Temporal structure of attosecond pulses from intense laser-atom interactions

Area of Science:

• Atomic, Molecular, and Optical Physics
• Quantum Optics
• Nonlinear Optics

Background:

• High harmonic generation (HHG) is a key process in nonlinear optics, producing coherent X-ray radiation.
• Understanding the underlying mechanisms of HHG is crucial for applications in ultrafast science.
• Previous models often attributed spectral features to bremsstrahlung or assumed filled bowlike structures.

Purpose of the Study:

• To investigate the spectral and temporal characteristics of high harmonic generation.
• To elucidate the physical mechanism responsible for the observed bowlike structures in the time-frequency domain.
• To identify conditions for efficient attosecond pulse generation via stimulated recombination.

Main Methods:

• Spectrotemporal analysis of emitted high harmonic radiation.
• Theoretical modeling of electron-atom reencounters and interference phenomena.
• Investigation of the role of ionization level and laser pulse duration.

Main Results:

• A power-law spectrum I(omega) ~ omega(-3.3+/-0.25) was observed for high harmonics.
• Clear bowlike structures were identified in the time-frequency (t,omega) plane, corresponding to electron-atom reencounters.
• These structures are not filled, ruling out bremsstrahlung and indicating a resonant process termed stimulated recombination (SR).
• Stimulated recombination leads to highly efficient resonant emission of attosecond pulses at a specific frequency.

Conclusions:

• Stimulated recombination is the dominant mechanism for the observed spectral and temporal features in HHG.
• SR is a resonant process involving electron self-interference with the ground state atom.
• Efficient SR and attosecond pulse generation are favored by low ionization levels and few-cycle laser pulses.