| In the quest for an electron beam suitable to the development of compact free-electron lasers, experiments to study electron emission from needle cathodes illuminated with CW and pulsed lasers were performed. In the first set of experiments tungsten needles were illuminated with a 514-nm CW argon laser at intensities up to 108 W/m2. The results show a strong dependence of the current on the electric field, and a non-linear behavior with laser intensity indicating that the photoexcited electrons rapidly relax to energy levels just above the Fermi level, and reside there for a seemingly long time before tunneling out. The observations seem to be particular to the complex band structure of tungsten, since no photoemission was observed using ZrC tips.; In the second set of experiments the tungsten needles were illuminated with ns pulses from a Q-switched Nd:YAG laser at 532 nm, 355 nm and 266 nm and intensities on the order of 1010–1012 W/m2. The experimental photocurrent at 355 and 266-nm wavelength is observed to be linear with laser intensity below the ablation threshold near 8 × 1011 W/m2. No direct or indirect photoemission is observed at the 532-nm wavelength for intensities below the ablation threshold. It was found that the threshold for ablation is wavelength independent, while the photocurrent in the linear regime (below the ablation threshold) shows a strong dependence on the laser wavelength. The experimental quantum efficiency is observed to approach unity for higher applied electric fields at the 266-nm wavelength. A simple model for the quantum efficiency as a function of the applied field based on the number of electrons in the metal accessible to the incident photons makes a good fit to the experimental data at the 266-nm wavelength. However, the model does not explain the wavelength dependence. The estimated brightness of an electron beam produced under these conditions is on the order of 1016 A/m2-steradian. With a brightness of this order, it is possible to build tabletop free-electron lasers operating in the far infrared and in the UV region. |
