Photon Pair Generation in Lithium Niobate Waveguide Periodically Poled by Femtosecond Laser
Experimental setup for femtosecond laser direct writing ferroelectric domain patterns in the Ti-indiffused LN channel waveguide.
Schematic of the experimental setup used for photon-pair measurements.
Integrated quantum photonic technology exhibits great potential for applications ranging from quantum communications and quantum computation to quantum metrology. Single-mode, high-purity, and integrated sources of single photons and/or entangled photon pairs are necessary for all these quantum protocol. Lithium niobite (LN) integration platforms provide second-order nonlinearity that enables photon-pair generation by spontaneous parametric down-conversion (SPDC). More specifically, periodically poling of LN, also known as ferroelectric domain engineering technique can be applied to realize quasi-phase-matching (QPM) SPDC for generation of correlated photons with higher efficiency.
The traditional fabrication method for QPM is the electric field poling. The external poling field is applied via patterned electrodes on the crystal surface and the ferroelectric domain switching process always begins with nucleation of inverted domains on the surface. In addition, the mask and lithography are needed to design the poling structure, which are feasible but not ideal for rapid prototyping.
The femtosecond laser provides a flexible method for the fabrication of QPM structures in ferroelectrics. The laser beam can be focused at multiple depths inside transparent medium for fabrication of three-dimensional nonlinear photonics crystals, which are big challenge if using the electric field poling. The femtosecond laser poling offers an ideal technique to construct periodic ferroelectric domain devices, enabling precise control of localized domains for advanced applications in nonlinear beam shaping and holography.
Recently, in collaboration with professor Sheng Yan from Australian National University, the research team of the School of Physics from Nankai University led by professor Li Yongnan demonstrate photon-pair sources based on Ti-indiffused LN waveguide periodically poled by a tightly focused femtosecond laser in Chinese Optics Letters, Volume 21, Issue 4, 2023 (F. Dai, et al., Photon Pair Generation in Lithium Niobate Waveguide Periodically Poled by Femtosecond Laser). The laser (central wavelength at 800 nm, pulse width 180 fs, repetition rate 76 MHz, and single pulse energy up to 5 nJ) is used to form the domain inversion in the Ti-indiffused LN waveguide. The average scan speed of the focal spot of the laser beam is about v = 10 μm = s through the waveguide from the −Z toward the Z-surface. The average QPM period is 2.74 μm along the x axis and 1.15 μm along the y axis. The propagation loss of the waveguide is measured to be about 0.1 dB/cm at the fundamental wavelength. A second-harmonic power of 4.4 mW is obtained for input power of 40 mW (peak power is 3.6 kW), and the conversion efficiency is 10.1%. The pair coincidence rate reaches ∼8000 counts per second for average pump power of 3.2 mW. This work first time prove the possibility of application of the nonlinear photonics structure fabricated by femtosecond laser to the integrated quantum source.
Prof. Li Yongnan from Nankai University believes that this method can be used for the periodically polling of the thin-film LN by optimizing the intensity distribution of the focal field of femtosecond laser. In addition, this method can be extended to three-dimensional domain structures, which provide a potential platform for steering of the spatial degree of freedom of the entangled two-photon states.
飞秒激光制备周期极化结构，实现光子对输出
图1 利用飞秒激光在钛扩散铌酸锂波导内制备周期极化结构实验装置示意图
图2 关联光子对产生及测量实验装置示意图
集成量子光子技术
集成光量子光技术在量子通信、量子计算及量子精密测量等领域都具有广阔的应用前景。单模、高纯度的集成单光子源或纠缠光源是完成这些量子协议的必要条件和关键技术之一。铌酸锂集成平台具有良好的二阶非线性，通过自发参量下转换过程可以有效产生光子对。借助于周期极化结构，也被称为铁电畴工程技术，可以实现准相位匹配，从而进一步提高量子光源的产生效率。
周期极化结构制备方法
传统的周期极化制备方法是电场极化，通过在晶体表面的图案化电极上施加外部极化场，从表面反向畴开始成核并逐渐形成周期性畴反转。这种技术需要掩模板和光刻技术来设计极化结构，工艺相对复杂。飞秒激光为铁电体晶中准相位匹配结构的制备提供了一种灵活的方法，激光束可以聚焦在透明介质内的不同深度，加工出三维非线性光子晶体，这对于电场极化是一个巨大的挑战。飞秒激光极化能够精确控制局域畴结构，在非线性光束整形和全息术中具有重要应用。
飞秒激光制备周期极化结构
南开大学物理科学学院李勇男教授研究组与澳大利亚国立大学盛艳教授合作，利用飞秒激光在钛扩散铌酸锂波导中制备了周期极化结构，并实现了关联光子对输出，该方法具有简单灵活优势，可用于集成量子光源。相关成果发表在Chinese Optics Letters第21卷第4期上(F. Dai, et al., Photon Pair Generation in Lithium Niobate Waveguide Periodically Poled by Femtosecond Laser)，并被选为当期封面。
封面中左端入射光为飞秒激光，透明状长方体为铌酸锂波导结构，右端的双光子为输出的关联光子对，绿色矩阵结构为制备的周期极化结构。封面主要展示了利用飞秒激光加工技术，在钛扩散铌酸锂波导中实现无掩模周期极化结构制备。