超低损耗铌酸锂光子学 
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1 引言
人类文明的进步很大程度上得益于信息通信与处理技术的发展。目前,光纤通信技术已使以接近光速传递信息成为可能,而半导体技术则促进了超大规模集成电路的发展,促使信息处理的速度和效率达到了前所未有的高度。然而,随着信息产业中大数据、人工智能、量子计算等新兴变革性高技术的不断涌现,人类对信息获取与处理能力提出了更高的需求。集成光路(PICs)有望成为一种极具竞争力的技术手段,可为信息产业提供具有更高速度与更低功耗的先进芯片的支撑平台。自1969年集成光路的概念被提出以来[1],科学家们就一直为实现大规模的光子芯片而进行不懈的努力,比较典型的技术途径有二氧化硅平面集成光路、硅基集成光路等。但标准化的制造路线至今仍未确立。其主要原因是光子芯片中的核心结构光波导,往往需要同时具备超低传输损耗、小弯曲半径、高调制效率这三项重要特征。对于传统光子材料和加工技术手段而言,同时满足这三方面要求极其困难。
表 1. 典型光子集成平台对比[23]
Table 1. Comparison between typical photonic integrated platforms[23]
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二氧化硅平面集成光路的工艺流程分以下步骤[2]:1)采用具有较高沉积速率的火焰水解沉积法在硅片上沉积一层白色的二氧化硅精细颗粒,这层颗粒经高温处理后变成透明石英玻璃膜,形成厚度可大于30 μm的波导下包层;2)利用火焰水解沉积和高温处理,在下包层上生长出掺杂GeO2的高折射率波导芯层,并在波导芯层上利用光刻与反应离子束刻蚀制备出光波导结构;3)采用火焰水解沉积和高温处理来制备包覆波导芯的上包层。由于上下包层都由纯熔融石英组成,而波导芯部分则因掺杂了GeO2而具有稍高的折射率,因此可以形成低损耗的光子回路。
该技术方案中,波导与单模光纤具有相同的结构设计和材料组分,全面继承了光纤所具有的优良特性,故可获得超低的传输与耦合损耗。此外,在加工波导的工艺中,离子束干法刻蚀可以将波导侧壁的粗糙度控制在纳米量级,远远小于波导中的传输光波长;再加上波导芯和包层之间的折射率差较小,可以有效抑制由表面粗糙度引起的散射损耗。尽管如此,二氧化硅平面集成光路仍存在两方面主要的缺陷:第一,二氧化硅的非线性系数和电光系数都较低,调控难度大,可实现的功能器件种类受限;第二,通过化学组分调控实现的波导芯结构同样具有折射率变化较小、波导弯曲半径过大的缺点,也不适用于大规模的光子集成应用。
硅基集成光路的加工通常是借助深紫外光刻和干法刻蚀等工艺来实现。1)将光刻胶旋涂在绝缘体上的硅(SOI)晶片上进行预烘;2)将晶片送到光刻机,通过带图案的掩模对光刻胶曝光;3)在平版印刷光刻之后,对光刻胶进行后烘,并进行显影;4)将显影后的光刻胶作为硅刻蚀的掩模,对顶部的硅薄膜层进行离子刻蚀;5)剥离光刻胶,留下紧贴在二氧化硅层上的纳米硅线波导结构。通常对刻蚀产生的纳米硅线还要进行一些后处理,如再沉积一层二氧化硅作为波导的上包层等[3-4]。这种硅线波导具有亚微米量级的模场尺寸和微米量级的弯曲半径,非常有利于实现高密度的光子集成。
得益于成熟的半导体光刻技术及其工业化产能,硅基集成光路具有大规模、低成本的工艺优势。此外,硅的折射率很高,在1550 nm波长约为3.5,硅和空气之间如此高的折射率差使得硅纳米线波导的弯曲半径可小到几个微米,从而可以实现极高密度的光子集成。利用该技术已制备了维度达15×15的可编程路径编码多维纠缠系统,能够以极高的精度、可控性和通用性实现全片上多维纠缠的产生、操控与测量[5]。尽管如此,硅基集成光路仍存在一些与硅材料自身特性相关的缺陷,如:硅晶体没有电光系数,因此通常只能通过热光效应对硅线波导进行调控,能耗较高且响应时间较慢,通常调制带宽为50~60 GHz;此类硅线波导的损耗仍然较大,典型的传输损耗约在1 dB/cm量级。
相当长的一段历史时期,种类繁多的光子芯片都得到了较为深入的研究,包括石英玻璃光子芯片[6-9]、聚合物光子芯片[10-12]、以硅为代表的半导体光子芯片[13-16]等。这些光子芯片技术均有各自的优势与不足。例如:石英玻璃光子芯片具有低成本与超低损耗的优势,然而,因其转弯半径过大,存在集成度难以提升的问题;聚合物光子芯片具有成本低廉、制备方便等优点,然而其调制效率与稳定性有待提高,损耗有待减小;以硅为代表的半导体光子芯片具有制造工艺成熟、成本较低等优势,然而对半导体材料进行高速光调制需借助载流子效应,故存在损耗与色散等问题。
以上示例表明,实现高质量的大规模集成光子芯片需要优越的材料特性和先进的加工手段完美结合。铌酸锂(LN)晶体具有宽的透射窗口(0.35~5.00 μm)、高的非线性系数(χ(2)=30 pm/V)、高的折射率(~2.2)和大的电光系数(r33=30.8×1012 m/V)等优异特性,是光子集成器件衬底材料的重要候选者[17-22],被誉为“光子学中的硅”。
2 飞秒激光直写结合聚焦离子束刻蚀
飞秒激光由于具有显著降低的热效应、可突破衍射极限的加工精度、可对透明材料内部进行三维加工等独特优势,被广泛用于微纳加工领域。2015年,利用飞秒激光直写结合聚焦离子束刻蚀来制备高品质铌酸锂微腔的技术路线被首次提出,同时成功地实现了品质因子(Q 值)超过105 的微盘腔的制备,突破了当时国际最高纪录[28-29]。该方案使用铌酸锂薄膜/二氧化硅/铌酸锂衬底的材料作为样品,如
在具体步骤上,飞秒激光烧蚀后获得直径约59 μm、厚度约15 μm的圆盘轮廓,其扫描电镜图(SEM)如
该项铌酸锂光子结构加工技术不仅可以加工单个光子结构,还可以实现多个光子结构的集成。
图 1. 利用飞秒激光直写结合聚焦离子束刻蚀技术制备的高品质因子铌酸锂微腔[28-29]。(a)铌酸锂微腔制备流程图;(b)飞秒激光烧蚀后形成的微柱体SEM图像;(c)聚焦离子束刻蚀后形成的微柱体SEM图像;(d)经过化学腐蚀和高温退火后,直径为55 μm微盘的SEM俯视图,插图为微盘SEM侧视图;(e)微腔在1553.83 nm谐振波长附近的透射光谱(虚线)及洛伦兹拟合曲线(实线)
Fig. 1. LN microresonator with high Q factor fabricated by the femtosecond laser writing combined with focused ion beam milling[28-29]. (a) Procedures of fabrication of an LN microresonator; (b) SEM image of a cylindrical post formed after femtosecond laser ablation; (c) SEM image of a cylindrical post formed after the FIB milling; (d) SEM image (top view) of the 55-μm diameter microresonator after the chemical etching and high temperature annealing, in which the inset is side view of the microresonator; (e) measured transmission spectrum (dotted line) and its Lorentzian fitting curve (solid line) around the resonant wavelength of 1553.83 nm
图 2. 利用飞秒激光直写结合聚焦离子束刻蚀技术制备的集成光子结构。 (a)~(c)铌酸锂微盘腔与波导的片上集成[40];(d)耦合铌酸锂微盘光子学分子[41];(e)垂直集成的双盘铌酸锂微腔[42]
Fig. 2. Integrated photonic structures fabricated by the femtosecond laser writing combined with focused ion beam milling. (a)-(c) Monolithic integration of an LN microdisk with a free-standing waveguide[40]; (d) coupled LN microdisk photonic molecules[41]; (e) vertical integration of double LN microdisks[42]
3 电子束曝光结合离子刻蚀
电子束曝光结合离子刻蚀技术于2014年由哈佛大学研究组提出[43]。该技术路线的主要工艺流程如
图 3. 电子束曝光结合离子刻蚀的铌酸锂光子芯片制造工艺流程图[43]
Fig. 3. Fabrication flow schematic of LN photonic chip by the electron beam exposure combined with ion milling[43]
采用该技术制备的直径为28 μm的铌酸锂微盘腔如
图 4. 制备的直径为28 μm的铌酸锂微盘腔的SEM图和透射谱[43]。(a)铌酸锂微盘腔的SEM图,其放大图显示侧壁非常光滑;(b)微盘腔的透射谱,显示多组谐振模式,其中插图为光纤锥耦合到微盘腔顶部的光学显微图;(c)~(e)二阶、一阶、三阶谐振模式的高分辨率透射光谱
Fig. 4. SEM image and transmission spectra of the fabricated LN microdisk resonators with 28-μm diameter[43]. (a) SEM image of the LN microdisk and its magnified view, revealing that the microdisk edge has smooth sidewalls; (b) representative transmission spectrum collected from the microdisk, revealing several sets of resonant modes, in which the inset shows the optical micrograph of tapered fiber coupling on top of the microdisk resonator; (c)-(e) high-resolution transmission spectra for 2nd, 1st and 3rd order resonant modes, respectively
该技术利用现有较为成熟的电子束曝光工艺与离子刻蚀技术,一旦找准工艺条件,就可以快速复制该技术路线,进行小批量光子原型器件的研发,因此被多个国内外研究团队所采纳[45-46]。
4 飞秒激光光刻辅助化学机械抛光
如上所述,飞秒激光直写与聚焦离子束刻蚀相结合,可以制备出Q值大于106的铌酸锂薄膜微盘腔。然而,人们通常需要更高Q值的回音壁模式(WGM)微谐振腔,以在低泵浦功率下实现高效的非线性过程。聚焦离子束刻蚀不可避免地会留下纳米量级的表面粗糙度,从而将铌酸锂薄膜微腔的Q值限制在107以下。鉴于此,飞秒激光光刻辅助化学机械抛光(PLACE)技术于2018年被提出并报道,实现了Q值高达4.7×107的铌酸锂微盘腔的制备[47];最近,通过进一步完善工艺,利用该技术已经能够将铌酸锂微腔的品质因子提高至108以上[48],这一品质因子的最新
图 5. 制备的波导耦合微环和跑道型微腔以及传输损耗测量[44]。(a)波导耦合微环腔和跑道型微腔的SEM图(上方)以及跑道型微腔的设计图(下方);(b)利用不同臂长的跑道型微腔获得的传输损耗,激发波长为1590 nm
Fig. 5. Fabricated waveguide-coupled microring and micro-racetrack resonators, and propagation loss measurement[44]. (a) SEM images of the microring and micro-racetrack resonators (top) and design of the micro-racetrack resonators (bottom); (b) extracted propagation loss for micro-racetrack resonators with different lengths at excitation wavelength of 1590 nm
图 6. 飞秒激光光刻辅助化学机械抛光(PLACE)的铌酸锂光子芯片制造流程图。(a)在LNOI上沉积铬(Cr)层;(b)飞秒激光烧蚀Cr薄膜形成掩模图案;(c)通过化学机械抛光将掩模图案从Cr膜转移到LNOI上;(d)移除Cr,并进行二次化学机械抛光; (e)化学机械抛光原理与实验装置图
Fig. 6. Fabrication flow schematic of LN photonic chip by the femtosecond laser photolithography assisted chemo-mechanical etching (PLACE). (a) Depositing a thin layer of Cr on the top of the lithium niobate on insulator (LNOI) wafer; (b) patterning the Cr layer by femtosecond laser ablation; (c) conducting chemo-mechanical polishing (CMP) on the sample to transfer the pattern from the Cr mask to the LNOI; (d) chemically removing the remaining Cr mask and performing a secondary CMP; (e) schematic illustration of the CMP principle and the instrument
纪录已经接近铌酸锂材料的本征吸收极限。
PLACE技术的独特优势在于该技术采用化学机械抛光(CMP)来选择性地去除或刻蚀衬底材料以完成芯片构图,而传统的CMP只是一种提高表面光滑度的手段。PLACE技术的主要步骤如
图 7. 制备的超高Q值铌酸锂微腔(直径约为1030 μm)[48]。(a)扫描电镜图;(b)光学显微图;(c)(d)图7 (a)中虚线框内微腔边缘扫描电镜放大图的俯视图和侧视图;(e)利用光纤锥耦合微腔的透射谱进行Q值测量,洛伦兹拟合曲线表明微腔负载Q值为7.5×107;(f)振铃测试获得谐振光子的寿命为 64.3 ns,表明微腔的负载Q值为7.8×107, 本征Q值为1.23×108
Fig. 7. Fabricated ultra-high Q LN microresonator with a diameter of ~1030 μm[48]. (a)(b) SEM image and optical micrograph of the microresonator; (c) zoom-in SEM image of microresonator periphery [the dashed box in Fig. 7 (a)] from the top view; (d) zoom-in SEM image of microresonator periphery [the dashed box in Fig. 7 (a)] from the side view; (e) Q factor measurement using the transmission spectrum from fiber taper coupling, indicating a loaded Q-factor of 7.5×107 of the microresonator; (f) ring-down measurement, indicating the lifetime of the resonant photon is 64.3 ns, corresponding to the loaded (intrinsic) Q factor of 7.8×107 (1.23×108)
研究发现,通过控制化学机械抛光过程的持续时间,可以连续改变铌酸锂悬空微盘的边缘楔角[49]。如
图 8. 不同边缘楔角对铌酸锂微盘的影响[49]。(a)~(f)不同边缘楔角的铌酸锂微盘的扫描电镜侧视图,角度分别为9°、14°、22°、30°、40°和51°;(g)不同边缘楔角的微盘腔的Q值
Fig. 8. Influence of different wedge angles on LN microdisk[49]. (a)-(f) SEM images of the fabricated LN microdisks from the side view with different wedge angles of 9°, 14°, 22°, 30°, 40°, and 51°, respectively; (g) Q factor of the fabricated LN microdisks with different wedge angles
图 9. 基于PLACE技术制备的铌酸锂微环腔和光波导[50]。(a)(b)铌酸锂微环腔的SEM俯视图及其局部放大图;(c) AFM测得微环表面的粗糙度为0.452 nm;(d)长度为11 cm的铌酸锂光波导的数码照片;(e)(f)铌酸锂光波导的光学显微图
Fig. 9. LN microring cavity and optical waveguide based on PLACE technique[50]. (a) Top-view SEM image of a LN microring resonator; (b) zoomed view of the ridge of the microring resonator in Fig. 9 (a); (c) AFM image of the ridge, revealing that the surface roughness of the microring is 0.452 nm; (d) picture of a chip consisting of an 11-cm-long waveguide captured by digital camera; (e)(f) zoomed images of the waveguides on the chip captured with an optical microscope
PLACE技术可以实现传输损耗低至0.027 dB/cm的脊型铌酸锂光波导的制备[50]。制备铌酸锂光波导的工艺流程与制备微盘腔的基本相同,区别在于飞秒激光直写光刻之后,保留金属铬掩模的铌酸锂薄膜区域是狭长的条形,从而在经过化学机械抛光、铬膜腐蚀等后续处理后,只留下脊型铌酸锂波导结构。
如
图 10. 铌酸锂单模脊形波导[38]。(a)覆盖氧化钽后的铌酸锂单模波导SEM图;(b)(c)测量与模拟的TE波输出空间模式;(d)(e)测量和模拟的TM波输出空间模式
Fig. 10. Single-mode ridge waveguide on lithium niobate[38]. (a) SEM image of LN waveguide covered with Ta2O5; (b)(c) measured and calculated TE mode profiles; (d)(e) measured and calculated TM mode profiles
5 超低损耗铌酸锂薄膜光子器件
利用以上的技术途径,目前已经在铌酸锂薄膜衬底上实现了一批高品质因子的铌酸锂光子原型器件,包括波导耦合分束器[51]、可重构多功能芯片[52]、光子延时线[53]、微腔激光器[54]、波导放大器[55]、高速光调制器[56]、微波光子芯片[5]等。
5.1 波导耦合分束器
5.2 可重构多功能集成光子芯片
图 11. 铌酸锂单模波导分束器[51]。(a)化学机械抛光制备的裸露(无覆盖)铌酸锂分束器的光学显微图;(b)沉积Ta2O5包层膜后的铌酸锂分束器的光学显微图;(c)分束比随耦合长度的变化呈正弦振荡关系;(d)不同耦合长度的分束器输出端口的模场分布
Fig. 11. LNOI single-mode beam splitters[51]. (a) Optical microscope image of the bare LNOI beam splitters fabricated with the chemo-mechanical polish lithography; (b) optical microscope image of the LNOI beam splitters covered with a layer of Ta2O5; (c) visibility as a function of the coupling length, showing a sinusoidal dependence of the splitting ratio on the coupling length; (d) near-field profiles captured at the output ports of the beam splitters with the different coupling lengths
图 12. 基于铌酸锂薄膜的可重构多功能集成光子芯片[52]。(a)多功能集成光子芯片设计示意图;(b)制备的芯片样品和硬币的数码照片;(c)图12 (b)虚线框对应的放大显微图;(d)1×6光开关波导布线示意图;(e)光开关的测试结果图;(f) 3×3均衡干涉分束器的波导布线示意图;(g)均衡干涉分束器的测试结果图
Fig. 12. Reconfigurable multifunctional photonic integrated chip on LNOI[52]. (a) Schematic of the multifunctional photonic integrated chip; (b) digital camera picture of the chip placed near by a 1 RMB coin; (c) zoom-in micrograph of the area [the dashed box in Fig. 12 (b)]; (d) schematic of waveguide wiring of a 1×6 light switch; (e) measurement result of the light switch; (f) schematic of waveguide wiring of a balanced 3×3 interference beam splitter; (g) measurement result of the balanced interference beam splitter
5.3 可重构光子延时线
大尺寸低损耗光波导组成的光学延迟线(OTDL)在光学陀螺、微波光子学、全光信号和量子信息处理等领域都引起了广泛的兴趣。2020年,利用PLACE加工技术,已经实现了长度超越1 m的波导低损耗光子延时线,如
图 13. 米级长度的铌酸锂波导光学延迟线[53]。(a)可重构光学延时线设计图;(b)加工完成的光学延时线器件数码照片;(c)光学延时线中测得的传输损耗值, 显示随延时线的长度呈线性依赖关系;(d)光脉冲经过不同长度的光学延时线后,示波器测得的时间相差2.2 ns的双脉冲峰,时间负值代表光脉冲先到达光探测器
Fig. 13. OTDL of a waveguide on LN with meter-scale lengths[53]. (a) Schematic of the reconfigurable OTDL; (b) digital camera photo of the fabricated OTDL device; (c) measured losses of OTDL, showing a linear dependence on the length of OTDL; (d) waveform recorded on the oscilloscope after the light pulse traveling through the OTDLs with different lengths, showing two peaks separated by a time delay of 2.2 ns, in which negative time means that the pulses arrive first at the photodetector
5.4 微腔激光器
利用PLACE技术,已经在厚度为600 nm的Z切掺铒铌酸锂薄膜上实现了微腔激光器的制备[54]。微腔的激光发射光谱如
图 14. 掺铒铌酸锂微盘腔激光器的激射阈值特性[54]。(a)泵浦功率为0.92 mW的掺铒激光器的光谱,插图显示出很强的上转换荧光;(b) 1560 nm附近的放大光谱,显示出多模激光光谱;(c)激光强度与976 nm泵浦光功率之间的依赖关系,圈圈为实验数据,实线为拟合曲线
Fig. 14. Lasing threshold characteristics of the Er3+-doped LN microdisk[54]. (a) Spectrum of the Er3+-doped laser with the pump power at 0.92 mW, in which the inset displays the strong upconversion fluorescence of the microdisk; (b) enlarged spectrum around 1560 nm, revealing the multimode lasing spectrum; (c) dependence of the intensity of all emission lines on the absorbed pump power at 976 nm, where the experimental data are shown as circles and the curve is a linear fitting
5.5 高增益波导放大器
最近,利用PLACE技术实现了高增益掺铒铌酸锂波导放大器[55]。
式中,Pon和Poff分别是在输出光纤处有无泵浦激光时收集到的信号光功率,而αL是掺铒铌酸锂波导的光传输损耗,L是光传输长度。
图 15. 制备的掺铒铌酸锂波导放大器及其性能测量[55]。(a)掺铒铌酸锂波导放大器示意图;(b)直波导的光学显微图;(c)掺铒铌酸锂波导横截面的SEM图(倾斜角:52°);(d)传输损耗曲线,插图为直径为400 μm的掺铒铌酸锂微环的显微图像;(e)不同泵浦功率下测得的1530 nm波长处的信号光光谱;(f)1530 nm信号光波长处测得的掺铒铌酸锂波导的净增益与泵浦光功率之间的依赖关系
Fig. 15. Fabricated Er3+-doped LN waveguide amplifier chip and characterization measurement[55]. (a) Schematic of the Er3+-doped LN waveguide amplifier chip; (b) optical micrograph of the straight waveguide; (c) SEM image of the cross section of the fabricated Er3+-doped LN waveguide (tilt angle: 52°); (d) propagation loss curves, in which the inset is micrography of a 400-μm-diameter Er3+-doped LN microring; (e) measured spectra at the signal wavelength of 1530 nm for the different pump powers; (f) net gain of the Er3+-doped LN waveguide as a function of launched pump power at the signal wavelength of 1530 nm
图 16. 单片高速电光调制器[31]。(a)由3个马赫-曾德尔干涉仪调制器构成的电光调制器芯片显微图,插图为调制器横截面示意图;(b) 70 Gbit/s速率下的数据传输实验;(c) CMOS驱动电路;(d)(e)峰峰值电压为200 mV和60 mV时,使用相干接收器和重构眼图获得的已测星座图
Fig. 16. Monolithic high-speed electro-optic modulator[31]. (a) Micrograph of electro-modulator chip consisted of three MZI modulators, in which the inset is cross-sectional schematic of the modulator; (b) data-transmission experiment at a rate of 70 Gbit/s; (c) CMOS driven circuit; (d)(e) measured constellation diagrams obtained with a coherent receiver and the reconstructed eye diagrams at peak-to-peak voltages of 200 mV and 60 mV
图 17. 高消光比级联MZI光子芯片[56]。(a)显微图;(b)单模脊形波导SEM图;(c)消光比与波长的关系
Fig. 17. High extinction cascaded MZI photonic chip[56]. (a) Micrograph; (b) SEM of single-mode ridge waveguide; (c) extinction ratio as a function of wavelength
5.6 高速光调制器
2018年,哈佛大学研究小组在包覆SiO2层的X切铌酸锂中,制备了基于马赫-曾德尔干涉仪的单片高速电光调制器[31]。如
单个马赫-曾德尔干涉仪调制器因其具有高消光比特性,能够很好地用于制备分光比为50∶50的Y型结构,然而,由于其存在不可避免的加工误差,制备具有50∶50完美分光比的Y型结构极其困难。级联马赫-曾德尔干涉仪能够克服此问题,可以提供更高的消光比。2019年,史蒂文斯理工学院研究小组通过感应耦合等离子反应刻蚀(ICP-RIE),可在X切铌酸锂薄膜上加工出级联马赫-曾德尔干涉仪(
图 18. 高速硅与铌酸锂混合集成电光调制器[36]。(a)调制器结构示意图;(b)混合波导的横截面结构示意图;(c)铌酸锂波导的横截面SEM图;(d)电极和波导的SEM图;(e)垂直绝热耦合器示意图;(f)垂直绝热耦合器不同位置(A、B、C)的横截面SEM图和理论计算的TE模式分布;(g)器件长度为3 mm时透射率随施加电压的变化关系,插图为对数坐标下的消光比;(h)器件长度为5 mm时透射率随施加电压的变化关系;(i)眼图测试结果,结果显示该器件的光信号调制速度可达100 Gbit/s
Fig. 18. High-speed hybrid silicon and LN electro-optic modulators[36]. (a) Schematic of the structure of the modulator; (b) schematic of the cross-section of the hybrid waveguide; (c) SEM image of the cross-section of the LN waveguide; (d) SEM image of the metal electrodes and the optical waveguide; (e) schematic of the VAC; (f) SEM images of the cross-sections of the VAC at different positions (A, B, C) and calculated mode distributions; (g) normalized transmission of the devices with 3-mm length as a function of the applied voltage, in which the inset is measured extinction ratio on a logarithmic scale; (h) normalized transmission of the devices with 5-mm length as a function of the applied voltage; (i) optical eye diagram for the signal at data rates of 100 Gbit/s
5.7 微波光子芯片
图 19. 铌酸锂微波光子芯片[57]。(a)光子芯片总体显微图;(b)含有薄膜声学腔的悬空光学跑道型微腔显微图;(c)由叉指换能器和光波导构成的薄膜声学腔的SEM伪彩图
Fig. 19. Microwave photonic chip on LNOI [57]. (a) Microscope image of the photonic chip; (b) microscope image of a suspended optical racetrack cavity with a thin-film acoustic resonator; (c) false-color SEM image of the thin-film acoustic resonator composed of an interdigital transducer (IDT) and an optical waveguide
6 铌酸锂薄膜光子器件的非线性光学效应
6.1 自然准相位匹配高效非线性谐波产生
高Q 值铌酸锂微盘腔能够产生诸多有趣的非线性光学特性。通过控制铌酸锂微腔的色散和光子偏振态,在尺寸为30 μm、厚度为600 nm的铌酸锂单晶微盘腔中,利用铌酸锂晶体最大的电光系数d33,实现了宽谱的、自然准相位匹配的二次谐波(SHG)、级联三次谐波(THG)产生[39]。在X切、晶体光轴沿Z方向的铌酸锂微盘腔中,沿微盘外围传播的TE波可以感受到晶体取向的不断变化。二阶非线性系数可以表示为
其中θ是波矢k与光轴之间的角度,如
图 20. X切铌酸锂微盘腔中宽带准相位匹配谐波的产生[39]。(a)微腔的有效非线性系数随方位角的变化;(b)二次谐波转换效率与耦合功率的依赖关系,插图为微腔二次谐波光学显微俯视图;(c)级联三次谐波功率与耦合功率三次方之间的依赖关系,插图为微腔二次谐波及级联三次谐波的光学显微俯视图
Fig. 20. Broadband quasi-phase-matched harmonic generation in an X-cut lithium niobate microdisk[39]. (a) Effective nonlinear coefficient of the microcavity varying with the azimuth; (b) conversion efficiency of SHG as a function of the in-coupled power, in which the inset is top-view optical micrograph of the SHG from the microresonator; (c) power of cascaded THG as a function of the cubic power of the in-coupled light, in which the inset is top-view image of the SHG and cascaded THG
其背后的非线性动力学机制为:泵浦光每绕微盘循环一周,有效非线性系数的符号改变一次,类似周期极化的铌酸锂结果,这提供了第一阶倒格矢去补偿倍频过程的相位失配;同时,有效非线性系数的振幅也在振荡,这提供了第三阶倒格矢去补偿级联三倍频过程的相位失配。
6.2 电光可调光机械力学效应
腔光机械力学是研究光与材料机械性能之间相互作用的一门学科,回音壁模式光学微腔由于能够在共振时形成一个光学弹簧,已成为研究光学-力学耦合作用的极佳平台,可以在腔量子电动力学、单分子探测等领域得到应用。
利用PLACE技术在厚度为700 nm的Z切铌酸锂薄膜材料上制备出与微电极同片集成的超高Q值铌酸锂微腔光机械系统[58]。该系统的主要工艺流程为:首先通过飞秒激光直写技术在镀铬膜的铌酸锂薄膜材料上刻写出微盘腔和两个电极的轮廓,然后通过一次化学机械抛光技术去除未被铬膜保护的铌酸锂,留下微盘腔和两个电极部分。为了保留铬金属镀层的调控电极,不采用铬金属腐蚀液,而再次采用飞秒激光直写技术选择性地去除微盘腔上的铬膜,最后将样品浸入稀的氢氟酸溶液中腐蚀掉微盘腔下的部分二氧化硅,得到悬空的微盘腔结构,其直径约为66 μm。
为表征该集成铌酸锂光机械系统的性能,采用如
图 21. 基于铌酸锂微腔的光机械力学系统[58]。(a)集成有Cr电极的66 μm 铌酸锂微腔的SEM俯视图;(b)微腔光机械的测试装置,左下插图为铌酸锂微腔中的模拟电场分布;(c)腔传输的射频谱, 右上插图为铌酸锂微盘中机械模式的仿真结果;(d)在100,200,300,400,500,600 V的直流(DC)电压下,机械频率呈线性下降,分别下降75,200,350,500,630,780 kHz
Fig. 21. Optomechanical system on LN microresonator[58]. (a) Top-view SEM of the 66-μm LN microresonator integrated with Cr electrodes; (b) experimental setup for characterizing the optomechanical system on microresonator, in which the inset (lower left) is simulated distribution of the electric field in LN microresonator; (c) radio frequency (RF) spectrum of the cavity transmission, in which the top-right inset is the simulation result of the mechanical mode in the LN disk; (d) mechanical frequency decreases linearly by 75, 200, 350, 500, 630, and 780 kHz at the DC voltages of 100, 200, 300, 400, 500, and 600 V, respectively
6.3 电光可调宽带光学频率梳
光学频率梳(简称光频梳)是在频域上表现为离散的、等间距的光学频率序列,是精密计时、光通信、光谱学等领域十分重要的精密光谱测量和频率计量的工具。小型化光频梳可以极大地拓展传统光频梳的应用范围,如光学原子钟、光通信、精密光谱测量和光量子计算等领域。通常情况下,频率梳是通过主动控制飞秒激光器的特性来实现的。最近研究表明,高Q值微谐振器中产生的级联三阶非线性光学过程,如四波混频、拉曼效应等,可以用于片上频率梳的产生。微腔频率梳具有数十甚至数百GHz的重复频率,大大减小了器件的尺寸,降低了器件的能耗。尽管在微腔中产生的光脉冲的零频率和重复频率有时候没有被锁定,但研究人员习惯将其也称为频率梳。下文使用术语“频率梳”来描述在铌酸锂微腔中获得的多频光源。
为了产生频率梳,在微谐振腔中通常需要有异常色散来平衡克尔效应。铌酸锂克尔频率梳最早在X切铌酸锂微环腔中得到证实[59]。如
图 22. 微环中产生的孤子频率梳[59]。(a)微环的SEM图;(b)模式的横截面示意图;(c)群速度色散;(d)孤子梳
Fig. 22. Soliton frequency comb generated from a microring[59]. (a) SEM image of the microring; (b) cross sectional schematic of mode; (c) GVD; (d) soliton comb
铌酸锂是具有几个强振动声子分支的拉曼活性晶体,通过拉曼散射可以产生拉曼激射和拉曼频率梳。如
图 23. 电光可调宽带铌酸锂微盘腔光频率梳[60]。(a)微盘和集成的Cr电极的光学显微图(透射照明);(b)产生的梳状光谱;(c)共振波长以~38 pm/100 V的电调制率随电压而线性变化;(d)光频梳随调控电压的变化
Fig. 23. Electro-optical tunable optical frequency comb on an LN microdisk[60]. (a) Optical micrograph of the disk and integrated Cr electrodes under transmission illumination; (b) optical spectra of comb generation; (c) resonant wavelength shifts linearly with an electrical tuning efficiency of ~38 pm/100 V; (d) comb line shift as a function of the applied voltage
图 24. 回音壁模式微腔中的多边形相干模式[61]。(a)直径为84 μm的铌酸锂微谐振腔的光学显微图;(b)微盘边缘的光学显微图;(c)三角形模式;(d)四边形模式;(e)五边形模式;(f)五角星模式;(g)(h)当耦合光纤与微盘的间距为900 nm和0 nm时,理论模拟获得的微盘腔内的相干模式
Fig. 24. Polygon coherent modes in a weakly perturbed whispering gallery microresonator[61]. (a) Optical micrograph of an 84-μm-diameter LN microdisk; (b) close-up view optical micrograph of the edge of the microdisk; (c) triangle mode; (d) square mode; (e) pentagon mode; (f) star mode; (g)(h) intensity distribution of the tapered fiber-microdisk resonance modes when the gap between the tapered fiber and microdisk is 900 nm and 0 nm, respectively
6.4 新型多边形模式物理效应
在超高Q值的铌酸锂微谐振腔中可以观测到多边形模式的物理效应,与此前在变形微腔中观测到的多边形模式相比,当前观测到的多边形模式微盘腔具有更高的Q值(1.4×106)[61]。
6.5 双腔光子学分子非线性效应
光子学分子是由两个微谐振腔耦合组成的一种光子结构,具有独特而复杂的模式特征,是实现相位匹配的非线性频率转换的一种有效结构。利用飞秒激光直写结合聚焦离子束刻蚀技术,已经实现了片上集成的铌酸锂双微盘腔光子学分子结构[
如
同时,利用二维时域有限差分法等数值模拟方法,分别对单个直径为35 μm大腔、单个直径为25 μm小腔,以及双腔耦合模的腔模分布进行了仿真计算。
图 25. 铌酸锂微盘光子学分子结构的非线性光学特性[41]。(a)泵浦功率分别为10.4, 14.0, 17.1, 21.9 mW时,双盘腔在二次谐波波段的非线性光谱;(b)泵浦功率为23.2 mW时的输出频谱,其中左插图为四波混频频谱,右插图为泵浦波长附近的光谱;(c)利用光子学分子分裂模式实现四波混频过程的相位匹配机制示意图;(d)强耦合引起模式劈裂
Fig. 25. Nonlinear optical characteristic in LN microdisk photonic molecules[41]. (a) Nonlinear spectra generated near the second harmonic (SH) wavelength at the pump powers of 10.4, 14.0, 17.1, and 21.9 mW; (b) spectra at the pump power of 23.2 mW, in which the left inset is four wave mixing (FWM) spectrum and the right inset is spectrum near the pump wavelength; (c) schematic illustration of the phase matching mechanism of the FWM process achieved by utilizing the splitting mode of the photonic molecules; (d) mode splitting due to strong coupling
图 26. 铌酸锂双层微盘腔[62]。(a)铌酸锂双层微盘腔结构示意图;(b)微盘腔的光学显微俯视图,两个不规则圆形为向内腐蚀的二氧化硅缓冲层;(c)微盘腔SEM侧视图,放大的伪彩图显示上下微盘层分离良好
Fig. 26. Double-layer LNOI microdisk[62]. (a) Schematic of the double-layer LNOI microdisk; (b) top-view optical microscopy image of the microdisk, in which the two inner irregularly shaped circles indicate the inward etched silica buffer layers; (c) SEM image of the microdisk, with enlarged false-colored images clearly showing the well-separated upper and lower layers
图 27. 铌酸锂双层微盘腔内外模式的能量分布[62]。(a)(c)铌酸锂双微盘内和微盘间的WGM横向强度分布(|E|2),箭头表示电场极化方向;(b)(d)相应模式沿X向的强度分布,其中灰色条对应铌酸锂微盘层
Fig. 27. Energy distribution of interior and exterior modes in the double-layer LNOI microdisk[62]. (a)(c) Transverse intensity profiles of interior WGMs and exterior slot WGMs, in which the arrows indicate the polarization of the electric field; (b)(d) intensity distribution of the respective mode along the X direction, in which the gray regions correspond to the LNOI microdisk layers
6.6 双层腔高Q值腔外回音壁模式效应
传统的回音壁模式光学微腔通过全内反射将光场局域在微腔内部,其光与物质相互作用局限于光与微腔介质本身之间的相互作用,只有一小部分的模式能量可以通过隐矢波形式延伸到腔外环境中用于光耦合或光传感,这极大地限制了它们在腔光力学、光学检测等重要领域中的应用。研究人员利用飞秒激光直写结合聚焦离子束刻蚀技术制备出双层铌酸锂微盘腔,并在理论上证实:该双层微盘腔不仅可以支持腔内高Q值回音壁模式振荡,而且可以支持狭缝回音壁模式振荡,同时双层微盘腔均具有较高的Q因子和很强的光局域性,可使光场在狭缝中得到增强[62]。
如
7 结束语
综上所述,利用飞秒激光直写结合聚焦离子束刻蚀技术、电子束曝光结合离子刻蚀技术、飞秒激光光刻辅助化学机械抛光技术,实现了超高品质因子微盘腔、超低损耗光波导等集成光路中的关键核心光子结构的制备。同时,基于这些光子结构构建了一批高性能铌酸锂光子器件,并实现了多种非线性光学效应。聚焦离子束技术的制造效率相对较低,目前不在产业界的考虑范围,另外两条技术途径目前都已产生了许多令人瞩目的成果,且都进入了产业界的视野。原理上来讲,化学机械抛光技术能够将光子结构表面的散射损耗降至极低水平,从而使得所制备的铌酸锂光子器件的损耗接近材料本征吸收损耗。该技术已逐渐从实验室走向实际应用,可以预见,未来该技术将会使光子集成领域的学术界和产业界受益匪浅。
当然,该技术也有一些问题有待解决,比如,受限于有限的铬与铌酸锂之间的刻蚀比率,铌酸锂微腔与平面集成波导之间存在较大的间隔,很难做到临界耦合;尽管微腔的Q值已突破108,但脊型波导的损耗却没有获得相应进展;大规模集成展露曙光,然而要实现复杂的功能集成,尚需时日;片上激光器已被实现,但还不是单模激光,且还没实现激光的片上非线性频率变换。铌酸锂光子集成技术的未来充满了机遇与挑战,尚有巨大发展空间等待着人们去探索。
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Article Outline
乔玲玲, 汪旻, 伍荣波, 方致伟, 林锦添, 储蔚, 程亚. 超低损耗铌酸锂光子学[J]. 光学学报, 2021, 41(8): 0823012. Lingling Qiao, Min Wang, Rongbo Wu, Zhiwei Fang, Jintian Lin, Wei Chu, Ya Cheng. Ultra-Low Loss Lithium Niobate Photonics[J]. Acta Optica Sinica, 2021, 41(8): 0823012.
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