer were achieved. Strong SHG peaks resulting from pump resonances of the metasurfaces were observed. Our nonlinear efficiency is more than 2 orders of magnitude larger than previously reported LN metasurfaces. The results inspire a way to improve the efficiency of nonlinear metasurfaces for ultracompact nonlinear light sources in applications of nonlinear holography, Li-Fi, beam shaping, etc.

Hybrid resonance metasurface for a lithium niobate electro-optical modulator.

Electrically tunable metasurfaces can realize two-dimensional pixelated spatial light modulation and have a wide range of applications in optical switching, free-space communication, high-speed imaging, and so on, arousing the interest of researchers. Here, a gold nanodisk metasurface on a lithium-niobate-on-insulator (LNOI) substrate is fabricated and experimentally demonstrated as an electrically tunable optical metasurface for transmissive free-space light modulation. Using the hybrid resonance formed by the localized surface plasmon resonance (LSPR) of gold nanodisks and the Fabry-Perot (FP) resonance, the incident light is trapped in the gold nanodisk edges and a thin lithium niobate layer to realize field enhancement. In this way, an extinction ratio of 40% is achieved at the resonance wavelength. In addition, the proportion of hybrid resonance components can be adjusted by the size of the gold nanodisks. By applying a driving voltage of ± 2.8 V, a dynamic modulation of 135 MHz is achieved at resonant wavelength. The highest signal-to-noise ratio (SNR) is up to 48 dB at 75 MHz. This work paves the way for the realization of spatial light modulators based on CMOS-compatible LiNbO3 planar optics, which can be used in lidar, tunable displays, and so on.

Dark mode-in-the-box for enhanced second-harmonic generation in corrugated waveguides

A numerical study of second-harmonic generation (SHG) in cavity resonator integrated grating filters (CRIGFs) shows extraordinary strong conversion rates in planar corrugated waveguides based on lithium niobate layers, not exceeding 625 µm in length. The key idea is to couple to the dark mode of the structure at the pump frequency that has a Q-factor much stronger than the bright mode commonly used. This is achieved by shifting the grating coupler between the enclosing Bragg mirrors, resulting in an asymmetric structure. Three asymmetric structures are proposed: without any matching, with impedance matching and with indirect phase matching through an additional pair of gratings. They offer respectively increasing conversion rates of η=4.31 × 10−3 W-1, 0.149 W-1 and 2.38 W-1, which compared to a symmetric CRIGF (η=7.12 × 10−6 W-1) offer respectively more than 2, 4 and 5 orders of magnitude improvement of the conversion rate.

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.

Origin of material removal mechanism in shear thickening-chemical polishing

A shear thickening-chemical polishing (ST-CP) approach exploiting the recombination mechanism of shear-thickening and chemical-physical friction is proposed for ultraprecision machining of optical materials. The ST-CP slurries with dynamic rheological behaviour are characterized, and the optimal preparation process is explored for high-efficiency polishing of workpieces. A critical shear rate (CSR) prediction model in the flow field of slurries is systematically investigated and experimentally verified in detail. A mathematical control of the material removal rate (MRR) is modelled and developed for ST-CP. The shear-thickening-induced micro-cutting and chemical-physical friction contribute to the material removal mechanism in the ST-CP process. A special chemical reaction layer consisting of Li2O and Nb2O7 evoked on the workpiece, which can soften the surface layer of lithium niobate (LiNbO3), increases the chemical-physical friction and material removal through micro-cutting and shearing. The material removal process in ST-CP is a dynamic equilibrium process in which atoms of the workpiece surface are continuously involved to form new substances or oxides to achieve a soft chemical reaction layer, accompanied by the shear-thickening-induced micro-cutting action. A series of ST-CP experiments validate that the maximal error between theoretical and experimental data is less than 11.5%, which shows the high degree of accuracy of the MRR prediction model. Measurements and calculations are performed to explore the effects of shearing velocities, Al2O3 content, pH value, and oxidant content on surface roughness and MRR. When the shear-thickening induced micro-cutting and chemical reaction reach a dynamic equilibrium, a maximum MRR of up to 65.8 mg/h is achieved, and surface roughness is significantly reduced within 120 min from Ra 36.04 nm–1.46 nm with low subsurface damage (<5 nm). This investigation reveals that ST-CP is a progressive ultra-precision manufacturing approach for optical polishing and finishing of crystal materials.

Electrical Tuning of Fresnel Lens in Reflection

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.

A 100 nm Thick, 32 kHz X-Cut Lithium Niobate Piezoelectric Nanoscale Ultrasound Transducer for Airborne Ultrasound Communication

In this letter we present the first demonstration of an ultra-thin (100 nm) lithium niobate (LN) flexural resonator for airborne ultrasound transduction. Due to the aggressively scaled thickness of the monocrystalline X-cut LN layer, the trasnsducers exhibit a low resonance frequency of 32 kHz while occupying an area of only $120\,\,\mu \text{m}\,\,\times 120\,\,\mu \text{m}$ . An innovative electrode design converts the applied electric field to out-of-plane deflections excited by the e21 piezoelectric coefficient. The displacement per unit of voltage at different in-plane rotation angles scales consistently with the theoretical e21 coefficients of X-cut LN. Overall, the transducer is capable of sensitivity per unit area that is significantly larger than the state-of-the-art. [2020-0340]

Simulation and Analysis of Microring Electric Field Sensor Based on a Lithium Niobate-on-Insulator

With the increasing sensitivity and accuracy of contemporary high-performance electronic information systems to electromagnetic energy, they are also very vulnerable to be damaged by high-energy electromagnetic fields. In this work, an all-dielectric electromagnetic field sensor is proposed based on a microring resonator structure. The sensor is designed to work at 35 GHz RF field using a lithium niobate-on-insulator (LNOI) material system. The 2.5-D variational finite difference time domain (varFDTD) and finite difference eigenmode (FDE) methods are utilized to analyze the single-mode condition, bending loss, as well as the transmission loss to achieve optimized waveguide dimensions. In order to obtain higher sensitivity, the quality factor (Q-factor) of the microring resonator is optimized to be 106 with the total ring circumference of 3766.59 μm. The lithium niobate layer is adopted in z-cut direction to utilize TM mode in the proposed all-dielectric electric field sensor, and with the help of the periodically poled lithium niobate (PPLN) technology, the electro-optic (EO) tunability of the device is enhanced to 48 pm·μm/V.

Structural and Optical Properties of nanostructured hybrid LiNbO3/Silicon wafer for Fabricating Optical Modulator

In this paper, a tattoo removal Q switching Nd: YAG laser, with a 6 nanosecond pulse duration, 2000 mJ power, frequency of pulses of 5 Hz was used to deposit of layer by layer of Silicone oxide, pure silicone, and lithium niobate for fabricated an optical modulator. The deposited layer by layer samples were analysed and characterized using the (XRD) X-ray diffraction, optical properties (UV-Vis spectroscopy) and photoluminescence (PL), and Atomic force microscopy AFM. Then we investigated of the deposited layers using. The XRD results showed the existence of different tops for all classes that have been deposited, Also, the optical and luminous results showed that only two peaks appeared, namely the top of the lithium niobate layer at the violet wavelength region with the limits of 368 to 381 nm, as well as the top of the silicon oxide layer within the middle of the visible wavelengths range and within the limits of 551 nm.

Metal based grating coupler on a thin-film lithium niobate waveguide.

Thin-film lithium niobate (LN) has been proved to be an excellent platform for building compact active and nonlinear photonic components on a chip. The coupling of a sub-micron sized LN waveguide and a single-mode fiber remains as one challenging issue. An efficient grating coupler made of Au stripes on an LN ridge waveguide is demonstrated here. The fabrication of the grating is convenient, using just a standard lift-off process of metal films. The peak coupling efficiency of an optimized structure reaches 50.4%, i.e., -3 dB coupling loss, at 1.55 µm wavelength for the fundamental transverse-electrical mode, where the 1-dB coupling bandwidth is 58 nm. Experimentally, fabricated devices, with buried oxide layer thicknesses slightly off the optimal values, exhibit coupling efficiencies of 43.8% and 33.7% for 400 nm and 600 nm thick LN layers.

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.

Efficient Cherenkov-type optical-to-terahertz converter with terahertz beam combining.

A nonlinear structure for efficient Cherenkov-type terahertz emission from ultrashort laser pulses is proposed, modeled, and experimentally demonstrated. The structure comprises a thin (a few tens of micrometers thick) layer of lithium niobate sandwiched between two silicon prisms. A focused-to-a-line laser pulse propagates in the layer and generates a Cherenkov wedge of terahertz radiation in the prisms. The radiation experiences total internal reflection in the prisms and emerges into free space as two adjacent beams collinear to the pump laser beam. The structure can generate a centimeter-wide terahertz beam with high transverse uniformity and a flat frequency spectrum. An optical-to-terahertz conversion efficiency as high as 0.35% is achieved with 10-µJ laser pulses. It can be further enhanced by reducing the thickness of the lithium niobate layer.

Hybrid plasmonic–phononic cavity design for enhanced optomechanical coupling in lithium niobate

A hybrid plasmonic–phononic cavity design which enables high vacuum coupling rate has been proposed in lithium niobate (LN) phononic crystals (Phncs) that have been perforated by air holes and coated with thin silver film. By tailoring the geometry, optomechanical interaction between the plasmonic modes (produced by the metal/insulator) and the phononic modes (confined by the phononic bandgap effect) is greatly enhanced. Numerical results based on finite-element method (FEM) reveal that in this hybrid plasmonic–phononic design, high vacuum coupling rate that predominantly contributed by moving boundary effect is on the order of 106 Hz, which is about one to two orders higher than that contributed by photoelastic effect shown in conventional phoxonic crystal designs. Results evidence how the vacuum coupling rate depends on geometrical parameters like the radius of the defect air hole, the thickness of silver layer, and LN layer. The simultaneous confinement and strong coupling, combined with other advantages as lack of constraint to the refractive index, and integration of piezoelectric material and metal in a chip, this hybrid design may be suitable for non-invasive biological sensing, optomechanically tunable plasmonic heater for drug release and lab-on-chip devices.

Linking microwaves to photons

Optics Communication networks seldom have all components operating at a single frequency. Components are individually designed to operate at optimum frequencies, with the signal then converted from one component to another by means of a transducer. Hybrid quantum systems, for example, will typically require links between microwaves, which operate the qubits, and photons that will transport the quantum information across the network. Shao et al. developed a thin-film acoustic resonator that allows the conversion of microwaves into optical signals. The acoustic resonators and optical wave-guide circuits are patterned within a suspended layer of lithium niobate. Control of the coupling between the microwaves driving the resonators and the resulting modulation of the optical signal demonstrates a platform to develop hybrid signal-processing technologies. Optica 6 , 1498 (2019).

Idealized Einstein-Podolsky-Rosen states from non–phase-matched parametric down-conversion

Entanglement of high dimensional states is becoming increasingly important for quantum communication and computing. The most common source of entangled photons is spontaneous parametric down conversion (SPDC), where the degree of frequency and momentum entanglement is determined by the non-linear interaction volume. Here we show that by reducing the length of a highly non-linear material to the micrometer scale it is possible to relax the longitudinal phase matching condition and reach record levels of transverse wavevector entanglement. From a micro-sized layer of lithium niobate we estimate the number of entangled angular modes to be over 1200. The entanglement is measured both directly using correlation measurements and indirectly using stimulated emission tomography. The high entanglement of the state generated can be used to massively increase the quantum information capacity of photons, but it also opens up the possibility to improve the resolution of many quantum imaging techniques.

Моделирование кривых дифракционного отражения рентгеновских лучей от протонообменных слоев монокристалла ниобата лития

In this work, within the framework of the dynamic theory of the X-ray scattering, a model is constructed that describes the diffraction reflection curves obtained from the proton-exchange layers of a lithium niobate monocrystal subjected to the post-exchange annealing. Planar proton-exchange waveguides based on the X-cut of the lithium niobate monocrystal are obtained experimentally. It was shown that the proton implantation leads to the formation of the new crystalline phases with a larger lattice parameter. The structure changes (by the method of the diffraction structural analysis) and the optical properties (by the method of the mode spectroscopy) of the obtained waveguides are investigated in the various temperature regimes. The microstrain of the crystal lattice caused by еру proton implantation was estimated by the analysis of X-ray diffraction line broadening. It is shown the thickness of the waveguide layer and the number of new crystalline phases depend not only on the temperature regime of the proton exchange, but also on the duration of post-exchange annealing. The simulation of the experimentally obtained curves of the diffraction reflection is carried out within the framework of the described model. As a result of the modeling, the depth of the waveguide layer was determined, which is consistent with the data obtained by the method of the mode spectroscopy. The models of the assumed profiles of the crystal lattice deformation caused by proton implantation are presented. The average values of the lattice microstrain and the phase composition of proton-exchange lithium niobate layers are determined.

Microscale Generation of Entangled Photons without Momentum Conservation.

We report, for the first time, the observation of spontaneous parametric down-conversion (SPDC) free of phase matching (momentum conservation). We alleviate the need to conserve momentum by exploiting the position-momentum uncertainty relation and using a planar geometry source, a 6 μm thick layer of lithium niobate. Nonphase-matched SPDC opens up a new platform on which to investigate fundamental quantum effects but it also has practical applications. The ultrasmall thickness leads to a frequency spectrum an order of magnitude broader than that of phase-matched SPDC. The strong two-photon correlations are still preserved due to energy conservation. This results in ultrashort temporal correlation widths and huge frequency entanglement. The studies we make here can be considered as the initial steps into the emerging field of nonlinear quantum optics on the microscale and nanoscale.