Fig. 2: Tunability of the emitted photon energy via the electron energy.

a Metasurface optimized to enhance the first five orders. The plasmon polaritons are excited by a laser pulse with an energy of 3.4 eV (370 nm), and electrons are shot parallel to the metasurface at a distance of 1 nm. The emission direction is described by φ and θ in the bottom right inset. b At a fixed electron energy, the on-axis output photon energy is higher for higher orders (for visibility, we only present orders n = 0, −1, −2, −3) and the source is brighter (intensity in units of number of photons/s/sr/1%BW). At high energies (denoted by the hatched region), our analytical formulation begins to differ from the results predicted by energy–momentum conservation (denoted by green lines), since the analytical formulation does not take quantum recoil into account. c–e Highly monochromatic harmonic peaks (fractional bandwidths (c) < 0.4% (d) < 0.2% (e) < 0.19%) for c soft X-rays from 550 keV electrons (already accessible with a TEM), d hard X-rays from 7.5-MeV electrons (can be obtained using a tabletop setup), and e γ-rays from 0.1-GeV electrons. Note that the line thickness has been artificially increased for clarity