RELATIVISTIC PULSE-COMPRESSION

Tunable radiation can be produced by Doppler upshifting light from a laser-generated moving ionization front. We analyze this technique as a method of generating ultrashort tunable light pulses in the UV to extreme UV (XUV) regions of the spectrum and demonstrate the possibility of generating subfemtosecond short-wavelength light pulses. Using realistic and technologically achievable experimental conditions and including dispersion of a pulse reflected from a finite-rise-time multiphoton-ionization-produced ionization front, we show that a single-cycle far-infrared (FIR) pulse can be compressed in time and frequency into a pulse substantially shorter than any light pulse demonstrated to date. We show that arbitrarily large upshifts are achievable using this technique, making it a potentially competitive way of producing tunable XUV radiation. The multiphoton ionization mechanism by which the ionization front is created is an important factor in determining the reflectivity of the ionization front.