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

Area of Science:

• Atomic Physics
• Quantum Optics
• Nonlinear Optics

Background:

• High-harmonic generation (HHG) is a key process for producing extreme ultraviolet and X-ray radiation.
• Understanding the influence of atomic and ionic resonances on HHG is crucial for controlling emitted spectra.
• Previous studies have explored resonances, but the role of specific ionic states and light-induced states in HHG spectra requires further investigation.

Purpose of the Study:

• To experimentally investigate the ionic resonance and satellite sidebands in high-harmonic generation in neutral helium.
• To elucidate the underlying mechanisms, including interference effects and the role of ionic states, in shaping the HHG spectrum.
• To identify the nature of the observed satellite sidebands and their relation to ionic energy levels and the driving laser field.

Main Methods:

• Experimental setup for high-harmonic generation using neutral helium gas.
• Precise measurement of harmonic spectra with a focus on the 40.8 eV region.
• Varying driving pulse intensities to study their effect on spectral features.
• Theoretical analysis to interpret the observed resonance, sidebands, and interference phenomena.

Main Results:

• Experimental observation of an ionic resonance at 40.8 eV and two adjacent satellite sidebands in HHG from helium.
• The central resonance line's amplitude and asymmetry were found to vary with driving pulse intensity.
• Interference between laser-dressed resonant emission from He+ 2p states and nonresonant harmonic generation was identified as the cause.
• Satellite sidebands were identified as virtual Floquet-like light-induced states near the He+ 2s state.
• The multipeak profiles of the sidebands were attributed to the structured spectrum of the driving laser pulse.

Conclusions:

• The study demonstrates the significant role of both bright and dark ionic states in shaping HHG spectra near resonances.
• The observed phenomena provide insights into the complex interplay between atomic/ionic species and intense laser fields.
• This work contributes to a deeper understanding of HHG physics and opens avenues for controlling attosecond pulse generation.