We generate high-order harmonics by using an 800-nm fundamental pulse whose polarization evolves with time. Controlling the ellipticity modulation of the fundamental field allows us to continuously confine the harmonic emission from an estimated minimum value of 7 fs $\left(1\mathrm{}\mathrm{fs}{=10\mathrm{}}^{-15}\mathrm{s}\right)$ up to more than the 35 fs input pulse duration. Depending on the observed harmonic, the harmonic spectrum can show either a narrowing or a broadening when the ellipticity is changed in good agreement with an effective confinement of the high-order harmonic generation.

• Received 10 April 2003

DOI:https://doi.org/10.1103/PhysRevA.68.043804