Second-harmonic generation at Lyman-alpha in atomic hydrogen

Area of Science:

• Atomic physics
• Nonlinear optics
• Quantum mechanics

Background:

• Second-harmonic generation (SHG) is a nonlinear optical process.
• Atomic hydrogen is a fundamental system for studying atomic processes.
• Two-photon resonance enhances nonlinear interactions.

Purpose of the Study:

• Investigate SHG in atomic hydrogen.
• Examine the influence of an applied electric field on SHG.
• Understand the origin of observed SHG at zero field.

Main Methods:

• Tuning laser radiation to the 2S-1S two-photon resonance in atomic hydrogen.
• Applying an external electric field.
• Measuring second-harmonic radiation intensity.
• Calculating ion density and charge-separation field.

Main Results:

• Second-harmonic radiation at 121.6 nm was observed even at zero applied electric field.
• The intensity of SHG is attributed to a charge-separation field.
• This field is caused by ions produced via three-photon ionization.
• Calculated SHG intensity matches measured values based on ion density.

Conclusions:

• A charge-separation field in atomic hydrogen can drive second-harmonic generation.
• Three-photon ionization plays a crucial role in this process.
• SHG in atomic hydrogen is observable without external fields under specific conditions.