When the first-order radially polarized vortex beam propagates in an uniaxial crystal, the spin and the orbital angular momentum parts can be separated. It is called the optical spin-orbit Hall effect. In this study, we investigate the tight focusing of the radially polarized vortex beam theoretically and find the spatial separation of the spin and the orbital angular momentum parts occurs in the focal plane when the polarization order equals 1 and the vortex charge equals 1 (or -1). Moreover, when the initial phase of the polarization state takes π/2, the spatial separation of intensity in the focal plane corresponds to the spatial separation of the spin and the orbital angular momentum parts. This phenomenon can be considered as a manifestation of the optical spin-orbit Hall effect in the tight focusing of radially polarized vortex beam. Also, we show that, when the polarization order is greater than 1, the initial phase change of polarization state just leads to the rotation of the focal field and the spin and the orbital angular momentum density in the focal plane. Our results provide the potential application in the field of optical micro-manipulation.

The focusing position of the laser affects the characteristics of the laser-induced plasma, which in turn influences the signal stability and the quantitative analysis of laser-induced breakdown spectroscopy. In this study, six standard alloy steels are used as the samples, and the trace elements including Si, Mn, Cr are quantitatively analyzed by internal standard method. The variations of line intensity, background noise intensity, signal-to-background ratio (SBR), detecting sensitivity and detection of limit (LOD) of the element with lens-to-sample distance (LTSD) were investigated. The results show that both the intensities of the analytical line and the background noise reached a maximum when the LTSD is 94 mm. While the corresponding SBR is relatively low. For quantitative analysis, it is observed that the change of the detection sensitivity with LTSD has the similar trend with that of SBR. That is, the detection sensitivity has a lower value when the LTSD is 94 mm. However, when the LTSD changes from 93 mm to 99 mm, LODs of the three elements are maintained at a small value. The present results indicate that for quantitative analysis of LIBS, the intensities of the analytical line and the background noise should be comprehensively considered when optimizing the laser focusing position.

Laser focusing is an important parameter that affects the characteristics of laser-induced plasma. Focusing lenses with different F-numbers form different energy density distributions near the surface of a sample, thus affecting the characteristics of plasma. In this study, the plasma generated by a nanosecond laser ablation of a micro-alloy steel certified sample at 1 atm of air was investigated. We compare the spectrally integrated plasma images obtained at different defocusing distances for short- and long-focus lenses and investigate the optical emission spectra of laser-induced plasma on steel alloy by using focusing lenses with different F-numbers. With an increase in the defocusing distance, the plasma plume changes from flat to hemispherical and then splitting occurs. The spectral line intensity increases first and then decreases, then increases slightly, and finally decreases gradually. For the long-focus lens, when the focal point is above the sample surface, the laser beam strongly interacts with air over a longer distance, leading to longer air plasma and weaker sample plasma compared with the short-focus lens. Thus, the relative intensity of the second peak in the spectral line intensity, according to the defocusing distance, gradually decreases with increasing F-number. We also obtain two-dimensional spatial distributions of the spectral line intensity according to the F-number and defocusing distance. The optimal defocusing distances for all focusing lenses occur when the focal point is below the sample surface. The relation between the optimal defocusing distance and F-number follows a single- exponential decay function.

Classification of flowers is required to promote their appreciation, value, and application. In this study, laser-induced breakdown spectroscopy (LIBS) was applied in conjunction with principal component analysis (PCA) to identify and classify flower species. Three flowers of similar color or shape were selected as the experimental samples. Laser pulses were directed at the sample surface to form plasma, from which light was collected, and the spectra were analyzed to identify the surface material. The spectral data of 240 groups of experimental samples were obtained using LIBS. Spectral data were pretreated, and the 28 characteristic spectral intensity ratios of six characteristic elements (Na, Mg, Al, K, Si, Ca) of flower species after normalization were chosen to constitute a matrix (240×28) of spectral data. The data were compressed, after which their dimensionality was reduced to reduce the number of output variables of the model for PCA from 28 to 3. Finally, cluster analysis of the flowers was performed, and a standard sample database of flowers was established. Subsequently, a template matching model (TMM) was used as a model for training. The accuracy of the entire TMM model was 97%.