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一直以来,低损耗硫系玻璃光波导的制备都是硫系集成光子器件的研究重点。研究人员开发了多种制备光波导的方法。主要有:湿法刻蚀[19]、干法刻蚀[20]、热压印[21]、剥离法[22]和激光直写[23]等。其中,在波导制备中利用湿法刻蚀技术很难控制波导的尺寸和形状,同时各向同性腐蚀导致的严重低切现象等问题也较难控制,且硫系玻璃在碱性溶液中易发生腐蚀,增加了波导侧壁和表面的粗糙程度。剥离技术相对简单操作,但是利用此法制备的波导同样存在侧壁相对粗糙的问题,因此没有被广泛采用。热压印技术在制备过程需要一定的高温、高压状态(即高于玻璃的转化温度),因此温度的不稳定会导致薄膜出现析晶的现象,由此制备的光波导性能就会受到严重的影响,同时也会存在压印后脱模困难等问题。激光直写技术在制备过程中会因激光输出功率的波动而导致难以获得理想尺寸和形貌的光波导,提高光子器件的集成度有限。为了更好地解决这些问题,研究人员采用干法刻蚀,通过控制波导芯层和光刻胶的刻蚀选择比,同时优化获得垂直的波导侧壁和良好的波导形貌,从而提高器件加工的精度,有助于提高器件的集成度。目前硫系光波导的干法刻蚀大多采用电感耦合等离子体刻蚀。国内外科研团队对硫系光波导的研究主要包括:常见的二元体系As2S(Se)3波导、三元体系中的GeAsS(Se)和GeSbS(Se)波导以及部分Te基硫系波导。
2007年,Madden等在As2S3薄膜上利用电感耦合等离子体反应离子刻蚀技术制备了长度为22.5 cm、截面为4 µm×2.6 µm的蛇形弯曲脊型波导(如图1(a)所示),并在1.55 µm处获得传输损耗为0.05 dB/cm[24]。2010年,Hu等通过对As2S3波导施加热回流处理,从而降低波导侧壁粗糙度并减小波导的辐射损耗,使得1.55 µm波长处波导的传输损耗在退火后减小到原来的50%[25],如图1(b)所示。
图 1 (a) 4 µm×2.6 µm的As2S3蛇形脊型波导的光学显微镜图[24]; (b) As2S3波导在退火后波导形貌的 AFM图[25]; (c) As20S80微盘谐振腔的形貌和截面图[26]; (d) As20S80微环谐振腔的表面和截面图[27]; (e) SiO2平台结构上沉积As2S3波导芯层以及波导截面的SEM图[28]
Figure 1. (a) Optical micrograph of 4 µm×2.6 µm As2S3 snakes strip waveguide[24]; (b) AFM picture of As2S3 waveguide sidewall roughness after thermal annealing[25]; (c) Scanning Electron Microscopy (SEM) and cross-section of As20S80 disk resonator[26]; (d) SEM and cross-section of As20S80 microring resonator[27]; (e) Depositing the As2S3 core material on the SiO2 platform structure, and SEM of cleaved cross-section of waveguide[28]
近年来,Shi等致力于研究利用成熟的硅光CMOS工艺得到硅波导及光滑的二氧化硅微槽结构,然后在二氧化硅微槽结构中沉积As20S80薄膜从而形成波导,通过进一步对As20S80薄膜进行高温退火,使微槽中的薄膜收缩形成侧壁光滑的保角波导,大幅降低波导侧壁带来的辐射损耗,最终获得了该波导在1.55 µm波长处的传输损耗分别为0.7 dB/cm和0.08 dB/cm[26-27],如图1(c)和(d)所示。2020年,Kim等利用在硅基上热氧化获得二氧化硅薄膜,并通过曝光刻蚀获得二氧化硅波导结构,最后在该结构表面通过热蒸发沉积As2S3薄膜,获得了Q值高达1.44×107的硫系微环谐振腔,这是迄今平面硫系光器件中报道的最高Q值,相应的波导在1.55 µm波长处的传输损耗小于0.03 dB/cm[28-29],如图1(e)所示。值得一提的是,上述通过在二氧化硅微槽结构上沉积获得硫系波导的模板化制备方法可以避免对硫系玻璃的直接刻蚀,从而获得高品质波导微腔器件,然而其热处理过程包含复杂的动力学过程,如扁平化、成核、回流和生长等过程,因此其波导结构尺寸难以精确控制,难以满足需要精细色散调控的高性能非线性应用。2021年,Zhang等提出了一种原位光诱导退火方法来提高As2S3薄膜的稳定性和鲁棒性,并在基于上述薄膜的波导制备工艺中引入聚合物热固化退火,制备得到了截面尺寸为2 000 nm×850 nm的As2S3波导,其传输损耗为0.1 dB/cm[30]。
为提高As2S3和As2Se3材料结构的稳定性,研究人员在二元结构中加入Ge元素,依靠其会在硫系玻璃网络结构中与其他元素成四配位的形式存在,可以提高硫系材料的结构稳定性以及增加玻璃的硬度和强度等机械性能。2010年,Gai等人通过设计亚微米结构尺寸(截面为630 nm×500 nm)的Ge11.5As24Se64.5脊型波导,并且在1.55 µm波长处获得了136 W−1m−1的高非线性系数和2.6 dB/cm的传输损耗[31]。2012年,Gai等又制备了偏振无关(截面为580 nm×580 nm)的Ge11.5As24Se64.5纳米线波导,当输入光为TM和TE模时,其波导在1.55 µm波长处的损耗分别为1.65 dB/cm和2.2 dB/cm[32]。2020年,Li等人利用等离子体刻蚀加工得到了Ge11.5As24Se64.5微盘谐振腔,在1.54 µm波长处获得了高达1.1×106的负载Q值,如图2(a)所示[33]。
图 2 (a) GeAsSe波导的截面SEM图[33]; (b) GeSbS微环谐振腔的电子显微镜图[36]; (c) Ge28Sb12Se60微盘谐振腔的SEM图[37]; (d) Ge28Sb12Se60微环的SEM图[38]; (e) 悬挂型GeAsSe微盘谐振器的俯视SEM图[33];(f) 谐振峰的洛伦兹拟合曲线[36];(g) 在1559.657 nm处的谐振峰的洛伦兹拟合曲线[37];(h) 谐振峰的洛伦兹拟合曲线[38]
Figure 2. (a) SEM cross-sectional view image of GeAsSe waveguide[33]; (b) SEM image of a GeSbS microresonator[36]; (c) SEM image of a Ge28Sb12Se60 microdisk resonator[37]; (d) SEM image of a Ge28Sb12Se60 microring[38]; (e) SEM top-view image of a suspended GeAsSe microdisk resonator[33]; (f) Lorentzian fit to the resonance dip[36]; (g) Lorentzian fit to the resonance dip at 1559.657 nm[37]; (h) Lorentzian fit to the resonance dip[38]
大多数科研团队采用含As元素硫系玻璃进行光波导制备研究,少部分科研团队尝试用Sb元素代替有毒的As元素,即在GeSbSe和GeSbS等硫系玻璃基质中开展光波导制备研究。2008年,美国麻省理工大学的Hu等利用CHF3和SF6刻蚀气体制备了3 µm和4 µm宽的Ge23Sb7S70波导,并获得了在1.55 µm波长处3~5 dB/cm的传输损耗[34]。2016年,Hu等进一步优化了波导的制备工艺,利用氟基等离子体刻蚀制备了低损耗的Ge23Sb7S70微环谐振腔和微盘谐振腔,其在1.53 µm和1.58 µm波长处的Q值分别为7.5×105和1.2×106,并由微环Q值计算得到该亚波长尺寸波导传输损耗仅为0.5 dB/cm,这是当时三元体系硫系玻璃光子器件中的最低损耗[35]。2021年,中山大学的Li等利用优化后的等离子体刻蚀工艺进一步降低了Ge25Sb10S65硫系波导损耗,并在1.55 µm波长处获得了2.2×106的超高Q值,如图2(b)所示[36]。
2016年,Krogstad等在高折射率Ge28Sb12Se60硫系玻璃(n~2.66)平台上利用标准的紫外光刻和电子束光刻分别制备了2 000 nm×90 nm和700 nm×340 nm的单模脊型波导,并在1.53 µm波长处得到了TE模式下4.0 dB/cm的传输损耗和TM模式下6.1 dB/cm的传输损耗,同时获得了波导非线性系数γ为6 W−1m−1和材料非线性因子n2/βλ为2.3[20]。2018年,厦门大学的Luo等制备了940 nm×400 nm的Ge22Sb18Se60脊型波导,获得了1.55 µm处的传输损耗为4.0 dB/cm,并且利用中心波长1560 nm、重复频率8.1 MHz、脉冲宽度800 fs的激光源泵浦长度2.1 cm的波导,产生了1.2~2.4 µm的宽带超连续谱[39]。2019年,PARK等对刻蚀后的Ge28Sb12Se60波导进行退火处理,在其玻璃转化温度以上实现热回流,波导在1.55 µm波长处的传输损耗从2.5 dB/cm降至1.0 dB/cm,但是波导形貌严重变形,使得波导的色散调控变得困难,不利于光学非线性应用[40]。2021年,宁波大学的Xu等制备了Ge28Sb12Se60硫系微盘和微环谐振腔,在1.55 µm波长附近分别获得了5×105和4.1×105的高Q值,如图2(c)、(d)所示,得益于高折射率硫系波导带来的亚微米尺寸光场约束,其非线性系数γ高达110 W−1m−1[37-38],使得亚微米约束的单模Ge28Sb12Se60波导在光学非线性应用方面极具潜力。表1列出了目前主要的几种1.55 μm工作波段硫系光波导的最新研究进展。
表 1 目前几种典型的1.55 μm工作波段硫系光波导
Table 1. Current several typical chalcogenide waveguides at 1.55 µm
Materials Refractive index Types of waveguides Dimension/μm2 Loss/dB·cm−1 Reference As2S3 2.43 Ring 10×1.3 0.03 [28] As2Se3 2.81 Waveguide 6.0×1.9 < 0.78 [41-42] Ge11.5As24S64.5 2.30 Waveguide 1.55×0.7 0.25 [43-44] Ge11.5As24Se64.5 2.55 Waveguide 2.0×1.0 0.48 [33] Ge23Sb7S70 2.22 Ring 0.75×0.63 0.84 [45] Ge25Sb10S65 2.2 Ring 2.4×0.8 0.19 [46] Ge28Sb12Se60 2.50 Waveguide 0.75×0.33 1.0 [40] Ge28Sb12Se60 2.80 Ring 0.8×0.3 1.3 [38] 随着硫系玻璃集成光波导制备技术的愈加成熟,研究人员同时也开展了中红外波段(2~20 μm)的硫系玻璃集成光子器件研究工作。2012年,澳大利亚国立大学的Gai等制备了长度为21 cm、截面为4.0 μm×2.5 μm的As2S3脊型波导,获得了波导在3.6 μm波长处的传输损耗为0.75 dB/cm[47]。2013年,麻省理工大学的Lin等利用剥离法制备了上下包层为Ge23Sb7S70、芯层为As2Se3的硫系微盘谐振腔,获得了在5.2 μm波长处2×105的高品质因子,相应的传输损耗为0.7 dB/cm,如图3(a)、(b)所示[22]。同年,澳大利亚国立大学的Ma等制备了下包层为Ge11.5As24S64.5、芯层为Ge11.5As24Se64.5的脊型波导,通过测量获得了3~7.4 μm的中红外波段内的TE模式平均损耗低于0.5 dB/cm,同时在5 μm波长处获得了0.3 dB/cm的低传输损耗[48],如图3(c)所示。
2015年,Ma等又报道了可在5.2 μm波长处工作的Ge11.5As24Se64.5跑道型微环谐振腔,其弯曲半径为180 μm,本征Q值达到1.45×105,并计算得到传输损耗为0.84 dB/cm[49]。表2总结了目前几种常见的中红外硫系玻璃集成光波导器件的研究进展。
表 2 中红外波段硫系玻璃集成光波导器件研究状况
Table 2. Recent research progress of chalcogenide optical waveguide performance in mid-infrared band
Materials Types of waveguides Dimension/μm2 Wavelength/μm Loss/dB·cm-1 Reference As2S3 Waveguide 4.0 × 2.5 3.6 0.75 [47] As2S3 Waveguide 1.2 × 0.6 2.0 1.447 [50] As2Se3 Microdisk 2.5 × 1.1 5.2 0.7 [22] As2Se3 Waveguide 3.0 × 1.35 5.27 2 ± 4 [51] Ge11.5As24Se64.5 Waveguide 4.0 × 1.25 5.0 0.3 [48] Ge11.5As24Se64.5 Ring 2.5 × 2.25 5.2 0.84 [49] Ge11.5As24Se64.5 Waveguide 4.0 × 2.2 3.8 - 5.0 ~0.6 [52] Ge23Sb7S70 Microdisk 3.0 × 1.8 5.2 0.21 [53] Ge23Sb7S70 Waveguide 2.0 × 1.2 3.31 7.0 [54] Ge23Sb7S70 Waveguide 2.0 × 1.0 3.31 8.0 [55] Ge28Sb12Se60 Waveguide 2.8 × 1.0 4.319 5.1 [56] 尽管中红外硫系玻璃集成光波导的研究已经取得了一些进展,但其波导传输损耗还比较高,可进一步利用硫系玻璃的光敏性和较低的玻璃转化温度发展波导热回流工艺,从而改善波导侧壁的粗糙度,同时优化波导的刻蚀参数,获得优良的中红外导波性能。也可以考虑从材料的角度出发,对硫系玻璃进行进一步提纯,去除玻璃中可能引起损耗的碳、氢、氧等杂质,降低硫系玻璃的本征损耗,获得低损耗的中红外硫系光波导器件。
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临床医疗、工业勘探、航空航天等精确作业的发展需求对气体探测的精确性与时效性提出了更高要求。近年来,红外光源、光电探测器等元器件的集成化发展也极大地推动了片上红外传感的实用化进程。2~20 μm波长范围内覆盖了许多痕量气体的特征吸收谱,尤其是3~5 μm和8~14 μm这两个重要的大气窗口[39, 55]。基于硫系玻璃波导的红外光传感因具有极宽的透明窗口范围(~20 μm)、制备工艺成熟、低损耗、易于集成等优点而备受关注,如检测大气中的微量气体[57],分析样品中的爆炸性残留物[58],或非侵入性监测患者的血糖水平[59]等。
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光子的能级作为一种非连续的状态,其能量与对应的频率相关。当一定频率范围的光与气体接触时,气体分子选择性吸收与其能级能量差相等的光子,进而分子能级之间的跃迁导致分子振动能级和转动能级的变化。数种带有O-H、C-H、N-O等化学键的痕量气体及污染物分子,其在电磁波频谱内均展现出较强的吸收特性。当光源波长覆盖待测痕量气体的特征吸收谱线时,输出光强会呈现出衰减趋势[60],通过不同物质分子的选择吸收特性来判断物质的成分及浓度。
2016年,Han等通过紫外线曝光及热蒸发工艺制备了一种用于检测甲烷(CH4)气体的Ge23Sb7S70螺旋波导,如图4(a)所示,其最大倏逝场能量占比达8%。当工作波长为3.31 μm时,针对甲烷气体,其检测阈值为2.5 vol.%[54]。 Gutierrez-Arroyo等在硅衬底上通过射频磁控溅射沉积、i线光刻和氟基反应离子蚀刻制备了基于双层锗锑硒的脊波导,如图4(b)所示,其芯层和下覆层分别是折射率系数为2.77和2.44的Ge12.5Sb25Se62.5和Ge28.1Sb6.3Se65.6,固有损耗为2.5 dB/cm,倏逝场能量占比为5%。当工作波长为7.7 μm时,甲烷和一氧化二氮(N2O)气体的检测阈值分别为14.2 ppm (1 ppm=10−6)和1.6 ppm[61]。2017年,Gutierrez-Arroyo等设计了一种芯层及下包层折射率系数分别为2.81和2.40的(GeSe2)100-x(Sb2Se3)x脊波导,倏逝场能量占比为8%,在固有损耗1 dB/cm的情况下,最优长度为4.3 cm。当工作波长为4.3 μm时,对于二氧化碳(CO2)气体的检测阈值为268 ppb (1 ppb=10−9);当工作波长为 3.31 μm和7.66 μm时,对甲烷气体的检测阈值分别为1.848 ppm和781 ppb;当波长为6.68 μm时,对甲苯(C7H8)溶液的检测阈值为26 ppb[62]。2018年,Mittal 等人提出并制备了ZnSe脊波导,如图4(c)所示,通过结合纸质流体学,在2.6~3.7 μm波段范围内测量了六种不同浓度异丙醇 (C3H8O,IPA)水溶液的波导透射光谱,并与理论模型取得良好的一致性[63]。2019年,Pi等提出了一种用检测于甲烷气体的As2Se3悬浮型槽波导。当工作波长为 3.291 μm时,倏逝场能量占比可达到85.77%,波导损耗为3 dB/cm,器件最优长度为1.45 cm。当最小可探测信噪比为10时,检测阈值低至1.7 ppm[65]。2020年,Zegadi等分析设计了一种芯层与下包层折射率系数分别为2.69和2.49的 (GeSe2)100-x(Sb2Se3)x槽波导,倏逝场能量占比为58%。当工作波长为4.3 μm时,对二氧化碳气体的灵敏度和检测阈值分别为35 nW·ppm−1和0.1 ppm;当工作波长为7.7 μm时,对甲烷气体的灵敏度和探测阈值分别为2.1 nW·ppm−1和1.6 ppm[66]。2021年,Wang等在硅衬底上提出了一种用于甲烷气体传感的GeSbSe(NBU-IR4)悬浮型狭缝波导。在3.0~4.4 μm波长范围内,倏逝场能量占比均在90%以上。当工作波长为3.67 μm时,倏逝场能量占比、灵敏度和检测阈值分别为93.81%、0.4578和18.17 ppm;当工作波长为3.291 μm时,倏逝场能量占比、灵敏度和检测阈值分别为91.98%、7.151和1.139 ppm[67]。Pi等人采用升空法制备了一种下覆层为MgF2的Ge28Sb12Se60矩形波导,如图4(d)所示。通过利用波长调制光谱技术抑制噪声,从而提高灵敏度。当工作波长为4.319 μm时,对于二氧化碳气体,在l cm的有效探测路径上,其检测阈值达到0.3%[56]。同年,该课题组提出并制备了一种基于银岛状膜的Ge28Sb12Se60表面增强型条波导,如图4(e)所示。该结构在1.8 nm厚的银岛状膜覆盖下达到性能最优状态。当工作波长为3.291 μm时,对于甲烷气体,当探测时间为50.6 s时,检测阈值可达0.61%;而当探测时间为0.2 s时,检测阈值为4.11%[64]。
图 4 (a) Ge23Sb7S70螺旋波导的示意图和SEM图[54];(b) 基于两种不同GeSbSe材料的脊波导截面图及其SEM图[61];(c) ZnSe脊波导示意图和截面SEM图[63]; (d) Ge28Sb12Se60条波导横截面SEM图和PDMS气室集成波导传感示意图[56];(e) 基于银岛状膜的Ge28Sb12Se60波导传感器的示意图[64]
Figure 4. (a) Schematic diagram and SEM image of the Ge23Sb7S70 spiral waveguide[54]; (b) Cross-sectional view and the corresponding SEM image of the ridge waveguide comprising two different compositions of GeSeSe glasses[61]; (c) Schematic diagram and cross-sectional SEM image of the the ZnSe rib waveguide[63]; (d) Cross-sectional SEM image of the Ge28Sb12Se60 strip waveguide and schematic diagram of the waveguide integrated with a PDMS gas cell[56]; (e) Schematic diagram of the Ge28Sb12Se60 waveguide sensor using the silver island film[64]
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谐振腔对环境因素的变化极为敏感,当介质输入或表面分子吸附等原因造成波导倏逝场空间内的折射率改变时,将引起谐振模式有效折射率的波动,即发生透射峰偏移现象,而通过量化偏移量可检测待测介质浓度。
2017年,Xu等提出了一种基于Ge11.5As24Se64.5的一维光栅型微桥腔,如图5(a)所示,可实现对折射率实部及虚部的同时探测。当共振波长为4.132 μm时,灵敏度及品质因子分别可达2280 nm/RIU和106以上[68]。2019年,Nalivaiko等提出了一种基于硫系玻璃的光栅波导传感器。当谐振波长为0.63 μm时,检测阈值可达1×10−5[69]。2021年,Huang等提出并制备了一种基于Ge28Sb12Se60微环结构的光传感,如图5(b)所示。当工作波长为1.55 μm时,波导传输损耗为4.3 dB/cm,固有品质因子和消光比分别为7.74×104和40 dB。通过检测分析不同浓度的氯化钠(NaCl)溶液,其灵敏度和检测阈值分别为123 nm/RIU和 3.24×10−4 RIU[70]。Zhang等提出并制备了一个Ge28Sb12Se60槽波导微环谐振结构,如图5(c)所示,槽区和包层区的倏逝场能量占比分别为36.3%和56.7%,当工作波长为1.55 μm时,微环谐振器的品质因子为 1 × 104,在内径为60 μm的情况下,其灵敏度和检测阈值分别为471 nm/RIU和3.3 × 10−4 RIU[71]。 为了进一步提升传感性能,可通过片上集成光源及光电探测器方式改善系统噪声。2018年,Du等演示了一种片上超连续谱光源集成光学传感器结构,如图6(a)所示。其中,Ge22Sb18Se60波导用于宽带超连续谱生成和传感检测,可实现1.38~2.05 μm范围的宽光谱探测[39]。2019年,Su等报道了一种Ge23Sb7S70螺旋波导光学传感器集成PbTe光电探测器,如图6(b)所示。当工作波长为3.291 μm时,对于甲烷气体,该集成传感器在0.078 Hz的噪声带宽下检测极限为1.0 vol.%,可通过改善制备工艺和消除激光功率波动噪声,将最大灵敏度达到330 ppmv[55]。
图 5 (a) 基于Ge11.5As24Se64.5波导的光栅谐振型传感器原理图、电场分布和能带图[68];(b) Ge28Sb12Se60微环谐振传感器的SEM图[70];(c) Ge28Sb12Se60槽型波导及微环谐振传感器的SEM图[71]
Figure 5. (a) Schematic, electric field distribution and band diagram of the Ge11.5As24Se64.5 grating resonance sensor [68]; (b) SEM images of the Ge28Sb12Se60 waveguide sensor using micro-ring resonance [70]; (c) SEM images of the Ge28Sb12Se60 slot waveguide and the Ge28Sb12Se60 slot micro-ring sensor [71]
硫系玻璃由于透明窗口宽、折射率可调、工艺成熟等优点在红外传感方面有着广泛的应用。目前,关于硫系玻璃波导片上红外传感的研究已有诸多报道,其中以光谱吸收和谐振偏移为主要研究方向,通过调节硫系材料成分及设计优化波导结构可以进一步优化灵敏度和检测阈值等传感性能。传感平台中的外置激光源及光电探测器不可避免地引入额外的系统噪声,片上光源及光电探测器的集成化发展将最大化抑制平台噪声,并实现基于光子芯片的小型化红外光谱传感。
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超连续光谱(SC谱)是一种具有高相干性、宽光谱范围和高亮度的光谱。SC谱产生的基本机制是在非线性介质中发生自相位调制、级联拉曼散射和四波混频、色散波发射和孤子自频移等非线性效应引起的高阶孤子裂变。随着SC谱技术的强劲发展,一系列基于SC谱光源的新型应用应运而生,如超短脉冲生成,光学相干断层扫描以及精确频率计量、光通信、光谱探测等。由于硫系玻璃的优异非线性特性,人们已经在多种硫系玻璃光纤中实现了SC谱产生。硫系玻璃集成光波导拥有极强的光场约束和灵活的色散调控能力,在SC谱产生方面也引起了研究人员的广泛兴趣。
在近红外SC谱产生方面,2008年,澳大利亚悉尼大学的Eggleton团队和澳洲国立大学的Davies团队合作制备了长6.0 cm、2.0 μm×0.87 μm的As2S3脊型波导,首次通过色散设计获得了+29 ps/nm/km的反常色散值和10 W−1m−1的非线性系数,并通过脉宽610 fs、峰值功率68 W的1.55 μm波长的光源泵浦该波导获得了750 nm带宽的SC谱输出[72]。同年,Beniamin J.Eggleton等在直径为950nm的纳米线波导中利用峰值功率仅为7.8 W的低阈值泵浦产生了1.1~1.7 μm的SC光谱[73]。2010年,澳大利亚国立大学的Gai等首先设计了亚微米尺寸的Ge11.5As24Se64.5波导,获得了当时硫系光波导中最高的非线性系数γ为136 W−1m−1,并且实现了1.2~1.7 μm的SC谱输出[31-32]。2012年,通过设计方形截面的偏振无关Ge11.5As24Se64.5波导,利用较低的泵浦功率实现了带宽覆盖1.1~2.2 μm的SC谱。2014年,M.R.Karim等在尺寸为700 nm×500 nm和775 nm×500 nm的波导中分别模拟获得了1.2~2.1 μm以及1.2~2.4 μm的SC光谱输出[74]。2016年,美国科罗拉多大学的Molly R等在730 nm×340 nm单模传输的Ge28Sb12Se60波导中实现了带宽覆盖250 nm的SC谱[20]。2020年,南开大学的Shang等在GeSbS波导平台上获得了带宽覆盖1300 nm的SC谱,并且实现了对CCl4浓度的探测[75]。2021年,Duk-Yong Choi等在刻蚀后形貌光滑的二氧化硅波导表面沉积了As2S3薄膜,获得了超低损耗硫系光波导[29],利用脉宽135 fs、重复频率100 MHz、中心波长1.55 μm的光源泵浦As2S3波导获得了1.5个倍频程的SC谱展宽,如图7(a)~(c)所示。
图 7 (a) 波导截面的SEM图[29];(b) 超连续光谱展宽模拟[29];(c) TM模式的超连续光谱产生的实验结果[29];(d) 波导截面的SEM图[52];(e) 在4.184 μm波长的泵浦下,不同泵浦功率获得的SC谱产生[52]
Figure 7. (a) Typical waveguide cross section under SEM inspection[29]; (b) The simulation of supercontinuum spectrum broadening[29]; (c) Experimental results of supercontinuum spectrum generation in TM mode[29]; (d) Typical waveguide cross section under SEM inspection[52]; (e) Experimental SC evolution with increasing powers at a pump wavelength of 4.184 μm[52]
在中红外SC谱产生方面,2012年,Gai等利用波长3.26 μm、脉宽7.5 ps的持续脉冲抽运6.6 cm长的 As2S3硫系脊形波导,产生了带宽覆盖2.9~4.2 μm的中红外SC谱。并且证明通过消除目前限制长波长拓展的包层吸收,其SC谱可以进一步展宽至6~8 μm[47]。2013年,Yu等在长度为7 cm、上下包层为GeAsS、芯层为GeAsSe的脊型波导结构中仿真获得了2.5~10 μm以上带宽的SC输出;并且利用中心波长5.3 μm、脉宽150 fs和峰值功率20 mW的光源泵浦厚度为5 mm的硫系玻璃获得了2.5~7.5 μm带宽的平坦SC输出[76]。2014年,Yu等又利用波长4 μm、脉宽320 fs的持续脉冲抽运1.0 cm长的Ge11.5As24Se64.5波导,获得了1.8~7.5 μm波段的宽带且平坦的中红外SC谱[77]。2016年,Yu等再次采用波长4.184 μm、脉宽330 fs的泵浦光抽运1.8 cm长的Ge11.5As24Se64.5,实现了带宽覆盖2.0~10.2 μm的SC谱,是目前在硫系光波导SC谱研究中获得的最大展宽[52],如图7(d)和(e)所示。2021年,Zhang等在GeAsSeTe波导中理论仿真获得了2~13 μm的宽带SC谱输出[78],由于中红外光源的限制未获得最终实验结果,但说明了基于硫系光波导的SC谱产生可以拓展到更远的中远红外波段。表3列出了基于硫系玻璃光波导的中红外SC谱产生的研究进展。
表 3 硫系波导片上中红外SC输出的研究报道
Table 3. Research progress of on-chip mid-infrared SC output in chalcogenide waveguides
目前硫系光波导中红外SC谱产生研究主要集中在SC谱的带宽展宽方面,中红外SC谱的相干性、输出功率及应用方面的研究工作相对较少。另一方面,由于Te基硫系玻璃具有硫系材料中最宽的透过范围以及最高的线性折射率和非线性折射率,未来可以选择Te基硫系玻璃作为片上SC谱产生的波导基质,进而获得更宽带的SC产生。同时,硫系玻璃光波导在中红外波段的传输损耗可以通过提高硫系玻璃基质材料的纯度、优化波导的侧壁等粗糙度来得到进一步降低;还可结合硫系玻璃组分可调节的灵活性来提高硫系波导的抗激光损伤性能,使得硫系光波导SC谱光源的带宽、相干性和输出功率等性能不断提高,具备更强的实用价值。
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受激布里渊散射(SBS)作为一种常见的非线性光学效应,描述了介质中光子与声子之间的相互作用引起的强大而灵活的光调控机制。当入射泵浦光功率较高时,由光波产生的电磁伸缩效应在介质内激发起前向传播的声波,入射光被声波散射而产生后向传播的光波,称为Stokes波,其光频率与泵浦光频梳相差一个声波频率,如图8(a)所示。
二氧化硅较小的折射率导致其对光模式的限制较弱,进而声光相互作用过程中得到的布里渊增益系数小,实现SBS的石英光纤长度一般需要拉至千米级来获取较大的SBS增益,这使得整个实验装置较为复杂,不利于器件的小型化和集成化。硅材料由于其较小的光弹系数和较差的声学模式约束,使得硅波导的布里渊增益系数很小,但近年来通过设计纳米尺寸波导结构产生辐射压力,并结合电致伸缩力,使硅波导的SBS效应得到了极大的增强[79-80]。硫系玻璃材料由于其较高的弹光系数(硅材料的10倍)以及极其优秀的声学模式约束能力,使得硫系光波导具有较大的SBS增益系数,被广泛用于SBS集成光子器件的研究中。
2011年,Pant等利用背散射信号和泵浦-探测技术,在长度为7 cm、截面为4 μm×850 nm的As2S3脊形波导中实现了SBS效应,测得的布里渊频移为7.7 GHz,布里渊线宽为34 MHz[81]。同时,通过信号光增益谱拟合得到布里渊增益系数为0.715×10−9 m/W,当泵浦功率为300 mW时,获得了16 dB的信号光增益。2013年,Kabakova等在长7 cm、宽4 μm的As2S3脊形波导中得到布里渊增益系数和布里渊频移分别为0.7×10−9 m/W和(7.5±0.2) GHz。利用该硫系光波导外接光纤回路构成谐振腔结构,首次实现了基于硫系光波导的片上窄线宽布里渊激光器,如图8(b)所示,测量得到的激光线宽比泵浦线宽窄15倍,比布里渊增益带宽窄300倍[82]。
SBS效应由于线宽极窄、频率稳定等显著优势,成为实现超窄带宽微波光子滤波器的最优选择之一。2016 年,悉尼大学采用 As2S3波导的窄带SBS效应实现了一种具有超高抑制比、带宽调谐范围为33~88 MHz高频率分辨率的微波光子带阻滤波,并实现了1~30 GHz的频率调谐[83]。2017年,Morrison等利用硅基混合集成方法(如图8(c)所示),将长5.8 cm的As2S3螺旋波导嵌入硅光波导中,从而在硅基器件中得到22.5 dB的高布里渊增益(净增益为18.5 dB);并进一步利用微环的谐振加强作用,当微腔的FSR等于布里渊频移SBS时,首次实现了平面光子器件中的布里渊激光出射[84]。
在过去的十年里,由于As2S3硫系玻璃波导的强布里渊相互作用已经实现了众多高性能片上光子器件。然而,As2S3硫系玻璃很容易被氧化,在器件制作过程中稳定性较差。同时,由于As2S3的玻璃化转变温度较低,采用现有的化学气相沉积(CVD)方法难以制备出传输损耗较低的二氧化硅包层As2S3波导。此外,基于As2S3硫系玻璃的光子器件在实际应用中往往会受到As毒性的限制。2021年,中山大学的Song等引入不含毒性元素的GeSbS硫系玻璃光子平台,获得了较大的片上SBS增益。该团队在长度为7 cm、截面为2.8 μm×850 nm的Ge25Sb10S65硫系螺旋波导中表征了SBS特性,测得布里渊频移和布里渊线宽分别为7.443 GHz和47.8 MHz。由于其较高的布里渊增益系数(338 m−1 W−1),在泵浦功率为200 mW时,得到信号增益为17.6 dB[85]。硫系光波导拥有很高的布里渊增益系数和低损耗特性,使得硫系光波导平台已经成为开发高性能SBS集成器件的理想平台。
图 8 (a) SBS的概述:泵浦波(ω1)从声子(Ω)中散射并增强声子,并产生Stokes波(ω2),结果是在距离泵浦的GHz处分离出一个狭窄的Stokes峰,该结构显示了后向布里渊散射[86];(b) 基于硫系玻璃光子芯片的布里渊激光器原理图[82];(c) 硅基混合集成As2S3螺旋环形谐振腔,谐振腔中的SBS 效应[84];(d) 基于SBS的集成微波光子滤波器,阻带中心频率调谐[83]
Figure 8. (a) Overview of SBS: A pump wave (ω1) scatters from and re-enforces an acoustic phonon (Ω) and is downshifted to a Stokes wave (ω2), the result is a narrow Stokes peak separated at a distance of GHz from the pump, this configuration shows backward Brillouin scattering[86]; (b) Schematic of a BL based on photonic chip[82]; (c) Schematic of the hybrid As2S3 ring resonator structure, concept figure for the lasing conditions[84]; (d) SBS-based integrated microwave photonic filter, stopband center frequency tuning[83]
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受激拉曼散射(SRS)是一种重要的非线性光学现象,指超过某一阈值(拉曼阈值)的强泵浦入射光入射到非线性介质中后,被称为斯托克斯光的较低频率的成分急剧增加,泵浦光的能量大部分转换到斯托克斯光的现象。SRS被认为是一种扩展传统半导体和稀土掺杂激光光源光谱覆盖的有效方法。基于SRS技术的集成SRS激光在光学放大、光谱传感、考古学和临床诊断等领域具有潜在的应用。近年来,光子集成拉曼激光器已经在硅[87]、金刚石[88]、氮化铝(AlN)[89]和铌酸锂(LN)[90]材料平台上被实现。然而在这些材料平台中获得的拉曼增益光谱带宽都比较窄,需要一种可产生宽带拉曼增益的材料平台。硫系玻璃由于具有显著的拉曼和克尔非线性,可透过到中远红外波段范围,同时又具有可忽略的双光子吸收(TPA)和自由载流子吸收(FCA)的特性,更加适合获得具有宽频带的拉曼增益光谱。
2013年,Francis Vanier等制备了半径为20 μm的As2S3微球,并且获得了7×107的高品质因子。当泵浦波长为1550 nm时,产生拉曼激光的泵浦功率阈值低至13 μW,同时获得了10.7%的转换效率[91],如图9(a)、(b)所示。2014年,Francis Vanier等再次在As2S3微球中获得了级联拉曼激光的输出。当泵浦波长为1557 nm和1880 nm时,分别获得了5阶和3阶的受激拉曼散射,而且泵浦阈值低至微瓦量级[92],如图9(c)~(e)所示。2021年,Alexey V. Andrianov等利用商用C波段窄线宽激光器在As2S3微球中泵浦产生了拉曼激光。当泵浦波长在1522~1574 nm范围内调节时,可获得1610~1663 nm的单模可调谐的拉曼激光。在泵浦功率明显超过阈值时,实现了4阶多模级联拉曼激光[93],如图9(f)所示。同年,中山大学的Zhang等在截面尺寸为2400 nm×800 nm集成的Ge25Sb10S65微环谐振腔中获得了大于106的高Q值,并且利用1550 nm波长的激光源泵浦该微腔,获得该材料的拉曼增益系数为7.37×10−12 m/W,泵浦阈值为3.25 mW[36],如图9(g)所示。
目前对于在片上集成硫系光波导中产生受激拉曼散射效应的材料还比较单一,主要集中在As2S3中,而且产生拉曼激光的波长主要在近红外波段,急需开展2 μm波长以上的拉曼激光的研究和应用。
图 9 (a) As2S3微球谐振峰的洛伦兹拟合曲线[91];(b)拉曼发射功率和耦合泵浦功率的关系[91];(c) 典型封装的As2S3微球图像[92];(d) 封装的As2S3微球谐振峰的洛伦兹拟合[92];(e) As2S3微球五阶级联SRS发射光谱[92];(f) 四级联拉曼激光实验光谱[93];(g) 当泵浦功率增加到30 mW时的拉曼测量光谱[36]
Figure 9. (a) Lorentzian fit to the resonance dip of As2S3 microsphere[91]; (b) Raman emission power versus coupled pump power[91]; (c) Image of a typical packaged As2S3 microsphere[92]; (d) Lorentzian fit to the resonance dip of typical packaged As2S3 microsphere[92]; (e) Spectrum of a 5 Raman orders cascaded SRS emission of an As2S3 microsphere[92]; (f) Experimental spectrum of four-cascade Raman lasing[85]; (g) Measured Raman spectrum when increasing the pump power to ~30 mW[36]
Review of chalcogenide glass integrated photonic devices (Invited)
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摘要: 硫系玻璃具有超宽的红外透过光谱范围、较高的线性折射率、极高的光学非线性和超快的非线性响应,近年来在集成光子器件研究领域备受关注。首先回顾了硫系玻璃集成光波导的制备,综述了硫系集成光子器件在红外传感和高性能非线性应用方面取得的进展,然后介绍了硫系相变光子器件在光开关、光存储和光计算等方面的前沿进展,最后对目前硫系玻璃光子器件研究存在的问题进行了归纳,并对未来的研究方向进行了展望。Abstract: In recent years, chalcogenide glasses have attracted much attention in the field of integrated photonic devices because of their ultra-wide infrared transmission spectrum, high linear refractive index, extremely high optical nonlinearity, and ultrafast nonlinear response. Firstly, the fabrication of chalcogenide glass integrated optical waveguides was reviewed, the progress of chalcogenide integrated photonic devices in infrared sensing and high-performance nonlinear applications was summarized. Then, the chalcogenide phase-change photonic devices in optical switching, optical storage, and optical computing were introduced. Finally, the current problems in chalcogenide glass photonic devices were summarized, and the future research directions were prospected.
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图 1 (a) 4 µm×2.6 µm的As2S3蛇形脊型波导的光学显微镜图[24]; (b) As2S3波导在退火后波导形貌的 AFM图[25]; (c) As20S80微盘谐振腔的形貌和截面图[26]; (d) As20S80微环谐振腔的表面和截面图[27]; (e) SiO2平台结构上沉积As2S3波导芯层以及波导截面的SEM图[28]
Figure 1. (a) Optical micrograph of 4 µm×2.6 µm As2S3 snakes strip waveguide[24]; (b) AFM picture of As2S3 waveguide sidewall roughness after thermal annealing[25]; (c) Scanning Electron Microscopy (SEM) and cross-section of As20S80 disk resonator[26]; (d) SEM and cross-section of As20S80 microring resonator[27]; (e) Depositing the As2S3 core material on the SiO2 platform structure, and SEM of cleaved cross-section of waveguide[28]
图 2 (a) GeAsSe波导的截面SEM图[33]; (b) GeSbS微环谐振腔的电子显微镜图[36]; (c) Ge28Sb12Se60微盘谐振腔的SEM图[37]; (d) Ge28Sb12Se60微环的SEM图[38]; (e) 悬挂型GeAsSe微盘谐振器的俯视SEM图[33];(f) 谐振峰的洛伦兹拟合曲线[36];(g) 在1559.657 nm处的谐振峰的洛伦兹拟合曲线[37];(h) 谐振峰的洛伦兹拟合曲线[38]
Figure 2. (a) SEM cross-sectional view image of GeAsSe waveguide[33]; (b) SEM image of a GeSbS microresonator[36]; (c) SEM image of a Ge28Sb12Se60 microdisk resonator[37]; (d) SEM image of a Ge28Sb12Se60 microring[38]; (e) SEM top-view image of a suspended GeAsSe microdisk resonator[33]; (f) Lorentzian fit to the resonance dip[36]; (g) Lorentzian fit to the resonance dip at 1559.657 nm[37]; (h) Lorentzian fit to the resonance dip[38]
图 4 (a) Ge23Sb7S70螺旋波导的示意图和SEM图[54];(b) 基于两种不同GeSbSe材料的脊波导截面图及其SEM图[61];(c) ZnSe脊波导示意图和截面SEM图[63]; (d) Ge28Sb12Se60条波导横截面SEM图和PDMS气室集成波导传感示意图[56];(e) 基于银岛状膜的Ge28Sb12Se60波导传感器的示意图[64]
Figure 4. (a) Schematic diagram and SEM image of the Ge23Sb7S70 spiral waveguide[54]; (b) Cross-sectional view and the corresponding SEM image of the ridge waveguide comprising two different compositions of GeSeSe glasses[61]; (c) Schematic diagram and cross-sectional SEM image of the the ZnSe rib waveguide[63]; (d) Cross-sectional SEM image of the Ge28Sb12Se60 strip waveguide and schematic diagram of the waveguide integrated with a PDMS gas cell[56]; (e) Schematic diagram of the Ge28Sb12Se60 waveguide sensor using the silver island film[64]
图 5 (a) 基于Ge11.5As24Se64.5波导的光栅谐振型传感器原理图、电场分布和能带图[68];(b) Ge28Sb12Se60微环谐振传感器的SEM图[70];(c) Ge28Sb12Se60槽型波导及微环谐振传感器的SEM图[71]
Figure 5. (a) Schematic, electric field distribution and band diagram of the Ge11.5As24Se64.5 grating resonance sensor [68]; (b) SEM images of the Ge28Sb12Se60 waveguide sensor using micro-ring resonance [70]; (c) SEM images of the Ge28Sb12Se60 slot waveguide and the Ge28Sb12Se60 slot micro-ring sensor [71]
图 7 (a) 波导截面的SEM图[29];(b) 超连续光谱展宽模拟[29];(c) TM模式的超连续光谱产生的实验结果[29];(d) 波导截面的SEM图[52];(e) 在4.184 μm波长的泵浦下,不同泵浦功率获得的SC谱产生[52]
Figure 7. (a) Typical waveguide cross section under SEM inspection[29]; (b) The simulation of supercontinuum spectrum broadening[29]; (c) Experimental results of supercontinuum spectrum generation in TM mode[29]; (d) Typical waveguide cross section under SEM inspection[52]; (e) Experimental SC evolution with increasing powers at a pump wavelength of 4.184 μm[52]
图 8 (a) SBS的概述:泵浦波(ω1)从声子(Ω)中散射并增强声子,并产生Stokes波(ω2),结果是在距离泵浦的GHz处分离出一个狭窄的Stokes峰,该结构显示了后向布里渊散射[86];(b) 基于硫系玻璃光子芯片的布里渊激光器原理图[82];(c) 硅基混合集成As2S3螺旋环形谐振腔,谐振腔中的SBS 效应[84];(d) 基于SBS的集成微波光子滤波器,阻带中心频率调谐[83]
Figure 8. (a) Overview of SBS: A pump wave (ω1) scatters from and re-enforces an acoustic phonon (Ω) and is downshifted to a Stokes wave (ω2), the result is a narrow Stokes peak separated at a distance of GHz from the pump, this configuration shows backward Brillouin scattering[86]; (b) Schematic of a BL based on photonic chip[82]; (c) Schematic of the hybrid As2S3 ring resonator structure, concept figure for the lasing conditions[84]; (d) SBS-based integrated microwave photonic filter, stopband center frequency tuning[83]
图 9 (a) As2S3微球谐振峰的洛伦兹拟合曲线[91];(b)拉曼发射功率和耦合泵浦功率的关系[91];(c) 典型封装的As2S3微球图像[92];(d) 封装的As2S3微球谐振峰的洛伦兹拟合[92];(e) As2S3微球五阶级联SRS发射光谱[92];(f) 四级联拉曼激光实验光谱[93];(g) 当泵浦功率增加到30 mW时的拉曼测量光谱[36]
Figure 9. (a) Lorentzian fit to the resonance dip of As2S3 microsphere[91]; (b) Raman emission power versus coupled pump power[91]; (c) Image of a typical packaged As2S3 microsphere[92]; (d) Lorentzian fit to the resonance dip of typical packaged As2S3 microsphere[92]; (e) Spectrum of a 5 Raman orders cascaded SRS emission of an As2S3 microsphere[92]; (f) Experimental spectrum of four-cascade Raman lasing[85]; (g) Measured Raman spectrum when increasing the pump power to ~30 mW[36]
图 10 硫系相变集成光开关。(a) 基于SiN覆盖相变GST的混合平台示意图[102];(b) 硅微环谐振器覆盖GST的混合平台示意图[103];(c) 基于GST的宽带低损耗定向耦合器光开关[104]
Figure 10. Integrated photonic chalcogenide phase-change switching. (a) Schematic of the integrated photonic SiN-on insulator platform for broadband switching operation[102]; (b) Spectral shift and loss characterization of GST using silicon microring resonators[103]; (c) Low-loss broadband directional coupler switches based on GST[104]
图 11 (a) 基于Si3N4微环谐振器实现的全光多级存储器的原理图[106];(b) 全光全集成片上多级存储器的工作原理;(c) 多级多位全光存储器[96]
Figure 11. (a) Schematic of all-optical multi-level memory based on Si3N4 microring resonator[106]; (b) Operation principle of an all-optical fully integrated on-chip multilevel memory; (c) A multibit and multiwavelength architecture[96]
图 12 硫系相变材料集成光矢量矩阵乘法(VMM)和神经网络: (a) 基于硫系相变材料-氮化硅平台的芯片级全光算盘[108]; (b) 硫系相变材料光子内存计算演示光学标量-标量乘法和矩阵向量乘法[110]; (c) 一种由光频率梳和内存计算单元阵列实现的集成光学张量核心[111]
Figure 12. PCM based optical VMM and neural networks: (a) A chip-scale all-optical abacus based on GST on Si3N4[108]; (b) Photonic in-memory computing demonstrating optical scalar-scalar multiplication and matrix-vector multiplication[110]; (c) An integrated photonic tensor core enabled by an optical frequency comb and in-memory computing cell arrays[111]
表 1 目前几种典型的1.55 μm工作波段硫系光波导
Table 1. Current several typical chalcogenide waveguides at 1.55 µm
Materials Refractive index Types of waveguides Dimension/μm2 Loss/dB·cm−1 Reference As2S3 2.43 Ring 10×1.3 0.03 [28] As2Se3 2.81 Waveguide 6.0×1.9 < 0.78 [41-42] Ge11.5As24S64.5 2.30 Waveguide 1.55×0.7 0.25 [43-44] Ge11.5As24Se64.5 2.55 Waveguide 2.0×1.0 0.48 [33] Ge23Sb7S70 2.22 Ring 0.75×0.63 0.84 [45] Ge25Sb10S65 2.2 Ring 2.4×0.8 0.19 [46] Ge28Sb12Se60 2.50 Waveguide 0.75×0.33 1.0 [40] Ge28Sb12Se60 2.80 Ring 0.8×0.3 1.3 [38] 表 2 中红外波段硫系玻璃集成光波导器件研究状况
Table 2. Recent research progress of chalcogenide optical waveguide performance in mid-infrared band
Materials Types of waveguides Dimension/μm2 Wavelength/μm Loss/dB·cm-1 Reference As2S3 Waveguide 4.0 × 2.5 3.6 0.75 [47] As2S3 Waveguide 1.2 × 0.6 2.0 1.447 [50] As2Se3 Microdisk 2.5 × 1.1 5.2 0.7 [22] As2Se3 Waveguide 3.0 × 1.35 5.27 2 ± 4 [51] Ge11.5As24Se64.5 Waveguide 4.0 × 1.25 5.0 0.3 [48] Ge11.5As24Se64.5 Ring 2.5 × 2.25 5.2 0.84 [49] Ge11.5As24Se64.5 Waveguide 4.0 × 2.2 3.8 - 5.0 ~0.6 [52] Ge23Sb7S70 Microdisk 3.0 × 1.8 5.2 0.21 [53] Ge23Sb7S70 Waveguide 2.0 × 1.2 3.31 7.0 [54] Ge23Sb7S70 Waveguide 2.0 × 1.0 3.31 8.0 [55] Ge28Sb12Se60 Waveguide 2.8 × 1.0 4.319 5.1 [56] 表 3 硫系波导片上中红外SC输出的研究报道
Table 3. Research progress of on-chip mid-infrared SC output in chalcogenide waveguides
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