Thin film lithium niobate electro-optic modulator with terahertz operating bandwidth

Design and fabrication of a sub-3 dB grating coupler on an X-cut thin-film lithium niobate platform

Thin-film lithium niobate (TFLN) based integrated photonic devices have been intensively investigated due to their promising properties, enabling various on-chip applications. Grating couplers (GCs) are wildly used for their flexibility and high alignment tolerance for fiber-to-chip coupling. However, achieving high coupling efficiency (CE) in TFLN GCs often requires the use of reflectors, hybrid materials, or extremely narrow linewidths of the grating arrays, which significantly increases the fabrication difficulty. Therefore, there is a demand for high-CE GCs on TFLN with simple structure and easy fabrication processes. In this paper, combining process capabilities, we demonstrate a highly efficient apodized GC by linearly optimizing the period length and the fill factor on a 600-nm-thick TFLN platform. Without any reflector or hybrid material, we achieve a remarkable coupling loss of -2.97 dB at 1555 nm on the 600-nm-thick X-cut TFLN platform with only a single lithography and etching step. Our work sets a new benchmark for CE among GCs on the 600-nm-thick TFLN platform.

Polarization-insensitive multimode interference coupler on an x-cut thin-film lithium niobate platform

Thin-film lithium niobate (TFLN) is a promising integrated photonics platform but currently lacks a polarization-insensitive multimode interference (MMI) coupler, a crucial component for polarization-related optical communication applications such as polarization management, polarization-division multiplexing, and polarization-insensitive modulation systems. This paper presents a novel, to the best of our knowledge, approach by rotating the MMI structure on an anisotropic x-cut TFLN at specific angles to compensate for the difference in the beat length between the two polarizations. A polarization-insensitive 1 × 2 MMI coupler is experimentally achieved with measured transmittances of -2.5 to -4 dB for both output ports and polarization modes in the wavelength range of 1520-1580 nm.

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.

Time-modulated near-field radiative heat transfer

Near-field radiative heat transfer has recently attracted increasing interests for its applications in energy technologies, such as thermophotovoltaics. Existing works, however, are restricted to time-independent systems. Here, we explore near-field radiative heat transfer between two bodies under time modulation by developing a rigorous fluctuational electrodynamics formalism. We demonstrate that time modulation can result in the enhancement, suppression, elimination, or reversal of radiative heat flow between the two bodies, and can be used to create a radiative thermal diode with an infinite contrast ratio, as well as a near-field radiative heat engine that pumps heat from the cold to the hot bodies. The formalism reveals a fundamental symmetry relation in the radiative heat transfer coefficients that underlies these effects. Our results indicate the significant capabilities of time modulation for managing nanoscale radiative heat flow.

Integrated lithium niobate microwave photonic processing engine

Integrated microwave photonics (MWP) is an intriguing technology for the generation, transmission and manipulation of microwave signals in chip-scale optical systems1,2. In particular, ultrafast processing of analogue signals in the optical domain with high fidelity and low latency could enable a variety of applications such as MWP filters3-5, microwave signal processing6-9 and image recognition10,11. An ideal integrated MWP processing platform should have both an efficient and high-speed electro-optic modulation block to faithfully perform microwave-optic conversion at low power and also a low-loss functional photonic network to implement various signal-processing tasks. Moreover, large-scale, low-cost manufacturability is required to monolithically integrate the two building blocks on the same chip. Here we demonstrate such an integrated MWP processing engine based on a 4 inch wafer-scale thin-film lithium niobate platform. It can perform multipurpose tasks with processing bandwidths of up to 67 GHz at complementary metal-oxide-semiconductor (CMOS)-compatible voltages. We achieve ultrafast analogue computation, namely temporal integration and differentiation, at sampling rates of up to 256 giga samples per second, and deploy these functions to showcase three proof-of-concept applications: solving ordinary differential equations, generating ultra-wideband signals and detecting edges in images. We further leverage the image edge detector to realize a photonic-assisted image segmentation model that can effectively outline the boundaries of melanoma lesion in medical diagnostic images. Our ultrafast lithium niobate MWP engine could provide compact, low-latency and cost-effective solutions for future wireless communications, high-resolution radar and photonic artificial intelligence.

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.

Space-qualifying silicon photonic modulators and circuits

Reducing the form factor while retaining the radiation hardness and performance matrix is the goal of avionics. While a compromise between a transistor's size and its radiation hardness has reached consensus in microelectronics, the size-performance balance for their optical counterparts has not been quested but eventually will limit the spaceborne photonic instruments' capacity to weight ratio. Here, we performed space experiments of photonic integrated circuits (PICs), revealing the critical roles of energetic charged particles. The year-long cosmic radiation exposure does not change carrier mobility but reduces free carrier lifetime, resulting in unchanged electro-optic modulation efficiency and well-expanded optoelectronic bandwidth. The diversity and statistics of the tested PIC modulator indicate the minimal requirement of shielding for PIC transmitters with small footprint modulators and complexed routing waveguides toward lightweight space terminals for terabits communications and intersatellite ranging.

Metal Electrodes for Filtering the Localized Fundamental Mode of a Ridge Optical Waveguide on a Thin Lithium Niobate Nanofilm

An approach for filtering the fundamental mode in an integrated optical modulator with multimode waveguides based on etched thin lithium niobate nanofilms is presented. It is shown that metal electrodes can be used as a modal filter to suppress high-order modes in wide multimode ridge waveguides and, consequently, to provide their quasi-single-mode regime of operation. The influence of the gap between the electrodes and its displacement relative to the waveguide symmetry axis is analyzed for various configurations of waveguides. The conditions for quasi-single-mode light propagation with suppression of high-order modes of more than 90 dB/cm are found. The influence of fabrication errors on the efficiency of modal filtering is discussed. Efficient electro-optical modulation with an equivalent voltage-length product of 4 V∙cm has been experimentally demonstrated on integrated optical phase modulator samples fabricated using conventional contact photolithography. The proposed topological solution can be further used for the fast and cheap fabrication of TFLN modulators by conventional contact photolithography. The proposed modal filtering can also be used in other waveguide topologies and in more complex waveguide devices.

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.

Elimination of the fundamental mode hybridization on an x-cut lithium-niobate-on-insulator by using a densely packed bent waveguide array

Lithium niobate (L i N b O 3, LN) is a promising material for integrated photonics due to its natural advantages. The commercialization of thin-film LN technology has revitalized this platform, enabling low-loss waveguides, micro-rings, and compact electro-optical modulators. However, the anisotropic birefringent nature of X-cut LN leads to mode hybridization of TE and TM modes, which is detrimental to most polarization-sensitive integrated optical waveguide devices. A novel structure, to the best of our knowldege, utilizing a densely packed bent waveguide array is presented in this paper to eliminate mode hybridization. The refractive index is modulated in a manner that eliminates the avoided crossing of the refractive index curves of the TE and TM fundamental modes; thus, mode hybridization is prevented. The structures are readily accessible in the full range of commercially available LN film thicknesses from 400 to 720 nm and in any etching depth. The proposed structures give a polarization extinction ratio of -30d B across all bend radii, while simultaneously maintaining low excess loss of less than -1d B after reaching a 100 µm bend radius.

High-efficiency thin-film lithium niobate modulator with highly confined optical modes

We demonstrate a low-loss, high-efficiency lithium niobate electro-optic (EO) modulator with optical isolation trenches to achieve stronger field confinement and reduced light absorption loss. The proposed modulator realized considerable improvements, including a low half-wave voltage-length product of 1.2 V·cm, an excess loss of ∼2.4 dB, and a broad 3-dB EO bandwidth of over 40 GHz. We developed a lithium niobate modulator with, to the best of our knowledge, the highest reported modulation efficiency of any Mach-Zehnder interferometer (MZI) modulator.

Manipulating Coherence of Near-Field Thermal Radiation in Time-Modulated Systems

We show that the spatial coherence of thermal radiation can be manipulated in time-modulated photonic systems supporting surface polaritons. We develop a fluctuational electrodynamics formalism for such systems to calculate the cross-spectral density tensor of the emitted thermal electromagnetic fields in the near-field regime. Our calculations indicate that, due to time-modulation, spatial coherence can be transferred between different frequencies, and correlations between different frequency components become possible. All these effects are unique to time-modulated systems. We also show that the decay rate of optical emitters can be controlled in the proximity of such time-modulated structure. Our findings open a promising avenue toward coherence control in thermal radiation, dynamical thermal imaging, manipulating energy transfer among thermal or optical emitters, efficient near-field radiative cooling, and engineering spontaneous emission rates of molecules.

Correlated twin-photon generation in a silicon nitride loaded thin film PPLN waveguide

Photon-pair sources based on thin film lithium niobate on insulator technology have a great potential for integrated optical quantum information processing. We report on such a source of correlated twin-photon pairs generated by spontaneous parametric down conversion in a silicon nitride (SiN) rib loaded thin film periodically poled lithium niobate (LN) waveguide. The generated correlated photon pairs have a wavelength centred at 1560 nm compatible with present telecom infrastructure, a large bandwidth (21 THz) and a brightness of ∼2.5 × 10⁵ pairs/s/mW/GHz. Using the Hanbury Brown and Twiss effect, we have also shown heralded single photon emission, achieving an autocorrelation g H(2)(0)≃0.04.

Cost-effective fiber-to-lithium niobate chip coupling using a double-side irradiation self-written waveguide

In recent years, integrated lithium niobate (LN) chips have been widely used for developing a variety of photonic devices, such as high-speed electro-optical (EO) modulators and frequency comb generators. A major challenge for their practical applications is the high coupling loss between micrometer-scale LN waveguides and optical fibers. Lensed fibers and special taper structures are commonly used to tackle the coupling issue. However, in some situations, these approaches may increase the overall complexity and cost of design, fabrication, and alignment. Here, we propose using the self-written waveguide (SWW), an optical waveguide induced by light irradiation, to cope with this coupling issue. The approach can apply in connecting a single-mode fiber (SMF) to any waveguide surface in principle, even with a large mode-field mismatch, significantly alleviating the tight alignment requirements typically needed for end-fire coupling into integrated waveguides. Our study demonstrates that the coupling loss between a SMF with a mode-field diameter (MFD) of 4.4 µm and a sub-micrometer LN rib waveguide could be dramatically reduced from an initial value of -14.27 dB to -5.61 dB, after double-side irradiated SWW formation. Our proposed approach offers a potential solution for achieving a cost-effective and flexible fiber-to-LN chip optical interconnect.

Lithium niobate photonics: Unlocking the electromagnetic spectrum

Lithium niobate (LN), first synthesized 70 years ago, has been widely used in diverse applications ranging from communications to quantum optics. These high-volume commercial applications have provided the economic means to establish a mature manufacturing and processing industry for high-quality LN crystals and wafers. Breakthrough science demonstrations to commercial products have been achieved owing to the ability of LN to generate and manipulate electromagnetic waves across a broad spectrum, from microwave to ultraviolet frequencies. Here, we provide a high-level Review of the history of LN as an optical material, its different photonic platforms, engineering concepts, spectral coverage, and essential applications before providing an outlook for the future of LN.

Electro-optic control of the external coupling strength of a high-quality-factor lithium niobate micro-resonator

Controlling the optical coupling between a micro-resonator and waveguide plays a key role in on-chip photonic circuits. Here, we demonstrate a two-point coupled lithium niobate (LN) racetrack micro-resonator that enables us to electro-optically traverse a full set of the zero-, under-, critical-, and over-coupling regimes with minimized disturbance of the intrinsic properties of the resonant mode. The modulation between the zero- and critical-coupling conditions cost a resonant frequency shift of only ∼344.2 MHz and rarely changed the intrinsic quality (Q) factor of 4.6 × 10⁵. Our device is a promising element in on-chip coherent photon storage/retrieval and its applications.

High-performance lithium-niobate-on-insulator optical filter based on multimode waveguide gratings

A high-performance optical filter is proposed and realized with multimode waveguide grating (MWG) and two-mode multiplexers on the x-cut lithium-niobate-on-insulator (LNOI) platform for the first time, to the best of our knowledge. The present optical filter is designed appropriately to avoid material anisotropy as well as mode hybridness, and has a low excess loss of 0.05 dB and a high sidelobe suppression ratio (SLSR) of 32 dB in theory with Gaussian apodization. The fabricated filters show a box-like response with 1-dB bandwidth of 6-23 nm, excess loss of ∼0.15 dB, sidelobe suppression ratio of >26 dB. The device performance is further improved with a sidelobe suppression ratio as high as 48 dB and a low excess loss of ∼0.25 dB by cascading two identical MWGs.

Membrane multiple quantum well electro-optical modulator employing low loss high-k radio-frequency slot waveguides

A membrane multiple quantum well (MQW) electro-optical (EO) modulator exploiting low loss high-k radio-frequency (RF) slot waveguides is proposed for sub-terahertz bandwidth. By employing high-k barium titanate (BTO) claddings in place of doped InP cladding layers in traditional InP-based MQW modulators, the proposed modulator exhibits enhanced modulation efficiency and bandwidth as well as reduced insertion loss. A low half-wave voltage-length product of 0.24 V·cm is estimated, together with over 240 GHz bandwidth for a 2-mm-long modulation region, thus allowing sub-terahertz operation.

Compact thin film lithium niobate folded intensity modulator using a waveguide crossing

A small footprint, low voltage and wide bandwidth electro-optic modulator is critical for applications ranging from optical communications to analog photonic links, and the integration of thin-film lithium niobate with photonic integrated circuit (PIC) compatible materials remains paramount. Here, a hybrid silicon nitride and lithium niobate folded electro-optic Mach Zehnder modulator (MZM) which incorporates a waveguide crossing and 3 dB multimode interference (MMI) couplers for splitting and combining light is reported. This modulator has an effective interaction region length of 10 mm and shows a DC half wave voltage of roughly 4.0 V, or a modulation efficiency (Vπ ·L) of roughly 4 V·cm. Furthermore, the device demonstrates a power extinction ratio of roughly 23 dB and shows .08 dB/GHz optical sideband power roll-off with index matching fluid up to 110 GHz, with a 3-dB bandwidth of 37.5 GHz.

Phase-polarization ensemble modulation effect of a BaTiO3 crystal film waveguide on Mach-Zehnder interferometer electrooptic intensity modulators

In this work, based on the nonlinear electrooptic (EO) modulation model, the relationship between the polarization states and the birefringence modulation caused optical phase of c-axis barium titanate (BaTiO₃) crystal film is investigated first. Then, under an ensemble effect of phase-polarization modulation (PPM), the photon polarization behavior of a lightwave in a BaTiO₃ film waveguide and its dynamic conversion process is clearly presented. Further, the optical output and optical phase changes of a Mach-Zehnder interferometric (MZI) type EO intensity modulator based on a dynamic nonlinear optical birefringence modulation are theoretically modeled. As a result, the systematic simulations show that, for one-wave (2π) optical phase modulation (OPM), the full width at half-maximum of the output signal is efficiently compressed 30% more than the current OPM. Finally, the improvements of the MZI output signal by the PPM scheme are in accord with the experimental results.

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.

High-Production-Rate Fabrication of Low-Loss Lithium Niobate Electro-Optic Modulators Using Photolithography Assisted Chemo-Mechanical Etching (PLACE)

Integrated thin-film lithium niobate (LN) electro-optic (EO) modulators of broad bandwidth, low insertion loss, low cost and high production rate are essential elements in contemporary interconnection industries and disruptive applications. Here, we demonstrated the design and fabrication of a high performance thin-film LN EO modulator using photolithography assisted chemo-mechanical etching (PLACE) technology. Our device shows a 3-dB bandwidth over 50 GHz, along with a comparable low half wave voltage-length product of 2.16 Vcm and a fiber-to-fiber insertion loss of 2.6 dB. The PLACE technology supports large footprint, high fabrication uniformity, competitive production rate and extreme low device optical loss simultaneously, our result shows promising potential for developing high-performance large-scale low-loss photonic integrated devices.

On-chip erbium-doped lithium niobate microring lasers

Lithium niobate on insulator (LNOI), regarded as an important candidate platform for optical integration due to its excellent nonlinear, electro-optic, and other physical properties, has become a research hotspot. A light source, as an essential component for an integrated optical system, is urgently needed. In this Letter, we reported the realization of 1550 nm band on-chip LNOI microlasers based on erbium-doped LNOI ring cavities with loaded quality factors higher than 1 million at ∼970nm, which were fabricated by using electron beam lithography and inductively coupled plasma reactive ion etching processes. These microlasers demonstrated a low pump threshold of ∼20µW and stable performance under the pump of a 980 nm band continuous laser. Comb-like laser spectra spanning from 1510 to 1580 nm were observed in a high pump power regime, which lays the foundation of the realization of pulsed laser and frequency combs on a rare-earth ion-doped LNOI platform. This Letter effectively promotes the development of on-chip integrated active LNOI devices.

Monolithically integrated electro-optic modulator fabricated on lithium niobate on insulator by photolithography assisted chemo-mechanical etching

Integrated electro-optic (EO) modulators are one of the building blocks of photonic integrated circuits. Here, we design and fabricate an EO Mach–Zehnder waveguide modulator on lithium niobate on insulator using photolithography assisted chemo-mechanical etching technology. We optimize the performance of multi-mode interferometer which serves as the 3 dB splitter as well as that of the inverse taper to achieve efficient fiber-waveguide coupling, resulting in a fiber-to-fiber insert loss of 7.6 dB for the fabricated device, with a half wave voltage (HWV) (Vπ ) of 0.84 V and a HWV-length product (Vπ × L) of 3.4 V cm. The all-optical-lithography fabrication approach holds the promising potential for mass production of EO modulators of cost-effectiveness and low Vπ .

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.

2D-3D integration of hexagonal boron nitride and a high-κ dielectric for ultrafast graphene-based electro-absorption modulators

Electro-absorption (EA) waveguide-coupled modulators are essential building blocks for on-chip optical communications. Compared to state-of-the-art silicon (Si) devices, graphene-based EA modulators promise smaller footprints, larger temperature stability, cost-effective integration and high speeds. However, combining high speed and large modulation efficiencies in a single graphene-based device has remained elusive so far. In this work, we overcome this fundamental trade-off by demonstrating the 2D-3D dielectric integration in a high-quality encapsulated graphene device. We integrated hafnium oxide (HfO₂) and two-dimensional hexagonal boron nitride (hBN) within the insulating section of a double-layer (DL) graphene EA modulator. This combination of materials allows for a high-quality modulator device with high performances: a ~39 GHz bandwidth (BW) with a three-fold increase in modulation efficiency compared to previously reported high-speed modulators. This 2D-3D dielectric integration paves the way to a plethora of electronic and opto-electronic devices with enhanced performance and stability, while expanding the freedom for new device designs.

Here, three-dimensional hafnium oxide and two-dimensional hexagonal boron nitride are integrated in the insulating section of double-layer graphene optical modulators, leading to a maximum bandwidth of 39 GHz and enhanced modulation efficiency.

High-efficiency chirped grating couplers on lithium niobate on insulator

High-efficiency chirped grating couplers (GCs) with coupling efficiencies (CE) approaching 90%/coupler were designed by using a particle swarm optimization algorithm. These GCs were fabricated on $z$-cut lithium niobate on insulator (LNOI) with an Au layer on the lithium niobate substrate. The maximum CEs for transverse electric and transverse magnetic polarization input were measured up to ${\sim}{72.0}\%$/coupler and ${\sim}{61.6}\%$/coupler, respectively, which are the state-of-the-art values for LNOI GCs as far as we know. These GCs contribute to the realization of high-efficiency LNOI on-chip integrated optics.

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.

Fully reconfigurable coherent optical vector-matrix multiplication

Optics is a promising platform in which to help realize the next generation of fast, parallel, and energy-efficient computation. We demonstrate a reconfigurable free-space optical multiplier that is capable of over 3000 computations in parallel, using spatial light modulators with a pixel resolution of only 340×340. This enables vector-matrix multiplication and parallel vector-vector multiplication with vector size of up to 56. Our design is, to the best of our knowledge, the first to simultaneously support optical implementation of reconfigurable, large-sized, and real-valued linear algebraic operations. Such an optical multiplier can serve as a building block of special-purpose optical processors such as optical neural networks and optical Ising machines.

Polarization-independent, lithium-niobate-on-insulator directional coupler based on a combined coupling-sections design

We have proposed a polarization-independent directional coupler (DC) by using sections with optimal coupling strength designs on the lithium-niobate-on-insulator platform. With this design, arbitrary polarization-independent coupling ratio ranging from 0% to 100% can be achieved by tuning the length of the identical coupling region. The DC exhibits ultralow excess losses (<0.1dB) and polarization-dependent losses (<0.05dB for the complete coupling) over a bandwidth of 100 nm. Moreover, the proposed DC is compact in size, simple in structure, and easy to fabricate.

High-temperature-resistant silicon-polymer hybrid modulator operating at up to 200 Gbit s-1 for energy-efficient datacentres and harsh-environment applications

To reduce the ever-increasing energy consumption in datacenters, one of the effective approaches is to increase the ambient temperature, thus lowering the energy consumed in the cooling systems. However, this entails more stringent requirements for the reliability and durability of the optoelectronic components. Herein, we fabricate and demonstrate silicon-polymer hybrid modulators which support ultra-fast single-lane data rates up to 200 gigabits per second, and meanwhile feature excellent reliability with an exceptional signal fidelity retained at extremely-high ambient temperatures up to 110 °C and even after long-term exposure to high temperatures. This is achieved by taking advantage of the high electro-optic (EO) activities (in-device n3r33 = 1021 pm V-1), low dielectric constant, low propagation loss (α, 0.22 dB mm-1), and ultra-high glass transition temperature (Tg, 172 °C) of the developed side-chain EO polymers. The presented modulator simultaneously fulfils the requirements of bandwidth, EO efficiency, and thermal stability for EO modulators. It could provide ultra-fast and reliable interconnects for energy-hungry and harsh-environment applications such as datacentres, 5G/B5G, autonomous driving, and aviation systems, effectively addressing the energy consumption issue for the next-generation optical communication.

Lithium niobate photonic-crystal electro-optic modulator

Modern advanced photonic integrated circuits require dense integration of high-speed electro-optic functional elements on a compact chip that consumes only moderate power. Energy efficiency, operation speed, and device dimension are thus crucial metrics underlying almost all current developments of photonic signal processing units. Recently, thin-film lithium niobate (LN) emerges as a promising platform for photonic integrated circuits. Here, we make an important step towards miniaturizing functional components on this platform, reporting high-speed LN electro-optic modulators, based upon photonic crystal nanobeam resonators. The devices exhibit a significant tuning efficiency up to 1.98 GHz V⁻¹, a broad modulation bandwidth of 17.5 GHz, while with a tiny electro-optic modal volume of only 0.58 μm³. The modulators enable efficient electro-optic driving of high-Q photonic cavity modes in both adiabatic and non-adiabatic regimes, and allow us to achieve electro-optic switching at 11 Gb s⁻¹ with a bit-switching energy as low as 22 fJ. The demonstration of energy efficient and high-speed electro-optic modulation at the wavelength scale paves a crucial foundation for realizing large-scale LN photonic integrated circuits that are of immense importance for broad applications in data communication, microwave photonics, and quantum photonics.

Lithium niobate (LN) devices are promising for future photonic integrated circuits. Here, the authors demonstrate an electro-optic LN modulator with a very small modal volume based on photonic crystal resonator architecture.

Wafer-scale low-loss lithium niobate photonic integrated circuits

Thin-film lithium niobate (LN) photonic integrated circuits (PICs) could enable ultrahigh performance in electro-optic and nonlinear optical devices. To date, realizations have been limited to chip-scale proof-of-concepts. Here we demonstrate monolithic LN PICs fabricated on 4- and 6-inch wafers with deep ultraviolet lithography and show smooth and uniform etching, achieving 0.27 dB/cm optical propagation loss on wafer-scale. Our results show that LN PICs are fundamentally scalable and can be highly cost-effective.

High-performance coherent optical modulators based on thin-film lithium niobate platform

The coherent transmission technology using digital signal processing and advanced modulation formats, is bringing networks closer to the theoretical capacity limit of optical fibres, the Shannon limit. The in-phase/quadrature electro-optic modulator that encodes information on both the amplitude and the phase of light, is one of the underpinning devices for the coherent transmission technology. Ideally, such modulator should feature a low loss, low drive voltage, large bandwidth, low chirp and compact footprint. However, these requirements have been only met on separate occasions. Here, we demonstrate integrated thin-film lithium niobate in-phase/quadrature modulators that fulfil these requirements simultaneously. The presented devices exhibit greatly improved overall performance (half-wave voltage, bandwidth and optical loss) over traditional lithium niobate counterparts, and support modulation data rate up to 320 Gbit s-1. Our devices pave new routes for future high-speed, energy-efficient, and cost-effective communication networks.

Thin-film lithium niobate electro-optic modulator on a D-shaped fiber

We propose a low-insertion-loss electro-optic modulator formed with LNOI bonded on a D-shaped SMF. The proposed modulator employs high-performance Mach-Zehnder interferometer (MZI) formed with ridge LNOI waveguides and driven by travelling-wave electrodes. The light from the fiber core is coupled into a thin strip LNOI waveguide and then launched into the MZI via a ridge LNOI waveguide with tapered slab height and vice versa. Such all-fiber configuration exempts the need of the butt-coupling with an SMF. The calculated results show that our proposed modulator is capable of achieving a low insertion loss of less than 1.5 dB, an EO modulation efficiency (Vπ·L) of 2.05 V·cm, and a 3-dB modulation bandwidth of larger than 80 GHz. Our all-fiber LNOI modulator is feasible in practice and opens a new door to realize high-speed fiber devices by the integration of an optical fiber and thin film LN.

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.

Proposal for an ultra-broadband polarization beam splitter using an anisotropy-engineered Mach-Zehnder interferometer on the x-cut lithium-niobate-on-insulator

We propose and theoretically demonstrate an integrated polarization beam splitter on the x-cut lithium-niobate-on-insulator (LNOI) platform. The device is based on a Mach-Zehnder interferometer with an anisotropy-engineered multi-section phase shifter. The phase shift can be simultaneously controlled for the TE and TM polarizations by engineering the length and direction of the anisotropic LNOI waveguide. For TE polarization, the phase shift is -π/2, while for TM polarization, the phase shift is π/2. Thus, the incident TE and TM modes can be coupled into different output ports. The simulation results show an ultra-high polarization extinction ratio of ∼47.7 dB, a low excess loss of ∼0.9 dB and an ultra-broad working bandwidth of ∼200 nm. To the best of our knowledge, the proposed structure is the first integrated polarization beam splitter on the x-cut LNOI platform.

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.

A heterogeneously integrated silicon photonic/lithium niobate travelling wave electro-optic modulator

Silicon photonics is a platform that enables densely integrated photonic components and systems and integration with electronic circuits. Depletion mode modulators designed on this platform suffer from a fundamental frequency response limit due to the mobility of carriers in silicon. Lithium niobate-based modulators have demonstrated high performance, but the material is difficult to process and cannot be easily integrated with other photonic components and electronics. In this manuscript, we simultaneously take advantage of the benefits of silicon photonics and the Pockels effect in lithium niobate by heterogeneously integrating silicon photonic-integrated circuits with thin-film lithium niobate samples. We demonstrate the most CMOS-compatible thin-film lithium niobate modulator to date, which has electro-optic 3 dB bandwidths of 30.6 GHz and half-wave voltages of 6.7 V×cm. These modulators are fabricated entirely in CMOS facilities, with the exception of the bonding of a thin-film lithium niobate sample post fabrication, and require no etching of lithium niobate.

Design and Optimization of Proton Exchanged Integrated Electro-Optic Modulators in X-Cut Lithium Niobate Thin Film

In this study, we designed, simulated, and optimized proton exchanged integrated Mach-Zehnder modulators in a 0.5-μm-thick x-cut lithium niobate thin film. The single-mode conditions, the mode distributions, and the optical power distribution of the lithium niobate channel waveguides are discussed and compared in this study. The design parameters of the Y-branch and the separation distances between the electrodes were optimized. The relationship between the half-wave voltage length production of the electro-optic modulators and the thickness of the proton exchanged region was studied.

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 coupling efficiency grating couplers on lithium niobate on insulator

We demonstrate monolithically defined grating couplers in Z-cut lithium niobate on insulator for efficient vertical coupling between an optical fiber and a single mode waveguide. The grating couplers exhibit ∼ 44.6%/coupler and ∼ 19.4%/coupler coupling efficiency for TE and TM polarized light respectively. Taperless grating couplers are investigated to realize a more compact design.

Nanostructuring of LNOI for efficient edge coupling

We present the design, fabrication and characterization of LNOI fiber-to-chip inverse tapers for efficient monolithic edge coupling. The etching characteristics of various LNOI crystal cuts are investigated for the realization of butt-coupling devices. We experimentally demonstrate that the crystal cut limits the performance of mode matching tapers studied in this work. We report a butt-coupling loss of 2.5±0.5 dB/facet across the C/L-band and 6 dB/facet (at 1550 nm) by implementing 200 nm tip mode matching tapers in +Z-cut LNOI and X-cut MgO:LNOI, respectively. We anticipate that these results will provide insight into the nanostructuring of LNOI and into the further development of efficient butt-coupling in this platform.

Reconfigurable frequency coding of triggered single photons in the telecom C-band

In this work, we demonstrate reconfigurable frequency manipulation of quantum states of light in the telecom C-band. Triggered single photons are encoded in a superposition state of three channels using sidebands up to 53 GHz created by an off-the-shelf phase modulator. The single photons are emitted by an InAs/GaAs quantum dot grown by metal-organic vapor-phase epitaxy within the transparency window of the backbone fiber optical network. A cross-correlation measurement of the sidebands demonstrates the preservation of the single photon nature; an important prerequisite for future quantum technology applications using the existing telecommunication fiber network.

Low-loss fiber-to-chip interface for lithium niobate photonic integrated circuits

Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion, and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly >10  dB/facet) has dominated the total insertion losses of typical LN photonic integrated circuits, where on-chip losses can be as low as 0.03-0.1 dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetrically guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7 dB/facet with high fabrication tolerance and repeatability. Our results open the door for practical integrated LN photonic circuits efficiently interfaced with optical fibers.

Low-loss waveguides on Y-cut thin film lithium niobate: towards acousto-optic applications

We investigate the dependence of photonic waveguide propagation loss on the thickness of the buried oxide layer in Y-cut lithium niobate on insulator substrate to identify trade-offs between optical losses and electromechanical coupling of surface acoustic wave (SAW) devices for acousto-optic applications. Simulations show that a thicker oxide layer reduces the waveguide loss but lowers the electromechanical coupling coefficient of the SAW device. Optical racetrack resonators with different lengths were fabricated by argon plasma etching to experimentally extract waveguide losses. By increasing the oxide layer thickness from 1 µm to 2 µm, we were able to reduce propagation loss of 2 µm (1 µm) wide waveguide from 1.85 dB/cm (3 dB/cm) to as low as 0.37 dB/cm (0.77 dB/cm). Resonators with a quality factor greater than 1 million were demonstrated as well. An oxide thickness of approximately 1.5 µm is sufficient to significantly reduce propagation loss, due to leakage into the substrate and simultaneously attain good electromechanical coupling in acoustic devices. This work not only provides insights on the design and realization of low-loss photonic waveguides in lithium niobate, but also, most importantly, offers experimental evidence of how the oxide thickness directly impacts losses and guides its selection for the synthesis of high-performance acousto-optic devices in Y-cut lithium niobate on insulator.

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.

High-extinction electro-optic modulation on lithium niobate thin film

Integrated nanophotonics using lithium-niobate-on-insulator promises much-awaited solutions for scalable photonics techniques. One of its core functions is electro-optic modulation, which currently suffers limited extinction (<30  dB) due to inevitable fabrication errors. We exploit a cascaded Mach-Zehnder interferometry design to offset those errors, demonstrating up to 53 dB modulation extinction for a wide range of wavelengths between 1500 nm and 1600 nm. Together, its favorable features of chip integration, high extinction, good stability, and being broadband may prove valuable in a plethora of flourishing photonics applications.

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.