Nonlinear photonic structures with a modulated second-order nonlinearity are used widely for quasi-phase-matched parametric processes. Creating three-dimensional (3D) nonlinear photonic structures is promising but still challenging, since standard poling methods are limited to two-dimensional structures. Light-induced quasi-phase matching (QPM) can overcome this issue by a depletion of the second-order nonlinearity with focused femtosecond laser pulses. We report, to the best of our knowledge, the first integration of a 3D QPM structure in the core of a lithium niobate waveguide applying light-induced fabrication. Depressed-cladding waveguides and embedded QPM structures are fabricated by femtosecond laser lithography. The 3D capability is exploited by splitting the QPM gratings in the waveguide core into two or four parts, respectively. These monolithic nonlinear waveguides feature parallel multi-wavelength frequency conversion. Finally, we demonstrate a concept for second-harmonic beam shaping taking advantage of a helically twisted nonlinear structure. Our results open new avenues for creating highly efficient advanced QPM devices.

中文翻译：

---

波导集成的三维准相位匹配结构

具有调制的二阶非线性的非线性光子结构被广泛用于准相位匹配的参数过程。由于标准极化方法仅限于二维结构，因此创建三维（3D）非线性光子结构是有希望的，但仍具有挑战性。光诱导准相位匹配（QPM）可以通过使用飞秒激光脉冲聚焦消除二阶非线性来解决此问题。据我们所知，我们首次将3D QPM结构集成到铌酸锂波导的芯中，并进行了光诱导制造。压层波导和嵌入式QPM结构是通过飞秒激光光刻技术制造的。通过将波导芯中的QPM光栅分别分为两部分或四部分来利用3D功能。这些单片非线性波导具有并行的多波长频率转换功能。最后，我们演示了利用螺旋扭曲的非线性结构进行二次谐波波束成形的概念。我们的结果为创建高效的高级QPM设备开辟了新途径。