Schematic of the proposed frequency-conversion scheme for LiTaO3.

Abstract: We investigate efficient fourth-harmonic generation in a single two-dimensional ({2D}) quadratically nonlinear photonic crystal. We propose a novel parametric process that starts with phasematched generation of a pair of symmetric second-harmonic waves, which then interact to produce a fourth-harmonic wave that is collinear to the fundamental. We show that this process is more efficient than conventional fourth-harmonic-generation schemes by a factor that reaches 4 at low intensities and discuss how to design and optimize the nonlinear {2D} photonic crystals that are implemented in {{LiNbO}$_3$} and {{LiTaO}$_3$}.

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