Laser-induced fluorescence spectroscopy (LIFS) is a novel spectroscopic technique with several advantages, such as multi-elemental simultaneous detection, rapid response, and real-time online monitoring; hence, it is a powerful tool for remote sensing. However, single-beam laser filamentation remains limited, including the limitation of fluorescence signal intensity and detection sensitivity owing to the influence of intensity clamping. Several methods have been proposed to address these problems. For instance, the fluorescence signal can be enhanced by the interaction between two or more filaments, which is crucial in improving the sensitivity of substance detection. Previous studies about the interactions between co-propagation filaments often adopted multiple femtosecond pulses to form plasma gratings in the medium and increase the fluorescence signal intensity. However, such methods rely on the spatial interference effect of an ultrafast light field. They have high requirements for parameters such as the excitation beam’s spatial angle and pulse delay. To circumvent the aforementioned problems, we study the interaction mechanism of collinear counter-propagating filaments and their influence on fluorescence properties. Experiment results demonstrate that although its enhancement effect on fluorescence can be comparable to that of the co-propagating filament, it is almost not affected by the laser polarization. Applying this technology to the spectroscopic detection of metal ions in compounds is expected to provide a novel approach to detecting trace contaminants by studying the linear relationship between characteristic spectral intensity and substance concentration.

We establish a collinear counter-propagating filaments (CPF) system, which comprises two collinear beams propagating in the opposite direction and focusing on the same focus through a lens, thereby forming two spatially overlapping counter-propagating filaments near the focus. We make precise adjustments using the electronic translation platform to better control the relative pulse delay of the two light beams. The atomized sample interacts with the filaments to excite the fingerprint fluorescence and the fingerprint fluorescence is transmitted to a spectrometer using an optical fiber. In addition, we configure brine solution samples of KCl, Na₂SO₄, and MgSO₄ with different mass fractions to establish calibration curves and analyze the detection sensitivity of this system.

First, we compare the signal intensities induced by the CPF and single filament (SF) under the same total excitation pulse energy and successfully obtain an enhancement factor of approximately 4. Furthermore, we compare the relationship between the intensity of induced fluorescence and the pulse energy of the two filamentation systems. When the laser energy increases from 0.5 mJ to 1.5 mJ, the increase in the CPF excited signal is significantly higher than the increase in the SF excited signal; this indicates that the collinear counter-propagating filaments can obtain higher fluorescence intensity. In addition, we study the evolution of fluorescence signal intensity with time and infer that the filament interaction will prolong the fluorescence decay time of excited molecules. Simultaneously, by changing the relative pulse delay of the two lasers, fluorescence enhancement can be observed in the pulse delay range of -3-3 ps, with an excitation pulse width of 50 fs. Finally, we obtain the detection sensitivity of CPF by establishing the calibration curve of metal ions (K⁺, Na⁺, and Mg²⁺) in the brine solution. The experiment result demonstrates that the detection sensitivities of K⁺, Na⁺, and Mg²⁺ are 9.3×10^-6, 4.3×10^-6, and 16.7×10^-6, respectively, and their corresponding determination coefficients of the calibration curves are 0.99 (KCl), 0.98 (Na₂SO₄), and 0.97 (MgSO₄), respectively; this implies that the method adopted in this study exhibits optimal linearity in concentration measurement and can be utilized for the quantitative analysis of compound samples within a specific concentration range.

This study introduces a fluorescence spectroscopy detection technique induced by collinear propagation filaments. The interaction of counter-propagating filaments prolongs the fluorescence decay time of excited state molecules and enhances fluorescence signals. The fluorescence intensity excited by counter-propagating filaments is 4 times stronger than the fluorescence intensity excited by the single filament under the same pulse energy. Compared with the co-propagation filament-induced fluorescence enhancement technology, the proposed method has loose requirements for the delay control of the two excitation pulses. More importantly, this method can effectively improve the detection sensitivity of metal ions in compounds and exhibits optimal linearity; hence, it provides a novel approach to detecting trace contaminants and other applications.