Laser-Induced Plasma Radiation with gigahertz frequency could be used as an input energy source for a photocathode RF gun to replace the conventional input process. The resonant frequency of our target photocathode RF gun is 2.856GHz which means, a plasma radiation with a frequency of 2.856GHz has to be used in order to achieve the resonance condition.
Radiation frequency within the range of 2.3-2.6GHz had been successfully detected with a 1064 nm fundamental wave at pulse duration of 460 picoseconds, and a second harmonic wave (wavelength = 532mn) is generated from the fundamental wave with a LBO at a shorter pulse duration of 330 picoseconds. It is expected that, with a shorter pulse width, higher frequency radiation (hopefully can be near 2.856GHz) in the S-band region could be achieved.
Due to detection limitation of our ring-antenna, we cannot actually measure such a high frequency of plasma radiation created by a second harmonic wave. But if what we predicted is true, by replicating our current experiment with an enhanced ring-antenna, a kilowatt or even megawatt radiation power with an S-band frequency may be able to achieve and could possibly be recognized as a perfect substitution for the tradition powering method of a photocathode RF gun

Reference
[1] Hamster H., Sullivan A., Gordon S., White W. and Falcone R.W. "Subpicosecond electromagnetic pulses from intense laser-plasma interaction". Phys. Rev. Lett. 71, 2725 (1993).
[2] Kress M., Loffler T., Susanne E., Mark T., and Roskos H. G. ” Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves”. Optics Letters. Vol. 29, Issue 10, pp. 1120-1122 (2004).
[3] Kreb, M. et al. “Determination of the carrier-envelope phase of few-cycle laser pulses with terahertzemission spectroscopy”. Nature Phys. 2, 327–331 (2006)
[4] Andrey S., Yong Y., Young C. and Alexander F. “Non-equilibrium plasma in liquid water: dynamics of generation and quenching”. Plasma Sources Sci. Technol. 20 024003.
[5] Hache´, A. et al. “Observation of coherently controlled photocurrent in unbiased, bulk GaAs”. Phys. Rev. Lett. 78, 306–309 (1997).
[6] AjayNahata, N., Weling, A. S. and Heinz, T. F. “A wideband coherent terahertz spectroscopy system using optical rectification and electro‐optic sampling”, Appl. Phys. Lett. 69, 2321 (1996).
[7] Fitzgerald, A. J., Berry, E., Zinovev, N. N., Walker, G. C., Smith, M. A. and Chamberlain, J. M. “An introduction to medical imaging with coherent terahertz frequency radiation”, Phys. Med. Biol. 47 R67 (2002).
[8] Koechner, W., “Solid state laser engineering”, Springer, 6th, (2006).
[9] Huang, Y. C. “Nonlinear Optics”, (2003).
[10] Li Z. C. and Jian Z. “Terahertz radiation from a wire target irradiated by an ultra-intense laser pulse”, PHYSICS OF PLASMAS, 14, 054505 (2007).
[11] Cheng D. K. “Field and wave electromagnetics”, second edition.
[12] Kirtley D. E. “Study of the Synchronous Operation of an Annular Field Reversed Conguration Plasma Device”, PHD thesis, University of Michigan (2008).
[13] Brignon A., Feugnet G., Huifnard J. -P. and Pocholle J. -P., “ Compact Nd:YAG and Nd: YVO4 Amplifiers End-Pumped by a High-Brightness Stacked Array”, IEEE Journal of Quantum Electronics, Vol. 34, No. 3, (1998).
[14] Saleh B. E. A., Teich M. C., “Fundamentals of Photonics”, Wiley, 2nd, (2007).
[15] Kim1 K. Y., Taylor1 A. J., Glownia1 J. H. and Rodriguez1 G., “Coherent control of terahertz supercontinuum generation in ultrafast laser–gas interactions “, Nature Photonics 2, 605 - 609 (2008).
[16] Mainfray G., and Manus G., “Multiphoton Ionization of Atoms”, Rep. Prog. Phys. 54 1333, (1991)