  The spectrum of higher harmonics in atoms calculated with a uniformized
semiclassical propagator is presented and it is shown that higher harmonic
generation is an interference phenomenon which can be described
semiclassically. This can be concluded from the good agreement with the quantum
spectrum. Moreover, the formation of a plateau in the spectrum is specifically
due to the interference of irregular, time delayed, trajectories with regular
orbits without a time-delay. This is proven by the absence of the plateau in an
artificial semiclassical spectrum generated from a sample of trajectories from
which the irregular trajectories (only a few percent) have been discarded.
