Surface plasmon-enhanced x-ray ultraviolet nonlinear interactions

We propose and demonstrate a method to manipulate x-ray emission rates and directions by coupling them to surface plasmon polaritons via x-ray-to-ultraviolet down-conversion in a metal. This mechanism generates highly correlated photon pairs, each consisting of one x-ray and one ultraviolet photon, enabling fine control of x-ray angular and spectral characteristics through the ultraviolet photons resonant with the surface plasmons. Our approach bridges a previously insurmountable gap between x-ray and plasmonic energy and momentum, enabling the use of engineered plasmonic structures to control x-ray emission. Furthermore, our work unlocks opportunities to explore plasmonic physics at the microscopic scale with atomic-scale resolution.