"This achievement has led to a revolutionary technological breakthrough, that is, the introduction of an electric field to control the femtosecond laser filamentation process in the air, resulting in a stable supercontinuous light source with high repetition rate and high intensity.
It is a major milestone in the field that has had a profound impact on the development of fluorescence microscopy, lidar, biomedical imaging, optical coherence tomography, and attosecond pulse technology. ”
One of the reviewers gave this high recognition to the new paper published in Light: SCience & Applications by the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences.
（来源：Light: SCience & Applications）
In this study, the research group proposed a method to effectively improve the pointing stability of both high-repetition femtosecond filaments and supercontinuum (SC) white light sources.
Through the manipulation of the external electric field, the team solved the international problem of filament self-jitter in the atmospheric filamentation process of high-repetition frequency femtosecond laser.
That is, a high-energy, high-intensity, high-repetition frequency (1kHz) supercontinuum white light laser light source with stable pointing and intensity is generated in the air.
Figure | Position distribution of filaments (a-d) and corresponding forward white light (e-h) spots at different high pressures (Source: Light: SCience & Applications)
According to reports, by applying an external DC electric field, the plasma recombination in the filament can be effectively suppressed, thereby significantly prolonging the life of the plasma.
In this way, the heat deposited in the air due to plasma recombination can be significantly reduced between adjacent pulses, which in turn reduces the intensity of the disturbance of the thermally induced gas flow.
Under this high electric field environment, the stable ionic wind generated by corona discharge can also effectively overcome the self-disturbing airflow of the filament.
In the experiment, the spatial pointing stability of the high-repetition frequency filament and the resulting forward supercontinuum white light was improved by at least two times when a high-voltage DC electric field was applied to the 1kHz femtosecond laser air filament.
This not only confirms the important role of external DC electric field in the control of high repetition rate optical filaments, but also provides new opportunities for high repetition frequency optical filaments in spectroscopy, processing, and other associated radiation sources.
Overall, this study demonstrates cutting-edge results on the effect of direct current electric field on laser filaments in air. The research group found that with the increase of the DC electric field strength, the spatial jitter phenomenon of the laser filament was significantly stabilized.
During this time, they also delved into the modulation effect of electric fields on femtosecond filaments, and revealed a fascinating new phenomenon through experimental research.
In doing so, they not only succeeded in identifying a unique mechanism, but also found that it can significantly reduce the filamentation process and the airflow disturbance caused by the self-heating effect during pulse propagation.
Moreover, the team cleverly constructed a theoretical model to explain the observed phenomena, thus revealing the physical nature behind them.
Electric field-assisted high-repetition frequency femtosecond filaments and spatially stable supercontinuum radiation are expected to play an important role in several fields:
First, it can be used for high-precision optical measurements.
With electric field-assisted femtosecond filaments, precise measurements can be made on very small temporal and spatial scales. This provides a powerful tool for temporal resolution measurements in physics, chemistry, and biology experiments.
Second, it can be used for material processing.
Electric field-assisted femtosecond filaments are suitable for fine material processing due to their extreme precision and control. It enables higher machining accuracy without destroying the area around the material.
Third, it can be used for substance detection.
Supercontinuum radiation, second harmonics, third harmonics, air lasers, THz light sources based on high repetition frequency filaments can be used for spectral analysis of multi-phase and multi-material components.
The high-repetition frequency gas filamentation controlled by the external electric field can improve the stability and transmission efficiency of the above-mentioned associated radiation sources, thereby improving the sensitivity and accuracy of related applications.
It is understood that the principle of supercontinuous white light is to shoot ultrashort laser pulses into a transparent medium. Due to a series of nonlinear effects, the outgoing laser spectrum is greatly broadened, resulting in white light with a continuous spectral distribution.
Due to its ultra-wide spectrum and coherent characteristics, supercontinuum white light laser has been widely used in optical parameter amplification, optical pulse compression, dynamic characterization of laser-induced structural transitions, atmospheric remote sensing detection and analysis, and white light lidar.
There are many media that can produce supercontinuous white light, among which solid materials or photonic crystals are more common. However, due to the limitation of the damage threshold of solid media, it is not possible to produce high-energy superCW white light lasers.
This limitation can be overcome by ionizing filaments in a gaseous medium with an ultrafast and ultra-intense pulsed laser. In the process of gas filamentation, the laser spectrum can be significantly broadened due to the nonlinear effect mechanisms such as Kerr effect self-phase modulation, self-steepening effect and four-wave mixing.
However, with the development of high repetition rate laser technology, the advantages of high repetition frequency filament have gradually emerged.
For example, the use of high-repetition frequency filaments to achieve super-continuous white light radar detection not only has a higher response speed, but also can easily achieve high frame rate scanning for spatial positions.
However, for high-repetition frequency gas filaments, due to the deposition of laser energy in the gas, the thermal diffusion of gas molecules will cause the air flow to move, so that there will be turbulence in the laser transmission path, resulting in the beam deflection of the next laser pulse.
This in turn leads to poor filament directivity, which seriously affects the intensity and spatial stability of supercontinuous white light generated by gas filamentation, and also adversely affects related applications.
Based on this, in order to explore the wider application of femtosecond filament, the team initiated research on the intersection of light field and high-voltage electric field, and carried out a large number of experimental work on femtosecond filament-induced corona discharge.
To their surprise, the electric field has a significant effect on improving the stability of the filament-guided forward white light, a phenomenon that was observed by chance in experiments. Upon discovering this, they quickly decided to delve into the phenomenon.
Later, they began with an in-depth literature survey to learn about the interactions between light fields, plasma, and electric fields.
Subsequently, a large number of experiments were carried out to explore how to accurately control the electric field, how to observe and measure optical filaments and supercontinuum radiation, etc.
With the accumulation of data, they found some new patterns: as the applied DC high voltage increases, the spatial stability of the optical filament and the forward supercontinuum radiation will continue to improve. However, when the high pressure reaches a certain threshold, it does not change.
In this process, the spectral intensity of supercontinuum radiation is also increased. Therefore, starting from the origin of femtosecond laser air filamentous thermoinduced jitter, the research group established a theoretical model to explain the physical mechanism of this phenomenon.
最终，相关论文以《通过电场辅助飞秒激光在空气中成丝产生 1 kHz 稳定、强烈的超连续谱光》（Stable, intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air）为题发在 Light: SCience & Applications（IF 19.4）。
图 | 相关论文（来源：Light: SCience & Applications）
Yaoxiang Liu is the first author, and researchers Tiejun Wang, Yuxin Leng and Ruxin Li from the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences serve as co-corresponding authors [1].
图 | 王铁军（来源：王铁军）
It is also reported that when the ultra-strong ultrafast laser passes through the nonlinear gas medium, it will induce strong nonlinear optical effects, such as harmonic generation and parametric amplification, which can be used to generate secondary radiation.
It is important to note that the characteristics of these secondary radiations are determined by the filament properties, which can be precisely controlled by the electric field.
Therefore, in the next step of the study, they plan to use the interaction between electric field and light field, based on the existing research results of the research group, to further explore how to use the electric field to control the characteristics of high repetition frequency optical filament as a secondary radiation source, and explore related new phenomena, new mechanisms and new applications.
Resources:
1.Liu, Y., Yin, F., Wang, TJ.et al. Stable, intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air. Light SCi Appl 13, 42 (2024). https://doi.org/10.1038/s41377-023-01364-3
Operation/Typesetting: He Chenlong