Enhanced phase-matching for generation of soft X-ray harmonics and attosecond pulses in atomic gases

We theoretically investigate the generation of high harmonics and attosecond pulses by mid-infrared (IR) driving fields. Conditions for coherent build-up of high harmonics are revisited. We show that the coherence length dictated by ionization-induced dephasing does not constitute an ultimate limitation to the coherent growth of soft X-ray (> 100 eV) harmonics driven by few-cycle mid-IR driving pulses: perfect phase-matching, similar to non-adiabatic self-phase-matching, can be achieved even without non-linear deformation of the driving pulse. Our trajectory-based analysis of phase-matching reveals several important advantages of using longer laser wavelengths: conversion efficiency can be improved by orders of magnitude, phase-matched build-up of harmonics can be achieved in a jet with a high gas pressure, and isolated attosecond pulses can be extracted from plateau harmonics. (c) 2007 Optical Society of America.