Experimental Study On Enhancement Of Femtosecond Laser-induced Breakdown Spectroscopy By Gold Nanostructures

Laser induced breakdown spectroscopy(LIBS)is a very popular element analysis technique.Because LIBS has many unique advantages,LIBS technology has been widely used in various fields.By combining the advantages of femtosecond laser,the femtosecond laser induced breakdown spectroscopy(fs-LIBS)has become a hot research topic.However,compared with the nanosecond LIBS and other analysis techniques,the ordinary fs-LIBS technology has obvious disadvantages: weak signal strength.Based on the principle of spectrum signal of LIBS enhanced by metal nanoparticles(NELIBS),LIBS spectrum signal of insulator is enhanced by combining femtosecond double pulses.What's more,LIBS signal enhanced by other metal nanostructures is studied.The innovations and main results of this thesis are summarized as follows:(1)The enhanced LIBS signal of crystal silica samples by metal nanoparticles made by thermal dewetting is studied.By considering the Keldysh theory and the Boltzmann formula,the enhancement mechanism is discussed.The experimental results show that the gold nanoparticles prepared by the 7.5 nm gold film have the best enhancement effect,and the enhancement effect decreases with the increase of laser energy.The estimation of the plasma temperature by the Boltzmann diagonal line method and plasma electron density by Stark broadening further interprets the mechanism of LIBS signal enhancement of gold nanoparticles.(2)Significant enhancement of LIBS signal of the crystal silica sample is obtained by combining nanoparticles and femtosecond double pulses,which is explained by re-heating theory.The experimental results show that,under our experimental conditions,the spectral line intensity and the enhancement effect are the best when the pulse delay is 120 ps,and the enhancement effect decreases with the increase of laser energy.It is also found that the distribution and size of the gold nanoparticles have little effect on the LIBS signal in the case of double pulses.The images of plasma under different conditions are collected directly in our experiment,which is more intuitive to show the enhancement of the double pulse and gold nanoparticles to the plasma.(3)The fs-LIBS signal is enhanced by gold nanofilms on the dielectric and silicon samples for the first time.The results show that,except metal sample,the LIBS signal is enhanced for the silica,ceramic and silicon sample.The experimental results show that the LIBS signal intensity is the strongest for gold film less than 10 nm.Compared with the enhancement of LIBS signal by gold nanoparticles,the enhancement effect of gold nanoparticles is better at low laser fluence,while the enhancement effect of Au films is better at high laser fluence.