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Harmonic generation in the Kramers-Henneberger stabilization regime.

We show how the Kramers-Henneberger (KH) frame can be used to interpret the generation of high-order harmonics in very strong high-frequency laser fields in which the ionization of the atom is suppressed. We show that although a single KH eigenstate produces little harmonic generation in a constant-amplitude laser field, a wave packet of bound KH states with a low ionization rate can result in a long comb of high-order harmonics. We verify that there is a close link between the production of strong even harmonics and the use of ultrafast laser-field turn-on times, and that the even-harmonic strength is reduced for larger and, therefore, perhaps more realistic rise times.