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We investigate the effects of lattice spacing and domain-filling ratio on the wavelength dependence in second-harmonic generation (SHG) from two-dimensional /spl chi//sup (2)/ nonlinear photonic crystals (2-D NPC) on lithium niobate. Spectral coverage of 150 and 200 nm around 1.5 and 1.9 /spl mu/m in quasi-phase-matched (QPM) SHG is obtained by angular rotation of the NPC with a lattice periodicity of 20/spl times/20 and 29.5/spl times/29.5 /spl mu/m/sup 2/, respectively. The SHG signals are shown to have off-axial propagation directions determined by the incident wavelength and the QPM vector. The intensity of the high-order reciprocal-lattice-vector (G/sub mn/)-assisted SHG signal is shown to depend on the domain-filling ratio. These observations are attributed to the unique dispersion of G/sub mn/ together with the nonzero /spl chi//sup (2)/(G/sub mn/) nonlinearity in the 2-D QPM-NPC having a tetragonal lattice structure.