Irregular Orbits Generate Higher Harmonics

The spectrum of higher harmonics in atoms calculated with a uniformized semiclassical propagator is presented and it is shown that the 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.