First Observation of Photoemission Enhancement from Copper Cathode Illuminated by Z-polarized Laser Pulse

Since 2006, we have been developing a novel photocathode gun gated by the laser-induced Schottkyeffect. This new type of gun utilizes laser coherency to aim at a compact femtosecond laser oscillator as an IR laser source using Z-polarization on a metal photocathode. This Z-polarization scheme reduces the laser photon energy to excite the cathode with a longer wavelength by reducing its work function due to the Schottky effect. Before this epoch-making scheme emerged, photocathode guns had never utilized a laser’s coherency itself. We applied a hollow laser incidence scheme with a hollow convex lens that is focused after passing the beam through a radial polarizer. Based on our calculations (convex lens: NA=0.15; 60% hollow ratio), a Z-field of 1 GV/m needs 1.26 MW at peak power for the fundamental wavelength (792 nm) of a Ti: Sapphire laser. To confirm photoemission enhancement effects due to a Z-field of a few GV/m compared with copper’s conventional photoemission, we used third harmonic generation (THG: 264 nm) as a pilot experiment and generated a Bessel beam with a hollow axicon lens in a vacuum to relax the phase matching condition of the wavefront on the cathode’s focus point. We observed the first Z-field emission enhancement with a copper cathode at THG (264 nm). The enhancement factor was 1.4 times at 1.6 GV/m of the averaged Z-field of the central laser spot (Bessel beam) on the cathode surface.