Effect Of Laser Polarization And Sample Temperature On Femtosecond Laser Induced Breakdown Spectroscopy

Laser induced breakdown spectroscopy(LIBS)is a kind of elemental detection and analysis technology based on the interaction between Laser and matter.The basic principle of LIBS is that high-energy pulsed laser converge on the sample surface through a focusing lens,which rapidly heats up the sample surface and evaporates microgram sample.These microgram samples generate high temperature and high pressure plasma in the instantaneous melting and gasification process.These plasma rapidly expands and radiates optical signals.Finally,these optical signals are collected by optical fibers and analyzed to obtain the elemental information of the sample.LIBS technology has many advantages,such as: convenient operation,low cost,wide detection range,etc.,which make LIBS technology play an important role in material element analysis.However,LIBS is not a perfect technology for the detection and analysis of material elements,and there are still some shortcomings,such as low detection sensitivity and low detection accuracy,which make the application of LIBS technology in some fields have certain limitations.At present,there have been many research achievements on improving the detection sensitivity of LIBS,such as preheated LIBS,nano-particle LIBS,spatially constrained LIBS,double-pulse LIBS,polarization enhanced LIBS,etc.Among them,preheating LIBS is a widely used spectral intensity enhancement method at present.It’s basic principle is that preheating target material can reduce the sample ablation threshold and the sample surface reflectivity,improve the coupling efficiency of laser and sample,increase the plasma injection density,and thus enhance the intensity of plasma emission spectrum line.In addition,changing the polarization of pulsed laser can also enhance the spectral intensity of plasma,because circularly polarized and linearly polarized pulsed laser have different acceleration mechanisms for free electrons.Linearly polarized femtosecond laser can alternately accelerate and decelerate free electrons periodically,while circularly polarized femtosecond laser can continuously accelerate free electrons.The same free electrons can gain higher velocity and kinetic energy in the femtosecond laser field with circular polarization.Thus,the spectral intensity of the plasma subjected to a circularly polarized femtosecond laser is stronger.In conclusion,LIBS spectrum can be enhanced by changing laser polarization and preheating target material.However,there are few experiments to study the effect of the combination of the two methods on LIBS spectrum.Therefore,the influence of laser polarization and preheating target material on LIBS spectrum is studied in this paper.This thesis consists of four chapters.The first chapter introduces the introduction of LIBS technology,research status,advantages and applications,and methods to enhance LIBS.The second chapter introduces the basic theory of LIBS,including the formation and mechanism of plasma,and two basic parameters describing plasma.In chapter 3,femtosecond laser-induced copper plasma spectra at different laser polarization and sample temperature are introduced.The results show that the enhancement effect of increasing sample temperature and changing laser polarization is much stronger than that of increasing sample temperature alone or changing laser polarization only.The fourth chapter summarizes the experimental results of this paper,and puts forward some suggestions and prospects for the future work of this research direction.