| Due to nonlinear effects such as Kerr self-focusing and intensity clamping,longer plasma filaments can be formed by the strong femtosecond lasers in the air.Plasma filaments can be used atmospheric remote sensing,artificial rainfall,and laser-induced lightning.Among them,the optimal control of plasma filament length and lifetime is a hot topic in current research.The control of plasma filament length and lifetime is very important for the practical application of laser-induced lightning.The variation of the length and lifetime of femtosecond laser filaments was investigated by adjusting the photon angular momentum parameters in air.Specifically,we controlled and adjusted the spin angular momentum and orbital angular momentum parameters of the photon angular momentum by using a λ/4 wave plate and a vortex half wave plate(topological charge number l =1,2).Plasma filaments were formed by focusing a femtosecond pulse laser with 2.2 m J energy using a convex lens,and then the length and life time of the filaments were studied based on time resolution and space resolution measurements of the 357.7 nm spectral line intensity of the plasma filament fluorescence radiation spectrum.The effects of photon spin and orbital angular momentum on the length and lifetime of femtosecond laser filaments are investigated systematically research the influence of photon orbital angular momentum on filament lifetime.First,it is concluded that when first order(l=1)or second order(l =2)vortex half wave plate is inserted in front of the lens under weak focusing(f = 50 cm)conditions,the spectral line strength of the filament is significantly weakened,and the filament lifetime is approximately shortened by 0.6 ns when second order(l =2)vortex half wave plate is used.However,under tight focusing(f =10 cm)conditions,there is no reduction in spectral line strength or filament lifetime after inserting the vortex half wave plates.It may be due to the lens focusing effect dominates the distribution of the filament energy pool under tight focusing conditions.Furthermore,we compare the length of filaments under laser of photon orbital angular momentum topological charge l = 0(using a lens alone)and topological charge l = 1,2(inserting a vortex half wave plate in front of the lens).The result show that when a first order(l = 1)or second order(l = 2)a vortex half wave plate is inserted in front of the lens,the intensity of the filament spectral line is weakened.Additionally,the main filamentation region moves significantly backward,while a weaker filament appears in front of it,leading extended filaments,possibly due to the conversion of the initial Gaussian beam into a hollow beam by the vortex half wave plate.Finally,we compare the formation of filaments under two incident conditions with photon spin angular momentum quantum numbers of 0(linearly polarized)and ±1(circularly polarized).The spatial resolution measurement results show that when the spin angular momentum quantum numbers s = 0,the intensity of the spectrum decreases by about27 %,while the length of the filament increases by 19.1 ～ 32.4 % compared to the case where s = ±1 for the spin angular momentum of the incident laser.We attribute this to the lower threshold for ionization of air when the incident laser is linearly polarized(s = 0)lower than the circularly polarized laser(s = ±1)condition. |
