We show that interaction of an atomic target with a perturbative nonionizing ultraviolet (UV) pulse creates a short-lived virtual state (VS), which can be involved in the nonperturbative interaction of a target with an intense infrared (IR) field. In the example of the high-harmonic generation (HHG), it is shown that this VS provides an alternative HHG channel, which is coherently implicated in the IR-induced HHG amplitude and thereby leads to interference fringes in the HHG yield as a function of the time delay between UV and IR pulses. We discuss the VS-based enhancement of HHG yield and the use of interference fringes for retrieving the UV-pulse waveform.

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• Atomic & molecular processes in external fields
• High-order harmonic generation
• Nonlinear optics
• Ultrafast optics