Tunable Ultracompact High‐Index Gallium Phosphide Nanowire Directional Couplers

Gallium phosphide (GaP) is emerging as a highly compelling platform for next‐generation photonic integrated circuits (PICs) due to its unique combination of a high refractive index, strong nonlinearities, and a broad transparency window extending into the visible spectrum where silicon (Si) is opaque. The development of this platform hinges on the realization of fundamental building blocks, with the directional coupler being an indispensable component for advanced circuits. In this work, we report the comprehensive design, fabrication, and experimental characterization of a 2 × 2 directional coupler constructed from mechanically assembled GaP nanowires. The fabricated devices exhibit the characteristic wavelength‐dependent power exchange between adjacent waveguides. We introduce and validate a computationally efficient 2D design methodology that accurately predicts device performance, significantly reducing the optimization cost compared to full 3D simulations. Critically, we provide a proof‐of‐concept for mechanical control over optical properties by activating the coupling effect in situ through physical manipulation of the nanowire interaction length. This result leverages the remarkable elasticity of GaP nanowires and establishes a foundational step toward complex, reconfigurable PICs for future applications in quantum information processing, nonlinear optics, and sensing.