Broadband enhancement of Ĉerenkov second harmonic generation in a sunflower spiral nonlinear photonic crystal

We present an experimental study on the Cerenkov second harmonic emission in a novel sunflower spiral array of ferroelectric domains in LiNbO₃ crystal. The spiral patterns offer a diffusive, circularly symmetric distribution of reciprocal lattice vectors, thereby enabling enhanced emission of the Cerenkov beam in a broad spectral range. Instead of the traditional electric field poling, the sunflower spiral patterns are fabricated here by using our pioneering method of ferroelectric domain engineering with ultrafast light. This all-optical method gives access to high quality domain structures with short periods, which is beneficial for efficient Cerenkov harmonic generation.