Wavelength-selective second-harmonic splitting and focusing in engineered χ⁽²⁾ photonic crystals

We demonstrate a multifunctional engineered χ⁽²⁾ photonic crystal that integrates second-harmonic generation, wavelength selectivity, beam splitting, and focusing within a single χ⁽²⁾-modulated ferroelectric photonic structure. Rather than encoding a single nonlinear wavefront, the engineered spatial χ⁽²⁾ modulation multiplexes wavelength-dependent phase-mismatch compensation and wavefront-encoding conditions, allowing different pump wavelengths to access distinct second-harmonic focusing responses. As a proof of concept, a 1050 nm pump is converted to 525 nm second-harmonic light and focused into two spatially separated spots, whereas a 1100 nm pump is converted to 550 nm second-harmonic light and focused into three spots. The measured multi-spot patterns and deflection angles agree well with the numerical design, while wavelength-resolved measurements confirm selective excitation of the encoded double-focus and triple-focus channels. These results establish a compact strategy for multiplexing independently prescribed nonlinear output functions within a single engineered χ⁽²⁾ structure.