The performance of many optical devices based on frequency conversion critically depends on spatial modulation of the nonlinear optical response of materials. This modulation ensures efficient energy exchange between optical waves at different frequencies via quasi-phase matching¹. In general, quasi-phase-matching structures, also known as nonlinear photonic crystals^2,3,4, offer a variety of properties and functionalities that cannot be obtained in uniform nonlinear crystals^5,6,7,8,9. So far, nonlinear photonic crystals have been restricted to one- or two-dimensional geometries owing to a lack of fabrication technologies capable of three-dimensional (3D) nonlinearity engineering. Here, we provide an experimental example of a 3D nonlinear photonic crystal, fabricated in ferroelectric barium calcium titanate, by applying an ultrafast light domain inversion approach. The resulting full flexibility of 3D nonlinearity modulation enables phase matching of nonlinear processes along an arbitrary direction, thereby removing constraints imposed by low-dimensional structures.

许多基于频率转换的光学设备的性能关键取决于材料的非线性光学响应的​​空间调制。该调制确保了通过准相位匹配¹在不同频率的光波之间进行有效的能量交换。通常，准相位匹配结构（也称为非线性光子晶体^2,3,4）具有各种特性和功能，而均匀的非线性晶体^5,6,7,8,9则无法获得这些特性和功能。。到目前为止，由于缺乏能够进行三维（3D）非线性工程的制造技术，非线性光子晶体已被限制为一维或二维几何形状。在这里，我们提供了通过应用超快光域反转方法，在铁电钛酸钡钙中制造的3D非线性光子晶体的实验示例。由此产生的3D非线性调制的完全灵活性使非线性过程沿任意方向的相位匹配成为可能，从而消除了低维结构带来的约束。