Description
When ultrafast laser pulses with sufficient peak power propagate through transparent media, laser filamentation arises from the dynamic interplay between self-focusing and photoionization-induced plasma defocusing. The plasma can be described as an initial primary photoionization-driven plasma and a secondary plasma, governed solely by plasma kinetics. As modern laser systems reach higher repetition rates, interpulse times approach plasma-kinetic timescales, and accumulation effects may arise [1]. As such, simulation and characterization of plasma-chemical kinetics in laser-induced filamentation are becoming increasingly important to account for the plasma’s molecular composition and its influence on optical properties.
In this work, we investigate how accumulation effects at elevated repetition rates affect photochemical reactions and alter the molecular composition of filament-induced plasmas. We measure the time-dependent plasma dynamics and key parameters—including gas temperature, electron temperature, electron density, and species-specific decay times—for filaments generated at a repetition rate of 10 kHz.
The experimental setup employs a Yb-based thin-disk regenerative amplifier (DIRA 500-10, TRUMPF Scientific Lasers) delivering sub-picosecond pulses at a central wavelength of 1030 nm and pulse energies of 2.5 mJ, 5 mJ, and 10 mJ. The pulses are tightly focused to promote filamentation and plasma ignition. Plasma characterization is performed using optical emission spectroscopy with an echelle spectrometer coupled to an intensified charge-coupled device (ICCD) camera, providing a resolution of 500 ps.
Analysis of the vibrational distributions and temporal evolution of molecular nitrogen emission bands enables identification of the excitation mechanisms responsible for nitrogen photoemission [2]. Electron density, electron temperature, and gas temperature are quantitatively determined from emission spectra and optical filament images within a collisional-radiative modeling framework, which was adapted from the diagnosis of electrically driven plasmas [2].
This diagnostic approach complements conventional optical probing techniques commonly used in filamentation studies [3], offering a comprehensive characterization of the plasma wake from a single measurement. It provides insight into changes in optical properties caused by the formation and extinction of molecular species. To our knowledge, this represents the first study of laser-filament-driven plasma at a 10 kHz repetition rate, where plasma wake characterization becomes particularly important due to reduced interpulse times and potential accumulation effects.
References
[1] T.-J. Wang, M. H. Ebrahim, I. Afxenti, D. Adamou, A. C. Dada, R. Li, Y. Leng, J.-C. Diels, D. Faccio, A. Couairon, C. Milián, and M. Clerici, "Cumulative Effects in 100 kHz Repetition‐Rate Laser‐Induced Plasma Filaments in Air," Adv. Photonics Res. 4, 2200338 (2023).
[2] S. Gröger, M. Fiebrandt, M. Hamme, N. Bibinov, and P. Awakowicz, "Characterization of a transient spark micro-discharge in nitrogen using simultaneous two-wavelength diagnostics," Meas. Sci. Technol. 31, 075501 (2020).
[3] Y.-H. Chen, S. Varma, T. M. Antonsen, and H. M. Milchberg, "Direct Measurement of the Electron Density of Extended Femtosecond Laser Pulse-Induced Filaments," Phys. Rev. Lett. 105, 215005 (2010).