Omnidirectional phase matching of arbitrary processes by radial quasi-periodic nonlinear photonic crystal

We investigate second-harmonic generation in a nonlinear photonic crystal having radial quasi-periodic order and continuous rotational symmetry. This device enables us to simultaneously phase match different nonlinear interactions in any arbitrary direction of propagation. We have fabricated such a crystal by electric field poling of a magnesium-doped stoichiometric LiTaO3 and demonstrated frequency doubling of two different pump wavelengths at three different angles. Fourier coefficients were 10 times higher than that of a lattice-based multidirectional frequency doubler.