Papers by Carlos Hernandez Garcia
In the quest for an electron beam suitable to the development of compact free-electron lasers, ex... more 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 x 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.
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Papers by Carlos Hernandez Garcia