Generation of attosecond pulses in macroscopic media

We describe theoretically the generation of ultrashort (subfemtosecond) pulses using high-order harmonics of a laser pulse with a time-dependent degree of ellipticity. The single-atom response is calculated by using a low-frequency strong-field approximation. Propagation effects are taken into account using a method going beyond the slowly varying envelope approximation. Propagation modifies significantly the results obtained in the single-atom response and, in certain conditions, makes the generation of one attosecond pulse possible. We discuss prospects for the observation of these ultrashort pulses.