Generation of Ultrafast VUV Radiation by Reflection from a Relativistic Ionization Front

Herein we propose a method of producing short-wavelength radiation, which has the potential to produce optical pulses shorter than any that have been generated to date. This method is based on the process of upshifting light by reflecting it off a moving ionization front. This process has been considered in detail as a method for upshifting microwave radiation1. We show that by using an intense femtosecond light pulse to generate an ionization front through multiphoton ionization (MPI), it is possible to upshift a single-cycle far-infrared light pulse into the ultraviolet or vacuum ultraviolet. Since the ionization front created by MPI can be very sharp, the reflection process may add little dispersion to the pulse, making it possible to generate nearly single-cycle light pulses in the VUV. In this paper, we demonstrate that the process is experimentally realizable using the current generation of high energy femtosecond (0.1 Joule in 100 fsec) lasers. We discuss several physics issues unique to the generation of short-wavelength light using this technique. More detailed calculations of multiphoton ionization rates are necessary to determine the reflected pulse energy achievable using this technique, as will be discussed.