Laser-dressed ionic states in high-harmonic generation in helium

We report the experimental observation of an ionic resonance at 40.8 eV and two adjacent satellite sidebands in high-harmonic generation in neutral helium gas. The amplitude and asymmetry of the central line vary with different driving pulse intensities, which arises from the interference between the laser-dressed resonant emission from the 2p state of He⁺ ions and the nonresonant harmonic generation. The two satellite sidebands are identified as the virtual Floquet-like light-induced states located at ±ℏω_d from the 2s state of He⁺ ions (ω_d denotes the driving laser frequency), whose multipeak profiles are imprinted by the structured spectrum of the driving laser pulse. These results demonstrate the roles of both bright and dark states of ionic species in shaping high-harmonic spectra in the vicinity of resonances.