Gradient enhanced third harmonic generation in a femtosecond filament

Area of Science:

• Nonlinear Optics
• Laser Physics
• Plasma Physics

Background:

• Femtosecond laser filamentation in gases is a key phenomenon in nonlinear optics.
• Third harmonic generation (THG) is a crucial process for frequency conversion using intense laser pulses.
• Controlling phase-matching is essential for efficient THG in extended nonlinear media.

Purpose of the Study:

• To investigate the effect of controlled filament truncation on third harmonic generation.
• To explore methods for enhancing THG efficiency in femtosecond laser filaments.
• To understand the role of phase-matching in THG within a pressure-gradient environment.

Main Methods:

• Generating femtosecond laser filaments in air with a controlled pressure gradient from atmospheric to vacuum conditions.
• Abruptly truncating the filament at specific positions along its propagation.
• Measuring the generated third harmonic radiation's intensity, spectral shape, and conversion efficiency.

Main Results:

• An enhancement of the generated third harmonic radiation by 3 orders of magnitude was observed.
• The observed enhancement is attributed to improved on-axis phase-matching conditions.
• The spectral characteristics and conversion efficiency of THG were analyzed during filament propagation.

Conclusions:

• Controlled filament truncation using a pressure gradient is an effective method to significantly enhance third harmonic generation.
• Optimizing phase-matching conditions via pressure gradients is crucial for maximizing THG efficiency in laser-induced filaments.
• This study provides insights into controlling nonlinear optical processes in gas-phase systems.