【摘要】 自非线性光学诞生以来,非线性频率转换一直是研究焦点。相较于高阶非线性效应,二阶非线性参量过程可在较低泵浦功率下激发介质非线性响应,产生不同频率谐波。至今,这种频率转换已成为突破原子能级限制、获取多种频率激光光源的重要手段,在量子光学、红外探测及全光通信中发挥着重要作用。准相位匹配是实现高效非线性频率转换的重要方法之一,通常依赖外加电场极化。然而,电场极化方法常受限于复杂工艺流程,其面临的挑战在近年来各类光子器件微型化发展趋势下愈发突显。因此,探索无电场极化准相位匹配的灵活实现方式,对下一代低成本、高效率大规模集成非线性光子芯片的实现具有重要意义。铌酸锂（LiNbO₃）作为一种关键非线性光学材料,以其卓越电光特性闻名。随着集成光子学发展,薄膜铌酸锂近年逐渐显露其独特优势。本文综述了近年来基于铌酸锂的准相位匹配器件研究进展与应用,除传统电场极化外,还涵盖了一系列新型准相位匹配方案及高转换效率、高带宽的实现。此外,重点介绍了基于自发准相位匹配的宽带倍频工作,并展望了其在集成光通信系统与光量子芯片中的潜在应用前景。更多还原

【Abstract】 Significance Since the inception of nonlinear optics,nonlinear frequency conversion has been a focal point for researchers.Compared to higher-order nonlinear effects,second-order nonlinear parametric processes can induce nonlinear responses in media at relatively low pump powers and generate harmonics of different frequencies.To this day,such nonlinear frequency conversion has become a crucial means to overcome atomic-level restrictions and obtain laser sources of multiple frequencies.It plays a significant role in fields such as quantum optics,infrared detection,and all-optical communication.Quasi-phase-matching is an essential method for achieving efficient nonlinear frequency conversion,traditionally relying on externally applied electric field poling.However,this conventional approach is often constrained by complex fabrication processes,with challenges becoming increasingly pronounced under the recent trend of miniaturizing photonic devices.Consequently,exploring flexible implementations of quasi-phase-matching without external electric fields holds significant potential for realizing low-cost,high-efficiency integrated nonlinear photonic chips in nextgeneration applications.To achieve efficient nonlinear processes,in addition to phase matching,the corresponding nonlinear materials and optical structures are equally important.As a key material in nonlinear optics,lithium niobate（LiNbO₃） is renowned for its exceptional electro-optic properties.In recent years,with advancements in integrated photonics,thin-film lithium niobate has emerged as a promising candidate due to its unique advantages,including high second-order nonlinearity,high refractive index,low loss,and fast electro-optic response,earning it the moniker "optical silicon".Currently,the fabrication of integrated photonic devices on lithium niobate substrates predominantly relies on lithium niobate-on-insulator（LNOI） technology,wherein a thin lithium niobate layer is heteroepitaxially integrated atop an insulating substrate,mimicking the structural paradigm of silicon-on-insulator（SOI） technology.This structure leverages the excellent electro-optic properties of lithium niobate while the insulating layer isolates interference between different devices,enhancing the stability of signal transmission.Despite its outstanding comprehensive optical performance,lithium niobate itself is a hard crystal that is difficult to process,with a Mohs hardness of 5,significant anisotropy,tendency to fracture,and insensitivity to most etching processes.Compared to siliconbased materials,lithium niobate waveguides typically have smaller tilt angles（currently,FIB can achieve～80° while thinning the material,dry etching method yields～65°,and CMP results in even lower values.） and rougher sidewalls,which are key factors limiting the quality factor of resonant cavities.Therefore,how to achieve high-quality optical structures on a thin-film lithium niobate platform has become a key focus of current research.Excellent on-chip structures can significantly enhance nonlinear interactions at the nanoscale,enabling efficient and compact frequency mixing and frequency generation.Hence,this study reviews recent progress and applications of quasi-phase-matching devices based on LiNbO₃,encompassing traditional electric field poling methods as well as novel quasi-phase-matching schemes aimed at achieving high conversion efficiency and broad bandwidth.Additionally,it highlights research on broadband second harmonic generation using spontaneous quasi-phasematching techniq ues and provides insights into potential future applications in integrated optical communication systems.Progress Among the prevalent phase-matching techniques for ridge waveguide structures on LNOI,modal phase matching and quasi-phase matching stand out,with the latter offering distinct advantages by maximizing mode overlap between fundamental modes and leveraging the largest nonlinear coefficient d₃₃.Electric-field poling remains the predominant method for achieving QPM,as exemplified by periodically poled lithium niobate（PPLN） structures（Fig.7）.In 2018,Loncar’s group demonstrated PPLN-based nanowaveguides on LNOI,exhibiting a conversion efficiency two orders of magnitude higher than bulk counterparts.By 2020,further advancements in PPLN microring resonators pushed SHG efficiency to an unprecedented 5000000 %/W,accompanied by a singlephoton nonlinear efficiency of 1%.Most recently,the introduction of nonlinear critical coupling（NCC） theory by Xiao’s team at Peking University has optimized experimental conditions for second harmonic generation（SHG）,setting new benchmarks in conversion efficiency.Despite its efficacy,electric-field poling faces inherent limitations,including high fabrication costs,the complex fabrication process,and stringent precision requirements,hindering large-scale wafer-level manufacturing.These challenges have spurred the development of poling-free quasi-phase-matching（QPM） alternatives,primarily categorized into two innovative approaches.The first approach is structural periodicity-induced nonlinear modulation.This strategy modulates refractive index or nonlinear coefficients via periodic structural perturbations（Fig.8）.In 2017,Loncar’s group pioneered periodically grooved waveguides on X-cut LNOI,achieving SHG efficiency of～4.6 %/（W·cm²） through refractive index modulation.Concurrently,femtosecond laser direct writing emerged as a versatile tool for nonlinear coefficient engineering.In 2015,Imbrock et al.integrated linear refractive index modification with nonlinear coefficient tuning to fabricate Type Ⅲ cladding waveguides in Z-cut bulk LiNbO₃,yielding a normalized SHG efficiency of～0.0637%/（W·cm²）.The second approach is based on the crystal anisotropy.This approach exploits angular variations in the effective nonlinear coefficient(d_eff) relative to the optical axis.In 2019,Lin et al.fabricated 15 μm-radius microdisk resonators on X-cut LNOI,utilizing modal dispersion in higher-order modes to compensate phase mismatch,thereby achieving first-order natural quasi-phase-matching.In 2023,the spontaneous quasi-phase-matching（SQPM） scheme was proposed,which theoretically achieves a maximum conversion efficiency of 4000 %/W at a quality factor of 10⁵（Fig.10）.Bandwidth represents another critical metric for nonlinear devices,reflecting frequency-domain tolerance during nonlinear processes.The micro-nano scale dispersion engineering of LNOI enables efficient utilization of d₃₃ for broadband second harmonic generation（Fig.12）.In 2017,Li et al.from Shanghai Jiao Tong University observed broadband SFG and SHG with 3 dB bandwidths of 9 nm and 15.5 nm respectively.In 2020,the Fejer team achieved ultra-broadband SHG with a 3 dB bandwidth of 110 nm by designing the cross-sectional structure of a ridge waveguide.Additionally,merely one year following the theoretical proposition of SQPM,the 3 dB bandwidths of SQPM based on resonant cavities and bent waveguides are theoretically extended to 50 nm.Conclusions and Prospects Electric field poling,a core efficient frequency conversion technology,faces limitations in process complexity,precision,and large-scale fabrication.With lithium niobate evolving from bulk to thin-film platforms,poling-free quasiphase matching schemes,especially LNOI-based SQPM,enable efficient poling-free frequency conversion,supporting large-scale integrated photonic chip manufacturing.Future advancements in material understanding and processing will expand applications in optical communication,quantum information,and nonlinear imaging,driving the commercialization of integrated photonics.更多还原