Large Electro-optic Coefficient Research Articles

London Dispersion Effects on the Structure and Properties of Nonlinear Optical BiB3O6 Crystal.

a-BiB3O6(BiBO) is an important nonlinear optical (NLO) material with high efficiency for applications in harmonic generations and quantum technology. Owing to its low symmetry and cooperative Bi3+lone pair arrangement, it has also exceptional large piezoelectric and electro-optic coefficients and strong anisotropies on other material characteristics. Previous theoretical calculations on its physical properties often gave confusing results. It is found here that London dispersion (LD) tends to stabilize structures with closer packBi3+with large polarizabilities, which is ignored in most previous density functional theory (DFT) calculations. Present study shows that without considering the LD effect, the structure of a-BiB3O6(BiBO) was predicted with an over-estimated (by over 10%) uniqueb-axis while underestimatesaand overestimatescin a less amount. Consequently it is not possible to use the calculated structure to obtain meaningful properties of this important material. By applying a modified post-DFT LD correction,the experimental structure is well reproduced with the theoretical optimized one. Important material property tensors of BiBO crystal are calculated in unprecedented precisions, including: dielectric constants (static and in THz range), elastic and elasto-optic constants, piezoelectric constants, refractive indices, NLO and electro-optic (EO) coefficients.

Nanoimprinting Solution-Derived Barium Titanate for Electro-Optic Metasurfaces.

Electro-optic metasurfaces have demonstrated significant potential in enhancing the modulation speed and efficiency for fast and large-scale free-space optical devices. Barium titanate has a strong electro-optic Pockels coefficient, but its availability in thin-film form is restricted due to costly growth processes or low thickness. Here, we fabricated active metasurfaces using an etch-free bottom-up process with sol-gel-based polycrystalline barium titanate with a large electro-optic coefficient similar to bulk lithium niobate. We achieve strong hybrid Mie/surface lattice resonances with a quality-factor of 200 at 633 nm wavelength, enhancing the light-matter interaction and therefore the Pockels effect. The metasurface transmission is electro-optically modulated with up to 5 MHz driving frequency at low voltages of less than 1 V thanks to resonant enhancement of the modulation amplitude by 2 orders of magnitude. This successful demonstration of electro-optic modulation in nanoimprinted barium titanate structures paves the way for low-cost and large-scale free-space modulators or tunable metalenses.

The Effect of Sputtering Target Density on the Crystal and Electronic Structure of Epitaxial BaTiO3 Thin Films

Barium titanate (BaTiO3) is a promising material for silicon-integrated photonics due to its large electro-optical coefficients, low loss, high refractive index, and fast response speed. Several deposition methods have been employed to synthesize BaTiO3 films. Magnetron sputtering is one of these methods, which offers specific advantages for growing large-scale films. However, there is a scarcity of studies investigating the effect of sputtering target density on the quality of BaTiO3 films. Therefore, this study aims to uncover the effect of sputtering targets on the crystal and electronic structures of epitaxial BaTiO3 thin films. Two BaTiO3 ceramic targets were sintered at different densities by altering the sintering temperatures. The crystal structure and chemical composition of the targets were then characterized using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Subsequently, BaTiO3 epitaxial films were grown by magnetron sputtering using these two targets. The crystal and electronic structures of the BaTiO3 films were analyzed using high-resolution X-ray diffraction, X-ray photoemission spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Notably, the BaTiO3 films grown with high-density targets show superior quality but contain oxygen vacancies, whereas those films synthesized with low-density targets display high surface roughness. These findings provide insights into the effect of sputtering target density on the crystal and electronic structures of epitaxial BaTiO3 thin films.

Thin-film lithium niobate electro-optic terahertz wave detector

The design, fabrication, and validation of a thin-film lithium niobate on insulator (LNOI) electro-optic (EO) time-domain terahertz (THz) wave detector is reported. LNOI offers unprecedented properties for the EO detection of freely propagating THz wave radiation pulses and transient electric fields because of the large EO coefficient of the material, engineering of the velocity matching of the THz wave and optical wave, and much reduced detector size. The proof-of-concept device is realized using thin-film lithium niobate optical waveguides forming a Mach–Zehnder interferometer with interferometer arms electrically poled in opposite directions. THz waves are coupled effectively to the fully dielectric device from free space without using antennas or plasmonics. The detection of THz waves with frequencies up to 800 GHz is successfully demonstrated. The detector allows for the detection of THz frequency electric fields up to 4.6 MV/m. The observed frequency response of the device agrees well with theoretical predictions.

Combined effect of piezoelectric, elastic, dielectric, and electro-optical properties on single-domain potassium tantalate niobate single crystals

The complete matrix parameters of the elastic, dielectric, and piezoelectric constants of high-quality Fe-doped KTa1-xNbxO3 (KTN) single crystals poled along [001]PC were successfully confirmed. The piezoelectric coefficients d33 and k33, which are of interest for various applications, are 137 pC/N and 50.4 %, respectively. The orientation dependence of these properties for the single-domain Fe:KTN samples was calculated, and it was found that the [001]PC direction is the optimal orientation for the highest performance with a single-domain state, at which the sample exhibits good transparency. Further, the single-domain sample exhibits a large effective electro-optical coefficient (γc = 126.6 p.m./V), and the combined piezoelectric and electro-optical effects provide a 0.4 % electro-optical coefficient. This is beneficial for applying high-response speed modulators with an effective electro-optical working mode and a low contribution of the inverse piezoelectric effect, owing to slower frequency response characteristics.

Vibrational contributions to the electro-optic effect of BBO, KTP and RTP crystals

Barium borate (Ba3(B3O6)2, low temperature phase, BBO), potassium titanyl phosphate (KTiOPO4, KTP) and rubidium titanyl phosphate (RbTiOPO4, RTP) are not only best performing nonlinear optical materials but also show preferable electro-optic (EO) Pockels effect that serve as practical EO modulators widely applied in laser technology. Disparity to the repeatedly measurements of their properties related to the EO effects, the mechanism leads to their large EO coefficients have never been fully studied theoretically from first-principles calculations. In this paper, using an ab initio program CRYSTAL based on a linear combination of atomic orbitals approach, the vibrational spectra of the title crystals were firstly obtained. Then the electronic and vibrational contributions to the EO effect were calculated through coupled perturbed Kohn–Sham approximation and variations of atomic displacements. The results show that the EO coefficients, as well as other properties of dielectric constants, SHG coefficients and IR and Raman spectra of the three crystals are in excellent agreement with the experimental values. It is also found that the EO coefficients of all the crystals studies are dominantly contributed from few vibrational modes at low frequencies and the vibrational contributions amount to about 60% in BBO, 90% in KTP and RTP. General rules of finding crystals with large EO effects are finally proposed based on present study and previous studies on LiNbO3 crystals.

Continuous control of polarization state and tunable dual-channel optical communication based on highly transparent PMN-PT electro-optic ceramics

Transparent ferroelectric materials have excellent electro-optic properties, suitable to be used to develop high-performance optical communication devices. La-doped 88Pb(Mg1/3Nb2/3)O3–12PbTiO3 (88PMN-12PT) transparent ceramics with excellent comprehensive performances, including high transparency of 70.5% at near-infrared wavelength (close to its theoretical value), large quadratic electro-optic coefficient of 23.6 × 10−16 m2 V−2, low half-wave voltage (Vπ) of 146 V, and good extinction ratio of 34 dB, are successfully prepared in this paper. Based on the electro-optic effect of PMN-PT transparent ceramics, laser polarization state is transformed continuously from linear polarization to elliptical polarization, later to circularly polarization by applying electric field. Furthermore, dual-channel optical communication with a tunable laser intensity ratio is first reported here. Results indicate that PMN-PT transparent ceramic is a promising candidate for electro-optic material in polarization controller, optical switch, and information monitor in optical communication.

Wafer-scale heterogeneous integration of thin film lithium niobate on silicon-nitride photonic integrated circuits with low loss bonding interfaces.

Silicon nitride (Si3N4) is a versatile waveguide material platform for CMOS foundry-based photonic integrated circuits (PICs) with low loss and high-power handling. The range of applications enabled by this platform is significantly expanded with the addition of a material with large electro-optic and nonlinear coefficients such as lithium niobate. This work examines the heterogeneous integration of thin-film lithium-niobate (TFLN) on silicon-nitride PICs. Bonding approaches are evaluated based on the interface used (SiO2, Al2O3 and direct) to form hybrid waveguide structures. We demonstrate low losses in chip-scale bonded ring resonators of 0.4 dB/cm (intrinsic Q = 8.19 × 105). In addition, we are able to scale the process to demonstrate bonding of full 100-mm TFLN wafers to 200-mm Si3N4 PIC wafers with high layer transfer yield. This will enable future integration with foundry processing and process design kits (PDKs) for applications such as integrated microwave photonics and quantum photonics.

Fano resonances generation in pyramidal lithium niobate metasurfaces with vertical symmetry-breaking

Fano resonance exhibits an asymmetric spectral profile with a sharp change between a dip and a peak. However, high quality factor Fano resonance is extremely difficult to be fabricated because of its ultra-sensitivity to geometrical imperfections. Here, we demonstrate the Fano resonances generation in all-dielectric pyramidal lithium niobate metasurface with vertical symmetry-breaking, which does not require tight geometrical tolerances and can be easily fabricated. The quality factor of the generated Fano resonance can reach up to 107 due to the reserve of horizontal symmetry. Our work provides a new method of achieving Fano resonances, which can find potential applications in a large variety of prominent optical devices, such as slow-light devices, susceptible sensors and ultrafast switches. Due to the large electro-optic coefficient of lithium niobate and the enhanced Pockels effect in the slow photon system, it has great potential for realizing tunable Fano resonance with extremely low voltage. These tuned Fano resonances, with a low voltage, will find many applications such as high-resolution spectroscopy, biomolecular detection, and tunable laser.

Large linear and nonlinear electro-optic coefficients in two-dimensional ferroelectrics

Analytical derivations and first-principles calculations are performed to investigate the electro-optic (EO) response of the two-dimensional ferroelectric SnS in its monolayer form. In addition to derive the formula for EO coefficients for two-dimensional systems, large linear and nonlinear electro-optic coefficients are discovered, along with a strongly nonlinear electro-optic response as a function of electric field. The origin of these electro-optic responses is further found and involves a few specific phonon modes, in general, and the evolution of their frequencies and atomic characteristics under electric fields, in particular. Large linear and nonlinear electro-optic coefficients can be found in other two-dimensional ferroelectrics, paving the way for potential optical device applications.

Exploration of the mechanisms enabling high performance in relaxor ferroelectric crystals

<p indent="0mm">Relaxor ferroelectric single crystals, exhibiting excellent dielectric, piezoelectric, electro-optic, and pyroelectric properties, are important functional dielectric materials in electronic technology. Since the 1990s, relaxor ferroelectric crystals have attracted considerable attention in the ferroelectric and piezoelectric research fields and improved the performance of numerous piezoelectric devices, especially medical ultrasonic imaging transducers. In this paper, we briefly introduce the history of relaxor ferroelectric crystals and then focus on the research progress of our group in recent years, as listed below. (1) To elucidate the contribution of polar nanoregions (PNRs)<italic> </italic>to piezoelectricity, we performed closely integrated experimental and computational studies on relaxor ferroelectric crystals. In this work, we obtained key experimental evidence (low-temperature dielectric relaxation) for the contribution of PNRs to the piezoelectric activity in relaxor-PbTiO₃ (PT) crystals via cryogenic measurements, and more importantly, for the first time, we quantified the contribution of PNRs accounting for 50%–80% of the room-temperature dielectric and piezoelectric properties. Based on the phase-field simulations, we proposed a mesoscale mechanism wherein the PNRs are aligned in a ferroelectric matrix facilitating the polarization rotation, which successfully explains the contribution of PNRs to dielectric/piezoelectric properties. This work opens up a new material design paradigm that introduces local heterogeneities to achieve enhanced macroscopic responses. (2) Based on the origin of high piezoelectric activity in relaxor ferroelectric crystals, we proposed a theoretical approach, i.e., judiciously introducing local structure heterogeneity, to engineer the interfacial energies, thereby increasing the piezoelectricity of ferroelectric materials. According to this method, we designed and fabricated a series of rare earth element-doped relaxor ferroelectric crystals with piezoelectric coefficients <italic>d</italic>₃₃ in the 3400–4100 pC/N, double the values of the undoped counterparts. The newly developed piezoelectric materials offer potential benefits for piezoelectric devices, especially high-frequency medical transducers, where ultrahigh clamped dielectric constant (~3000) improves the electrical impedance matching of the transducers, thereby increasing the sensitivity and decreasing the insertion loss in the ultracompact transducers. (3) Using phase-field simulations and experimental measurements, we transformed the originally opaque [001]-poled rhombohedral PMN-PT crystals into a transparent one by using an AC electric field to engineer the domain structure. These crystals possess ultrahigh piezoelectric coefficient <italic>d</italic>₃₃ (&gt;2100 pC/N), outstanding electromechanical coupling factor <italic>k</italic>₃₃ (~94%), and large electro-optical coefficient <italic>γ</italic>₃₃<sc>(~220 pm/V),</sc> far beyond the performance of the state-of-the-art transparent ferroelectric crystal LiNbO₃, showing great potential in various hybrid device applications, including photoacoustic medical transducers and self-energy-harvesting touchscreens. Furthermore, we discovered that the larger domain sizes lead to higher piezoelectricity in [001]-poled rhombohedral perovskite ferroelectric crystals, opposing the long-standing belief that a smaller domain size always results in higher piezoelectricity in ferroelectric crystals. This observation may benefit the future design of piezoelectric materials. Finally, the applications of our newly designed relaxor ferroelectric crystals in electromechanical and electro-optical devices are introduced, and the future perspectives of relaxor ferroelectric crystals are discussed.

Advances in lithium niobate photonics: development status and perspectives

Lithium niobate (LN) has experienced significant developments during past decades due to its versatile properties, especially its large electro-optic (EO) coefficient. For example, bulk LN-based modulators with high speeds and a superior linearity are widely used in typical fiber-optic communication systems. However, with ever-increasing demands for signal transmission capacity, the high power and large size of bulk LN-based devices pose great challenges, especially when one of its counterparts, integrated silicon photonics, has experienced dramatic developments in recent decades. Not long ago, high-quality thin-film LN on insulator (LNOI) became commercially available, which has paved the way for integrated LN photonics and opened a hot research area of LN photonics devices. LNOI allows a large refractive index contrast, thus light can be confined within a more compact structure. Together with other properties of LN, such as nonlinear/acousto-optic/pyroelectric effects, various kinds of high-performance integrated LN devices can be demonstrated. A comprehensive summary of advances in LN photonics is provided. As LN photonics has experienced several decades of development, our review includes some of the typical bulk LN devices as well as recently developed thin film LN devices. In this way, readers may be inspired by a complete picture of the evolution of this technology. We first introduce the basic material properties of LN and several key processing technologies for fabricating photonics devices. After that, various kinds of functional devices based on different effects are summarized. Finally, we give a short summary and perspective of LN photonics. We hope this review can give readers more insight into recent advances in LN photonics and contribute to the further development of LN related research.

Solution epitaxy of single-crystal and single-domain KNbO3 film with a great photovoltaic current

As a classic ferroelectric material, orthorhombic perovskite KNbO3 demonstrates a large electro-optical coefficient, high refractive index, and excellent nonlinear optical property. These make single-crystal KNbO3 films being long-pursued for lead-free sensor or optoelectronic device applications. However, it remains a challenge to obtain high-quality single-crystal KNbO3 films because of the volatility of potassium at a high growth temperature. In this work, single-crystal KNbO3 films with a tunable thickness have been successfully fabricated by a low-temperature (∼220 °C) hydrothermal epitaxy on cubic single-crystal Nb:SrTiO3 substrates. Interestingly, the KNbO3 film was investigated to adopt a single-domain structure with a polarization direction pointing to the substrate. In-depth characterization reveals an intriguing disordered region of approximately 0.6–1.2 nm because of partial Nb substitution of Ti near the interface, giving rise to the coexistence of Ti3+ and Ti4+ ions. Such region is proposed to release a large interface strain energy from a large lattice misfit (∼2.3%) between {011}KNbO3 and {100}Nb:SrTiO3 and reduce interfacial electrostatic energy via screening of the positive polar surface of KNbO3. This allows to drive a hydrothermal epitaxy of single-crystal and single-domain KNbO3 without forming a typical dislocation. The films demonstrate a great photovoltaic short-circuit current of 15.65 μA/cm2 under the 375 nm laser irradiation of 500 mW/cm2 light intensity, and the corresponding photoresponsivity is the highest among the emerged KNbO3-based materials. Such improvement in the photoresponsivity has been attributed to a successful low-temperature solution epitaxy of single-crystal and single-domain KNbO3 films, where K deficiency can be significantly suppressed. These findings suggest that a solution epitaxy strategy is effective in producing high-quality ferroelectric films for lead-free optoelectronic or self-powered photodetector device applications.

Modeling temperature, frequency, and strain effects on the linear electro-optic coefficients of ferroelectric oxides

An electro-optic modulator offers the function of modulating the propagation of light in a material with an electric field and enables a seamless connection between electronics-based computing and photonics-based communication. The search for materials with large electro-optic coefficients and low optical loss is critical to increase the efficiency and minimize the size of electro-optic devices. We present a semi-empirical method to compute the electro-optic coefficients of ferroelectric materials by combining first-principles density-functional theory calculations with Landau–Devonshire phenomenological modeling. We apply the method to study the electro-optic constants, also called Pockels coefficients, of three paradigmatic ferroelectric oxides: BaTiO3, LiNbO3, and LiTaO3. We present their temperature-, frequency-, and strain-dependent electro-optic tensors calculated using our method. The predicted electro-optic constants agree with the experimental results, where available, and provide benchmarks for experimental verification.

Tunable microcavities coupled to rare-earth quantum emitters

Electro-optical control of on-chip photonic devices is an essential tool for efficient integrated photonics. Lithium niobate on insulator (LNOI) is an emerging platform for on-chip photonics due to its large electro-optic coefficient and high nonlinearity. Integrating quantum emitters into LNOI would extend their versatility in classic photonics to quantum computing and communication. Here, we incorporate rare-earth ion (REI) quantum emitters into electro-optical tunable lithium niobite (LN) thin films and demonstrate control of LN microcavities coupled to REIs over a frequency range of 160 GHz with 5 µs switching speed. Dynamic control of the cavities enables modulation of the Purcell enhancement of REIs with short time constants. Using Purcell enhancement, we show evidence of detecting single Y b 3 + ions in LN cavities. Coupling quantum emitters in fast tunable photonic devices is an efficient method to shape the waveform of the emitter. It also offers a platform to encode quantum information in the integration of a spectral–temporal–spatial domain to achieve high levels of channel multiplexing, as well as an approach to generate deterministic single-photon sources.

Systematic study on the optimization of a bis(N,N-diethyl)aniline based NLO chromophore via a stronger electron acceptor, extended π-conjugation and isolation groups

New chromophores which contains the optimal structure of the same bis(N,N-diethyl)aniline donor were synthesized and the hyperpolarizability β can be effectively translated into large electro-optic coefficients in poled polymers.

Determining the Quadratic Electro-Optic Coefficients for Polycrystalline Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) Using a Polarization-Independent Electro-Optical Laser Beam Steerer

A polarization-independent electro-optical (EO) laser beam steerer based on a bulk relaxor ferroelectric polycrystalline Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) was developed in this study to steer light ranging from visible to mid-IR wavelengths. A large number of the resolvable spots was achieved with this EO steerer. A Fourier-transform infrared (FTIR) spectroscopy was employed to determine the refractive index of the polycrystalline PMN-PT over a wide range of optical wavelengths. Besides measuring the transmission of this material, the capacitance bridge analysis was used to characterize the effect of temperature on the dielectric constant of PMN-PT. The performance of the steerer over a variety of wavelengths was simulated using COMSOL Multiphysics. The deflection angle for the wavelengths of 532, 632.8, 1550, and 4500 nm was measured in the lab in terms of mrad.mm/kV at two different temperatures and compared to the simulation results. The quadratic Kerr electro-optic coefficient and the halfwave electric field were determined for those four wavelengths at two different temperatures. The results showed polycrystalline PMN-PT has a large quadratic EO coefficient for visible light, almost as large in the near IR, but drops significantly in the mid-IR. No significant temperature dependency for the EO coefficients was observed for any of those four wavelengths.

Photonic devices based on thin-film lithium niobate on insulator

Lithium niobate on insulator (LNOI) is rising as one of the most promising platforms for integrated photonics due to the high-index-contrast and excellent material properties of lithium niobate, such as wideband transparency from visible to mid-infrared, large electro-optic, piezoelectric, and second-order harmonic coefficients. The fast-developing micro- and nano-structuring techniques on LNOI have enabled various structure, devices, systems, and applications. In this contribution, we review the latest developments in this platform, including ultra-high speed electro-optic modulators, optical frequency combs, opto-electro-mechanical system on chip, second-harmonic generation in periodically poled LN waveguides, and efficient edge coupling for LNOI.

Electro-optic response in epitaxially stabilized orthorhombic mm2 BaTiO3

$\mathrm{BaTi}{\mathrm{O}}_{3}$ (BTO) is an emergent material in the field of silicon-integrated photonics, as its thin films have been demonstrated to have a very large electro-optic (Pockels) coefficient that can be used for optical modulators. However, BTO grown directly on $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO)-buffered Si (STO is required for epitaxial growth) initially grows with a polarization direction not suitable for the geometries currently used in photonic devices. Here, we grow BTO on a $\mathrm{BaSn}{\mathrm{O}}_{3}$-buffered STO substrate to form orthorhombic $mm2$ BTO, which has in-plane polarization orientation needed for photonic devices. Extensive crystalline characterization is done to confirm the high quality of the films, along with electro-optic measurements. Theoretical simulations coupled with the experimental results provide a foundational understanding of the properties of strain-stabilized orthorhombic BTO. Large electro-optic coefficients of 121 pm/V are observed in films as thin as 40 nm.

Design and synthesis of organic optical nonlinear multichromophore dendrimers based on double-donor structures

Two-arm dendrimer C2 and three-arm dendrimer C3 with large electro-optic coefficients and stability were synthesized and evaluated compared to one-chromophore-containing dendritic material C1.