薄膜铌酸锂光电器件与超大规模光子集成（特邀） 
下载： 1854次封底文章创刊五十周年特邀
近些年来，由于大数据、人工智能等信息技术发展的迫切需求，光子集成技术得到了极大关注。目前可以用作光子集成的材料体系有很多，受关注的主要有Si/SiN、Ⅲ-Ⅴ族材料（以InP为代表）、以及铌酸锂（LN）等。材料的选取要考虑两个关键指标：波导传输损耗和片上电光调制功耗。铌酸锂以其大透明窗口、低传输损耗、良好的电光/压电/非线性等物理性能，以及优良的机械稳定性等，在实现这两个指标方面，有其特有的优势。
在《中国激光》创刊50周年之际，华东师范大学程亚教授应邀为该刊撰写了《薄膜铌酸锂光电器件与超大规模光子集成》一文，该文详细回顾了薄膜铌酸锂光子技术的发展历程，介绍了重要光子器件如电光调制器、延迟线、电光频率梳、量子光源、非线性超构表面、激光器与放大器等近期的研究进展。由于超大规模光子集成在量子信息、人工智能和大数据等方面有重要应用，程亚教授也介绍了其团队在薄膜铌酸锂大规模光子集成技术方面的探索以及所取得的进展。该综述内容虽然广泛但针对性强，无论是正从事薄膜铌酸锂光电器件研发的人员，还是关注光子集成技术发展的读者，阅读该文后都会有所收益。
目前薄膜铌酸锂光电器件与规模光子集成普遍使用的还是电子束曝光或深紫外（DUV）光刻结合离子束刻蚀技术，程亚教授针对铌酸锂特有的物理与化学性质，提出了飞秒激光直写辅助化学机械抛光（PLACE）的薄膜铌酸锂加工新技术。其团队采用这种技术制备出了多种高性能薄膜光电器件，实现了传输损耗低至0.027 dB/cm的波导、
该综述在“总结与展望”部分指出，虽然薄膜铌酸锂光子技术已经实现了一系列突破，但目前所研制的薄膜器件在性能上距其应达到的物理极限尚有很大距离，薄膜铌酸锂集成光子技术发展仍有很大的提升空间。硅光技术有微电子技术可以“借鉴”，而薄膜铌酸锂的精细加工，还需根据材料自身的特性，继续进行探索和创新。与硅光等相对成熟的技术相比，薄膜铌酸锂集成光子技术整体正处于快速发展阶段。如何让薄膜铌酸锂材料更好、更快地助力光电子技术的发展，最终实现产业化，学术界与产业界仍需共同努力！
2024-01-19
近年来，薄膜铌酸锂光子集成技术发展极为迅速，其背后有着深刻的物理、材料、技术原因。单晶薄膜铌酸锂为解决光子集成芯片领域长期存在的低传输损耗、高密度集成以及低调制功耗需求提供了至今为止综合性能最优的解决方案。面向未来的新一代高速光电器件与超大规模光子集成芯片应用，本文回顾了薄膜铌酸锂光子技术的起源及其近期的快速发展，讨论了若干薄膜铌酸锂光子结构的加工技术，并展示了一系列当前性能最优的薄膜铌酸锂光子集成器件与系统，包括超低损耗可调光波导延时线、超高速光调制器、高效率量子光源，以及高功率片上放大器与片上激光器。这些器件以其体积小、质量轻、功耗低、性能好的综合优势，将对整个光电子产业产生难以估量的影响。
The recent advancement in thin film lithium niobate photonic integration technology has been rapid, driven by profound physical, material, and technological factors. Single crystal thin film lithium niobate is particularly noteworthy for offering the most comprehensive performance solution to date, addressing long-term challenges in low transmission loss, high-density integration, and low modulation power consumption within the realm of photonic integrated circuits. This paper provides an overview of the origin and recent swift development of thin film lithium niobate photonic technology, focusing on its potential for the future generation of high-speed optoelectronic devices and ultra-large-scale photonic integrated circuit applications. Various processing technologies for thin film lithium niobate photonic structures are discussed, accompanied by the introduction of current high-performance devices and systems. These include ultra-low loss tunable optical delay lines, ultra-fast light modulators, high-efficiency quantum light sources, as well as high-power on-chip amplifiers and lasers. These devices, distinguished by their unprecedented advantages of small size, light weight, low power consumption, and high performance, are poised to make a tremendous impact on the entire optoelectronic industry.
Since the laboratory production of the lithium niobate single crystal, lithium niobate has emerged as a crucial material in the fields of electro-optics and nonlinear optics. Optical modulators based on the linear electro-optic effect, utilizing lithium niobate, offer notable advantages including high modulation rates, a substantial extinction ratio, low chirp, and high linearity. In the 21st century, integrated photonics technology has gained increasing attention for two primary reasons. Firstly, the demand for photonic devices with attributes such as low energy consumption, high-performance computing, and reconfiguration capabilities has surged due to the explosive growth in applications for information technology, including big data, artificial intelligence, high-speed networks, virtual reality, and quantum information processing. Secondly, integrated photonics technology itself has been continuously advancing. The enhancement of on-chip photonic device performance and cost reduction have reached a threshold that is propelling the emergence of a new photonic industry.
Traditional fabrication techniques, such as ion diffusion and ion implantation, impose limitations on the modulation efficiency and power consumption of lithium niobate modulators, impeding the progress of lithium niobate photonic integration technology. Overcoming this challenge requires addressing both material platforms and device fabrication. In this context, thin film lithium niobate has emerged as a pivotal material, paving the way for integrated photonics. China has long been engaged in research on lithium niobate materials and photonics, contributing significantly to the field's advancement through several crucial milestones. Therefore, it is essential to summarize the recent revolution to provide guidance for future development.
The fabrication flow diagram for the electron beam lithography combined with ion etching process is presented (Fig. 1). A schematic diagram of the photolithography-assisted chemo-mechanical etching (PLACE) technique is provided (Fig. 2). The report includes an ultra-high-speed high-resolution laser lithography system for lithium niobate integrated photonics (Fig. 3). Through the integration of thin film lithium niobate with advanced fabrication techniques, substantial progress has been achieved in the development of lithium niobate photonic devices, encompassing delay lines (Fig. 4), high-speed electro-optical modulators (Fig. 5), optical frequency combs (Fig. 6), quantum light sources (Fig. 7), metasurfaces (Fig. 8), waveguide lasers (Fig. 9), and amplifiers (Fig. 10). Additionally, the article outlines some integrated photonics applications. Specifically, the achievement of electro-optically 4×4 programmable photonic circuits enabled by wafer-scale integration on thin film lithium niobate is illustrated (Fig. 11). This device, composed of cascaded MZIs, demonstrates a total on-chip power dissipation of only 1.5 mW when operated at a 100 MHz modulation rate. Furthermore, an on-chip arrayed waveguide grating (AWG) fabricated on thin film lithium niobate with on-chip loss as low as 3.32 dB is reported (Fig. 12). Finally, four-channel waveguide amplifiers fabricated on monolithically integrated active/passive thin film lithium niobate are also showcased (Fig. 11), demonstrating a robust low-loss optical interface for the monolithic integration of passive and active thin film lithium niobate photonics.
Significant enhancements have been achieved in critical parameters of photonic devices, including modulation bandwidth, power consumption, propagation loss, and active and passive functionalities, as well as advancements in large-scale integration. These technological strides are poised to benefit the evolution of integrated photonics applications. However, some exceptional performances have yet to reach the physical limits of lithium niobate photonics devices, necessitating further efforts in thin film lithium niobate integrated photonics technology. Notably, numerous thin film lithium niobate photonics devices have already approached or even attained optical performances suitable for industrial applications. This opens up abundant opportunities for the development of next-generation optical information technology.
1 引言
1893年，电光晶体材料由德国物理学家普克尔（F. Pockels）［1］首次发现。电光晶体是具有特殊功能的光学材料，其主要特点是在施加电压后晶体中不需要产生自由载流子就能够产生非常显著的光学折射率变化。这一现象背后的物理机制是材料在电场中被极化从而改变了晶体内的微观电磁响应，其宏观表现即为光折射率受到了电场的调制。1965年，Ballman［2］首次在实验室成功制备了具有高电光系数的铌酸锂单晶，并展示了铌酸锂单晶的制备方法及其物理特性的测量和表征。此后，铌酸锂单晶成为电光和非线性光学研究中的重要材料之一，并在光通信、光学传感和光子集成等领域发挥着重要作用。
我国很早就在铌酸锂材料与铌酸锂光子学方面开展了研究，并对该领域的发展作出了若干极其关键的里程碑式贡献。早在20世纪70年代，南京大学的冯端先生、闵乃本先生就开展了与激光有关的晶体生长、晶体缺陷与晶体物性的研究，并于1980年首次实现了周期极化铌酸锂晶体，即PPLN单晶材料，同时他们从实验上观察到了准相位匹配增强效应［3］。随后，闵乃本先生和祝世宁先生等通过进一步发展准周期相位匹配原理，利用准周期结构提供的多个独立倒格矢匹配多个光参量过程，在一块周期极化晶体上实现了红、绿、蓝三基色光的同轴输出［4-9］。在铌酸锂晶体光折变特性研究方面，王华馥先生等于1980年首次报道了可以通过掺镁来显著提升铌酸锂晶体光损伤阈值这一研究成果，并发现当铌酸锂中掺杂MgO的物质的量浓度达到4.6%时，其在可见光波段的抗光折变能力可以提高两个量级以上［10］。这些研究对于铌酸锂电光器件的发展极为重要。之后，国内研究人员在铌酸锂的弱光非线性、紫外光折变效应、光全息存储、光折变波导和孤子等方面取得了一系列研究成果［11-18］。这些长期的积累使得我国在铌酸锂光子学国际舞台上持续保持着强劲的竞争力。
铌酸锂光子学的发展离不开铌酸锂单晶材料与激光技术的融合。1960年，梅曼（T. H. Maiman）［19］首次在实验室中发明了激光这一相干光源。激光一经发明便快速获得了几乎所有重要科研领域与应用产业的青睐。这一众所周知的事实这里就不再赘述。值得一提的是，当时还未出茅庐的光通信领域也对激光情有独钟。1966年，贝尔实验室将激光光源与铌酸锂光调制器件相结合，开展了针对自由空间光通信的研究［20］。1973年，贝尔实验室又开展了光集成技术研究——在铌酸锂单晶衬底上利用离子扩散形成光波导回路［21-23］。这些研究的一项显著成果就是目前高速光通信领域不可或缺的铌酸锂光调制器。基于线性电光效应的铌酸锂光调制器具有高调制速率、高消光比、低啁啾、高线性度等优势。然而，由于离子扩散型波导天然地具有较大的波导模场尺寸，迫使波导两侧的行波电极之间存在较大的间隔，从而无法采用较低的调制电压来实现有效的光调制。这一特点严重限制了铌酸锂调制器的调制速度与功耗。同时，离子扩散型波导芯与波导包层间固有的较小的折射率差使得该类波导不可避免地要采用较大的弯曲半径来获得较低的弯曲损耗，从而阻碍了铌酸锂基光子集成技术的发展。这一局面势必要从材料制备和器件原理两方面来突破。
进入21世纪后，光子集成技术逐渐成为一个越来越受到关注的课题。其原因无外乎两方面：其一，信息技术的应用达到了新的爆炸点，诸如大数据、人工智能、高速网络、虚拟现实、量子信息等等都对具有低能耗、超高算力、可高速重构的光子器件提出了迫切需求；其二，光子集成技术自身也在不断突破，片上器件的性能和成本不断降低，达到了驱动新产业生态的门槛。在这一背景下，薄膜铌酸锂逐渐成为一种重要的光子集成材料。该材料最早由Levy等［24］于1998年通过离子注入剥离技术制备，当时Levy等获得了厚度约为9 μm的悬空薄膜。从那时起，该技术经历了不断的改进，以便更好地实现薄膜铌酸锂的厚度控制和低粗糙度表面，同时提供更多可选择的缓冲层材料（缓冲层位于薄膜铌酸锂材料和一层较厚的底座材料之间，厚度一般为数微米。缓冲层材料通常与薄膜铌酸锂协同作用构成功能层）用于与薄膜铌酸锂键合。如今，我国的济南晶正电子科技有限公司（NANOLN）、上海新硅聚合半导体有限公司等已经可以制备出表面粗糙度低于0.5 nm的亚微米膜厚的4 inch（1 inch=2.54 cm）及以上尺寸的商用薄膜铌酸锂晶圆。高质量薄膜铌酸锂晶圆的开发和产业化取得了巨大成功，这对于基于薄膜铌酸锂的光子集成技术的发展起到了关键推动作用。在过去的20年里，研究人员使用多种纳米加工技术，包括化学湿法刻蚀［25-27］、等离子体干法刻蚀［27-34］、机械划片和切割［35-40］以及化学机械抛光［41-43］等，在薄膜铌酸锂上制备了各种微纳光子结构，目标是在薄膜铌酸锂平台上实现高性能光子集成器件。
2 铌酸锂光子器件制备技术进展
在光子学领域，铌酸锂以其出色的光学性质成为高性能光子器件材料的理想选择。然而，铌酸锂作为一种化学惰性材料，具有较高的化学稳定性，难以通过传统的化学刻蚀方法进行加工。此外，铌酸锂材料本身的硬度较高，这使得采用机械划刻的方式也具有一定的难度。传统的铌酸锂波导通常采用金属扩散、质子交换以及离子注入等工艺，在铌酸锂块体材料上通过化学组分修饰的方式进行制备［44］。然而，这样制备的波导存在一些缺点，例如折射率差异小，模场束缚效应弱，波导弯曲曲率半径较大，因此不适合在光子集成芯片上实现光学元件的高密度集成。近年来，薄膜铌酸锂材料的发展为片上光子集成器件开辟了一条新途径。薄膜铌酸锂与空气包层及其下方的二氧化硅的折射率存在显著差异，这使得实现光场紧束缚波导成为可能。同时，薄膜铌酸锂降低了刻蚀深度要求，将波导结构从三维转变为二维［45-46］。早期的研究已经表明，可以使用纯机械方法（如金刚石切削）在薄膜铌酸锂上制备出脊形波导［47-48］。这种方法能够制备高纵横比、低光学损耗的直波导［35，46］，以及曲率半径较大的弯曲波导。但是对于更复杂光学器件的制备以及光子集成而言，这种方法仍然面临诸多挑战。
对于大规模光子集成应用，基本的单元器件（如光波导）需要同时具备超低传输损耗、小弯曲半径和高调制效率这三个关键特性。在早期研究阶段，为了制备具有低损耗特性的薄膜铌酸锂光子结构，研究人员尝试了比纯机械办法更灵活和更精细的加工方法，例如激光光刻和氩离子刻蚀以及聚焦离子束刻蚀等技术。尽管采用这些技术加工的结构尚未达到适用于高性能器件的标准，但这些开创性尝试证明了在薄膜铌酸锂上制造纳米光子结构的可行性，同时凸显了通过降低粗糙度来降低波导传输损耗的难度［47］。自2005年起，氩离子刻蚀方法开始用于在薄膜铌酸锂上制造光子器件，如脊形波导［49］和光学微腔［30］。在这一方法中，微腔品质因子（Q值）主要受限于表面和侧壁的粗糙度。为了降低散射损耗，研究人员采用了在薄膜铌酸锂上覆盖二氧化硅层的方法。通过这种方法制备了一种覆盖有二氧化硅层的由宽度为4 μm的多模波导和半径为100 μm的微环耦合的薄膜铌酸锂结构，实验测得铌酸锂微环的负载Q值为4.0×103，对应于脊形波导的传输损耗约为4 dB/cm。这一损耗值还远远达不到光子学应用的要求。除了氩离子刻蚀技术，2009年，国外研究人员尝试使用聚焦离子束刻蚀制备纳米光子结构，但由于再沉积效应的影响，结构表面仍然具有较大的粗糙度［31］。值得注意的是，光子器件的损耗性能主要受制于表面粗糙度，而非薄膜铌酸锂本身固有的材料吸收。这一认识为后续研究指明了方向，有助于研究人员集中精力克服粗糙度带来的传输损耗的限制。
2014年，为了克服以往制备技术的不足，中国科学院上海光学精密机械研究所（以下简称“上海光机所”）将飞秒激光直写与聚焦离子束刻蚀技术相结合，将表面粗糙度降低至1 nm以下，制备出了直径约为82 μm的回音壁模式微腔，其在1550 nm附近具有约2.5×105的Q值［50］。通过优化加工参数，Q值很快被提升至106以上［51-52］。上海光机所首次将薄膜铌酸锂光子器件的传输损耗降低到具有实际应用意义的指标范畴内，这表明通过优化干法刻蚀的条件，能够实现低粗糙度铌酸锂波导表面。
2014年，哈佛大学通过将电子束曝光和离子束刻蚀技术相结合，也实现了超高Q值的铌酸锂光学微腔［32］。哈佛大学发展的这一技术与长期被广泛应用于大规模集成电路制造领域的互补金属氧化物半导体（CMOS）兼容，因此更易获得推广应用。该制造技术路线的核心包括光刻胶曝光（光刻）和离子干法刻蚀两个主要步骤。首先，通过曝光将掩模板上的图形转移到光刻胶上。曝光过程中可以选用紫外光、电子束或X射线等光源，这取决于器件结构所需的分辨率。曝光结束后，光刻胶上形成了与掩模板相对应的图案。为了将这个图案转移到下方的薄膜铌酸锂材料上，接下来需要采用刻蚀工艺。刻蚀是采用物理、化学或物理化学相结合的方法，将未被光刻胶保护的部分薄膜铌酸锂材料去除，从而形成与掩模板相对应的图案。刻蚀工艺通常分为湿法刻蚀和干法刻蚀两种类型。通过氢氟酸或氢氧化钾对铌酸锂块体材料进行刻蚀的湿法刻蚀工艺具有悠久的历史，但这种方法存在一些不足，如：刻蚀表面粗糙度一般较高，刻蚀速率低，具有化学各向同性的特点（除-z面以外）。与湿法刻蚀相比，干法刻蚀利用等离子体将未被光刻胶覆盖的衬底材料去除，具有较好的各向异性，而且刻蚀深度易于精确控制。干法刻蚀的种类很多，主要有离子束刻蚀、反应离子刻蚀、电感耦合等离子体刻蚀等。在薄膜铌酸锂材料的干法刻蚀过程中，氩气是使用最广泛的刻蚀气体［44］。利用其纯粹的物理轰击去除材料，不会在刻蚀过程中生成难以挥发的反应物，因而不会出现反应物沉积导致的粗糙的刻蚀界面。该技术路线的简单示意图见
图 1. 利用电子束曝光结合离子束刻蚀的方法制备薄膜铌酸锂光子结构的工艺流程示意图［53］
Fig. 1. Fabrication flow schematic of photonic structures on thin film lithium niobate by the electron beam lithography combined with ion etching[53]
离子束刻蚀不可避免地会在表面留下纳米级别的粗糙度以及内部晶格损伤，从而使得铌酸锂微盘腔的最终负载Q值限制在约106量级或稍高，这与铌酸锂单晶材料的吸收极限（对应在1550 nm波长下的本征Q值为2×108）［54］相比，仍然存在一个数量级以上的差距。2018年，上海光机所与华东师范大学提出了飞秒激光直写辅助化学机械抛光（PLACE）技术，目的是在薄膜铌酸锂上实现超低损耗光子器件和集成系统的晶圆级光刻制造［55-57］。
图 2. 飞秒激光直写辅助化学机械抛光（PLACE）制备铌酸锂光子芯片的工艺流程图［56］。（a）在薄膜铌酸锂晶圆表面沉积金属铬膜；（b）使用飞秒激光直写技术对铬膜进行图案化处理；（c）对样品进行化学机械抛光；（d）化学腐蚀去除剩余的铬掩模并进行二次化学机械抛光；（e）化学机械抛光原理和装置示意图
Fig. 2. Fabrication flow schematic of LN photonic structures by the femtosecond laser photolithography assisted chemo-mechanical etching (PLACE)[56]. (a) Depositing a thin layer of Cr on the top of the lithium niobate on insulator wafer; (b) patterning the Cr layer by femtosecond laser ablation; (c) conducting chemo-mechanical polishing on the sample; (d) chemically removing the remaining Cr mask and performing a secondary chemo-mechanical polishing; (e) schematic illustration of the chemo-mechanical polishing principle and the instrument
利用PLACE方法在厚度为900 nm的铌酸锂薄膜材料上制备出了直径为105 μm的铌酸锂微盘腔，其Q值能够达到4.7×107，侧壁粗糙度降低至0.115 nm，侧壁倾角可在10°~55°之间连续调控［58］。利用该技术在厚度为700 nm的铌酸锂薄膜材料上制备了脊形波导，波导顶部宽度为1.44 μm。在波导顶部沉积一层厚度为2 μm的Ta2O5包层可以获得单模传输。实验测得该波导在1550 nm波段的光学损耗仅为0.029 dB/cm［55］。此外，采用该技术在厚度为400 nm的铌酸锂薄膜上制备了直径为160 μm的微环和宽度约为3 μm的多模波导，并实现了0.027 dB/cm的传输损耗［59-60］。若要进一步降低铌酸锂波导的损耗，需要对薄膜铌酸锂自身的缺陷吸收进行改进。2021年，上海光机所采用纯机械打磨的方式减薄薄膜铌酸锂，避免了离子剥离过程中的缺陷损伤，有效降低了薄膜本体的吸收损耗，将片上铌酸锂微腔的Q值进一步提高到108量级［61］。所制备的微盘腔的直径为1030 μm，侧壁的楔角为8°，模式体积比先前报道的相同直径的铌酸锂块体谐振腔小两个数量级［47，62-63］。通过振铃（ringdown）测试得到该微腔的光子寿命为64.3 ns，推断其本征Q值为1.23×108，这是目前文献报道的铌酸锂微腔的最高Q值。
PLACE是一种专门为在薄膜铌酸锂上实现高质量大规模光子集成而开发的技术。由于飞秒激光直写技术固有的串行逐点加工的特性，PLACE的制造效率本质上取决于紧聚焦飞秒激光的扫描速度。为此，针对PLACE加工工艺，华东师范大学专门研发了一种超高速高分辨率超快激光光刻制造系统［64］。
图 3. 超高速高分辨率飞秒激光光刻［64］。（a）超高速多边形激光扫描仪的工作原理；（b）基于多边形扫描仪的飞秒激光光刻系统实物图；（c）不同光栅周期下的色谱图；（d）4英寸晶圆上制备的彩色画作；（e）4英寸晶圆上制备的1960条MZI阵列图案
Fig. 3. Ultra-high-speed high-resolution laser lithography[64]. (a) Principle of the ultra-high-speed polygon laser scanner; (b) experimental setup of femtosecond laser lithography system based on the polygon scanner; (c) digital camera photograph of the color palettes at different grating periods; (d) color printing on 4-inch wafer; (e) an array of 1960 Mach-Zehnder interferers patterned on a 4-inch wafer
PLACE技术的优势主要在于突破了单次连续曝光区域的尺寸限制并可获得极低的波导传输损耗，这两项优势对于实现大规模铌酸锂光子芯片并推动其应用具有重要意义。此外，PLACE工艺的加工成本较低。与传统的高精度干法刻蚀工艺相比，PLACE工艺无法达到数十纳米的结构精度，因此无法被用于制备亚波长光子结构，例如超表面或光子晶体。然而，PLACE加工技术在制备低损耗光波导和高品质微腔等微米级光子结构方面展现出了不可取代的优势，所获得的损耗指标与微腔的Q值指标至今还未被干法刻蚀技术制备的同类结构超越。鉴于目前我国半导体工艺设备被“卡脖子”的现状，PLACE技术具有特殊意义，毕竟该技术的整体流程和加工设备都是我国自主研发的。
3 若干重要铌酸锂光电器件
微纳加工技术的进步为光子集成提供了器件的多样性以及集成规模的持续扩张潜力［45，65-71］。基于薄膜铌酸锂芯片，研究人员已成功实现了各种高性能的片上光学器件，如延时线［55，72-74］、波导放大器［75-83］、高速电光调制器［53，84-87］、光频梳［88-89］、量子光源［90-92］等，它们在经典光学和量子光学应用领域都具有巨大的潜力。在此列举一些示例，因认知高度与广度的局限，加上篇幅限制，这里选取的示例不一定全面合理。
3.1 片上低损耗光波导延时线
光子延时线具有低传输损耗、小尺寸、高调谐速度和调谐效率等优势。2020年，上海光机所和华东师范大学利用PLACE加工技术在薄膜铌酸锂上实现了超低波导损耗的米级光子延时线［55］。该单模延时线波导长度达到了111.26 cm，测量得到的传输损耗约为0.03 dB/cm。由于当时还不具备制备长电极的技术，该光子延时线延时量的动态切换是通过片上光开关实现的，因此无法实现连续的延时量调节。
近期，片上长电极的高效加工制备技术得以实现，其与低损耗光波导相结合可实现具有连续调谐能力和宽延时调谐范围的光子延时线［64］。如
图 4. 电光可调谐的薄膜铌酸锂光学延迟线［64］。（a）可调光学延迟线的器件设计图；（b）集成微电极的可调谐光学延迟线的显微图；（c）测量得到的10、20、30 cm长波导的传输损耗；（d）实验测得的延时量及其拟合曲线，曲线斜率即为延时量的光电调谐效率
Fig. 4. Electro-optically tunable optical delay line in thin film lithium niobate[64]. (a) Schematic diagram of tunable optical delay line; (b) micrograph of a tunable optical delay line integrated with microelectrodes; (c) measured transmission losses in the 10, 20, and 30 cm long waveguides; (d) measured time delay by experiment and its fitting curve, the slope of the curve is the electro-optic tuning efficiency of the delay
3.2 铌酸锂高速电光调制器
光调制器是一类广泛用于数字通信的器件，包括电光、声光、磁光等不同类型的调制器。铌酸锂材料以其优异的电光特性被认为是制备高性能电光调制器的理想选择。与硅基调制器［93-95］和磷化铟调制器［96］等采用非线性调制方法的器件不同，铌酸锂调制器的工作机制与载流子的移动无关。它利用线性电光效应将电调制信号加载到光载波上，调制速率主要由微波电极的性能决定，因此可以实现更高的调制速度和线性度以及更低的功耗。通过巧妙设计行波电极，薄膜铌酸锂器件可以很容易实现100 GHz以上的3 dB带宽［86，97］，这对于高速光通信、光互联和片上光计算等都将产生重大影响。
近些年，国内外对薄膜铌酸锂电光调制器的研究取得了许多突破性进展［84-87，98］。
图 5. 基于薄膜铌酸锂的双偏振同相-正交调制器［86］。（a）调制器的三维示意图；（b）半波电压Vπ的测量结果；（c）电光响应测量结果
Fig. 5. Dual-polarization thin film lithium niobate in-phase quadrature modulators[86]. (a) Three-dimensional schematic of the modulator; (b) measurement of half-wave voltage Vπ; (c) measurement of electro-optical response
3.3 铌酸锂电光频率梳
集成电光频率梳凭借其较为平坦的频率梳包络，成为未来光通信、光探测与测距、光计算、光传感和光谱学等应用中不可或缺的组件。薄膜铌酸锂光子平台的技术进步推动了电光频率梳的发展，使其能够在更高效、更紧凑、成本更低的芯片级系统中实现［34，58，98-101］。2023年，香港城市大学王骋研究组［89］展示了一种基于薄膜铌酸锂的宽带电光频率梳。基于28 dBm的射频驱动功率，该器件在25 GHz重复频率下产生了具有47个梳齿的宽带光学频率梳，而且重复频率和工作波长均可调谐。如
图 6. 高效率宽带电光频率梳发生器［89］。（a）器件布局；（b）在不同射频驱动频率下测量的电光梳状频谱；（c）在不同光泵浦波长下产生的电光频率梳
Fig. 6. High efficiency broadband electro-optic frequency comb generator[89]. (a) Device layout; (b) measured electro-optical comb spectra at different radio frequency driving frequencies; (c) electro-optical frequency comb generation at different optical pump wavelengths
3.4 铌酸锂非线性量子光源
量子光源是量子信息科学和量子技术的核心组成部分，对于量子通信、量子计算和量子传感等领域的进展至关重要。铌酸锂具有出色的非线性光学特性和电光性能，因而成为制备和控制量子纠缠态的理想选择，也是实现全功能单片集成光量子芯片的关键材料之一。常用于制备量子光源的方法是自发参量下转换，该方法利用铌酸锂晶体最大的二阶非线性系数，将高频光子有效地转化为低频纠缠光子对［90-92，102］。2014年，南京大学祝世宁院士团队［102］通过向质子交换波导中引入周期性极化结构，在Hong-Ou-Mandel干涉仪中产生了两个具有电光可控相移的独立光子对源，从而产生了确定分离的全同光子对。该工作为不同光子源的片上集成提供了一个案例，并为全功能光量子芯片技术铺平了道路。
高亮度纠缠光子源是下一代量子光学集成的关键要求之一，同时也是高数据速率量子比特产生、通信、处理的基础。2021年，南京大学在薄膜铌酸锂光芯片中实现了超高亮度能量-时间纠缠双光子源［92］，该光子源具有2.79×1011 Hz/mW的光子对产率和1.53×109 Hz/（nm·mW）的光谱亮度，这是其他技术途径难以达到的。实验中采用的薄膜铌酸锂波导横截面参数以及波导中的模式分布模拟结果如
图 7. 基于薄膜铌酸锂的片上多路复用能量-时间纠缠双光子源［92］。（a）波导截面和模场分布模拟；（b）薄膜铌酸锂波导和块体铌酸锂晶体的群速度色散模拟；（c）极化畴结构的扫描共聚焦激光显微镜照片
Fig. 7. Multiplexed energy-time-entangled photon generation from thin film lithium niobate on insulator chip[92]. (a) Cross section of waveguide and mode field simulation; (b) simulation of group velocity dispersion for thin film lithium niobate waveguide and bulk lithium niobate crystal; (c) picture of domain structure captured by confocal laser scanning microscopy
3.5 铌酸锂超构表面
铌酸锂是一种重要的非线性频率转换光学材料，具有宽带隙（4 eV）和出色的二阶非线性系数，覆盖从紫外到中红外的宽光谱范围［17，29，104-107］。近年来，铌酸锂超构表面取得了许多重要进展［34，108-115］。这些超构表面能够通过调整微纳米级的几何形态结构，精确地在亚波长尺度内控制光场，实现高度局域化的非线性光学过程。2022年，南开大学的许京军、任梦昕等［113］借助薄膜铌酸锂超构表面实现了转换效率高达2.0×10-4的二次谐波产生。
图 8. 薄膜铌酸锂超构表面中高效的二次谐波产生（SHG）［113］。（a）薄膜铌酸锂超构表面产生二倍频的示意图；（b）薄膜铌酸锂超构表面的扫描电子显微镜图像；（c）SHG转换效率与泵浦光功率之间呈线性关系（两个超构表面MS1和MS2具有相同的结构周期600 nm，圆孔直径分别为225 nm和250 nm）
Fig. 8. Giant second harmonic generation (SHG) from thin film lithium niobate metasurfaces[113]. (a) Schematic illustration of the SHG from the thin film lithium niobate metasurface; (b) SEM image of the thin film lithium niobate metasurface; (c) linearly relationship between SHG conversion efficiency and pump beam power (the two metasurfaces MS1 and MS2 have the same period of 600 nm but different air hole diameters of 225 nm and 250 nm, respectively)
3.6 铌酸锂激光器/放大器
通过在薄膜铌酸锂中掺杂稀土离子可以实现具有光学增益能力的波导放大器和激光器，更重要的是，将此类有源元件与无源器件在片上集成，可以满足损耗补偿、功率提升和功能集成等一系列片上集成光子系统的需求［116-118］。基于薄膜铌酸锂材料，华东师范大学、上海光机所、上海交通大学、南开大学均于2021年前后领先于国际成功制备了掺Er3+薄膜铌酸锂回音壁模式片上微激光器［119-124］。然而，受到光学微腔的模场体积限制，该类激光器的输出功率通常较低。为了获得大功率的激光输出，可以基于Fabry-Perot（FP）谐振腔来构筑激光器，通过增加增益区的长度来提升输出激光功率。低损耗铌酸锂光波导成为该类器件的核心单元。
图 9. 基于Sagnac环形反射器的掺Er3+薄膜铌酸锂单模激光器［125］。（a）掺Er3+薄膜铌酸锂FP谐振腔的光学显微图像（底部插图显示的是980 nm波长激光泵浦时掺Er3+薄膜铌酸锂FP谐振腔的绿色上转换荧光照片）；（b）1544 nm波长处的光谱（激光峰值符合洛伦兹线形，插图显示的是掺Er3+薄膜铌酸锂FP谐振腔输出端口的红外图像）；（c）掺Er3+薄膜铌酸锂FP谐振腔激光器的片上激光功率随泵浦光功率的变化
Fig. 9. Er3+-doped thin film lithium niobate single-mode laser based on Sagnac loop reflectors[125]. (a) Optical microscopy image of an Er3+-doped thin film lithium niobate FP resonator (the bottom inset shows the green upconversion fluorescence of the Er3+-doped thin film lithium niobate FP resonator pumped by 980 nm laser); (b) spectrum around 1544 nm wavelength (the lasing peak is fitted with a Lorentzian line shape, the inset shows the infrared optical microscopy image of the output port of the Er3+-doped thin film lithium niobate FP resonator); (c) on-chip laser power of Er3+-doped thin film lithium niobate FP resonator laser changes with increasing pump power
除了用于产生片上激光外，掺杂稀土元素的光学材料因其长激发态寿命和折射率稳定性等特点，也常被用作集成光波导放大器的基底材料。目前，华东师范大学、上海光机所、香港城市大学、南开大学、上海交通大学等多个研究机构已成功研制出了薄膜铌酸锂片上波导放大器［75-83］。单根波导放大器目前报道的最大增益已经达到了27 dB，输出功率已经达到了毫瓦量级［82］。
事实上，大功率薄膜铌酸锂片上波导放大器的工作机理非常复杂，其原因在于薄膜铌酸锂光波导的模场面积很小。因此，在较低的光功率条件下，波导中的光强可以达到引发非线性效应的程度，从而阻碍了泵浦光的长距离传输以及信号光的进一步有效放大。为此，华东师范大学近期开展了基于片上相干合束来提升薄膜铌酸锂波导放大器输出功率的实验研究［130］。
图 10. 掺Er3+薄膜铌酸锂波导放大器的片上相干合束［130］。（a）片上相干合束掺Er3+波导放大器示意图，插图为Er3+的能级图；（b）施加在3 mm微电极上的三角波对输出信号功率的电调制；（c）施加泵浦激光后，测量的1530 nm信号光的消光比
Fig. 10. On-chip coherent beam combination of waveguide amplifiers on Er3+-doped thin film lithium niobate[130]. (a) Schematic of the on-chip coherent beam combination of Er3+-doped waveguide amplifiers, where the inset shows the energy level diagram of Er3+; (b) electro-optic modulation of the output signal power by a triangular wave applied to a 3 mm microelectrode; (c) measured extinction ratio of 1530 nm signal light after applying the pump laser
4 基于薄膜铌酸锂的大规模光子集成
自1969年贝尔实验室首次提出光子集成技术（PIC）这一概念以来［131］，人们一直在为实现大规模光子集成而不懈努力。特别地，PIC技术为突破大规模光纤网络的通信容量瓶颈以及实现大规模光电融合提供了有效途径［132］。PIC被认为是支撑大数据、人工智能和量子计算等新兴应用的潜在技术平台，其发展将为未来信息处理和通信领域带来革命性改变。
近期，华东师范大学、上海光机所在薄膜铌酸锂衬底上实现了4×4可编程线性光子运算器［133］。该器件在损耗、功耗和运算速度方面显示出巨大优势。
图 11. 基于薄膜铌酸锂平台制备的4×4可编程线性光子电路［133］。（a）任意SU（4）变换矩阵；（b）MZI单元的内部结构；（c）200个随机矩阵的测量保真度的直方图；（d）一个随机矩阵的理论与实验结果的对比
Fig. 11. 4×4 programmable photonic circuits based on thin film lithium niobate[133]. (a) Arbitrary SU(4) transformation matrix; (b) internal structure of MZI-unit; (c) histogram of the measured fidelity of 200 random matrices; (d) comparison between theoretical and experimental results of a random matrix
对于大规模光子芯片应用，这里再提供一个示例。在光通信行业，单片集成阵列波导光栅（AWG）常被用作波分复用数据发射机，以扩展光网络的容量并提高传输速率［134-136］。AWG是一种光子集成电路器件，可以将不同波长的多个光学信号组合到一个波导通道中。由于具有低插入损耗、高光谱分辨率、低成本、小尺寸和单片集成特性，AWG还可以用于微型光谱仪或用于片上光学相干断层扫描［137-138］。基于PLACE加工技术，华东师范大学制备了一个片上八通道薄膜铌酸锂AWG［139］，
图 12. 基于薄膜铌酸锂制备的片上阵列波导光栅（AWG）［139］。（a）制备的八通道薄膜铌酸锂AWG的显微图像；（b）薄膜铌酸锂AWG芯片，右插图为不同输入波长下红外相机拍摄的输出图像；（c）8个通道的频谱测量结果；（d）单通道频谱测量结果
Fig. 12. On-chip arrayed waveguide grating (AWG) fabricated on thin film lithium niobate[139]. (a) Micrograph of the fabricated 8-channel thin film lithium niobate AWG; (b) thin film lithium niobate AWG chip and the output images captured by the infrared camera at different input wavelengths; (c) measured spectra on eight channels; (d) spectrum measured at one of eight channels
大规模光子集成还需要解决有源/无源光子器件结合的问题。传统的有源无源集成需要引入额外的耦合元件，例如模斑变换器和透镜光纤，这不仅显著增加了制造的复杂性和成本，还降低了集成器件的性能。上海光机所、华东师范大学通过预先制备无源（即不掺杂活性离子）和有源（即掺杂稀土离子）薄膜铌酸锂的拼接衬底，实现了一种稳定的低损耗光学界面，其界面损耗仅为0.26 dB［81］。借助飞秒激光连续直写过程中自动保障的波导对准条件，在该有源无源拼接衬底上实现大规模光波导阵列集成将变得非常简单直接。
图 13. 单片集成有源/无源薄膜铌酸锂片上四通道波导放大器［81］。（a）器件设计图；（b）四通道波导放大器的实物图；（c）采用980 nm波长激光泵浦时，拍摄的四通道光波导放大器阵列照片；（d）四通道掺Er3+波导中1550 nm波长信号的模式分布（插图）和强度分布；（e）（f）1550 nm和1530 nm信号波长下，四通道掺Er3+铌酸锂波导阵列的增益特性
Fig. 13. Four-channel waveguide amplifiers fabricated on the monolithically integrated active/passive thin film lithium niobate[81]. (a) Illustration of the device design; (b) digital camera captured picture of the four-channel waveguide amplifiers; (c) photo of the four-channel waveguide amplifier array with 980 nm wavelength laser pumping; (d) mode profile (insets) and intensity distribution of the 1550 nm wavelength signal in the four-channel Er3+-doped waveguide; gain characterization of the four-channel Er3+-doped lithium niobate waveguide array for the signal wavelengths at 1550 nm (e) and 1530 nm (f)
5 总结与展望
集成光子技术长期受到片上光波导高传输损耗和高调制功耗的制约，无法持续扩大集成规模。近10年来，薄膜铌酸锂光子学的快速发展为改变这一现状提供了可能。如上所述，薄膜铌酸锂结合先进的微纳光子制备技术，已经实现了光子集成领域的一系列突破，使得光子器件在调制带宽、调制功耗、传输损耗、有源无源集成、超大集成规模等方面都有显著提升。值得一提的是，目前这些性能大都还未达到铌酸锂光子器件的物理极限，因此，薄膜铌酸锂集成光子技术仍具有广阔的发展空间。同时，很多薄膜铌酸锂光子器件都已接近甚至具备了产业应用的条件，这为下一代光信息技术发展提供了强大驱动力。
这里笔者提出一个可供探讨的建议：光子集成技术的发展模式可以选择两条道路。其中一条是复制微电子大规模集成的发展模式，尽可能用光子器件取代电子器件的一些功能，以提高信息处理的速度和效率；另一条是复制光纤光子学的发展模式，将一系列传统的光纤光子器件的体积缩小、能耗降低，以获得大规模片上器件与系统的集成能力，从而提升信息传递容量和信息处理能力。硅光子由于与微电子集成技术存在天然血缘，在很大程度上走的是第一条道路。薄膜铌酸锂光子学可以同时选择这两条道路发展，特别是第二条发展道路为铌酸锂光子器件快速融入现有的光纤网络提供了一条捷径。在不久的将来，薄膜铌酸锂光调制器有望部分取代现有光纤网络中的铌酸锂块体光调制器，或者铌酸锂片上光放大器有望部分取代现有的掺铒光纤放大器。
当前薄膜铌酸锂光子学研究及其产业化推进所面临的最大困难仍然是整体生态处于建设初期，相关的设计平台、制备平台、测试平台、标准器件库、应用平台等都还没有达到高度专业化的程度，还有待深入磨合。因此，其生态规模远小于硅光子产业，与微电子集成产业相比更是不可同日而语。改变这一现状没有捷径，唯有通过基础研究与市场应用的协同驱动，持续改进和创新，才能完善和壮大这一新的产业生态。可喜的是，薄膜铌酸锂光子器件已经逐渐走出实验室，坚实地迈出了通往未来的第一步。
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Article Outline
程亚. 薄膜铌酸锂光电器件与超大规模光子集成（特邀）[J]. 中国激光, 2024, 51(1): 0119001. Ya Cheng. Thin Film Lithium Niobate Electro-Optic Devices and Ultralarge-Scale Photonic Integration(Invited)[J]. Chinese Journal of Lasers, 2024, 51(1): 0119001.
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