Studies Of Phase Vortices In Femtosecond And Speckle Fields Based On Modulation Of Spiral Multi-Pinhole Plate

Optical vortices are the light waves possessing the helical wave fronts along the direction of propagation.The amplitude vanishes and the phase is undetermined in the vortex core,where is a phase singularity point.The phase circulation around the singularity point is an integer multiple of 2?,and the phase profile is characterized by a factor of exp(il?),where the integer l is called the topological charge of the vortex.Per photon in optical vortex has the orbital angular momentum of l?.Based on the unique properties of optical vortices,the study has attracted enormous attentions and extended to many fields,such as atom Bose–Einstein condensates,micromanipulation,optical communications,super-resolution microscopy,quantum information processing,and nonlinear spectroscopy.One of the major achievements of the optics in recent decades has been the generation of light pulses in extremely short duration,down to a few femtoseconds and tens of attoseconds in the past few years.In recent years,femtosecond optical vortices have been reforming traditional femtosecond technology.The development of the optical vortices in ultrashort pulse brings the spatiotemporal properties into the singularity optics,molecular reaction dynamics,and other vortex-based strong-field physics.Femtosecond vortices have shown tremendous potential in a wide range of applications,including femtosecond micro–nano manipulation,subwavelength nonlinear microscopy,femtosecond special-structure processing,and filament optics.In this paper,we mainly depend on the wave modulation of spiral multi-pinhole plate to generate femtosecond vortices and reform the phase vortices in speckle field.The theory of the spiral multi-pinhole plate is based on the even variable radius in azimuthal angle to modulate the phase variations of diffracting waves.The characteristic vortices can be generated by different spiral plates with specific designs.We use the pinhole plate with Fermat spiral arrangement to modulate femtosecond pulse,and successfully investigate the spatiotemporal evolutions of femtosecond vortices;under the phase modulation of ratchet spiral mode,the orbital angular momentum of femtosecond vortices can be flexibly selected in ultrashort optical field;high-order vortices in femtosecond pulse are produced by the modulation of multiple spiral arrays;when the speckle field is modulated by the spiral plate,we observed the twin-vortices in the generated clustered speckles;the locally-enhanced light scattering of four concentrated intensities is observed in the light field scattering by the random surface structure that is replicated monocrystalline silicon wafer by a transparent material.The details are in seven chapters in our paper.Chapter one: We generally describe the optical vortices in the introduction.The development of optical vortex and the overview of the mathematical description of vortex are firstly introduced.We secondly review the methods of vortex generations in lab,including spiral phase plate,computer-generated holograms,cylindrical lens,and multiple beam interference.Third,we show the methods of detecting orbital angular momentum in optical vortices,including fork-grating,interferometries,interference of vortex with plane wave.Furthermore,we briefly describe the developments and backgrounds of the femtosecond vortices and the speckle vortices.Chapter two: We use the spiral multi-pinhole plate to generate ultrashort vortex pulses and study their spatiotemporal evolutions involving intensity,phase,orbital angular momentum,and energy current.In the experiment,a Mach–Zehnder-type interferometer is employed to perform the investigation of ultrashort vortices.Combining the experimental results and the theoretical analyses,we discuss the spatiotemporal evolutions of ultrashort vortex pulses in femtosecond regime.The results show that the distribution of orbital angular momentum in the cross-section of the vortex pulse is maintained almost invariable in the pulse duration,while both the intensity and the energy current obey a Gaussian-like distribution.With time evolution,the phase contour lines of such vortex pulses rotate around the propagation axis.Chapter three: We put forward an approach for orbital-angular-momentum mode selection in an ultrashort laser pulse.This approach utilizes the optical mask of a ratchet array consisting of multiple spiral arrays arranged in a rotationally symmetric distribution as the ratchet model.Both theoretical analyses and experimental work in the context of ultrashort vortex pulses demonstrate that the ratchet array enables flexible generation of structured orbital-angular-momentum waves.This optical mask shows the advantages of damage resistance,without medium-dispersion,and ease of handling in an ultrafast optical field.Furthermore,the ratchet array can be applied to the analysis and generation of other matter vortex beams within an axially scalar system.Chapter four: We propose the multi-spiral arrays to modulate the phase distribution of ultrafast optical field and generate high-order femtosecond vortices.The multi-spiral arrays are consisted of multiple same spirals with rotationally symmetric arrangement,which have the quasi-centrel symmetric distribution.We firstly analyze the ability of multi-spiral arrays for generating high-order vortices in theory,and then,the simulations are achieved the symmetric optical field of femtosecond vortices.We extract the data of orbital angular momentum and energy current from the simulated data to compare the distribution with the case of single spiral,and the results indicate that the optical field with the modulation of multi-spiral arrays has more symmetric distribution.Moreover,we experimentally perform the multi-spiral arrays in the context of femtosecond field and demonstrate that it can efficiently generate the high-order vortices.The achievement is an improvement for the method of spiral plate generating vortices and will be significant for the modulation of optical field and vortex-based physical studies.Chapter five:We report an experimental study of a new vortex state in clustered speckles generated by modulating the scattering waves with a spiral multi-pinhole plate.This vortex state can be termed a twin-vortex state with two spatially neighboring singularities nested in a common vortex core.In our experiment,the spiral multi-pinhole plate with N pinholes arranged along a spiral curve introduces a phase variation of an integer multiple of 2? into the waves.With different phase modulations,the waves scattered from random surfaces can be modulated to produce various forms of twin-vortices in clustered speckles.Based on the experimental data,we conduct a statistical analysis of both the density of twin-vortices and the distance between the neighboring singularities.The results indicate that the twin-vortex has the properties of low density and closely neighboring singularities.The particular vortices with two singularities in speckle fields are experimentally studied for the first time.Chapter six:We study the optical properties of light scattering by a monocrystalline silicon wafer,by using transparent material to replicate its surface structure and illuminating the fabricated sample with a laser source.The experimental results show that the scattering field contains four spots of concentrated intensity with high local energy,and these spots are distributed at the four vertices of a square with lines of intensity linking adjacent spots.After discussing simulations of and theory about the formation of this light scattering,we conclude that the scattering field is formed by the effects of both geometrical optics and physical optics.Moreover,we calculate the central angle of the spots in the light field,and the result indicates that the locally-enhanced intensity spots have a definite scattering angle.These results may possibly provide a useful method for improving energy efficiency within mono-Si based solar cells.Chapter seven: We conclude the work and the achievement of our researches in this paper,and briefly introduce the preconceived work in future.