FIGS. 17A-17B show the emission intensity when electrons having energy of 3.7 MeV and 100 eV, respectively, are used and when the SPP has a free space wavelength of 10 μm. The main difference in radiation output—compared to the λ=1.5 μm case for the same confinement factor—lies in the output photon energy, which is smaller for a given electron energy due to the larger spatial period of the surface plasmons. More specifically, in FIG. 17A, it can be seen that highly-directional, monoenergetic (0.23% FWHM energy spread), few-keV X-rays are generated by 3.7 MeV electrons, which may be obtained readily from a compact RF electron gun. In FIG. 17B, 100 eV electrons now generate near/mid-infrared photons at on-axis peak energies of 0.58 eV (0.2% energy spread) and 0.32 eV (0.3% energy spread). As before, the lack of radiative directionality in the 100 eV case is an inevitable result of the lack of relativistic angular confinement when non-relativistic electrons are used.
The resulting 20 keV photons in FIG. 16A are highly directional and monoenergetic, with an on-axis full-width at half-maximum (FWHM) energy spread of 0.25% and an angular spread of less than 10 mrad. The effect of electron beam divergence is discussed below.
Space Charge and Electron Beam Divergence
