Reconfigurable Geometric Phase Matching by Multilayered Nonlinear Thin‐Film Crystals

Phase matching is essential for efficient energy transfer in nonlinear wave‐mixing processes. Traditional methods, such as birefringent and quasi‐phase matching, have remained conceptually unchanged since their discovery over 60 years ago, each posing inherent constraints and limitations. Here, we demonstrate the concept of geometric phase matching as a dynamically reconfigurable approach for tunable nonlinear wave mixing, based on a multilayered platform of nonlinear thin‐film crystals. We leverage this concept to experimentally show reconfigurable and spin‐controlled phase matching for second‐harmonic generation (SHG), opening new avenues for dynamic manipulation of nonlinear interactions in photonic devices. We specifically demonstrate full modulation of SHG from a bilayer structure, nearly perfect and tunable geometric phase matching from an eight‐layer structure, and polarization tomography that reveals the evolution of the spin‐dependent interaction. This approach not only expands the design space for nonlinear optical processes but also paves the way for highly robust, tunable, and efficient frequency conversion for next‐generation adaptive nonlinear photonic, quantum photonic, and nonlinear optical metamaterial technologies based on thin‐film crystals.