  The vortex ratchet effect has been studied in Al films patterned with square
arrays of submicron antidots. We have investigated the transport properties of
two sets of samples: (i) asymmetrical antidots where vortices are driven by an
unbiased ac current, and (ii) symmetrical antidots where in addition to the ac
drive a dc bias was used. For each sample, the rectified (dc) voltage is
measured as a function of drive amplitude and frequency, magnetic field, and
temperature. As unambiguously shown by our data, the voltage rectification in
the asymmetric antidots is induced by the intrinsic asymmetry in the pinning
potential created by the antidots, whereas the rectification in the symmetric
antidots is induced by the dc bias. In addition, the experiments reveal
interesting collective phenomena in the vortex ratchet effect. At fields below
the first matching field ($H_1$), the dc voltage--ac drive characteristics
present two rectification peaks, which is interpreted as an interplay between
the one-dimensional motion of weakly pinned incommensurate vortex rows and the
two-dimensional motion of the whole vortex lattice. We also discuss the
different dynamical regimes controlling the motion of interstitial and trapped
vortices at fields slightly higher than $H_1$ and their implications for the
vortex ratchet effect.
