Heterogeneous lithium niobate photonics on silicon substrates

High-Speed Electro-Optic Modulators Based on Thin-Film Lithium Niobate

Electro-optic modulators (EOMs) are pivotal in bridging electrical and optical domains, essential for diverse applications including optical communication, microwave signal processing, sensing, and quantum technologies. However, achieving the trifecta of high-density integration, cost-effectiveness, and superior performance remains challenging within established integrated photonics platforms. Enter thin-film lithium niobate (LN), a recent standout with its inherent electro-optic (EO) efficiency, proven industrial performance, durability, and rapid fabrication advancements. This platform inherits material advantages from traditional bulk LN devices while offering a reduced footprint, wider bandwidths, and lower power requirements. Despite its recent introduction, commercial thin-film LN wafers already rival or surpass established alternatives like silicon and indium phosphide, benefitting from decades of research. In this review, we delve into the foundational principles and technical innovations driving state-of-the-art LN modulator demonstrations, exploring various methodologies, their strengths, and challenges. Furthermore, we outline pathways for further enhancing LN modulators and anticipate exciting prospects for larger-scale LN EO circuits beyond singular components. By elucidating the current landscape and future directions, we highlight the transformative potential of thin-film LN technology in advancing electro-optic modulation and integrated photonics.

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.

Differential phase-diversity electrooptic modulator for cancellation of fiber dispersion and laser noise

Bandwidth and noise are fundamental considerations in all communication and signal processing systems. The group-velocity dispersion of optical fibers creates nulls in their frequency response, limiting the bandwidth and hence the temporal response of communication and signal processing systems. Intensity noise is often the dominant optical noise source for semiconductor lasers in data communication. In this paper, we propose and demonstrate a class of electrooptic modulators that is capable of mitigating both of these problems. The modulator, fabricated in thin-film lithium niobate, simultaneously achieves phase diversity and differential operations. The former compensates for the fiber's dispersion penalty, while the latter overcomes intensity noise and other common mode fluctuations. Applications of the so-called four-phase electrooptic modulator in time-stretch data acquisition and in optical communication are demonstrated.

Efficient second- and higher-order harmonic generation from LiNbO3 metasurfaces

Lithium niobate (LiNbO3) is a material that has drawn great interest in nonlinear optics because of its large nonlinear susceptibility and wide transparency window. However, for complex nonlinear processes such as high-harmonic generation (HHG), which involves frequency conversion over a wide frequency range, it can be extremely challenging for a bulk LiNbO3 crystal to fulfill the phase-matching conditions. LiNbO3 metasurfaces with resonantly enhanced nonlinear light-matter interaction at the nanoscale may circumvent such an issue. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and HHG from a LiNbO3 metasurface enhanced by guided-mode resonance. We observe a high normalized SHG efficiency of 5.1 × 10-5 cm2 GW-1. Moreover, with the alleviated above-gap absorption of the material, we demonstrate HHG up to the 7th order with the shortest generated wavelength of 226 nm. This work may provide a pathway towards compact coherent white-light sources with frequency spanning into the deep ultraviolet region for applications in spectroscopy and imaging.

The Combined Spectral Response of a MEMS Metamaterial Absorber for the Mid-IR and Its Sub-Wavelength Fabrication Residual Array of Holes

Metasurface coatings on a free-standing SiN thin film membrane are fabricated on a Si substrate using masked lithography and CMOS-compatible surface micromachining. The result is a band-limited absorber for the mid-IR, which is part of a microstructure that is attached to the substrate by long and slender suspension beams to provide thermal isolation. As a residual of the fabrication, the regular pattern of sub-wavelength unit cells of 2.6 μm side length, which defines the metasurface, is interrupted by an equally regular array of sub-wavelength holes of 1-2 μm diameter and at 7.8-15.6 μm of pitch. This array of holes is essential for enabling access of the etchant and attack of the underlying layer during fabrication, which ultimately results in the sacrificial release of the membrane from the underlying substrate. As the plasmonic responses of the two patterns interfere, a maximum is imposed on the hole diameter and a minimum on the hole-to-hole pitch. However, the hole diameter should be sufficiently large to allow access of the etchant, while the maximum spacing between holes is set by the limited selectivity of the different materials to the etchant during sacrificial release. The effect of the parasitic hole pattern on the spectral absorption of a metasurface design is analyzed by simulations of the responses of combined holes-metasurface structures. Arrays of 300 × 180 μm² Al-Al₂O₃-Al MIM structures are mask-fabricated on suspended SiN beams. The results show that the effect of the array of holes can be disregarded for a hole-to-hole pitch larger than 6 times the side length of the metamaterial until cell, while the diameter of the hole should remain smaller than about 1.5 μm, and their alignment is critical.

The next generation of hybrid microfluidic/integrated circuit chips: recent and upcoming advances in high-speed, high-throughput, and multifunctional lab-on-IC systems

Since the field's inception, pioneers in microfluidics have made significant progress towards realizing complete lab-on-chip systems capable of sophisticated sample analysis and processing. One avenue towards this goal has been to join forces with the related field of microelectronics, using integrated circuits (ICs) to perform on-chip actuation and sensing. While early demonstrations focused on using microfluidic-IC hybrid chips to miniaturize benchtop instruments, steady advancements in the field have enabled a new generation of devices that expand past miniaturization into high-performance applications that would not be possible without IC hybrid integration. In this review, we identify recent examples of labs-on-chip that use high-resolution, high-speed, and multifunctional electronic and photonic chips to expand the capabilities of conventional sample analysis. We focus on three particularly active areas: a) high-throughput integrated flow cytometers; b) large-scale microelectrode arrays for stimulation and multimodal sensing of cells over a wide field of view; c) high-speed biosensors for studying molecules with high temporal resolution. We also discuss recent advancements in IC technology, including on-chip data processing techniques and lens-free optics based on integrated photonics, that are poised to further advance microfluidic-IC hybrid chips.

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.

Quantum prospects for hybrid thin-film lithium niobate on silicon photonics

Photonics is poised to play a unique role in quantum technology for computation, communications and sensing. Meanwhile, integrated photonic circuits-with their intrinsic phase stability and high-performance, nanoscale components-offer a route to scaling. However, each integrated platform has a unique set of advantages and pitfalls, which can limit their power. So far, the most advanced demonstrations of quantum photonic circuitry has been in silicon photonics. However, thin-film lithium niobate (TFLN) is emerging as a powerful platform with unique capabilities; advances in fabrication have yielded loss metrics competitive with any integrated photonics platform, while its large second-order nonlinearity provides efficient nonlinear processing and ultra-fast modulation. In this short review, we explore the prospects of dynamic quantum circuits-such as multiplexed photon sources and entanglement generation-on hybrid TFLN on silicon (TFLN/Si) photonics and argue that hybrid TFLN/Si photonics may have the capability to deliver the photonic quantum technology of tomorrow.

Thin film lithium niobate electric field sensors

We present our results for using thin film lithium niobate devices for electric field sensing applications. Micro-ring modulator and Mach-Zehnder modulator-based electric field sensors are demonstrated. Micro-ring resonator sensors can be used for low frequency (up to several GHz) electric field sensing applications and achieve a high sensitivity of 80 mV/(m Hz^1/2) with a very compact size of 300 μm, as limited by the intensity and phase noise of the used distributed feedback laser. A measurement bandwidth of 2.5 GHz is measured for these sensors and is limited by the detector bandwidth. Alternatively, Mach-Zehnder modulators allow for perfect phase matching between the radio frequency signals and optical signals, and they can be used for electric field sensing up to several THz. A sensitivity of 2.2 V/(m Hz^1/2) was obtained using our Mach-Zehnder electric field sensor with an interaction length of 600 μm. The Mach-Zehnder sensor can sense electric fields with frequencies reaching 0.6 THz based on the calculated results.

A Barium Titanate-on-Oxide Insulator Optoelectronics Platform

Electro-optic modulators are among the most important building blocks in optical communication networks. Lithium niobate, for example, has traditionally been widely used to fabricate high-speed optical modulators due to its large Pockels effect. Another material, barium titanate, nominally has a 50 times stronger r-parameter and would ordinarily be a more attractive material choice for such modulators or other applications. In practice, barium titanate thin films for optical waveguide devices are usually grown on magnesium oxide due to its low refractive index, allowing vertical mode confinement. However, the crystal quality is normally degraded. Here, a group of scandate-based substrates with small lattice mismatch and low refractive index compared to that of barium titanate is identified, thus concurrently satisfying high crystal quality and vertical optical mode confinement. This work provides a platform for nonlinear on-chip optoelectronics and can be promising for waveguide-based optical devices such as Mach-Zehnder modulators, wavelength division multiplexing, and quantum optics-on-chip.

Materials for emergent silicon-integrated optical computing

Progress in computing architectures is approaching a paradigm shift: traditional computing based on digital complementary metal-oxide semiconductor technology is nearing physical limits in terms of miniaturization, speed, and, especially, power consumption. Consequently, alternative approaches are under investigation. One of the most promising is based on a "brain-like" or neuromorphic computation scheme. Another approach is quantum computing using photons. Both of these approaches can be realized using silicon photonics, and at the heart of both technologies is an efficient, ultra-low power broad band optical modulator. As silicon modulators suffer from relatively high power consumption, materials other than silicon itself have to be considered for the modulator. In this Perspective, we present our view on such materials. We focus on oxides showing a strong linear electro-optic effect that can also be integrated with Si, thus capitalizing on new materials to enable the devices and circuit architectures that exploit shifting computational machine learning paradigms, while leveraging current manufacturing infrastructure. This is expected to result in a new generation of computers that consume less power and possess a larger bandwidth.

On-chip four-mode (de-)multiplexer on thin film lithium niobate-silicon rich nitride hybrid platform

A four-mode (de-)multiplexer with transverse electric field light (TE₀-TE₃) is experimentally demonstrated on a thin film lithium niobate-silicon rich nitride hybrid platform. Enabled by cascaded asymmetrical directional couplers, a (de-)multiplexer with low insertion loss (0.38 dB to 1.6 dB) and low cross talk (-18.46dB to -20.43dB) is obtained at 1550 nm. All channels have cross talk <-16dB from 1480 nm to 1580 nm. The transmission of 4×50 Gbps on-off keying signals is experimentally achieved on the proposed (de-)multiplexer. Experimental results show that the proposed (de-)multiplexer is a promising approach to enhance the transmission capacity in thin film lithium niobate based photonics integrated circuits.

Tunable dual-channel ultra-narrowband Bragg grating filter on thin-film lithium niobate

We demonstrate dual-channel phase-shifted Bragg grating filters in the telecom band on thin-film lithium niobate. These integrated tunable ultra-narrow linewidth filters are crucial components for optical communication and sensing systems, as well as future quantum-photonic applications. Thin-film lithium niobate is an emerging platform suitable for these applications and has been exploited in this Letter. The demonstrated device has an extinction ratio of 27 dB and two channels with close linewidths of about 19 pm (quality factor of ${8} \times {{10}^4}$), separated by 19 GHz. The central wavelength could be efficiently tuned using the high electro-optic effect in lithium niobate with a tuning factor of 3.83 pm/V. This demonstration can be extended to tunable filters with multiple channels, along with desired frequency separations and optimized tunability, which would be useful for a variety of complex photonic integrated circuits.

Multi-tip edge coupler for integration of a distributed feedback semiconductor laser with a thin-film lithium niobate modulator

Lithium niobate-on-insulator (LNOI) has been emerging as a popular integration platform for optical communications and microwave photonics. An edge coupler with high coupling efficiency, wide bandwidth, high fabrication and misalignment tolerance, as well as a small footprint is essential to couple light in or out of the LNOI chip. Some edge couplers have been demonstrated to realize fiber-to-chip coupling in the last few years, but the coupling with distributed feedback (DFB) semiconductor laser is rarely studied. In this paper, we propose a multi-tip edge coupler with three tips to reduce the mode size mismatch between the LNOI waveguide and the DFB laser. The tilted sidewall, fabrication tolerance, misalignment tolerance, and facet reflection due to the effective index mismatch are discussed. It shows that the proposed multi-tip edge coupler can be practically used in the production of effective LNOI integrated chips.

Microstructure and domain engineering of lithium niobate crystal films for integrated photonic applications

Recently, integrated photonics has attracted considerable interest owing to its wide application in optical communication and quantum technologies. Among the numerous photonic materials, lithium niobate film on insulator (LNOI) has become a promising photonic platform owing to its electro-optic and nonlinear optical properties along with ultralow-loss and high-confinement nanophotonic lithium niobate waveguides fabricated by the complementary metal-oxide-semiconductor (CMOS)-compatible microstructure engineering of LNOI. Furthermore, ferroelectric domain engineering in combination with nanophotonic waveguides on LNOI is gradually accelerating the development of integrated nonlinear photonics, which will play an important role in quantum technologies because of its ability to be integrated with the generation, processing, and auxiliary detection of the quantum states of light. Herein, we review the recent progress in CMOS-compatible microstructure engineering and domain engineering of LNOI for integrated lithium niobate photonics involving photonic modulation and nonlinear photonics. We believe that the great progress in integrated photonics on LNOI will lead to a new generation of techniques. Thus, there remains an urgent need for efficient methods for the preparation of LNOI that are suitable for large-scale and low-cost manufacturing of integrated photonic devices and systems.

Ultra-efficient and fully isotropic monolithic microring modulators in a thin-film lithium niobate photonics platform

The large electro-optic coefficient, r₃₃, of thin-film lithium niobate (LN) on insulator makes it an excellent material platform for high-efficiency optical modulators. Using the fundamental transverse magnetic optical mode in Z-cut LN enables isotropic in-plane devices; however, realizing a strong vertical electric field to capitalize on r₃₃ has been challenging. Here we present a symmetric electrode configuration to boost the vertical field strength inside a fully-etched single-mode LN waveguide. We use this design paradigm to demonstrate an ultra-compact fully isotropic microring modulator with a high electro-optic tuning efficiency of 9 pm/V, extinction ratio of 20 dB, and modulation bandwidth beyond 28 GHz. Under quasi-static operation, the tuning efficiency of the modulator reaches 20 pm/V. Fast, efficient, high-contrast modulation will be critical in future optical communication systems while large quasi-static efficiency will enable post-fabrication trimming, thermal compensation, and even complete reconfiguration of microring-based sensor arrays and photonic integrated circuits.

Foundry-compatible thin film lithium niobate modulator with RF electrodes buried inside the silicon oxide layer of the SOI wafer

Ever-increasing complexity of communication systems demands the co-integration of electronics and photonics. But there are still some challenges associated with the integration of thin film lithium niobate (TFLN) electro-optic modulators with the standard and well-established silicon photonics. Current TFLN platforms are mostly not compatible with the silicon photonics foundry process due to the choice of substrate or complicated fabrication requirements, including silicon substrate removal and formation of radio-frequency (RF) electrodes on the top of the TFLN. Here, we report on a platform where all the optical and RF waveguiding structures are fabricated first, and then the TFLN is bonded on top of the silicon photonic chip as the only additional step. Hence, the need for substrate removal is eliminated, and except for the last step of TFLN bonding, its fabrication process is silicon foundry compatible and much more straightforward compared to other fabrication methods.

Fundamental electro-optic limitations of thin-film lithium niobate microring modulators

We investigate the impact of waveguide curvature on the electro-optic efficiency of microring resonators in thin-film X-cut or Y-cut lithium niobate (in-plane extraordinary axis) and derive explicit relations on the response. It is shown that such microring modulators have a fundamental upper bound on their electro-optic performance (∼50% filling factor) which corresponds to a specific arrangement of metal electrodes surrounding the microring and yields nearly identical results for X-cut and Y-cut designs. We further show that this limitation does not exist (i.e., 100% filling factor is possible) with Z-cut microring modulators or can be circumvented (i.e., ∼100% filling factor is possible) in X-cut and Y-cut modulators that use a race-track configuration with segmented electrodes. Comparison of our analytical results with multiphysics simulations and measured electro-optic efficiencies of microring resonators in the literature demonstrates the validity and accuracy of our approach.

Subvolt electro-optical modulator on thin-film lithium niobate and silicon nitride hybrid platform

A low voltage operation electro-optic modulator is critical for applications ranging from optical communications to an analog photonic link. This paper reports a hybrid silicon nitride and lithium niobate electro-optic Mach-Zehnder modulator that employs 3 dB multimode interference couplers for splitting and combining light. The presented amplitude modulator with an interaction region length of 2.4 cm demonstrates a DC half-wave voltage of only 0.875 V, which corresponds to a modulation efficiency per unit length of 2.11 V cm. The power extinction ratio of the fabricated device is approximately 30 dB, and the on-chip optical loss is about 5.4 dB.

Photonic integration based on a ferroelectric thin-film platform

Photonic-integrated circuits (PICs) using ferroelectric materials are expected to be used in many applications because of its unique optical properties such as large electro-optic coefficients. In this study, a novel PIC based on a ferroelectric thin-film platform was designed and fabricated, where high-speed optical modulator, spot-size converters (SSCs), and a variable optical attenuator (VOA) were successfully integrated. A ferroelectric lanthanum-modified lead zirconate titanate (PLZT) thin film was epitaxially-grown by using a modified sol-gel method, and it exhibits large electro-optic coefficients (>120 pm/V) and low propagation loss (1.1 dB/cm). The optical modulator, a Mach-Zehnder type, exhibited a half-wave voltage (V_π) of 6.0 V (V_πL = 4.5 Vcm₎ and optical modulation up to 56 Gb/s. Also, the VOA (with attenuation range of more than 26 dB) was successfully integrated with the modulator. As a result, it is concluded that the developed ferroelectric platform can pave the way for photonic integration.

Actively-monitored periodic-poling in thin-film lithium niobate photonic waveguides with ultrahigh nonlinear conversion efficiency of 4600 %W-1cm-2

Chip-scale implementations of second-order nonlinear optics benefit from increased optical confinement that can lead to nonlinear interaction strengths that are orders of magnitude higher than bulk free-space configurations. Here, we present thin-film-based ultraefficient periodically-poled lithium niobate nonlinear waveguides, leveraging actively-monitored ferroelectric domain reversal engineering and nanophotonic confinement. The devices exhibit up to 4600 %W^-1cm^-2 conversion efficiency for second-harmonic generation, pumped around 1540 nm. In addition, we measure broadband sum-frequency generation across multiple telecom bands, from 1460 to 1620 nm. As an immediate application of the devices, we use pulses of picojoule-level energy to demonstrate second-harmonic generation with over 10% conversion in a 0.6-mm-long waveguide. Our ultracompact and highly efficient devices address growing demands in integrated-photonic frequency conversion, frequency metrology, atomic physics, and quantum optics, while offering a coherent link between the telecom and visible bands.

Improved second harmonic performance in periodically poled LNOI waveguides through engineering of lateral leakage

In this contribution, we investigate the impact of lateral leakage for linear and nonlinear optical waveguides in lithium niobate on an insulator (LNOI). Silicon nitride (SiN) loaded and direct patterned lithium niobate cross-sections are investigated. We show that lateral leakage can take place for the TE mode in LNOI ridge waveguides (X-cut lithium niobate), due to the birefringence of the material. This work gives guidelines for designing waveguides in LNOI that do not suffer from the lateral leakage effect. By applying these design considerations, we avoided the lateral leakage effect at the second harmonic wavelength of a nonlinear optical waveguide in LNOI and demonstrate a peak second harmonic generation conversion efficiency of ~1160% W^-1cm^-2.

High performance fully etched isotropic microring resonators in thin-film lithium niobate on insulator platform

We present our design, fabrication, and experimental results for very high-performance isotropic microring resonators with small radii (∼ 30 µm) based on single-mode strip waveguides and transverse magnetic (TM) polarization in a fully etched lithium niobate (Z-cut) thin-film on insulator. The loss of the devices is predicted to be < 10 dB/cm, and is measured to be ∼ 7 dB/cm. The measured optical responses of microring resonators exhibit an extinction of ∼ 25 dB (close to critical coupling), a 3 dB optical bandwidth of 49 pm (∼ 6 GHz) for all-pass structures, an extinction of ∼ 10 dB for add-drop structures, and a free spectral range of ∼ 5.26 nm, all of which are in excellent agreement with the design. This work is the first step towards ultra-compact and fully isotropic optical modulators in thin-film lithium niobate on insulator.

III-V on silicon avalanche photodiodes by heteroepitaxy

We demonstrate a III-V avalanche photodiode (APD) grown by heteroepitaxy on silicon. This InGaAs/InAlAs APD exhibits low dark current, gain >20, external quantum efficiency >40%, and similar low excess noise, k∼0.2, as InAlAs APDs on InP.

High modulation efficiency lithium niobate Michelson interferometer modulator

We demonstrate highly efficient lithium niobate thin film Michelson interferometer modulators with half-wave voltage length product of 1.4 V∙cm. Amorphous silicon grating couplers have been incorporated to achieve a 3.8-dB/port waveguide-fiber coupling loss. Devices with 1-mm phase shifter arms have a footprint of 2.5 mm × 1.7 mm. The demonstrated modulation data rates is up to 35 Gb/s.

Design of a hybrid chalcogenide-glass on lithium-niobate waveguide structure for high-performance cascaded third- and second-order optical nonlinearities

Dispersion engineering for efficient supercontinuum generation (SCG) is investigated in a hybrid nonlinear photonic platform that allows cascaded third- and second-order optical nonlinearities in transverse-electric (TE) guided modes. The highly nonlinear chalcogenide waveguides enable SCG spanning over 1.25 octaves (from about 1160 nm to more than 2800 nm at 20  dB below maximum power), while the TE polarization attained is compatible with efficient second-harmonic generation in a subsequent thin-film lithium niobate waveguide integrated monolithically on the same chip. A low-energy pump pulsed laser source of only 25 pJ with 250 fs duration, centered at a wavelength of 1550 nm, can achieve such wideband SCG. The design presented is suitable for the f-to-2f carrier-envelope offset detection technique of stabilized optical frequency comb sources.

Towards subterahertz bandwidth ultracompact lithium niobate electrooptic modulators

Achieving ultrahigh-speed electro-optic modulators (subterahertz modulation bandwidths) is shown to be feasible in the thin-film lithium niobate integrated photonic platform. Design guidelines for optimization of the main radio-frequency and optical parameters are presented, and 3-dB modulation bandwidth up to 400 GHz is proved attainable in 3-mm-long devices. Such unprecedented bandwidths pave the path towards utilizing the devices in advanced optical communication systems.

Tunable hybrid silicon nitride and thin-film lithium niobate electro-optic microresonator

This Letter presents, to the best of our knowledge, the first hybrid Si₃N₄-LiNbO₃-based tunable microring resonator where the waveguide is formed by loading a Si₃N₄ strip on an electro-optic (EO) material of X-cut thin-film LiNbO₃. The developed hybrid Si₃N₄-LiNbO₃ microring exhibits a high intrinsic quality factor of 1.85×10⁵, with a ring propagation loss of 0.32 dB/cm, resulting in a spectral linewidth of 13 pm, and a resonance extinction ratio of ∼27  dB within the optical C-band for the transverse electric mode. Using the EO effect of LiNbO₃, a 1.78 pm/V resonance tunability near 1550 nm wavelength is demonstrated.

Large Pockels effect in micro- and nanostructured barium titanate integrated on silicon

The electro-optical Pockels effect is an essential nonlinear effect used in many applications. The ultrafast modulation of the refractive index is, for example, crucial to optical modulators in photonic circuits. Silicon has emerged as a platform for integrating such compact circuits, but a strong Pockels effect is not available on silicon platforms. Here, we demonstrate a large electro-optical response in silicon photonic devices using barium titanate. We verify the Pockels effect to be the physical origin of the response, with r₄₂ = 923 pm V^-1, by confirming key signatures of the Pockels effect in ferroelectrics: the electro-optic response exhibits a crystalline anisotropy, remains strong at high frequencies, and shows hysteresis on changing the electric field. We prove that the Pockels effect remains strong even in nanoscale devices, and show as a practical example data modulation up to 50 Gbit s^-1. We foresee that our work will enable novel device concepts with an application area largely extending beyond communication technologies.

Integrated all-optical wavelength and polarization conversion of orbital angular momentum carrying modes

Wavelength division multiplexing (WDM) using higher-order spatial modes such as orbital angular momentum (OAM) through a channelized bandwidth provides enhanced capacity communication systems. An all-optical wavelength converter is a key function in implemented WDM networks to overcome the wavelength contentions. In addition, a polarization converter provides efficient control on the state of polarization for encoded data channels in the optical networks. This paper proposes a novel versatile-designed integrated optical device with Y_cut ridge lithium niobate photonic wire configuration that acts as a wavelength or polarization converter for data modulated on OAM. It is schemed in such a way that generates decomposed guided modes with a new wavelength and polarization via cascaded second harmonic generation/difference frequency generation (cSHG/DFG) and type-II DFG nonlinear interactions, respectively, where their desired relative phase is achieved by a linear electro-optical effect in the successive phase shifter part. The low loss ≤0.09  dB/cm, high purity (≥94%), and low voltage (≤4  V) of the high-speed proposed modulator enable its compatible operation in commercial wireless and fiber-based polarization-multiplexed WDM communication systems.

Photoinduced Enhanced Raman from Lithium Niobate on Insulator Template

Photoinduced enhanced Raman spectroscopy from a lithium niobate on insulator (LNOI)-silver nanoparticle template is demonstrated both by irradiating the template with 254 nm ultraviolet (UV) light before adding an analyte and before placing the substrate in the Raman system (substrate irradiation) and by irradiating the sample in the Raman system after adding the molecule (sample irradiation). The photoinduced enhancement enables up to an ∼sevenfold increase of the surface-enhanced Raman scattering signal strength of an analyte following substrate irradiation, whereas an ∼threefold enhancement above the surface-enhanced signal is obtained for sample irradiation. The photoinduced enhancement relaxes over the course of ∼10 h for a substrate irradiation duration of 150 min before returning to initial signal levels. The increase in Raman scattering intensity following UV irradiation is attributed to photoinduced charge transfer from the LNOI template to the analyte. New Raman bands are observed following UV irradiation, the appearance of which is suggestive of a photocatalytic reaction and highlight the potential of LNOI as a photoactive surface-enhanced Raman spectroscopy substrate.

Vertical mode transition in hybrid lithium niobate and silicon nitride-based photonic integrated circuit structures

This Letter presents an optical mode transition structure for use in Si₃N₄/LiNbO₃-based hybrid photonics. A gradual modal transition from a Si₃N₄ waveguide to a hybrid Si₃N₄/LiNbO₃ waveguide is achieved by etching a terrace structure into the sub-micrometer thick LiNbO₃ film. The etched film is then bonded to predefined low pressure chemical vapor deposition Si₃N₄ waveguides. Herein we analyze hybrid optical devices both with and without the aforementioned mode transition terrace structure. Experimental and simulated results indicate that inclusion of the terrace significantly improves mode transition compared to an abrupt transition, i.e., a 1.78 dB lower mode transition loss compared to the abrupt transition. The proposed transition structure is also applied to the design of hybrid Si₃N₄-LiNbO₃ micro-ring resonators. A high-quality factor (Q) resonator is demonstrated with the terrace transition which mitigates undesired resonances.

Dispersion engineered high quality lithium niobate microring resonators

Lithium niobate (LN) exhibits outstanding material properties with great potential for many applications. Recent advance in LN integrated photonics on chip-scale platforms has shown significant advantages in device engineering and functionality innovation. Precise engineering of group-velocity dispersion (GVD) is crucial for many important nonlinear photonic applications. In this paper, we demonstrate high-Q LN microring resonators, with optical Q above 1 million, whose GVD can be flexibly controlled in both normal and anomalous dispersion regimes, with a value between -0.128 ps²/m and 0.043 ps²/m in the telecom band, by controlling the device cross section and by utilizing the birefringence. We are able to achieve a small anomalous GVD of -0.015 ps²/m that is even smaller than that of a silica optical fiber. The flexible engineering of GVD paves a critical step towards broad nonlinear photonic applications in high-Q LN microring resonators.

Deuterated silicon nitride photonic devices for broadband optical frequency comb generation

We report and characterize low-temperature, plasma-deposited deuterated silicon nitride films for nonlinear integrated photonics. With a peak processing temperature less than 300°C, it is back-end compatible with complementary metal-oxide semiconductor substrates. We achieve microresonators with a quality factor of up to 1.6×10⁶ at 1552 nm and >1.2×10⁶ throughout λ=1510-1600  nm, without annealing or stress management (film thickness of 920 nm). We then demonstrate the immediate utility of this platform in nonlinear photonics by generating a 1 THz free-spectral-range, 900 nm bandwidth modulation-instability microresonator Kerr comb and octave-spanning, supercontinuum-broadened spectra.

Ultra-low loss ridge waveguides on lithium niobate via argon ion milling and gas clustered ion beam smoothening

Lithium niobate's use in integrated optics is somewhat hampered by the lack of a capability to create low loss waveguides with strong lateral index confinement. Thin film single crystal lithium niobate is a promising platform for future applications in integrated optics due to the availability of a strong electro-optic effect in this material coupled with the possibility of strong vertical index confinement. However, sidewalls of etched waveguides are typically rough in most etching procedures, exacerbating propagation losses. In this paper, we propose a fabrication method that creates significantly smoother ridge waveguides. This involves argon ion milling and subsequent gas clustered ion beam smoothening. We have fabricated and characterized ultra-low loss waveguides with this technique, with propagation losses as low as 0.3 dB/cm at 1.55 µm.

Ultra-low loss photonic circuits in lithium niobate on insulator

Lithium niobate on insulator (LNOI) photonics promises to combine the excellent nonlinear properties of lithium niobate with the high complexity achievable by high contrast waveguides. However, to date, fabrication challenges have resulted in high-loss and sidewall-angled waveguides, limiting its applicability. We report LNOI single mode waveguides with ultra low propagation loss of 0.4 dB/cm and sidewall angle of 75°. Our results open the route to a highly efficient photonic platform with applications ranging from high-speed telecommunication to quantum technology.

Modal birefringence-free lithium niobate waveguides

We investigate polarization-insensitive waveguide designs afforded by the interplay of material and waveguide birefringence in LiNbO₃-on-insulator photonic wires. Fundamental mode birefringence-free operation in the 0.8-1.8 μm spectral range is predicted for a suitable choice of waveguide widths in the 375-600 nm range. Optimized buried waveguide designs yield broadband (1350-1625 nm) index matching between TE₀₀ and TM₀₀ modes. Furthermore, simultaneous phase- and group-velocity matching at infrared wavelengths appears feasible for pulse durations as short as 100 fs.

Electronic Metamaterials with Tunable Second-order Optical Nonlinearities

The ability to engineer metamaterials with tunable nonlinear optical properties is crucial for nonlinear optics. Traditionally, metals have been employed to enhance nonlinear optical interactions through field localization. Here, inspired by the electronic properties of materials, we introduce and demonstrate experimentally an asymmetric metal-semiconductor-metal (MSM) metamaterial that exhibits a large and electronically tunable effective second-order optical susceptibility (χ⁽²⁾). The induced χ⁽²⁾ originates from the interaction between the third-order optical susceptibility of the semiconductor (χ⁽³⁾) with the engineered internal electric field resulting from the two metals possessing dissimilar work function at its interfaces. We demonstrate a five times larger second-harmonic intensity from the MSM metamaterial, compared to contributions from its constituents with electrically tunable nonlinear coefficient ranging from 2.8 to 15.6 pm/V. Spatial patterning of one of the metals on the semiconductor demonstrates tunable nonlinear diffraction, paving the way for all-optical spatial signal processing with space-invariant and -variant nonlinear impulse response.

Monolithic Mid-Infrared Integrated Photonics Using Silicon-on-Epitaxial Barium Titanate Thin Films

Broadband mid-infrared (mid-IR) photonic circuits that integrate silicon waveguides and epitaxial barium titanate (BTO) thin films are demonstrated using the complementary metal-oxide-semiconductor process. The epitaxial BTO thin films are grown on lanthanum aluminate (LAO) substrates by the pulsed laser deposition technique, wherein a broad infrared transmittance between λ = 2.5 and 7 μm is observed. The optical waveguiding direction is defined by the high-refractive-index amorphous Si (a-Si) ridge structure developed on the BTO layer. Our waveguides show a sharp fundamental mode over the broad mid-IR spectrum, whereas its optical field distribution between the a-Si and BTO layers can be modified by varying the height of the a-Si ridge. With the advantages of broad mid-IR transparency and the intrinsic electro-optic properties, our monolithic Si on a ferroelectric BTO platform will enable tunable mid-IR microphotonics that are desired for high-speed optical logic gates and chip-scale biochemical sensors.

Heterogeneous integration of lithium niobate and silicon nitride waveguides for wafer-scale photonic integrated circuits on silicon

An ideal photonic integrated circuit for nonlinear photonic applications requires high optical nonlinearities and low loss. This work demonstrates a heterogeneous platform by bonding lithium niobate (LN) thin films onto a silicon nitride (Si₃N₄) waveguide layer on silicon. It not only provides large second- and third-order nonlinear coefficients, but also shows low propagation loss in both the Si₃N₄ and the LN-Si₃N₄ waveguides. The tapers enable low-loss-mode transitions between these two waveguides. This platform is essential for various on-chip applications, e.g., modulators, frequency conversions, and quantum communications.

Second-harmonic generation in periodically-poled thin film lithium niobate wafer-bonded on silicon

Second-order optical nonlinear effects (second-harmonic and sum-frequency generation) are demonstrated in the telecommunication band by periodic poling of thin films of lithium niobate wafer-bonded on silicon substrates and rib-loaded with silicon nitride channels to attain ridge waveguide with cross-sections of ~2 µm². A nonlinear conversion of 8% is obtained with a pulsed input in 4 mm long waveguides. The choice of silicon substrate makes the platform potentially compatible with silicon photonics, and therefore may pave the path towards on-chip nonlinear and quantum-optic applications.

High-performance and linear thin-film lithium niobate Mach-Zehnder modulators on silicon up to 50 GHz

Compact electro-optical modulators are demonstrated on thin films of lithium niobate on silicon operating up to 50 GHz. The half-wave voltage length product of the high-performance devices is 3.1 V.cm at DC and less than 6.5 V.cm up to 50 GHz. The 3 dB electrical bandwidth is 33 GHz, with an 18 dB extinction ratio. The third-order intermodulation distortion spurious free dynamic range is 97.3  dBHz^2/3 at 1 GHz and 92.6  dBHz^2/3 at 10 GHz. The performance demonstrated by the thin-film modulators is on par with conventional lithium niobate modulators but with lower drive voltages, smaller device footprints, and potential compatibility for integration with large-scale silicon photonics.

Universal discrete Fourier optics RF photonic integrated circuit architecture

This paper describes a coherent electro-optic circuit architecture that generates a frequency comb consisting of N spatially separated orders using a generalised Mach-Zenhder interferometer (MZI) with its N × 1 combiner replaced by an optical N × N Discrete Fourier Transform (DFT). Advantage may be taken of the tight optical path-length control, component and circuit symmetries and emerging trimming algorithms offered by photonic integration in any platform that offers linear electro-optic phase modulation such as LiNbO_3, silicon, III-V or hybrid technology. The circuit architecture subsumes all MZI-based RF photonic circuit architectures in the prior art given an appropriate choice of output port(s) and dimension N although the principal application envisaged is phase correlated subcarrier generation for all optical orthogonal frequency division multiplexing. A transfer matrix approach is used to model the operation of the architecture. The predictions of the model are validated by simulations performed using an industry standard software tool. Implementation is found to be practical.

Design of nanobeam photonic crystal resonators for a silicon-on-lithium-niobate platform

We outline the design for a photonic crystal resonator made in a hybrid Silicon/Lithium Niobate material system. Using the index contrast between silicon and lithium niobate, it is possible to guide and confine photonic resonances in a thin film of silicon bonded on top of lithium niobate. Quality factors greater than 10⁶ at optical wavelength scale mode volumes are achievable. We show that patterning electrodes on such a system can yield an electro-optic coupling rate of 0.6 GHz/V (4 pm/V).

Electric-optical property of the proton exchanged phase modulator in single-crystal lithium niobate thin film

The electric-optical property of the proton exchanged phase modulator in an x-cut single-crystal lithium niobate thin film was studied. Proton exchanged waveguides generally suffered from a deteriorated electric-optical coefficient. By introducing a shallow proton exchange layer (thickness = 0.165 μm), most energy of the optical mode was allowed to guide in the untouched single-crystal lithium niobate film, making contribution to the effective electric-optical coefficient as high as 29.5 pm/V, which was very close to that of the bulk lithium niobate (r₃₃ = 31 pm/V). A 12 V voltage applied to the electrodes located on the two sides of the waveguide induced a 0.097 nm shift of the Fabry-Perot resonant peak. Considering the wavelength difference of the neighboring resonant peaks (0.228 nm) and the length of the electrodes (2.3 mm), the voltage-length product was as low as 6.5 V·cm, indicating the efficient electric-optical modulation.

Low loss ridge waveguides in lithium niobate thin films by optical grade diamond blade dicing

We report on the fabrication and characterization of ridge waveguides in lithium niobate thin films by diamond blade dicing. The lithium niobate thin films with a thickness of 1 µm were fabricated by bonding a He-implanted lithium niobate wafer to a SiO(2)-coated lithium niobate wafer and crystal ion slicing. Propagation losses of 1.2 dB/cm for TE and 2.8 dB/cm for TM polarization were measured at 1550 nm for a 9.28 mm long and 2.1 µm wide waveguide using the Fabry-Perot method.