| As the intensity and peak power of laser pulses continue to increase,modulation techniques for high-power lasers become increasingly important.Traditional optical components based on solid materials are no longer applicable for high-power lasers when the thermal damage threshold is exceeded.Compared with the traditional solid optical materials,plasma owns the advantages of high damage threshold,wide parameter ranges and excellent electrical and thermal conductivities,while numerous mechanisms of laser-plasma interaction(LPI)endow it with rich optical properties.Therefore,plasma-based optical devices have shown unique advantages in modulating high-power lasers in recent years,forming the research topic of plasma optics with wide application prospects in various fields like high-power laser science,laser inertial fusion,plasma astrophysics,etc.Laguerre-Gaussian(LG)mode is a special laser propagation mode that can be described by the complete set of eigensolutions of the Helmholtz equation in the cylindrical coordinate system under the paraxial condition.LG mode is jointly determined by the azimuthal mode index I and the radial mode index p.LG light with a nonzero I has the novel property of carrying orbital angular momentum and has a wide range of applications in cold atom physics,bio-photonics and optical communications,etc.Almost all the previous researches on the interaction between plasma and high-power LG lights focus on optical angular momentum related properties,which are characterized by the azimuthal mode index l,whereas the radial mode index p is seldom investigated.The intensity pattern of an LG beam with nonzero l and p consists of p+1 halos separated from each other by dark rings and a dark center.Here we propose a plasma zone plate(PZP)driven by the ponderomotive force of an LG beam with a nonzero p.Starting from the fluid equations,we theoretically derive the plasma density evolution driven by the ponderomotive force generated by an LG beam.The results show that an LG beam with a nonzero p can modulate the plasma to form an annular distribution of alternating high and low density rings.Using the modulated plasma as a PZP allows multi-focal focusing of high-power lasers.The functioning of PZP is theoretically analyzed with LPI theory and verified with 3D particle-in-cell(PIC)simulations.The simulation results are qualitatively consistent with the theoretical analysis:the laser is modulated by the PZP to form a multifocal field structure similar to that of a Fresnel zone plate,and the maximum magnification increases with the radial mode index p.Our work refines the theory of the plasma density evolution driven by the ponderomotive force and provides a new dimension for the design of plasma-based optical devices. |
