Photonic spectral horizons for spin-orbit conversion.

We present a theoretical and computational framework for metasurface mirrors that switch optical angular momentum as a function of wavelength. The approach is based on controlling the phase retardance and amplitude balance of anisotropic meta-atoms to create a sharp wavelength threshold, termed a spectral horizon, where the reflected light reverses its spin and orbital angular momentum. The paper develops an operational definition of this spectral horizon, supported by analytical modeling, large-scale Monte Carlo validation, and full-field simulations of spatial intensity and phase profiles incorporating fabrication variability. The reference design, based on TiO2 nanopillars operating near 1550 nm, exhibits a steep spectral transition, narrow switching bandwidth, and high efficiency that remain stable under realistic tolerances. These results support a passive and reproducible route to wavelength-selective angular momentum control with applications in wavelength-division multiplexing, quantum state routing, and spectrally encoded photonic logic. We focus on the formulation and statistical validation of a synthetic spectral horizon for angular momentum, which connects metasurface dispersion, device performance metrics, and fabrication tolerances in a unified framework.