Controlling X-ray emission with dispersion-engineered surface plasmon polaritons

We propose controlling the angular and spectral distribution of hard x-ray emission by entangling x-ray photons with ultraviolet surface plasmon polaritons (SPPs) whose dispersion is engineered by a metal-dielectric multilayer on a nonlinear crystal. Spontaneous parametric down-conversion of an x-ray pump produces a hard x-ray signal photon correlated with an ultraviolet SPP mode near its resonance. Engineering the SPPs dispersion reshapes the phase-matching landscape and imprints tunable angular-spectral structure on the emitted x-ray photons. The scheme enables compact, designable control of x-ray emission and extends surface-plasmon-assisted nonlinear x-ray optics.