  We report on the fabrication of what we believe is the first example of a two
dimensional nonlinear periodic crystal\cite{berger}, where the refractive index
is constant but in which the 2nd order nonlinear susceptibility is spatially
periodic. Such a crystal allows for efficient quasi-phase matched 2nd harmonic
generation using multiple reciprocal lattice vectors of the crystal lattice.
External 2nd harmonic conversion efficiencies > 60% were measured with
picosecond pulses. The 2nd harmonic light can be simultaneously phase matched
by multiple reciprocal lattice vectors, resulting in the generation of multiple
coherent beams. The fabrication technique is extremely versatile and allows for
the fabrication of a broad range of 2-D crystals including quasi-crystals.
