Wavelength tunability of second-harmonic generation from two-dimensional χ(2) nonlinear photonic crystals with a tetragonal lattice structure

The wavelength tunability of second-harmonic generation (SHG) in a two-dimensional nonlinear photonic crystal (2D NPC) on lithium niobate is examined by using a 1.55 μm-band optical parametric oscillator. We observed SHG signals that were generated off-axis from the fundamental pump beam and a multiple number of quasi-phase-matchable (QPM) wavelengths for a given crystal setting. The spacing between the phase-matched wavelengths increases with the azimuthal rotation angle (φ) in the x–y plane of the crystal. The peak intensity and propagation direction of the SHG signals are found to vary with the pump wavelength and crystal rotation angle. These observations are ascribed to a 2D distribution of high-order reciprocal lattice vectors Gmn(φ) and the corresponding χ(2)(Gmn) nonlinearities in 2D NPC having a tetragonal structural symmetry.