Hexagonally poled lithium niobate: A two-dimensional nonlinear photonic crystal

We report on the fabrication of what we believe is the first example of a t wo-dimensional (2D) nonlinear photonic crystal [Berger, Phys. Rev. Lett. 81 , 4136 (1998)], where the refractive index is constant but where the 2nd or der nonlinear susceptibility is spatially periodic. Such crystals allow for efficient quasi-phase-matched and harmonic generation using multiple recip rocal lattice vectors. External 2nd harmonic conversion efficiencies >60% w ere measured with picosecond pulses. The fabrication technique is extremely versatile and should allow for the fabrication of a broad range of 2D crys tals including quasicrystals.