Recent advances in solid state laser technology enable efficient generation of optical frequency laser pulses a few cycles in duration with focused intensities in excess of ${10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. When such pulses are propagated in neutral gases such as helium, ionization rates comparable to the optical frequency are induced. The very high ionization rates and the corresponding optical field strengths to which neutral atoms can be exposed with few cycle pulses present an intriguing possibility for coherent XUV generation. In this Letter, we will examine phase matching issues associated with this phenomenon.

• Received 7 March 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.2971