Second-harmonic generation (SHG) extensively employs periodically poled nonlinear crystals through forward quasi-phase-matching to achieve efficient frequency conversion. As poling periods approach sub-micrometers, backward quasi-phase-matching has also been demonstrated, albeit by utilizing pulsed laser drives. The realization of symmetric second-harmonic generation, characterized by counterpropagating pumps, however, has remained elusive despite theoretical predictions. The main challenge lies in achieving strong nonlinear coupling with the poling period below half the wavelength of the second-harmonic light. The recent emergence of high-quality ferroelectric lithium niobate thin films provides an opportunity for achieving precise domain control at submicron dimensions. In this paper, we demonstrate reliable control of ferroelectric domains in a thin film lithium niobate waveguide with a poling period down to 370 nm, thereby realizing highly efficient continuous-wave pumped symmetric SHG. This demonstration not only validates the feasibility of achieving subwavelength periodic poling on waveguides but could also enable submicron ferrolectric domain structures to be leveraged in integrated photonics and nonlinear optics research.

中文翻译：

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亚波长周期性极化薄膜铌酸锂的对称二次谐波产生


二次谐波发生（SHG）广泛采用周期性极化非线性晶体，通过正向准相位匹配来实现高效的频率转换。随着极化周期接近亚微米，后向准相位匹配也已被证明，尽管是通过利用脉冲激光驱动。然而，尽管有理论预测，但以反向传播泵为特征的对称二次谐波生成的实现仍然难以实现。主要挑战在于实现极化周期低于二次谐波光波长一半的强非线性耦合。最近出现的高质量铁电铌酸锂薄膜为实现亚微米尺寸的精确域控制提供了机会。在本文中，我们展示了对极化周期低至 370 nm 的薄膜铌酸锂波导中铁电畴的可靠控制，从而实现高效的连续波泵浦对称 SHG。该演示不仅验证了在波导上实现亚波长周期性极化的可行性，而且还可以使亚微米铁电畴结构在集成光子学和非线性光学研究中得到利用。