Pseudo-spin light circuits in nonlinear photonic crystals.

Guiding light forms the backbone of numerous photonic circuits that allow complex, robust, and miniaturized light control. Commonly, guiding is achieved by modifying linear permittivity, resulting in a non-homogeneous linear medium. Here, we propose and experimentally realize photonic circuits in a homogeneous refractive index medium, where the guiding is driven entirely by nonlinear interaction, enabling dual-wavelength light beam guidance and optical control. This mechanism is analogous to spin current transport in sharp magnetic domain walls, where magnetization texture constitutes a spin-dependent potential. Using narrow custom-poled nonlinear photonic crystals, we guide frequency superposition beams that act as pseudo-spins over more than four Rayleigh lengths. We show that guiding properties depend on the relative phase between participating wavelengths, which can be optically switched on and off with an optical pump. Additionally, using two parallel-poled structures, we experimentally realize a pseudo-spin directional coupler, paving the way for numerous waveguiding hallmarks in a single nonlinear crystal and offering robust control over frequency superposition states of light. Finally, our findings show that it is possible to experimentally emulate complex, precise 2D magnetization domain wall structures, opening avenues for exploration that remain challenging in magnetic materials.