Three-dimensional nonlinear photonic crystal in ferroelectric barium calcium titanate

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^2–4, offer a variety of properties and functionalities that cannot be obtained in uniform nonlinear crystals^5–9. 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.