Solution-Processed Barium Titanate Nonlinear Woodpile Photonic Structures with Large Surface Areas

Three dimensional (3D) nonlinear photonic crystals are one of the potentialcandidates for enabling photonic circuits. Most of the current top-down fabri-cation approaches for such crystals are time consuming and suffer from smallsurface areas and limitations to specific substrates. Herein, a bottom-up fabri-cation technique (UV-soft nanoimprint lithography) together with solutionprocessing of barium titanate (BaTiO 3 ) nanoparticles is used to create large-scale3D nonlinear woodpile photonic crystals. Opposed to other 3D photonic crystals,the woodpile structure can be fabricated layer by layer. This property makes itsuitable for roll-to-roll fabrication, which can be easily adapted in industry. Theprepared photonic crystals with up to eight layers have surface areas of 1.0 cm 2 .The linear and nonlinear optical behavior of these 3D woodpile structures ischaracterized. Their transmission stop band lies around 757 nm and a relativelinear minimum transmission of 48.4% for a four-layer sample is obtained.Images of diffraction patterns together with polarization-dependent secondharmonic generation measurements prove a cubic photonic crystal structure inthe linear and nonlinear regime. In addition to second-order nonlinearity, BaTiO 3exhibits electro-optic, elasto-optic, and thermo-optic effects. Therefore, it isespecially suited for future active photonic crystal applications.