| Typical quantum processes driven by intense laser fields, such as ionization and high-order harmonic generation, are difficult for theory because of the absence of a small parameter. Both the laser field and the atomic binding force are comparably strong. This problem is overcome by the so-called "Strong-Field Approximation" (SFA) and the associated concept of "quantum orbits," which occur naturally in a saddle-point evaluation of the transition amplitude. In effect, quantum orbits allow one to extract time-dependent information out of the S-matrix element. While each quantum orbit by itself is simple, their interference generates very complex spectra. The formalism of quantum orbits will be reviewed, and the results will be compared with experimental data. The formalism is not restricted to monochromatic fields but can also be applied to few-cycle pulses. |
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