Integrated lithium niobate photonic platform

Thin-film lithium niobate (TFLN) has rapidly emerged as a highly competitive integrated photonic platform in fields such as data communication, microwave photonics, and quantum information processing. However, as a critical interface for chip-fiber links, the TFLN edge coupler is limited by its mode field diameter, exhibiting extremely low assembly tolerance when coupling with a standard single-mode fiber (SSMF), which severely restricts its application. Here, we propose a TFLN edge coupler based on a silicon nitride trident structure. By converting the small mode field of the lithium niobate waveguide into the large mode field of the Si3N4 trident structure, a high-efficiency, high-tolerance, and process-compatible edge coupler is constructed. Three-dimensional simulation results show that the device exhibits a mode field diameter greater than 8 μm, with TE mode coupling loss less than 0.71 dB, TM mode coupling loss less than 0.91 dB in the C+L band, and polarization-dependent loss below 0.41 dB. This device provides a promising solution to the challenge of high-performance, high-tolerance interconnection between TFLN chips and SSMFs.

Multiplexing and demultiplexing of optical orbital angular momentum (OAM) are critical operations in mode-division multiplexing communications. Traditional Dammann gratings, spiral phase planes, and optical geometric transformations are regarded as convenient methods for OAM mode (de)multiplexing. However, crosstalk between the different modes and the difficulty of mode multiplexing greatly limit their application to mode-division multiplexing communications. Here, using a set of inversely-designed phase planes, we demonstrate an OAM (de)multiplexer based on multiphase plane light conversion that can enable perfect OAM multiplexing communication. The sorted patterns are Gaussian-like and can be coupled easily into single-mode fiber arrays. Inputs from the fiber array are turned into coaxial OAM modes after the phase planes. OAM mode crosstalk generated by the multiplexer is less than − 20 dB , with insertion loss of less than 2.6 dB. OAM modes are sorted by the demultiplexer with mode crosstalk below − 10 dB , and the sorting results are coupled to the fiber array. OAM modes carrying 10 Gbit/s on–off keying signals were transmitted in a 5 km few-mode fiber. The measured bit-error-rate curves have power penalties of less than 10 dB. The proposed configuration is highly efficient and convenient and will be beneficial for potential applications in quantum information, information processing, and optical communications.

Transfer and conversion of images between different wavelengths or polarization has significant applications in optical communication and quantum information processing. We demonstrated the transfer of images based on electromagnetically induced transparency (EIT) in a rubidium vapor cell. In experiments, a 2D image generated by a spatial light modulator is used as a coupling field, and a plane wave served as a signal field. We found that the image carried by coupling field could be transferred to that carried by signal field, and the spatial patterns of transferred image are much better than that of the initial image. It also could be much smaller than that determined by the diffraction limit of the optical system. We also studied the subdiffraction propagation for the transferred image. Our results may have applications in quantum interference lithography and coherent Raman spectroscopy.

Optical vortex beams, carrying orbital angular momentum (OAM), are of great interest for applications in optical communications, particle manipulation, and quantum information processing. However, achieving high‐efficiency generation and manipulation of vortex beams with a single compact element, particularly in the visible range, remains challenging. Here, we demonstrate laser‐written geometric phase fork gratings in silica glass that enable simultaneous generation and propagation control of vortex beams with an efficiency up to 98.4% at 515 nm. By tailoring the phase‐gradient period and topological charge ( l = 1–100), we achieve tunable control over the vortex beam's topological charge, diameter, and diffraction position. The fabricated gratings maintain high efficiency across a broad spectral range from 450 to 690 nm. Furthermore, we demonstrate spin–orbit coupling between incident vortices and fork gratings, verifying that the output OAM values follow the expected addition rule l out = l in ± l fork . In addition, the fork gratings exhibit excellent thermal stability (up to 800°C) and optical damage threshold (close to pristine silica glass). These results establish nanopore‐based geometric‐phase fork gratings as a scalable, broadband, high‐performance platform with outstanding thermal and optical tolerance for next‐generation OAM photonics.

We investigate the coherent control of dressing interaction in four-wave mixing (FWM) in a Y-type four-level atomic system based on the dressed states theory. We study the density matrix equation acted by two dressing fields simultaneously governing FWM signal intensity and the eigenvalues under the dressed state formalism. It is shown that the peak position and intensity of Autler–Townes splitting both originate from the dressing interaction as the detuning of the probe field is scanned. Moreover, it is observed that the interaction could be controlled by the dressing fields. Such flexibly controlled nonlinear signal has potential applications in optical communication and quantum information processing.

Low loss ridge waveguides on the hybrid single crystalline silicon and lithium niobate thin films

Thin film lithium niobate has been widely considered as a promising photonic integration platform. However, due to the non-vertical sidewall of lithium niobate etching, it is challenging to design an efficient fiber coupler for the submicron-sized lithium niobate waveguide. Here, a high-performance edge coupler to a cleaved fiber is introduced on the thin film lithium niobate platform using a sharp three-dimensional taper. The proposed taper structure, which ensures a nearly adiabatical transition of both transverse-electric (TE) and transverse-magnetic (TM) modes, is formed by two standard patterning steps for lithium niobate. A silica ridge waveguide is used as the intermediate transition structure between the taper and a high-numerical-aperture fiber. The simulated coupling losses are as low as −0.1 dB per facet, while the experimental mean values reach −0.29 and −0.24 dB for TE and TM modes, respectively. The coupling spectra exhibit a flat wavelength response, which almost coincides for both polarizations. The 0.5-dB coupling bandwidth is beyond 180 nm. Within this bandwidth, the worst coupling losses are −0.61 and −0.56 dB for TE and TM modes, respectively, and the polarization-dependent loss is below 0.17 dB. The present edge coupler also exhibits a good fabrication tolerance, and its fabrication can also facilitate wafer-scale processing without wafer dicing and end-facet polishing.

Electro-optic phase modulation is a core technology for signal processing in optical communication and microwave photonics, yet the current multifunctional integrated devices for high-bandwidth phase modulation and efficient frequency doubling suffer from critical bottlenecks including large crosstalk, excessive chip area, bandwidth-voltage trade-off and single material performance limitations. To address these issues, this study proposes a Si₃N₄-TFLN heterogeneous integrated design scheme by synergizing the low-loss property of silicon nitride and the strong electro-optic effect of thin-film lithium niobate. A dual-drive Mach-Zehnder modulator (DD-MZM) and a racetrack micro-ring resonator (MRR) with high quality-factor and large free spectral range are cascaded to realize the monolithic integration of high-precision phase modulation and microwave signal frequency doubling. Co-simulation results show that the designed device achieves an electro-optic modulation bandwidth of 72 GHz and a half-wave voltage-length product of 2.8 V·cm, which well reconciles the bandwidth-voltage trade-off. The device exhibits a frequency doubling efficiency of 15%, 30.4% higher than that of the single TFLN-based MRR. The cascaded coupling structure suppresses the functional crosstalk to -38 dB, and the total chip layout area is only 3.648 mm², over 60% smaller than that of the existing similar devices. This work provides a high-performance on-chip solution for high-density integrated systems in optical communication, microwave photonics and quantum information processing.

In this work, we present a free-space transmissive light amplitude modulator based on thin-film lithium niobate on an insulator platform with an indium tin oxide meta-grating. The design leverages guided mode resonances induced by the transparent conductive oxide layer, enabling efficient electrooptical modulation in the near-infrared region. By integrating transparent conductive oxide both as electrical contact and as the resonating structure, the device eliminates the need for complex alignment during fabrication and minimizes optical losses associated with metallic contacts. We experimentally demonstrate that the device achieves a fundamental mode resonance at 968.5 nm with a quality factor of 440. The electrooptical tuning efficiency is thoroughly investigated across different modes using measurements and simulations. A notable resonance shift of 0.38 nm is observed for the fundamental mode under a ±10 V bias, while a maximum modulation amplitude of 4.6% is achieved with a higher-order mode. Furthermore, the device exploits incident angle tuning as an additional degree of freedom, effectively splitting and sensitively shifting resonances. The new resonances can provide electrooptic tunability. These results highlight the potential of this compact and scalable design for applications in spatial light modulation, optical communications, and tunable optics.

Soliton microcombs are a cornerstone of integrated frequency comb technologies, with applications spanning photonic computing, ranging, microwave synthesis, optical communications, and quantum light generation. In nearly all such applications, electro-optic (EO) components play a critical role in generating, monitoring, stabilizing, and modulating solitons. Toward building photonic integrated circuits for next-generation applications that will simultaneously maximize system performance and minimize size, weight, and power consumption metrics, achieving soliton microcombs and efficient EO modulation on a chip is essential. X-cut thin-film lithium niobate (TFLN) has emerged as a leading photonic platform for the realization of high-performance integrated EO devices and systems. However, despite extensive research, soliton microcombs have remained elusive to X-cut TFLN due to its multiple strong Raman-active modes, in-plane refractive index anisotropy, and photorefractive effects. Here, we address this long-standing challenge and demonstrate versatile soliton microcombs on X-cut TFLN, with repetition rates spanning from the gigahertz (∼26GHz, single soliton) up to the millimeter-wave (∼0.156THz, soliton crystal) regime. The combs feature exceptional long-term stability, maintaining a direct injection-locked state for over 90 min (manually terminated), with repetition-rate phase noise closely tracking that of a high-quality electronic microwave synthesizer. Our finding broadly advances both the fundamental science and practical applications of integrated comb sources by enabling efficient EO modulation and broadband coherent solitons to be monolithically combined on the same chip.

Thin-film lithium niobate (TFLN) electro-optic modulators (EOMs) offer distinct advantages, including high speed, broad bandwidth, and low power consumption. However, their large size hinders the density of integration, which trades off with the half-wave voltage. Photonic crystal (PC) structures can effectively reduce the device footprint via the slow-light effect; however, they experience significant losses due to fabrication defects and sharp corners. Here, we theoretically demonstrate an ultracompact Mach–Zehnder interferometer (MZI) EOM based on a TFLN valley photonic crystal (VPC) structure. The design can achieve a high forward transmittance (>0.8) due to defect-immune unidirectional propagation in the VPC, enabled by the unique spin-valley locking effect. The EOM, with a small footprint of 21 μm × 17 μm, achieves an extinction ratio of 16.13 dB and a modulation depth of 80%. The design can be experimentally fabricated using current nanofabrication techniques, making it suitable for broad applications in optical communications.

The escalating capacity limitations of conventional near-infrared telecommunication bands have spurred urgent investigations into wide-band optical communication systems spanning from the near-infrared to mid-infrared regimes. This has motivated the development of optical components combining broadband bandwidth with high-speed operation. The state-of-the-art modulators face challenges in achieving broad operational bandwidth due to waveguide dispersion and velocity mismatch. Here we demonstrate a thin-film lithium niobate (TFLN) electro-optic (EO) modulator with an unprecedented 800-nm operational bandwidth, covering the full O-U telecom bands and extending into the 2-μm regime. The TFLN modulator exhibits >67 GHz EO bandwidth across O-U bands (~100 GHz at O-/S-/C-/L-bands) and >50 GHz at 2-μm band. It enables single-lane exceeding 240 Gbps PAM-4 transmission across O-U bands and a record 170 Gbps PAM-4 transmission at 2-μm band. This breakthrough establishes TFLN as a compelling platform for multispectral photonics, bridging conventional telecom infrastructure with emerging 2-μm technologies for next-generation optical communications.

Thin-film lithium niobate (TFLN) has been widely used in electro-optic modulators, acoustic--optic modulators, electro-optic frequency combs, and nonlinear wavelength converters owing to the excellent optical properties of lithium niobate. The performance of these devices is highly dependent on the fabrication quality of TFLN. Although state-of-the-art TFLN microrings with an intrinsic quality factor (Q-factor) exceeding 1 × 107 have been realized by inductively coupled plasma-reactive ion etching (ICP-RIE) and chemical mechanical polishing (CMP), ICP-RIE has moderate throughput, moderate reproducibility, and high cost in etching TFLN, while CMP features moderate throughput and low cost in etching TFLN. Here, a wet etching method for TFLN, leading to the fabrication of a micro-racetrack with an intrinsic Q-factor of over 9.27 × 106 is developed. The suitability of this method to fabricate a narrow coupling gap between the bus waveguide and microring enables the coupling conditions of the microring to be customized. This method features a high throughput, a high reproducibility, and a low cost in etching TFLN, showing the potential to boost the mass production of integrated LN photonic devices with high fidelity and affordabilitydramatically.

Nonlinear frequency conversion offers powerful capabilities for applications in telecommunications, signal processing, and computing. Thin-film lithium niobate (TFLN) has emerged as a promising integrated photonics platform due to its strong electro-optic effect and second-order nonlinearity, which can be exploited through periodic poling. However, conventional poling techniques in x-cut TFLN are constrained to minimum period sizes on the order of microns, restricting access to highly phase-mismatched interactions such as counter- and backward-propagating frequency conversion. In this work, we demonstrate scalable periodic poling of x-cut TFLN with domains periods as short as 215 nm and realize devices that support both counter- and back-propagating phase matching. We estimate conversion efficiencies of 1,474 %/W/cm2 and 45 %/W/cm2 for the two interaction types, respectively. Sum frequency generation measurements confirm that the nonlinear generation takes place in the desired direction. Furthermore, we report spontaneous parametric down conversion for the counter-propagating configuration and, for the first time, for a backward propagating device. This breakthrough provides unprecedented control over engineering of ferroelectric domain geometries in TFLN, leading into the generation of photon pairs with precisely tailored spatial and spectral characteristics. Such capabilities hold strong potential for advancing quantum signal processing, scalable quantum computing architectures, and precision quantum metrology.

Despite the growing interest in thin film lithium niobate (TFLN) as a material platform for photonic integrated circuits (PIC), its moderate refractive index, CMOS-incompatible fabrication processes and inherent material anisotropy still raise questions about achieving dense integration comparable to mature platforms like silicon photonics while preserving the superior properties of lithium niobate. Here we show a photonic inverse design method to enable miniaturization and dense integration of lithium niobate PIC components. As proofs-of-concept, we experimentally demonstrate ultra-compact mode-division (de)multiplexer (19 × 25 μm²), multimode waveguide crossing (15 × 15 μm2), and waveguide bends (30 μm bending radius). The fabricated components are used to construct multimode photonic circuits for large-capacity data communications, demonstrating dense integration of over 10 waveguide elements within a 0.06 mm² chip area. By integrating electro-optic modulators on the same chip, high-speed data modulation is demonstrated with 120 Gbps data rate per channel alongside multimode signal transmission. This work is expected to advance 10-fold higher area density of passive components and optical path design in TFLN.