Analytical model for THz emissions induced by laser-gas interaction

Area of Science:

• Plasma Physics
• Laser-Matter Interactions
• Terahertz (THz) Science

Background:

• Laser-driven ionization of gases can generate broadband electromagnetic radiation.
• Understanding THz emission mechanisms is crucial for applications in spectroscopy and imaging.
• Previous models often simplify the complex interplay of ionization and plasma dynamics.

Purpose of the Study:

• To develop a one-dimensional model for THz emissions in laser-driven hydrogen gas.
• To analyze the scaling of THz fields with laser and gas parameters.
• To elucidate the physical mechanisms behind THz wave generation and spectral characteristics.

Main Methods:

• Development of a 1D theoretical model for laser-induced ionization and current oscillations.
• Derivation of analytical expressions for transmitted and reflected THz fields.
• Comparison of model predictions with results from particle-in-cell and propagation codes.

Main Results:

• Complex, non-monotonic scaling of THz fields with respect to laser parameters identified.
• Backward THz wave primarily driven by electron current oscillations below plasma frequency.
• Transmitted THz wave shows contributions from plasma currents and photo-ionization across different frequency ranges.

Conclusions:

• The model provides analytical insights into THz spectra observed in simulations.
• The interplay between plasma current oscillations and photo-ionization governs THz emission.
• The ratio of propagation length to plasma skin depth is critical for understanding emission characteristics.