High-harmonic generation and parametric amplification in the soft X-rays from extended electron trajectories

Area of Science:

• Quantum optics
• Attosecond science
• Soft X-ray generation

Background:

• High-order harmonic generation (HHG) typically produces odd-numbered harmonic lines.
• Existing HHG theory involves electron trajectories lasting less than one optical cycle.

Purpose of the Study:

• To report the first observation of a novel high-order harmonic spectrum.
• To investigate the underlying quantum mechanical mechanisms responsible for this unusual spectral property.

Main Methods:

• Generation of high-order harmonics in the 200 eV to 1 keV spectral range.
• Analysis of spectral properties and comparison with theoretical predictions.
• Investigation of electron trajectory dynamics and quantum path interference.

Main Results:

• Observation of a high-harmonic spectrum with only every 4th harmonic line (4i + 1).
• Identification of two dominant extended electron trajectories contributing to the radiation.
• Demonstration that these trajectories satisfy the resonance condition for X-ray parametric amplification.

Conclusions:

• The unusual spectral property arises from quantum path interference of extended electron trajectories.
• Extended electron trajectories, accelerated over multiple optical cycles, are crucial for this phenomenon.
• This finding opens new avenues for controlling and generating tailored soft X-ray radiation.