Systematic study of amorphous ABC heterostructures at the atomic scale as a second-order nonlinear optical metamaterial

A systematic exploration of amorphous ABC heterostructures revealed that nanoscale morphological modifications markedly improved their artificial bulk second-order susceptibility. These amorphous birefringent heterostructures were fabricated using plasma-enhanced atomic layer deposition of three oxides, thereby breaking centrosymmetry at the nanoscale. We observe that the optical nonlinearity depends on the thickness variation of the three constituent materials, SiO<inf>2</inf>, TiO<inf>2</inf>, and Al<inf>2</inf>O<inf>3</inf>, ranging from tens of nanometers to the atomic scale, and where the thin films exhibit second-order susceptibility at their interfaces. Our findings reveal that the enhancement of nonlinear optical properties is strongly correlated with a high interface density and superior interface quality, where the interface second-order nonlinearity transitions to bulk-like second-harmonic generation. An effective bulk second-order susceptibility of χ<inf>zzz</inf> = 2.0 ± 0.2 pm/V at 1032 nm is achieved, comparable to some conventional monocrystalline nonlinear materials.