  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 optical
properties of a 2D photonic crystal and 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 5. 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 the second-harmonic generation in the
near-UV by a factor of 18 illustrating the potential in the flexible
fabrication technique for barium titanate photonic devices.
