Third harmonic generation in self-focused filaments in liquids

Area of Science:

• Nonlinear Optics
• Laser Physics
• Materials Science

Background:

• Femtosecond laser-matter interactions are crucial for understanding light propagation in media.
• Third harmonic generation (THG) is a key nonlinear optical process.
• Self-focusing influences high-intensity laser beam propagation and nonlinear phenomena.

Purpose of the Study:

• To investigate the relationship between third harmonic generation and self-focusing in liquids.
• To characterize the behavior of THG output power with varying input power.
• To explore continuum generation dynamics above the self-focusing threshold.

Main Methods:

• Utilizing 1300 nm wavelength femtosecond laser pulses for pumping.
• Employing the z-scan technique to measure nonlinear refractive index (n(2)) and deduce self-focusing thresholds.
• Observing and analyzing third harmonic generation and continuum generation phenomena.

Main Results:

• Third harmonic generation onset coincides with the self-focusing threshold in various liquids.
• Self-phase modulation and continuum generation observed above the threshold.
• Third harmonic output power demonstrated independence from input power in the THG regime.
• Measured conversion efficiency for THG was approximately 10^-6.

Conclusions:

• The observed THG in liquids is strongly correlated with self-focusing.
• The behavior aligns with phase-locking models typically applied to gas-phase nonlinear optics.
• Femtosecond laser-induced nonlinear phenomena in liquids exhibit complex dynamics beyond simple harmonic generation.