Simultaneous type-I and type-II phase matching for second-order nonlinearity in integrated lithium niobate waveguide

Second-order optical nonlinearity is widely used for both classical and quantum photonic applications. Due to material dispersion and phase matching requirements, the polarization of optical fields is pre-defined during the fabrication. Only one type of phase matching condition is normally satisfied, and this limits the device flexibility. Here, we demonstrate that phase matching for both type-I and type-II second-order optical nonlinearity can be realized simultaneously in the same waveguide fabricated from thin-film lithium niobate. This is achieved by engineering the geometry dispersion to compensate for the material dispersion and birefringence. The simultaneous realization of both phase matching conditions is verified by the polarization dependence of second-harmonic generation. Correlated photons are also generated through parametric down conversion from the same device. This work provides a novel approach to realize versatile photonic functions with flexible devices.