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微腔光频梳是一种利用连续波单频光泵浦产生的宽谱光频梳,它利用高Q值光学微腔中三阶光学非线性效应引起的四波混频(FWM)过程实现等间隔光谱梳齿的产生。与传统光频梳相比,微腔光频梳具有体积小、功耗低、自由光谱范围(FSR)大等特点,在光频率合成[1]、测距与雷达[2-3]、光谱分析技术[4-5]、微波光子学[6-7]以及天体物理[8-9]等领域有着广阔的应用前景。
表1比较了常用于制作微腔光频梳的材料平台特性,包括氮化硅(Si3N4)、铝镓砷(AlGaAs)、铌酸锂(LiNbO3)、氮化铝(AlN)和氮化镓(GaN)等。Si3N4是最早用于微腔光梳产生的材料,也是目前最为成熟的微腔光频梳材料,Si3N4微环谐振腔的品质因子(Q)值高达107。通常利用低压化学气相沉积(LPCVD)来制造高质量的Si3N4薄膜。然而,在绝缘体上硅(SOI)晶片上生长时会产生较大的应力,难以获得较厚的氮化硅薄膜。采用大马士革镶嵌工艺可避免大应力产生,但是该工艺需要对SOI刻蚀后进行热回流,会导致一定的工艺误差。近年来,AlGaAs因其具有较高的非线性系数(n2 =2.6×10−17 m2W−1)获得了极大的关注。同时, AlGaAs在中红外波段具有极低的损耗系数,有望产生中红外波段光梳。然而,外延生长的AlGaAs折射率小于GaAs衬底,难以形成光波导。为了解决这一问题,可以利用晶片键合工艺,将AlGaAs与SOI晶片直接键合,从而形成较大的折射率差,实现有效的光限制。但是这会增加工艺复杂度,而且工艺造成的缺陷限制了器件Q值的提升。此外,AlGaAs的热光系数较高,导致产生孤子较为困难。虽然可以利用低温制冷的方式产生孤子,但这限制了AlGaAs微腔的实际应用[10]。LiNbO3因其独特的光学特性受到广泛关注,特别是绝缘体上铌酸锂(LNOI)出现之后,更是成为研究热点,基于LNOI的微腔光梳也有诸多报道。但与AlGaAs一样,LNOI微腔光频梳器件也需要利用键合工艺实现有效的光限制。
表 1 通信波段微腔光梳材料平台
Table 1. Properties of microcomb material platforms at telecom wavelengths
Material n χ(2) /pm·V−1 n2/10−18m2·W−1 λTPA/nm Mode area/μm2 FSR/GHz Qint/×106 Pth/mW Remarks Al0.2Ga0.8As[11] 3.3 180 26 1483 0.28 1000 1.5 ~ 0.03 Bonding Si3N4[12] 2 − 0.25 460 ~1 99 ~10 < 1 − AlN[13-14] 2.1 6 0.23 440 2.3 435 0.8 25 MOCVD growth Diamond[15] 2.4 − 0.82 450 0.81 925 0.97 20 − LiNbO3[16] 2.2 54 0.18 635 1 200 ~4 4.2 Bonding GaN[17] 2.3 −9 1.4 729 1.6 324 1.8 6.2 MOCVD growth AlN和GaN属于非中心对称晶体,同时具有二阶和三阶光学非线性系数,有望实现电调谐光频梳。AlN的禁带宽度高达6.2 eV,其透明窗口覆盖深紫外到中红外,而GaN则在729 nm~6 μm范围内均保持较低的吸收系数。此外,在通信波段,AlN的三阶非线性系数与Si3N4、LiNbO3等相当,而GaN的非线性系数n2 约为1.4×10−18 W−1,是Si3N4、LiNbO3和AlN等材料的数倍。较高的非线性系数有助于降低微腔光梳产生的阈值,从而实现低功耗微腔光频梳。随着半导体照明产业的不断成熟,可以利用金属有机化合物气相外延(MOCVD)在蓝宝石衬底上生长高质量且厚度可控的GaN和AlN薄膜。高晶体质量的薄膜有助于降低材料的光学损耗,从而实现高Q值光学微腔。同时,AlN和GaN与蓝宝石衬底具有较大的折射率差,可以形成良好的光学限制,如图1所示。因此,蓝宝石上的AlN和GaN薄膜非常适于开展集成化非线性光子器件的研究。
Advances in III-nitride-based microresonator optical frequency combs (Invited)
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摘要: 微腔光频梳在光谱测量、微波光子学、光学原子钟和相干光通信等领域具有重要的应用。宽禁带氮化物半导体材料,如氮化铝(AlN)和氮化镓(GaN)等属于非中心对称晶体,具有二阶和三阶光学非线性系数,宽带的透明窗口以及与蓝宝石衬底较高的折射率差,使其成为研究非线性光子器件的理想平台。文中介绍了氮化物微腔的特性,同时对基于氮化物微腔光梳的相关研究进展,包括AlN微腔中的宽谱光频梳产生和光学参量振荡、GaN微腔中的孤子光频梳产生等进行了介绍和展望。Abstract: Chip-scale optical frequency combs based on microresonators have great potentials in spectroscopy, microwave photonics, optical atomic clocks and coherent optical communications. The non-centrosymmetric wurtzite crystal structure of aluminum nitride (AlN) and gallium nitride (GaN) allows them to exhibit both second- and third-order nonlinear optical coefficients, together with wide transparency window and large refractive index contrast against sapphire substrate, making III-nitrides an attractive platform for nonlinear photonics. The basic properties of AlN and GaN microresonators as well as recent advances in III-nitride-based microresonator frequency combs are presented, including broadband frequency comb generation and optical parametric oscillation in AlN microresonators, and soliton microcomb generation in GaN microresonators.
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Key words:
- aluminum nitride /
- gallium nitride /
- optical frequency combs /
- optical microresonators
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图 3 晶体AlN微环谐振腔的结构示意图(a), 色散曲线(b) ,微环波导(c) , 耦合波导端面扫描电镜照片 (d)[14] ;(e)、(f)宽谱近红外和近可见光波段光频梳产生[21]
Figure 3. (a) Device schematic, (b) dispersion profile, and SEM images of (c) microring waveguide and (d) bus waveguide facet of a crystalline AlN microring resonator[14] ; (e),(f) Broadband NIR and near visible band optical frequency comb generation[21]
表 1 通信波段微腔光梳材料平台
Table 1. Properties of microcomb material platforms at telecom wavelengths
Material n χ(2) /pm·V−1 n2/10−18m2·W−1 λTPA/nm Mode area/μm2 FSR/GHz Qint/×106 Pth/mW Remarks Al0.2Ga0.8As[11] 3.3 180 26 1483 0.28 1000 1.5 ~ 0.03 Bonding Si3N4[12] 2 − 0.25 460 ~1 99 ~10 < 1 − AlN[13-14] 2.1 6 0.23 440 2.3 435 0.8 25 MOCVD growth Diamond[15] 2.4 − 0.82 450 0.81 925 0.97 20 − LiNbO3[16] 2.2 54 0.18 635 1 200 ~4 4.2 Bonding GaN[17] 2.3 −9 1.4 729 1.6 324 1.8 6.2 MOCVD growth -
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