Research On Controllable Filament Arrays Generated By Femtosecond Vector Beams Based On Polarization Filtering

When intense femtosecond laser pulses propagate in transparent media,multiple filaments with disordered distribution will be generated due to the random noise and competition of background energy around the filamentation.How to control the filaments has become a hot issue,because regular filaments is highly desired in many applications,such as the compression of laser pulse,white-light radar and microwave guiding.The realization of organized filaments in transparent media is a great challenge in the filamentation field,which generally needs complicated design on the light field,and is also difficult to control the filament arrays rotating and determine the filaments number at the same time.The amazing traits of vortex beams provide a new inspiration to the research of various optical and physical phenomena,and many meaningful applications of vortex beams have been put into practice,including optical tweezers,quantum entanglement,multi-waveguide,and optical communication.In recent years,the filamentation phenomenon of vortex beams has also attracted a lot of attention.Compared with Gaussian beams,the filaments generated from the femtosecond vortex beams embody some differences,especially in the on-axis fluorescence intensity and the white-light supercontinuum generation.It is commonly believed that the structured beams should have a lower intensity of filamentation and supercontinuum radiation than that of Gaussian beams due to their weaker ability to anti-turbulance.However,in our previous work,we find that the Gaussian beam has a longer filament,weaker fluorescence intensity and narrower supercontinuum extension than the 1-order vector polarized beams under a specific circumstance.It is conflict with the common belief,the reason of which is still unclear.In this work,we propose a convenient method to generate regularly distributed multiple filaments of femtosecond vector laser beams in fused silica by simply filtering the polarization of the vector beams with a polarizer.The filaments are found to be evenly distributed in the ring of the vector beam,forming a filament-necklace which remains stable along the propagation direction.The filaments number is determined by the topological charges of the vector beams.Furthermore,the pattern of filament-necklace can be continuously rotated by rotating the polarizer.This method provides a simple and effective way to generate stable filament-necklace in transparent media and to rotate the pattern to any angle through exterior regulation.By polarization filtering,we subtly obtain uniformly distributed filament arrays and achieve stable transmission of not less than 20 mm without complex optical field design and complicated optical instruments,and the filament arrays can also be controlled to rotate at arbitrary angles based on its polarization properties.We systematically measured the filamentation intensity and the resulting white light supercontinuum spectrum of strong Gaussian femtosecond laser pulses with first-,second-,and third-order vortex femtosecond laser pulses at different input energies and different focusing distances in the experiment.The filaments generated by Gaussian beams have a relatively longer but weaker propogation compared with that of vortex beams.Furthermore,a broader supercontinuum radiation can be obtained by vortex beams rather than Gaussian beams when input energy is large enough.Related theoretical studies also show that in the case of self-focusing,the vortex beam is able to produce filaments with higher peak intensity than the Gaussian beam,which is consistent with our experimental results.Our work provides a result completely differing from the common sense that the structured beams tend to have a weaker filamentation and narrower supercontinuum radiation than that of Gaussian beams,and will be helpful to the understanding of the propagation of femtosecond laser beams in air.