Laser-Induced Domain Engineering in Potassium Tantalate Niobate Crystals for Ultra-Broadband Second-Harmonic Generation

Nonlinear optics serves as a pivotal enabler for on-demand laser sources, advancing fundamental science and enabling transformative technologies across biomedicine, industry, and environmental monitoring. Here we demonstrate that arbitrary structuring of ferroelectric domains in three dimensions represents a significant chance to overcome this limitation, by flexible matching of the speeds of light waves of different colors and by trapping and enhancing light waves in such structures via Bragg resonance. This universal strategy for programmable second-order nonlinear susceptibilities in potassium tantalate niobate crystals becomes feasible by exploiting femtosecond laser direct writing to locally induce paraelectric-to-ferroelectric phase transitions. This laser-induced domain engineering enables the direct fabrication of tailored nonlinear structures. The resulting domain-engineered structures exhibit a rich phase-matching landscape, simultaneously supporting nonlinear Raman-Nath diffraction and Čerenkov-type emission. Crucially, these structures demonstrate highly efficient second-harmonic generation across an ultra-broadband 300 nm wavelength range in the infrared, enabling robust conversion into the visible spectrum. Our findings establish laser-induced spatial phase transition engineering as a versatile and powerful platform for the creation of multi-functional nonlinear photonic devices. This work provides a promising solution for broadband frequency conversion, with a significant impact on all fields relying in tailored laser light.