Pseudospin light circuits in nonlinear photonic crystals

We present a new guiding mechanism in which light confinement is achieved entirely through nonlinear interaction, without any change in the refractive index of the medium. By spatially modulating the second-order nonlinearity to form a narrow, periodically poled strip within an otherwise monodomain KTP crystal, we realize dual-wavelength guiding that is all-optically controlled by the pump beam. The mechanism is based on the sum-frequency generation process and is analogues to spin transport across magnetic domain walls, where the signal and idler fields form an effective pseudo-spin whose guiding behavior depends on their relative phase. We experimentally demonstrate these concepts in several guiding configurations, including a pseudo-spin slab waveguide, directional coupler, and Y-splitter, enabling robust control of frequency-superposition states. Our findings pave the way for numerous waveguiding hallmarks within a single nonlinear crystal and open new possibilities for integrated and quantum photonics.