Objective Laser-induced breakdown spectroscopy (LIBS) is a novel atomic-emission spectroscopic technique that has been widely used for the elemental analysis of materials. Investigators in the field of LIBS research are actively working to improve the sensitivity of this technique, and many methods have been proposed to enhance its spectral intensity. With the development of chirped-pulse-amplification technology, femtosecond pulse lasers have been introduced into the study of LIBS. Compared with nanosecond lasers, femtosecond lasers have many advantages for LIBS, but it is important to improve their spectral intensity. In general, the output laser beam of a femtosecond laser system is linearly polarized. In a linearly polarized laser field, electrons undergo alternating acceleration and deceleration in each optical period of the laser pulse. However, femtosecond lasers with circular polarization can accelerate electrons continuously, so they attain higher energies. The energies of electrons after irradiation with a circularly polarized femtosecond laser beam are different from those of electrons after irradiation with a linearly polarized femtosecond laser beam. This makes the optical signal from a plasma induced by a circularly polarized femtosecond laser beam different from that induced by a linearly polarized femtosecond laser beam. It is therefore necessary to compare the plasma emission induced by a femtosecond laser under both linear and circular polarizations.
Methods We focused a femtosecond laser beam onto the surface of a brass sample to produce plasmas, and we analyzed the resulting plasma spectra. To compare the effects of linear and circular polarizations on the spectral-emission intensity, we adjusted the polarization of the femtosecond laser beam from linear polarization to circular polarization by using a quarter-wave plate. The sample was placed on a three-dimensional translation table to avoid over-ablation. We collected the light emission from the laser-induced plasma and focused the collected light into an optical fiber, which transmitted it to a spectrometer. The light dispersed by the spectrometer was detected with an intensified charge-coupled device (ICCD). The ICCD was synchronized using an electrical signal from the synchronization-and-delay generator of the femtosecond laser system. Each spectral datum was an average of 50 laser shots. The whole experiment was carried out in air.
Results and Discussions We first compared the time-integrated spectra from femtosecond-laser-induced brass plasmas obtained under circular and linear polarizations. The spectral intensities of Zn (I) and Cu (I) obtained with a circularly polarized laser beam were higher than those obtained with a linearly polarized laser beam, and the spectral intensity increased by about 15%. An important difference between linear polarization and circular polarization in the femtosecond laser field is that the kinetic energies of electrons subject to differently polarized laser fields are different. Under linear polarization, electrons undergo alternating acceleration and deceleration in each optical period of the pulse, so they attain low kinetic energy. In contrast, electrons are always accelerated under circular polarization, so they attain high kinetic energy. Second, we measured the time-resolved spectra of femtosecond LIBS. The time-resolved peak intensities of Cu (I) at 510.55nm and Zn (I) at 472.21nm under circularly polarized laser are higher than that under linearly polarized laser, and the atomic lines in circularly polarized laser-induced plasmas persist longer duration. We thus find that circularly polarized femtosecond pulsed laser irradiation can produce stronger plasmas, which emitting stronger time-resolved spectra during the process of plasma decay. Laser polarization thus plays an important role in femtosecond LIBS. Finally, we calculated the time-resolved electron temperature and density under irradiation with circular and linear polarizations based on the Boltzmann plot and Stark broadening. The changes in the electron temperature and density are similar to the changes in the spectral intensity. Electrons under circularly polarized laser collide with atoms or ions, leading to higher electron temperature and density than those under linearly polarized laser. The higher-energy electrons transfer more energy to the lattice to produce stronger plasmas.
Conclusions We produced plasmas on the surfaces of brass samples by using focused femtosecond pulse laser, and we measured the spectral lines of Zn (I) and Cu (I) emitted from the plasmas under linear and circular polarizations. The results show that the spectral intensities under circularly polarized femtosecond laser were higher than those under linearly polarized femtosecond laser, and the spectral intensities increased by about 15%. We also measured the time-resolved spectra of linearly and circularly polarized femtosecond-laser-induced brass plasmas. For the same laser energy, atomic lines in the plasma produced by the circularly polarized laser persisted longer duration than those produced by the linearly polarized laser. Compared with linear polarization, the electron temperature and density obtained with circular polarization were also higher. This is because a circularly polarized laser can accelerate the electrons continuously, so the kinetic energy of electron produced by a circularly polarized femtosecond laser are higher than that produced by a linearly polarized femtosecond laser. Electrons with higher kinetic energy collide in the plasma to produce higher electron temperature and density, which thus emit a higher spectral intensity. Therefore, we can improve the signal intensity of femtosecond LIBS by adjusting a femtosecond laser from linear polarization to circular polarization. We expect this work to be useful for the study of femtosecond LIBS.
