Phase matching of high-order harmonics in hollow waveguides

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