A generalized scheme for phase-conjugate resonant 2n-wave mixing, which has a high efficiency and is easy for phase matching, is proposed. As a new type of coherent laser spectroscopy this approach can be employed for studying highly excited atomic states or states with a high angular momentum. To demonstrate its feasibility we have studied the doubly excited autoionizing Rydberg states of Ba by phase-conjugate six-wave mixing, and have furthermore achieved eight-wave mixing in Na. This method may find wide application in related areas such as coherent transient spectroscopy, Autler-Townes spectroscopy and electromagnetically induced transparency. In particular, it may provide new insights into the nature of highly excited states.
Using a formal scattering theoretical approach, we develop a nonperturbative quantum electrodynamics theory to describe high-order harmonic generation ͑HHG͒. This approach recovers the semiclassical interpretation that HHG results from the recombination of photoelectrons, ionized by the laser field, with the parent ions, and gives the same phenomenological cutoff law. The HHG emission rate is expressed as an analytic closed form. We also discuss the connection between HHG and the above threshold ionization from the formal scattering viewpoint.
Based on the full quantum theory of high-order harmonic generation (HOHG) (Lewenstein et al 1994 Phys. Rev. A 49 2117-32), we study the effects of the static electric field on HOHG. It is found that the presence of the static electric field breaks the inversion and reflection symmetry, as a result, the HOHG spectrum exhibits a double-plateau structure. Furthermore, the cut-off of the HOHG no longer corresponds to the maximum kinetic energy the ionized electron can obtain from the laser field. Instead, it corresponds to the maximum kinetic energy of the electron with one return. Finally, for ultra-high static electric field, we find a monotonic decrease of the harmonic intensity as the harmonic order is increased.
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