Irregular orbits generate higher harmonics

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