  Several correlation and interference effects in strong-field physics are
investigated. We show that the interference of continuum wave packets can be
the dominant mechanism of high-harmonic generation (HHG) in the
over-the-barrier regime. Next, we combine HHG with resonant x-ray excitation to
force the recolliding continuum electron to recombine with a core hole rather
than the valence hole from that it was previously tunnel ionized. The scheme
opens up perspectives for nonlinear xuv physics, attosecond x-ray pulses, and
spectroscopy of core orbitals. Then, a method is proposed to generate
attochirp-free harmonic pulses by engineering the appropriate electron wave
packet. Finally, resonant photoionization mechanisms involving two atoms are
discussed which can dominate over the direct single-atom ionization channel at
interatomic distances in the nanometer range.
