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摘要:
作为三维超构材料的衍生物，具有亚波长厚度的人工超构表面结构能够在紧凑的平台上灵活操纵光与物质的相互作用，有利于多功能、超紧凑光子器件的研发，对于微纳光子学和集成光子学具有重要意义。铁电晶体铌酸锂凭借其跨越可见光至中红外波段的宽透明窗口以及较大的非线性光学、电光系数，被认为是最有前途的多功能集成光子平台之一。近年来，基于铌酸锂薄膜(lithium-niobate-on-insulator，LNOI)的集成光子学器件研究也得到了迅猛发展。本文总结了几种有潜力制备高质量铌酸锂超构表面的微纳加工技术，同时介绍了近年来铌酸锂超构表面结构的研究进展，并对其未来的研究方向进行了展望。
Abstract:As derivatives of 3D metamaterials, artificial metasurface structures with sub-wavelength thicknesses can flexibly manipulate light-matter interactions in a compact manner, which is beneficial for the fabrication of multi-functional and ultracompact photonic devices. Therefore, metasurface structures are of great significance for micro-nano photonics and integrated photonics. The ferroelectric crystal lithium niobate is regarded as one of the most promising multifunctional integrated photonic platforms due to its wide transparent window spanning from the visible to the mid-infrared band as well as large nonlinear optical and electro-optic coefficients. In recent years, research on integrated photonics devices based on lithium-niobate-on-insulator (LNOI) thin films has also been developed rapidly. In this paper, several micro-nano processing technologies that have the potential to prepare high-quality lithium niobate metasurfaces are summarized. At the same time, the research progress of lithium niobate metasurface structures in recent years is introduced, and its future research directions are prospected.
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Key words:
- metasurface /
- lithium-niobate /
- micro-nanofabrication /
- nonlinear optics
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Overview: Two-dimensional metasurfaces constructed from sub-wavelength size meta-atoms can flexibly control the local distribution of electromagnetic fields, which has attracted extensive attention in recent years. The polarization, phase and amplitude of electromagnetic waves can be controlled with subwavelength resolution by properly designing the nanostructure of the metasurfaces. Compared with 3D metamaterials, 2D metasurfaces can not only greatly alleviate the high resistance losses accumulated in traditional metamaterials, but also avoid the manufacturing requirements of complex 3D nanostructures. In addition, the sub-wavelength thickness of the metasurfaces has significant integration advantages, which makes it possible to develop ultra-compact photonic devices with a variety of optical functions, which is of great significance to micro/nano photonics and integrated photonics. Especially in the field of nonlinear optics, metasurfaces can alleviate or even completely overcome the requirement of phase matching to some extent, thus showing a strong nonlinear optical response. Metal materials exhibit significant ohmic losses in other wavebands except microwave, resulting in a relatively low optical quality factor of traditional plasmonic metasurfaces, which also limits their application in many functional nano-photonic devices. Furthermore, some precious metals (such as gold and silver, etc.) are not only expensive to manufacture, but also incompatible with traditional Complementary Metal Oxide Semiconductor (CMOS) processes. In view of this, dielectric metasurfaces compatible with semiconductor technology have gradually become a research focus. Ferroelectric lithium niobate (LiNbO3) is known as "optical silicon" because of its transparent window from visible to mid-infrared band (0.35-5 μm), relatively high refractive index, excellent electro-optic (EO) and second-order nonlinear optical properties, as well as excellent acoustooptic and piezoelectric properties. These unique properties make lithium niobate one of the most widely used materials in photonics, and it is an ideal substrate material for realizing efficient dielectric metasurfaces.
With the rapid development of lithium-niobate-on-insulator (LNOI) thin film technology and related surface micro-nano manufacturing technology in recent years, a series of high-quality and high-performance photonic functional devices on lithium niobate chip have been realized, such as compact modulators with ultra-high performance, broadband frequency combs, as well as high-efficiency optical frequency converters and single-photon sources. Great progress has been made in nonlinear optical frequency conversion, electro-optic modulation and optical passivity. In this paper, we briefly introduce several micro-nano processing technologies that have the potential to produce high-quality lithium niobate metasurfaces, and summarize the recent research progress in optical frequency conversion, electro-optic modulation, optical passivity and other aspects of lithium niobate metasurfaces, and prospected the potential research directions in the field of micro-nano optics.
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1. 引 言
超构表面是由亚波长尺寸的超构单元构建的二维表面，能够灵活地对电磁场的局域分布进行调控，在近年来受到了广泛的关注[1-2]。通过对超构单元的纳米结构进行合理设计，二维超构表面能够以亚波长分辨率控制电磁波的偏振、相位和振幅等特性[3]。相较于三维超构材料，二维超构表面不仅能够极大地缓解传统超构材料中积累的高阻损耗，并且规避了复杂的三维纳米结构的制造要求。此外，亚波长厚度的超构表面具有显著的集成优势，使研制具备多种光学功能的超紧凑光子器件成为可能，对于微纳光子学与集成光子学具有重要意义。特别是在非线性光学领域，超构表面可以在一定程度上缓解甚至完全克服相位匹配的要求，从而表现出强烈的非线性光学响应[4-5]。
超构表面的早期研究主要围绕等离激元(金属)材料展开[6-7]，材料中的光子-电子模式的集合振荡能够提升局域电场限制并增强光与物质的相互作用，因而在非线性增强[8-10]、高灵敏度传感[11-13]、量子光学[14-15]等方面取得了一系列振奋人心的成果。然而，金属材料在除微波之外的其它波段表现出显著的欧姆损耗，导致传统等离激元超构表面的光学品质因子(quality factor，Q-factor)相对较低，这也限制了它们在多种功能性纳米光子器件中的应用。此外，一些贵金属(如金和银等)不仅制作成本较高，其微纳加工工艺也与传统的半导体(complementary metal oxide semiconductor，CMOS)工艺不兼容。鉴于此，与半导体工艺相兼容的介电超构表面逐渐成为研究热点[16-17]。
介电超构表面通常由具有高折射率、高非线性系数的介质或半导体材料(如Si、GaAs、TiO2等)构成，这些材料通常在光学频率波段损耗较低，并且与周围介质之间具有较高的折射率对比度，能够激发除电场共振模式之外的磁场共振模式，进而可以通过电磁共振来调控光的性质并且实现局域电磁场增强。因此，介电超构表面在诸多领域都展示出了优异的光学性能[18-22]。然而，在非线性光学应用方面，以Si为代表的诸多半导体材料由于具有中心对称的晶体结构而不表现出倍频、差频等二阶非线性光学性质，仅仅应用于三阶非线性光学性能增强。而以GaAs为代表的具有非中心对称晶体结构的半导体材料大多带隙较窄，因而不适用于可见光波段的应用。因此，寻找功能更为全面的介电材料尤为重要。
铁电晶体铌酸锂(LiNbO3)具有从可见光到中红外波段(0.35 μm~5 μm)的透明窗口、相对较高的折射率、优异的电光(electro-optic，EO)和二阶非线性光学性能以及出色的声光和压电特性[23-24]，被誉为“光学硅”[25]。这些独特的性质使铌酸锂成为光子学中应用最广泛的材料之一，是实现高效介电超构表面的理想基底材料。随着近几年来绝缘体上铌酸锂(lithium-niobate-on-insulator，LNOI)薄膜技术[26-27]以及相关表面微纳制造技术的快速发展，一系列高质量、高性能的铌酸锂片上光子功能性器件得以实现[28-31]，例如具有超高性能的紧凑型调制器[32-35]、宽带频率梳[36-38]、以及高效率的光学频率转换器[39-41]和单光子源[42-43]等。其中，铌酸锂片上超构表面结构在非线性光学频率转换、电光调制、光无源等方面的研究取得了巨大进展。
本文简要介绍了几种有潜力制备高质量铌酸锂超构表面的微纳加工技术，同时总结了近期铌酸锂超构表面在光频转换、电光调制、光无源等方面的研究进展，并对其在微纳光学领域有发展潜力的研究方向进行了展望。
2. 铌酸锂超构表面的制备
高质量的LNOI薄膜是利用“离子切片”工艺研制的，并已在近几年实现商业产品化。具体的制备步骤可参考已经发表的综述论文[44-45]，本文不再赘述。这里简要介绍LNOI片上超构表面的制备方法。
事实上，LNOI片上超构表面结构的制备工艺与其它LNOI片上微纳光子学结构(如波导、微腔等)类似。制备过程可以按照有无掩膜的情况加以区分，典型的制备流程如图1所示。有掩膜情况主要分为两种：1) 首先利用光刻技术实现光刻胶的图案化，此处光刻胶可以直接作为掩膜，也可以在光刻后沉积一层金属作为掩膜，再结合剥离(lift-off)工艺完成图案化处理；掩膜制备完成后，结合干法刻蚀或者化学机械抛光(chemical-mechanical polishing，CMP)技术去除多余的铌酸锂，完成图案转移；随后进行后处理过程，利用湿法刻蚀去除残留掩膜，实现微纳结构的初步制备。2) 在沉积一层金属掩膜后采用飞秒激光烧蚀技术对掩膜进行图案化处理；利用CMP技术实现图案转移；通过后处理过程去除残留掩膜。无掩膜直接刻蚀铌酸锂可以通过聚焦离子束(focused ion beam，FIB)、CMP或者飞秒激光烧蚀技术实现，其中以FIB技术最为常见。此外，对于需要做后处理的微盘腔、纳米梁等特殊结构，可以再结合湿法刻蚀去除底部多余的衬底结构。在制备过程中，CMP也可作为降低表面或侧壁粗糙度的后处理过程。
图 1. LNOI片上微纳光子学结构制备的主要流程图：图案化处理；图案转移；后处理过程Figure 1. The main flow chart of fabrication of photonic structure on the LNOI chip: patterned processing; pattern transfer; post-processing接下来本文将对上述制备过程中所涉及的几种关键技术进行简单介绍。
用于图案化处理的光刻技术主要分为紫外光刻技术(ultraviolet photolithography)[46]和电子束曝光技术 (electron beam lithography，EBL)[47-48]两种类型。与紫外光刻技术相比，EBL技术的优势在于加工精度较高，有利于铌酸锂薄膜上复杂图案的制备。但紫外光刻技术具有更高的制造效率，适合超构表面光学器件的大规模量产。此外，紫外光刻技术能够与CMOS加工工艺相兼容，而CMOS工艺有助于实现晶圆级的单片集成，降低封装成本。
在图案转移过程中，相比于其他刻蚀方法，干法刻蚀具有各向异性、可灵活控制刻蚀深度、适于转移复杂二维图案并兼容多层处理等优点，在微纳结构加工中备受青睐，也更适用于超构表面的制备。在过去的几十年里，包括反应离子刻蚀(reactive ion etching, RIE)、感应耦合等离子体反应离子刻蚀(inductively coupled plasma reactive ion etching，ICP-RIE)[49-51]以及Ar+等离子体纯物理刻蚀(即氩刻)[52-55]等在内的多种干法刻蚀方法都得到了广泛的应用。与Si和SiNx等大多数集成光子学平台不同的是，铌酸锂缺乏合适的反应离子刻蚀配方，在高质量微纳结构的制造上存在困难。例如，基于氟化物的RIE虽然可以通过形成具有挥发性的氟化铌(NbF4)有效去除铌酸锂[56]，但同时该方法也会在表面形成难挥发的氟化锂(LiF)颗粒，导致严重的二次沉积问题[57-58]。针对此，研究人员对刻蚀参数(如气体比例、功率等)进行了不断优化[59-63]，同时采用湿法刻蚀技术去除多余反应物，从而最大限度地增加侧壁光滑度，减少散射损耗。2020年，德国耶拿大学Setzpfandt教授课题组通过采用多步骤反应离子刻蚀技术，制备出具有光滑侧壁的高质量铌酸锂超构表面，在非共振波段该结构的透过率高达97％，详细的制备流程及SEM图如图2所示[64]。相比于氟基刻蚀，Ar+等离子体纯物理刻蚀可以直接从根源上避免LiF的形成，从而实现更优的侧壁光滑度，这也是目前LNOI最常用的干刻方法之一。该方法可以结合不同的方式进行，例如感应耦合等离子体(inductively coupled plasma，ICP)、电子回旋共振(electron-cyclotron resonance，ECR)或离子束刻蚀(ion-beam etching，IBE)系统等。2017年，哈佛大学Lončar教授课题组结合EBL和氩刻技术制备出传输损耗低至2.7 dB/m的脊形多模波导，同时通过设计具有完美宽度和直线段长度的跑道型微谐振腔，实现了Q值大于107的超高质量光学微腔[28]。截至目前，同种方法已被成功应用于柱状超构表面的制备(Q值在776.6 nm达到129)[65]。然而，纯物理刻蚀铌酸锂也面临一些挑战。首先，对于可用的光刻抗蚀剂而言，它的刻蚀选择性较差，从而导致有限的刻蚀深度和较低的模式限制。其次，Ar+刻蚀铌酸锂结构也会由于二次沉积效应造成侧壁粗糙问题，但与LiF二次沉积(以颗粒形式)不同的是，Ar+刻蚀的二次沉积形成的表面较为平滑，不会引入很高的散射损耗。采用Ar+刻蚀的最显著缺点是会形成过切的波导轮廓(即横截面呈梯形)，其侧壁倾角通常在40°~80°范围内，这与化学刻蚀过程相比并不占优势，并且会对相邻结构之间的最小特征尺寸和间距造成限制。通常来说，较低的真空压力和较高的等离子体功率有助于产生更陡峭的侧壁[66]。此外，在干法刻蚀工艺完成后可使用湿化学清洗去除二次沉积物和其他污染物，从而进一步降低光学损耗[67-69]。
图 2. (a) 铌酸锂超构表面SHG示意图；(b) 制备工艺流程示意图；(c) 所制备超构表面的SEM图像，其中纳米谐振腔由截断金字塔和下面的残余层组成[64]Figure 2. (a) A schematic of the SHG from the lithium niobate metasurface; (b) Schematic illustration of the process flow of fabrication; (c) SEM image of the fabricated metasurface in which the nanoresonator consists of a truncated pyramid and a residual layer underneath[64]除干法刻蚀外，聚焦离子束(focused ion beam，FIB)技术作为目前最精确的无掩膜微纳结构加工方法之一，允许制造高纵横比及陡峭侧壁的微纳结构，已应用于许多复杂片上光子器件的制备。由FIB制备的第一个铌酸锂薄膜微谐振腔在2015年被提出，Q值为2.5×105。在制备过程中对飞秒激光烧蚀形成的圆柱连续进行两次FIB铣削，以光滑其粗糙外围，图3显示了FIB铣削前后微谐振腔侧壁的SEM图像[70]。随后，同一组研究人员通过优化铣削电流(1 nA)和退火后的持续时间(500 °C ~ 4 h)，成功地将Q值提高到2.45×106 [71]。除微谐振腔外，FIB的灵活简便性及其高精度加工特性有助于充分发挥铌酸锂超构表面的潜力，也是目前制备铌酸锂超构表面的常用方法。2019年，南开大学许京军、任梦昕教授课题组采用FIB铣削技术，通过选择性地轰击与去除铌酸锂分子，在几百纳米厚度的铌酸锂薄膜上成功制备了周期纳米线阵列，实现了具有优异光学功能的铌酸锂超构表面[72]。2021年，意大利米兰理工大学Celebrano教授课题组基于FIB铣削技术制备了铌酸锂纳米柱阵列超构表面，侧壁倾角可达83.6°[73]。其中，在进行FIB铣削之前，研究人员创新性地通过射频磁控溅射法沉积了一层Cr膜，从而避免了FIB加工过程中的充电效应，进一步优化对纳米结构高度的控制，确保所制备超构表面的尺寸均匀性。同时，Cr膜作为牺牲层还可以防止Ga+的注入，从而减少铌酸锂柱内缺陷。尽管FIB技术非常适合制造需要高分辨率的结构，但其操作面积通常为百平方微米，无法满足大规模片上光子器件的研制，这也大大限制了其在实际应用中的发展。
作为FIB铣削的替代方案，化学机械抛光(chemical-mechanical polishing，CMP)技术[74]不仅摆脱了尺寸限制，而且能够作为后处理过程，显著改善片上光学结构表面和侧壁的粗糙度，降低结构的散射损耗，从而成为在铌酸锂薄膜上加工超构表面的另一种有力备选方案。2017年，德国弗赖堡大学Buse教授课题组结合紫外光刻与RIE技术制备了微环结构，制备过程如图4(a)所示。在此基础上，采用CMP技术抛光侧壁，使其侧壁粗糙度降低至4 nm，Q值>3×106。图4(c)和4(d)显示了CMP前后微环侧壁的放大SEM图像[75]。此外，CMP技术本身可以作为制造过程中图案转移的有效手段，利用CMP技术制备片上光学结构通常包括四个步骤：1) 在铌酸锂薄膜上沉积一层Cr作为刻蚀掩膜；2) 通过光刻技术或者飞秒激光烧蚀将Cr掩膜图案化；3) 采用CMP技术去除未被Cr掩膜覆盖的铌酸锂；4) 利用湿法刻蚀去除表面Cr掩膜。在薄膜制备和波导、微谐振腔制造中应用CMP技术可以缓解由离子注入引起的晶格损伤，将平均表面粗糙度降至亚纳米级别以下[29, 76]。2021年，华东师范大学程亚教授课题组结合飞秒激光烧蚀与CMP技术，先后成功实现了Q值高达108(波长为1550 nm)的LNOI微盘、微环谐振腔，接近了铌酸锂的本征材料吸收极限，这也是目前文献报道的最高Q值[77-78]。不可否认的是，飞秒激光烧蚀与CMP技术的结合为未来制备铌酸锂片上超低损耗光子系统和器件开辟了一条新的道路，但也存在明显的劣势。首先，CMP技术的加工特性限制了其加工效率；其次，由于制造过程中金属掩模和铌酸锂的选择性有限，导致经由 CMP 技术制备的纳米结构的纵横比低于1.5[79]，这为制造间隙小于2 μm的耦合器件带来了挑战。因此，在未来应该付出更多努力来实现更高的纵横比，这对于构建高密度光子集成电路至关重要。
图 4. 采用紫外光刻结合RIE技术制备微环腔，然后用CMP抛光侧壁。(a) 制备工艺流程示意图；(b) 微环腔SEM图像；CMP前(c)后(d)微环腔侧壁的放大SEM图像[75]Figure 4. Microring fabricated by UV lithography and RIE, followed by sidewall polishing by the CMP. (a) Schematic illustration of the process flow of fabrication; (b) False-color SEM image of the microring; and enlarged SEM image of the sidewall (c) before and (d) after the CMP[75]3. 铌酸锂超构表面的应用
3.1 非线性光学频率转换
二次谐波产生(Second harmonic generation，SHG)是最常见也是最简单的非线性光学效应之一。在利用LNOI实现SHG的探索研究过程中，III−V半导体材料GaAs和AlGaAs因其较大的二阶非线性光学系数受到广泛关注，成为研究非线性光学超构表面的理想材料[80-82]。然而，这些半导体材料在可见光波段具有很高的光学吸收，其可见光SHG转换效率很低。与之相比，铌酸锂具有较宽的带隙和较高的二阶非线性光学系数，能够在紫外到中红外的宽波段范围内实现多种高效的非线性效应[38, 83-85]，包括SHG效应。
依靠着LNOI平台提供的高折射率差和紧凑型结构，LNOI微纳光子学结构(例如光波导、回音壁模式微腔、光子晶体谐振腔)的SHG效应是近年来研究的热门课题[38-39, 86-89]，并取得了显著成果。周期性极化铌酸锂(periodically poled lithium niobate，PPLN)波导的准相位匹配SHG归一化转换效率通常由η = P2/P12L2给出，其中P是功率，L为波导长度，下标1和2分别表示基波和二次谐波。迄今为止，研究人员已通过准相位匹配在SiNx加载波导和单片铌酸锂脊波导中实现了超高的归一化转换效率。在前一种情况下，通过设计波导横截面限制了基模TE模式向TM平板模式的泄漏，在PPLN上的5 mm长度SiNx加载波导中实现了1160 %/W·cm2的高归一化转换效率[90]；在后一种情况下，通过对二次谐波信号进行主动监测以实现最佳极化，在300 µm长的铌酸锂脊形波导中实现了4600%/W·cm2的超高归一化转换效率[91]。与在波导中不同的是，回音壁模式微腔及光子晶体腔的SHG归一化转换效率通常由η = P2/P12给出。2019年，程亚教授课题组提出了一种无需周期性极化即可在LNOI微腔中实现高效倍频的新机制，其在LNOI微盘中实现的SHG归一化转换效率高达9.9%/mW[83]。同年，美国史蒂文斯理工学院Huang教授课题组及耶鲁大学Tang教授课题组先后制备出具有双重共振的周期性极化的LNOI微环形谐振腔，SHG归一化转换效率分别高达230000%/W和250000%/W[40-41]。2020年， Tang教授课题组利用最大非线性极化率张量d33在LNOI微环谐振腔中实现了高达5000000%/W的归一化转换效率，这也是目前所报道的基于LNOI微谐振腔SHG转换效率的最高值[92]。此外，基于LNOI光子晶体谐振腔的SHG也已经在实验中实现，但由于基波和谐波无法同时处于谐振状态，其转换效率较低(当前记录为0.078%/W[93])。令人欣慰的是，有关双共振光子晶体腔的最新进展有潜力解决这一问题[94]，有望实现超高效的非线性转换，甚至实现单光子非线性。
除以上LNOI微纳光子学结构外，由亚波长超构单元组成的超构表面为设计纳米尺度的非线性响应提供了一个革命性的概念。充当光学谐振腔的超构单元能够将电磁能量压缩到超越衍射极限的空间内，提供很高的局域场增强，极大地提升光与物质的相互作用[17, 95-96]，从而使二维超构表面在提高非线性光频转换效率方面具有很大潜力，基于LNOI薄膜的铌酸锂超构表面结构也有望为实现新型高效纳米级SHG光源提供一种新的思路。
铌酸锂超构表面结构在非线性共振光子学中具有巨大优势。研究人员首先从理论上指出铌酸锂超构表面的Mie共振模式和Fano共振模式对SHG具有增强作用[97-98]，随后，瑞士苏黎世联邦理工学院Grange教授课题组通过化学合成法制备了具有Mie共振模式的铌酸锂纳米立方体结构，并在360 nm处观测到了高效的SHG增强，该数据相比于块状铌酸锂提高了107倍[99]。然而，由于自下而上的化学合成方法相对复杂，Mie谐振腔的几何结构仅限于立方体。为克服这一限制，中山大学刘进教授课题组利用飞秒激光烧蚀技术制备了单个纳米球，该纳米球结构支持Mie共振模式，并在750 nm飞秒激光激发下显示出紫外(ultraviolet，UV) SHG，转换效率可达4.45 × 10−8 [100]。Mie谐振腔作为一种新型的高效全介质超构单元，在非线性光学超构表面方面具有巨大的潜力。
在对单个谐振腔SHG研究的基础上，为进一步提高转换效率，研究人员开始致力于对谐振腔阵列即超构表面结构的研究。2020年，德国耶拿大学Setzpfandt教授课题组结合EBL和IBE技术，制备出由铌酸锂截断金字塔阵列构成的共振超构表面，该器件在1550 nm波长处表现出较强的Mie共振模式。利用铌酸锂较大的对角二阶非线性极化率张量，在垂直于超构表面的方向上观察到增强的SHG。此外，通过分析非线性极化率张量的不同元素对总二次谐波信号的贡献，发现d33在其中起主要作用，为有效利用d33需使泵浦光沿晶体光轴方向入射，实验测得这种由电贡献主导的共振模式所产生的最大SHG转换效率可达10−6 [64]。2021年，许京军教授课题组进一步利用FIB铣削技术研制了铌酸锂纳米光栅超构表面结构，并实现了在可见光范围内的可调SHG特性。图5(a)给出了非线性铌酸锂超构表面SHG的原理图，并在插图中展示了所制备的超构表面截面的典型扫描电子显微镜(SEM)图像。通过调整几何参数从而调整超构表面的共振来选择性地提高不同波长的 SHG 效率，在强度为2.05 GW cm−2的s偏振光泵浦下，SHG转换效率约为2 × 10−6，为未加工的薄膜区域的两倍，如图5(b)所示[101]。
图 5. (a) 非线性铌酸锂超构表面的SHG示意图。左下插图为D=600 nm的超构表面截面的典型SEM图像，右下插图显示了研究中使用的铌酸锂薄膜的测量二阶极化率；(b) 超构表面SHG效率的光谱依赖性[101]Figure 5. (a) A schematic of the SHG from the nonlinear lithium niobate metasurface. Left inset gives a typical SEM image of cross section of the metasurface with D=600 nm. Right inset presents the measured second-order susceptibility of the lithium niobate film used in this study; (b) Spectral dependence of SHG efficiencies from metasurfaces[101]虽然铌酸锂超构表面在SHG增强方面取得如此多的进展，但LNOI薄膜的生产成本相对过高。2021年，意大利米兰理工大学Celebrano教授课题组利用FIB铣削技术制备了第一个基于铌酸锂的单片非线性纳米柱阵列超构表面，在可见光范围内低至0.5 GW/cm2的泵浦强度下实现了2.40 × 10−8的SHG转换效率。与此同时，该结构允许将 SHG 衍射到第一衍射级模式，其信号比零级强 2个数量级，从而产生约 20 dB 的消光比。此外，在超构表面的作用下，所产生的二次谐波被优先重定向到沿泵浦偏振方向的衍射级，因此可以通过控制泵浦偏振实现对衍射图案的操纵，从而为偏振编码非线性光学开辟了新的机会[73]。
表1总结了三种不同超构单元组成的铌酸锂超构表面SHG的主要性能参数。其中PT为截断金字塔阵列超构表面的周期，L为纳米谐振腔的边长；D为光栅超构表面的周期，d和h分别为脊的宽度和高度；PC为纳米柱阵列超构表面的周期，R为纳米柱的半径。
表 1. 超构表面SHG主要性能参数Table 1. Main performance parameters of metasurface SHGMeta-atoms Size Process technology FH intensity FH wavelength SH wavelength Conversion efficiency Truncated pyramid[59] PT=870 nm
L=700 nmEBL+IBE 4.3 GW/cm2 ~1550 nm ~775 nm ~10−6 Ridge waveguide[86] D=600 nm
d=85 nm
h=235 nmFIB 2.05 GW/cm2 740 nm~1000 nm 370 nm~500 nm 2×10-6 Cylinder[65] PC=590 nm
R=175 nmFIB 0.5 GW/cm2 825 nm 412 nm 2.9×10−8 | Show Table
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铌酸锂超构表面优异的光频转换性能还体现在利用自发参量下转换(spontaneous parametric down-conversion，SPDC)高效制备光子对上。近年来，量子光学在很多领域具有广泛的应用，包括超高速和强大的安全量子通信[102-103]、快速准确的量子计算[104-105]以及高分辨率计量、成像和传感[106-107]等。通过非经典光源产生高质量的纠缠单光子对是上述大多数应用的重要组成部分，并且一直是现代量子光学技术长期追求的目标。利用非线性晶体中的SPDC过程产生纠缠和相关单光子对是目前最通用的技术之一，对于开展量子光学研究及研发高效量子光源具有重要意义。在SPDC过程中，拥有较高频率(ωp)的泵浦光子以一定概率转换成具有较低频率的下转换关联光子对(信号光子与空闲光子，频率分别为ωs和ωi)。然而，作为一个典型的自发量子非线性过程，其效率极低。最近，人们广泛致力于利用基于非线性晶体、波导、光子晶体和环形谐振腔的传统设计来增强这种效应[108-109]，但仍存在体积庞大或不适合自由空间应用的问题。相比之下，基于LNOI平台的理论和实验均已证明在信号和空闲光子频率处具有共振的单个纳米谐振腔能够有效提高SPDC的效率[110-111]。在此基础上，由非线性纳米谐振腔阵列组成的超薄超构表面有望产生更高的光子对产生率，实现紧凑型高效单光子源。2021年，德国耶拿大学Chekhova教授课题组制备出在信号和空闲光子频率处具备基本电磁共振的铌酸锂截断金字塔阵列超构表面，利用该结构进行SPDC过程的原理如图6(a)所示。通过测量来自超构表面的SPDC光谱(如图6(b)所示)发现，在共振频率附近的窄带宽内，光子对生成率相比于未加工的薄膜区域提高了两个数量级(130倍)。实验结果还证明，发射光子对的光谱宽度可以通过电共振波长与SPDC简并波长之间的失谐来控制[112]。该结构使纠缠光子的平面光学源成为可能，并有望成为一种新的有前途的量子光学实验平台。
图 6. (a) 铌酸锂超构表面的SPDC：泵浦光从基板侧入射，光子对在反射中收集。泵浦和SPDC光子都沿铌酸锂光轴z偏振；(b) 从量子光学超构表面测量的SPDC光谱。灰色星显示来自未图案化铌酸锂薄膜的SPDC光谱[112]Figure 6. (a) SPDC from a lithium niobate metasurface: the pump is incident from the substrate side, photon pairs are collected in reflection. Both the pump and the SPDC photons are polarized along the lithium niobate optic axis z; (b) Measured SPDC spectra from quantum optical metasurfaces. Gray stars show the SPDC spectrum from the unpatterned lithium niobate film[112]3.2 电光调制
在过去的几十年中，超构表面在光场调控领域展现出了非凡能力。但目前的超构表面在本质上大多是静态的，其光学特性在制造过程结束后就被固定下来。对超构表面特性进行调制可以为光场调控提供新的机会，从而促进向动态光学器件的过渡[2, 113-116]。因此，超构表面特性的动态调控一直是研究的热点，许多不同的动态调谐机制也已经趋于成熟，例如光泵浦[117]、热加热 [118]、化学反应[119]和电刺激[120]。在所有这些调控机制中，电场调控技术因有望将超构表面与其它片上光电器件集成而引起了人们的广泛关注。
铌酸锂晶体具有较宽的透明窗口(0.35 μm~5 μm)，较高的折射率(765 nm处n0 = 2.26)以及优异的电光系数(r33 = 34 pm/V)，在电光调制研究方面具有广泛的应用[32, 121]。迄今为止，LNOI已经成为超紧凑光子器件的一个有前途的平台，包括电光调制器在内的各种高质量、高性能的功能性器件被成功演示。得益于LNOI的结构优势(铌酸锂薄膜和衬底(如SiO2)之间的大折射率对比度)，光学模式被紧密限制在纳米厚度的铌酸锂层内，从而进一步提高了电光调制效率。通过使用不同的LNOI微结构，如马赫-曾德尔干涉波导[33]、光子晶体[122]、微环[36]或微盘[123]等，具有数十到数百GHz调制速度的各种片上EO调制器单元已经得以实现。
2020年，苏黎世联邦理工学院Grange教授课题组展示了具有高效EO调制性能的铌酸锂周期阵列的初步设计[124]。2021年许京军教授课题组首次通过实验证明了铌酸锂超构表面的EO调制特性，其利用纳米光栅内部连续谱中的准束缚态(quasi-bound states in the continuum, QBIC)共振模式，在可见光范围内实现了对透射光相位的动态调控，调制强度相比于未加工的薄膜区域提升了1.46倍[125]。同年，Grange教授课题组通过ICP-RIE技术制备出由线性EO效应调谐的铌酸锂纳米柱阵列超构表面。该超构表面由两侧的金电极驱动，其示意图和SEM图像如图7(a)所示。图7(b)展示了驱动电压为2 VPP时不同波长下的透射谱及调制增强因子，结果表明EO调制幅度与波长有关，同时在超构表面的光共振处观察到透射光的调制强度增强了80倍，与未加工的薄膜区域相比增强了两个数量级，这也是迄今为止最快和最强的EO调制超构表面[65]。这一概念证明工作向使用铌酸锂超构表面进行自由空间调制迈出了重要的第一步。
图 7. (a) 由金电极驱动的超构表面结构。左下插图为超构表面柱结构的SEM图像，右下插图显示了电极(黄色)之间几个超构表面(紫色)的伪色SEM；(b) 半径为 135 nm、周期为 500 nm 的超构表面的透射率(蓝色线)，橙色线表示 2 VPP和 180 kHz 的交流电压下的调制增强(定义为超构表面的调制幅度除以未图案化区域的调制幅度)[65]Figure 7. (a) Metasurface driven by Au electrodes. The lower left inset shows the SEM image of the metasurface pillar structure. The lower right inset shows a false-color SEM of several metasurfaces (purple) between the electrodes (yellow); (b) Measured transmission (blue) of a metasurface with radius 135 nm and period 500 nm, normalized by the transmission of an unstructured area. The orange line shows the modulation enhancement, defined as the modulation amplitude of the metasurface divided by the modulation amplitude of an unpatterned area, for an AC voltage of 2 Vpp and 180 kHz[65]3.3 光无源功能
在LNOI片上光学器件中，非线性相位匹配条件通常是通过双折射或铁电畴的周期性反转来实现的[126-127]。然而，这两者都需要额外的色散调控，并且通常是窄带的[39, 128]。例如SHG的转换带宽通常在10 nm以内，而SPDC的带宽虽然可以通过设计不同的波导长度达到100 nm以上，但在实际应用方面仍然有很大的局限性。针对这一局限性，在LNOI集成光子学中引入由周期性分布的纳米天线组成的光学超构表面结构，可以规避相位匹配要求。其基本方案是在片内波导的顶部表面绘制一个梯度超构表面结构，如图8(a)所示。通过合理设计天线阵列和波导结构，可以任意控制波导内的光传播，从而实现非完美的相位匹配条件。这种方案支持TE和TM偏振的光学元件，在非线性光学研究中具有显著优势。图8(b)显示了基于超构表面的非线性集成光子器件的工作原理，在被超构表面图案化的波导区域中，光功率首先从泵浦频率下的基模TE00(ω)耦合到SH频率下的基模TE00(2ω)，然后在梯度超构表面的帮助下耦合到SH频率下的高阶波导模式TEmn(2ω)和TMmn(2ω)。梯度超构表面提供的单向波矢量使得从TEmn(2ω)和TMmn(2ω)模式返回到模式TE00(2ω)或TE00(ω)模式的光功率耦合效率非常低，这种单向的光功率传输确保了SHG功率作为传播距离的函数的有效积累，即所谓的无相位匹配(phase-matching-free)非线性产生，因此转换效率对泵浦频率和器件几何形状的变化都不敏感。基于该方案，2017年，哈佛大学Loncar教授课题组在纳米光子LNOI波导上构图了a-Si纳米棒天线的相控阵列，实验结果证明该结构在广泛的泵浦波长范围内表现出高效的SHG，归一化转换效率为1660%/W·cm2[88]。2020年，南京大学李涛教授课题组基于LNOI片上波导顶部图案化的光栅超构表面，演示了SHG以及生成的SH波的面内操纵。此外，为了操纵波导内SH信号的波前，在光栅超构表面中引入了全息设计。因此，非线性光束整形的强大功能得以展示，包括双聚焦和Airy光束生成，从而为灵活的片上多通道路由提供了机会[129]。相比于LNOI片上铌酸锂作为超构单元的超构表面结构，这种超构表面与铌酸锂波导的耦合体系能够在频率转换的同时对产生的倍频光导模实现灵活的波前调控，从而将空间光到导模的耦合、基波光到倍频光的频率转换以及谐波输出波前操控三个过程合为一，实现了高度集成的非线性光束调控功能。在此基础上，2021年该课题组在LNOI平台上开发了一个集成的几何超构表面，用作导波和自由空间辐射光之间的接口。通过在LNOI波导的顶面上排布不同旋转角度的亚波长天线图案，可以将导波操纵成空间中所需要的波前，从而实现聚焦、多通道涡旋波束生成和全息成像等多种复杂的自由空间光操纵功能[130]。
图 8. (a) 集成梯度超构表面的LiNbO3片上脊波导，用于实现无相位匹配的二次谐波产生; (b) 基于超构表面无相位匹配的二次谐波产生原理图[88]Figure 8. (a) Schematic of the LiNbO3 on-chip ridge waveguide integrated with a well-designed gradient metasurface for achieving phase-matching-free second harmonic generation; (b) Conceptual diagram of the metasurface-based phase-matching-free second harmonic generation[88]除波前调控外，铌酸锂超构表面在光无源方面的应用还体现在灵活分束和高灵敏传感上。2018年，许京军教授课题组报道了一种基于梯度超构表面的可见光和近红外光分束器。该超构表面由两排圆柱体组成，它们显示出相反方向的相位梯度，从而将传输的光束折射到两个方向。此外，该分束器的分流比可以通过有选择性地调整某排铌酸锂圆柱体的损耗水平来进行灵活调节[131]。基于此，纳米级尺寸分束器可广泛应用于制造小型光子器件，如微型干涉仪、集成光学电路的多路复用器等。随后，该课题组在负载SiO2的铌酸锂波导上设计了微棒阵列超构表面的复合结构，展示了其作为太赫兹传感通用设计的潜力。片上局域表面等离子体的近场耦合可以使表面波模式的约束更强，沿波导的相互作用长度更长，这将有效地增加分子吸收，从而能够检测到薄乳糖层。当固有特征频率为0.529 THz且乳糖层较薄时，透射光谱的选择性显著，与正常通过相同厚度的乳糖层透射太赫兹波时相比透射光谱强度增强了20倍。实验与模拟结果均表明，该结构可以作为一种高灵敏度片上太赫兹传感器，用于微量物质的检测[132]。
4. 总结与展望
本文综述了LNOI薄膜片上光子学器件—铌酸锂超构表面的最新研究进展，包括有潜力的制备方案以及铌酸锂超构表面在光频转换、电光调制、光无源等方面的应用现状。随着近年来晶圆级、高质量的LNOI薄膜制造技术的突破，基于LNOI 薄膜的微纳光学和集成光子学正处于快速发展阶段。各种高性能铌酸锂光子学器件的应用已经不仅限于线性和非线性光学，甚至已经扩展到量子光学、腔电光学和压电光机械等新兴领域。众多研究结果证明，铌酸锂超构表面有利于制造具有高灵活性的超紧凑光子器件，同时展现出优异的光学功能。在非线性光学领域，铌酸锂超构表面的应用不仅限于谐波与光子对产生，也有望应用于其它非线性过程，如四波混频、和频产生、参数下转换等，在生物传感、量子光通信等领域都具有广泛的应用前景。在电光调制领域，未来更多的工作应致力于将器件调制范围扩展到 GHz 范围内，以及通过优化设计达到更好的电场和光场重叠或更高的Q因子共振，实现电光调制幅度的更强增强，从而为空间光调制器在波前调控、脉冲整形、偏振控制等领域的发展奠定坚实基础。此外，超构表面与铌酸锂波导的耦合体系也有望实现高效耦合器、分束器、传感器等多种光无源功能器件。总之，基于超构表面对光的灵活操纵特性及铌酸锂独特的材料性质，铌酸锂超构表面结构在未来具有巨大的应用潜力。
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图 1 LNOI片上微纳光子学结构制备的主要流程图：图案化处理；图案转移；后处理过程
Figure 1. The main flow chart of fabrication of photonic structure on the LNOI chip: patterned processing; pattern transfer; post-processing
图 2 (a) 铌酸锂超构表面SHG示意图；(b) 制备工艺流程示意图；(c) 所制备超构表面的SEM图像，其中纳米谐振腔由截断金字塔和下面的残余层组成[64]
Figure 2. (a) A schematic of the SHG from the lithium niobate metasurface; (b) Schematic illustration of the process flow of fabrication; (c) SEM image of the fabricated metasurface in which the nanoresonator consists of a truncated pyramid and a residual layer underneath[64]
图 4 采用紫外光刻结合RIE技术制备微环腔，然后用CMP抛光侧壁。(a) 制备工艺流程示意图；(b) 微环腔SEM图像；CMP前(c)后(d)微环腔侧壁的放大SEM图像[75]
Figure 4. Microring fabricated by UV lithography and RIE, followed by sidewall polishing by the CMP. (a) Schematic illustration of the process flow of fabrication; (b) False-color SEM image of the microring; and enlarged SEM image of the sidewall (c) before and (d) after the CMP[75]
图 5 (a) 非线性铌酸锂超构表面的SHG示意图。左下插图为D=600 nm的超构表面截面的典型SEM图像，右下插图显示了研究中使用的铌酸锂薄膜的测量二阶极化率；(b) 超构表面SHG效率的光谱依赖性[101]
Figure 5. (a) A schematic of the SHG from the nonlinear lithium niobate metasurface. Left inset gives a typical SEM image of cross section of the metasurface with D=600 nm. Right inset presents the measured second-order susceptibility of the lithium niobate film used in this study; (b) Spectral dependence of SHG efficiencies from metasurfaces[101]
图 6 (a) 铌酸锂超构表面的SPDC：泵浦光从基板侧入射，光子对在反射中收集。泵浦和SPDC光子都沿铌酸锂光轴z偏振；(b) 从量子光学超构表面测量的SPDC光谱。灰色星显示来自未图案化铌酸锂薄膜的SPDC光谱[112]
Figure 6. (a) SPDC from a lithium niobate metasurface: the pump is incident from the substrate side, photon pairs are collected in reflection. Both the pump and the SPDC photons are polarized along the lithium niobate optic axis z; (b) Measured SPDC spectra from quantum optical metasurfaces. Gray stars show the SPDC spectrum from the unpatterned lithium niobate film[112]
图 7 (a) 由金电极驱动的超构表面结构。左下插图为超构表面柱结构的SEM图像，右下插图显示了电极(黄色)之间几个超构表面(紫色)的伪色SEM；(b) 半径为 135 nm、周期为 500 nm 的超构表面的透射率(蓝色线)，橙色线表示 2 VPP和 180 kHz 的交流电压下的调制增强(定义为超构表面的调制幅度除以未图案化区域的调制幅度)[65]
Figure 7. (a) Metasurface driven by Au electrodes. The lower left inset shows the SEM image of the metasurface pillar structure. The lower right inset shows a false-color SEM of several metasurfaces (purple) between the electrodes (yellow); (b) Measured transmission (blue) of a metasurface with radius 135 nm and period 500 nm, normalized by the transmission of an unstructured area. The orange line shows the modulation enhancement, defined as the modulation amplitude of the metasurface divided by the modulation amplitude of an unpatterned area, for an AC voltage of 2 Vpp and 180 kHz[65]
图 8 (a) 集成梯度超构表面的LiNbO3片上脊波导，用于实现无相位匹配的二次谐波产生; (b) 基于超构表面无相位匹配的二次谐波产生原理图[88]
Figure 8. (a) Schematic of the LiNbO3 on-chip ridge waveguide integrated with a well-designed gradient metasurface for achieving phase-matching-free second harmonic generation; (b) Conceptual diagram of the metasurface-based phase-matching-free second harmonic generation[88]
表 1 超构表面SHG主要性能参数
Table 1. Main performance parameters of metasurface SHG
Meta-atoms Size Process technology FH intensity FH wavelength SH wavelength Conversion efficiency Truncated pyramid[59] PT=870 nm
L=700 nmEBL+IBE 4.3 GW/cm2 ~1550 nm ~775 nm ~10−6 Ridge waveguide[86] D=600 nm
d=85 nm
h=235 nmFIB 2.05 GW/cm2 740 nm~1000 nm 370 nm~500 nm 2×10-6 Cylinder[65] PC=590 nm
R=175 nmFIB 0.5 GW/cm2 825 nm 412 nm 2.9×10−8
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