Application Of Ultrafast Laser Technology In Laser-induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy（LIBS）is a material composition analysis technology,which has been widely used in aerospace,environmental testing,biomedicine,and industrial metallurgy.Compared to traditional material analysis techniques,LIBS technology has many advantages such as fast,real-time,non-contact,multi-element simultaneous analysis,etc.,but it is also affected by matrix effect and plasma shielding effect during analysis,and has the disadvantage of low detection sensitivity.At present,there are various auxiliary methods to improve the LIBS technology,among which the plasma-grating-induced breakdown spectroscopy（GIBS）technology based on ultrafast laser technology is a novel and excellent improvement method.In this paper,the GIBS technology was further studied and compared with the Filament-induced breakdown spectroscopy（FIBS）technology.In the process of soil analysis,we first observed that the spectral line signal intensity of the GIBS technique is about 2 times higher than that of the FIBS technique,but the enhancement effect will gradually disappear as the inter-pulse delay increases over 200 fs.By studying the influence of the sample position on the spectral line signal,it was proved that the spatial size of the plasma grating is shortened compared with that of the single filament.But the optimal excitation position in the GIBS system was always kept in the same region.In contrast,the optimal excitation position in the FIBS system moved toward the lens about 0.5 mm as the laser energy increased from 0.5 m J to 1 m J.Finally,in the study of quantitative detection of Cr element concentration in soil,the limit of detection（LOD）in FIBS is calculated to be 55 ppm,while that of GIBS is 29 ppm,which proves that GIBS has better detection ability of trace elements.In addition,the multidimensional-plasma-grating-induced breakdown spectroscopy（MIBS）technique was proposed to improve the analysis effect.First,it was proved that three non-collinear and non-coplanar filaments in the constructed MIBS system successfully interact simultaneously to form a two-dimensional plasma grating.And it was proved that the two-dimensional plasma grating acts in the excitation process of the sample,rather than the simple addition of the three filaments alone to excite the sample.Compared with the GIBS system,it was found that the MIBS system can further increase the spectral line intensity by about 2 times,and prolong the plasma lifetime by 20%,and the excitation effect of the sample is effectively improved.Finally,in the quantitative detection of Mn in soil,it was calculated that MIBS technology can reduce the detection limit from 394 ppm of GIBS to 306 ppm.Finally,we proposed the filament-plasma-grating induced breakdown spectroscopy（F-GIBS）technique,and used the liquid jet method to detect liquid samples.Firstly,we determined the detection delay of 100 ns by multi-dimensional reference with parameters such as the spectral line intensity,signal-to-noise ratio（SNR）and relative standard deviation（RSD）.In the research on the influence of the inter-pulse delay of the plasma grating and the filament on the spectral line intensity,an optimal inter-pulse delay of ±50 ps was found to maximize spectral line intensity.In addition,the intensity of spectral line obtained at the 0 ps inter-pulse delay is much higher than the sum of the spectral line signals of FIBS and GIBS.Finally,by diagnosing the air plasma fluorescence,we found that when the three coplanar filaments interacted synchronously,a new plasma grating structure would appear,and the maximum power was obtained at this time.With the increase of the inter-pulse delay,the power density decreases,but it will rebound with an inter-pulse delay of around ±50 ps.Therefore,there are two mechanisms of double-pulse excitation and nonlinear coupling of plasma gratings to optical filaments in the excitation of liquid samples by F-GIBS technology.