RESUMEN
Spatiotemporal (ST) wave packet carrying pure transverse orbital angular moment (OAM) with subwavelength spatial size has attracted increasing attentions in recent years, which can be obtained by tightly focusing a linear superposition of ST vortices with different topological charges. In this work, numerical models are proposed to explore the impact of the pulse width of the ST vortex on the characteristics of its focal field. We demonstrate that the rigorous model for calculating the focused ST wave packet is essential for ultrashort optical pulse, while the simplified model has the advantage of high efficiency but can only provide credible results when the pulse width of the illumination is long enough. Specifically, when the pulse width decreases from 100 fs to 5 fs, the accuracy of the simplified model would decrease significantly from 99% to 65.5%. In addition, it is found that the pulse duration would still lead to the collapse of transverse OAM structure near the focus of a high numerical aperture lens, even though the ST astigmatism has already been corrected. To analyze the physical mechanism behind this distortion, Levenberg-Marquardt algorithm is adopted to retrieve the OAM distribution of the focal field. It is shown that the contributions from undesired OAM modes would become nontrivial for short pulse width, leading to the formation of the focal field with hybrid OAM structures. These findings provide insight for the focusing and propagation studies of ultrashort ST wave packets, which could have wide potential applications in microscopy, optical trapping, laser machining, nonlinear light-matter interactions, etc.
RESUMEN
As the essential properties of organisms, detection and characterization of chirality are of supreme importance in physiology and pharmacology. In this work, we propose an optical technique to sort chiral materials by use of longitudinal polarization vortex (LPV) structures, which is generated with tightly focusing Pancharatnam-Berry tailored Laguerre-Gaussian beam. The nonparaxial propagation of the focusing field leads to the creation of multiple pairs of dual LPV structures with arbitrary topological charge and location, which can be independently controlled by the spatial phase modulation applied on the illumination. More importantly, the opposite spin angular momentums carried by each pair of dual foci lead to different energy flow directions, making it suitable to sort nanoparticles by their handedness. In addition, the LPV structures would also bring different dynamic behaviors to the enantiomers, providing a feasible route toward all-optical enantiopure chemical syntheses and enantiomer separations in pharmaceuticals.
