  High-order harmonic generation by few-cycle 800 nm laser pulses in neon gas
in the presence of a strong terahertz (THz) field is investigated numerically
with propagation effects taken into account. Our calculations show that the
combination of THz fields with up to 12 fs laser pulses can be an effective
gating technique to generate single attosecond pulses. We show that in the
presence of the strong THz field only a single attosecond burst can be phase
matched, whereas radiation emitted during other half-cycles disappears during
propagation. The cutoff is extended and a wide supercontinuum appears in the
near-field spectra, extending the available spectral width for isolated
attosecond pulse generation from 23 to 93 eV. We demonstrate that phase
matching effects are responsible for the generation of isolated attosecond
pulses, even in conditions when single atom response yields an attosecond pulse
train.
