Initiation of THz Spherical Wave of Electron Bunch near Metallic Boundary

Spatio-temporal measurement of THz spherical wave generated by the electron bunch passing across a metallic foil was conducted. Transverse THz electric field around a relativistic sub-picosecond electron bunch was measured using electro-optic (EO) sampling based on balance detection. The shape of the spherical wavefront was changed by adjusting the longitudinal position of the metallic foil which reflects THz electric field around the electron bunch before passing across the foil. Experimentally obtained field map was compared with numerical simulations.