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Controlling the Near-Field of Metasurfaces for Free-Electron Multi-Harmonic Hard X-Ray Generation

Metasurfaces are subwavelength spatial variations in geometry and material, and offer us an ingenious way of manipulating far-field radiation. Here, we show that their ability to manipulate the near-field is also an extremely powerful lever in controlling nanophotonic photon-electron interactions. The metasurface near-field contains high-order spatial harmonics that can be leveraged to generate substantial high harmonic components, whose properties are directly tunable through the metasurface geometry, the electron energy and the incidence angle of the input laser. By developing an analytical theory that produces results in excellent agreement with our ab initio simulations, we show how the design of metasurface-enhanced plasmons can be leveraged to produce intense and highly-directional multi-harmonic emission in the X-ray and the gamma-ray range. Furthermore, we show that the use of few-cycle laser pulses is instrumental in enhancing the intensity and bandwidth of the radiation output while avoiding damage to the metasurface. As an example, we present the design of a four-color X-ray source.