We investigate the phase matching of a high-harmonic generation by intense, 20 fs laser pulses in a gas-filled capillary waveguide. We identify several regimes in which the harmonic field can build up coherently: a balance of atomic and waveguide dispersions, noncollinear Cerenkov phase matching, and a balance of atomic and plasma dispersions. The role of atomic dispersion is examined by measuring and calculating the harmonic signal for several gases. We also predict and provide preliminary evidence for a regime where phase matching occurs only at specific fractional ionization levels, where the harmonic signal is sensitive to the absolute phase of the carrier wave.

• Received 9 March 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.2187