Sol–Gel Barium Titanate Nanohole Array as a Nonlinear Metasurface and a Photonic Crystal

Abstract The quest of a nonlinear optical material that can be easily nanostructured over a large surface area is still ongoing. Here, we demonstrate a nanoimprinted nonlinear barium titanate 2D nanohole array that shows the optical properties of a 2D photonic crystal and a metasurface, depending on the direction of the optical axis. The challenge of nanostructuring the inert metal‐oxide is resolved by direct soft nanoimprint lithography with sol–gel derived barium titanate enabling critical dimensions of 120 nm with aspect ratios of five. The nanohole array exhibits a photonic bandgap in the infrared range when probed along the slab axis, while lattice resonant states are observed in out‐of‐plane transmission configuration. The enhanced light‐matter interaction from the resonant structure enables to increase in the second‐harmonic generation in the near‐ultraviolet by a factor of 18 illustrating the potential in the flexible fabrication technique for barium titanate photonic devices.