Grating transition radiation a new source of monochromatic light using relativistic electrons

The production of quasi-monochromatic light by relativistic electrons impinging on the surface of an optical grating is investigated at electron energies between 3 and 120 MeV. In analogy to Smith-Purcell (SP) radiation generated by electrons travelling close and parallel to the grating surface the wavelength depends on the angle of observation. In addition, in grating transition radiation (GTR) the wavelength depends also on the angle of incidence of the electron. At a fixed wavelength the angular distribution is concentrated in hollow cones centered around the main directions of emission of the different orders of diffraction. These hollow cones are similar to the well-known distribution of transition radiation from flat surfaces (TR). The experiments have been carried out at the Gent linear accelerator in the energy range 3-15 MeV and at the Geel linear accelerator at 20-120 MeV. Several gratings were used, ruled in SiC with 1800 lines/mm and 2000 1/mm and with different profiles. The angular distribution and polarization of GTR have been measured in zeroth and first diffracted order for several wavelengths between 400 and 700 nm. The intensity observed in zero order near specular reflection is compared to TR observed from a flat mirror of the same surface material. We calculated the GTR intensity expanding the Coulomb field of the electron into plane waves. These waves are diffracted by the grating leading to a "grating problem" as in the case of SP radiation. The formalism converges towards TR in the limit of vanishing groove depths.