Radio frequency (RF) emissions in the range of 30 ~ 800 MHz from laser induced air plasma by a 532 nm nanosecond laser are investigated. The RF emissions from air plasma induced by different laser energies and laser polarization are obtained. It is noted there is no consistency of the radio frequency emission with the change of laser energy. Unlike the optical emissions from plasma, which origins from electron transition between energy levels, RF radiation generates from oscillation of electric dipoles in plasma. The space distribution of the electric dipoles in plasma is not symmetrical along the laser propagation direction. As the laser parameters change, the distribution of the electric dipoles varies, so the radio frequency emissions do not change continuous. The RF signal of air plasma is found to depend on laser polarization directions and laser energy. The amplitudes of RF emissions are observed first increase and then decrease with further increase of laser energy, which is due to higher of ionization degree and electron density at larger laser energy, thus made the RF radiation quickly decay. The dominant frequencies and amplitude of RF emissions were observed vary with the laser polarization direction, and it is found that the maximum amplitude of the output of RF emissions were detected when the polarization direction of laser beam is along the axis of the antenna and minimum when the polarization direction of the laser beam is perpendicular to the axis of the antenna. Potential physical mechanism responsible for laser parameter dependent on RF emission, rich emission lines of air plasma was discussed.
Compared with the traditional spectral analysis methods, such as inductively coupled plasma mass spectrometry method, atomic absorption spectrometry method, the analysis sensitivity, accuracy and spectral resolution of the laser induced breakdown spectroscopy technology is relatively lower. Due to the advantages o f low ablation thresholds, high-spatial resolution, minimal invasion, high-efficiency transportation of femtosecond laser, the femtosecond laser induced breakdown spectroscopy method (fs-LIBS) has become an active topic in recent years. In order to further improve the analysis performance of fs-LIBS, the spatial confinement method is proposed. In this paper, the cavity confinement enhancing effect of fs-LIBS is discussed. Based on the local thermal equilibrium condition (LTE) assumption, the plasma temperature and electron density is obtained. The results shown that the plasma emission intensity, plasma temperature and electron density are improved under the given cavity constraints. In effect, the plasma generated shock wave encounters cavity barriers during its expansion, the shock wave is reflected back to the plasma center. One hand is improved the plasma temperature and electron density, on the other hand is increased the number of particles in the upper energy level, which leads to an increase in the intensity of the plasma emission spectrum. In general, the spatial confinement method combined with the fs-LIBS showed its great potential in improving the figures-of-merit of ultrafast optical LIBS technology.