Efficient collinear fourth-harmonic generation by two-channel multistep cascading in a single two-dimensional nonlinear photonic crystal

We investigate efficient fourth-harmonic generation in a single two-dimensi onal (2D) quadratically nonlinear photonic crystal. We propose a novel para metric process that starts with phase-matched generation of a pair of symme tric second-harmonic waves, which then interact to produce a fourth-harmoni c wave that is collinear to the fundamental. We show that this process is m ore efficient than conventional fourth-harmonic-generation schemes by a fac tor that reaches 4 at low intensities and discuss how to design and optimiz e the nonlinear 2D photonic crystals that are implemented in LiNbO3 and LiT aO3. (C) 2001 Optical Society of America.