Study On Space-time Tight Focusing Property Of Ultrafast Vector Light Fields

Space-time(ST)shaping of ultrafast pulse laser is considered as a powerful tool for the development of high-efficiency laser trapping,ultrafast optical spanner,precise timeresolution measurement,ultrafast spectroscopy,integrated optical chip and high-resolution imaging.Currently,these shaping techniques devote mainly to achieve the specific spatial modulation and temporal encoding of light fields.However,a number of works usually focus on the single-functional scalar light wave shaping or on ST coupling of low-order vectorial light fields and fully overlook the variation details of light fields within an ultrashort time regime.Thus,how to realize the ultrafast multi-target control of light fields via abovementioned shaping approach,and to realize the ultrafast trapping and rotation upon a given particle by combining the high-order vector-vortex(spatial)traits with the ultrafast time(temporal)variations remains to be elusive until now,which hinders not only instructive insights into the ultrafast light-matter interactions but also the applications in the novel optical tweezer settings.To solve these problems,based on the time-dependent vectorial diffraction theory and fast Fourier transformation,this work firstly builds the ST focused light field model in which a femtosecond light pulse(in the absence/presence of the vortex phase)with distinct amplitude and manifold polarization configurations through a high numerical aperture objective lens,respectively.In this respect,we detailly study the underlying mechanisms of not only the control of ultrafast multi-targets light fields but the ultrafast trapping and manipulation of particles under high-order vector-vortex tightly focused light fields.These works have laid a solid foundation for the future theoretical works and practical industrial applications.The main results and conclusions with respect to this work are as follows.(1)This work presents a new concept for realizing ultrafast modulation of multi-target focal fields based on the facile combination of time-dependent vectorial diffraction theory with fast Fourier transform.It is achieved by focusing femtosecond pulsed light carrying vector-vortex by a single objective lens under tight focusing condition.It is uncovered that the ultrafast temporal degree of freedom within a configurable temporal duration(?400 fs)plays a pivotal role in determining the rich and exotic features of the focused light field at one time,namely,bright-dark alternation,periodic rotation,and longitudinal/transverse polarization conversion.The underlying control mechanisms have been unveiled by the creation of zero or? phase variation,time-dependent Gouy phase shift,and energy flux redistribution,respectively.Especially,the radially polarized light with vortex phase can be regard as an optimal candidate to achieve the ultrafast multi-target control of light fields.Additionally,the initially experimental results demonstrated by this work are well in agreement with our proposed theoretical predictions and numerical analyses.Besides being of academic interest in diverse ultrafast spectral regimes,these peculiar behaviors of the space-time evolutionary beams may underpin prolific ultrafast-related applications such as multifunctional integrated optical chip,high-efficiency laser trapping,microstructure rotation,super-resolution optical microscopy,precise optical measurement,and liveness tracking.(2)This work theoretically studies the dependence of the light field distributions,optical force and spin torque of high-order vector-vortex(HOVV)beams focused by a high numerical aperture objective on ultrafast variable time(0 ? 400 fs).It is accomplished by the fast Fourier transform,time-dependent vectorial diffraction theory and Rayleigh scattering model.Firstly,it is shown that the light field patterns of tightly focused HOVV beams can be quasi-periodically reshaped by adjusting the intrinsic fleeting time.We further examine the three-dimensional(3D)distributions of optical force on a Rayleigh particle(with a size much smaller than the light wavelength)in the focal volume induced from tightly focused HOVV beams,which reveals that the distinct vector-vortex light fields as a function of ultrafast time endow the capability of the selective trapping for particles.Beyond that,we proceed to the 3D spin torque on the given particle,which makes the conversions between transverse and longitudinal spin torques possible,resulting in the 3D spatial rotation within an ultrafast timescale.Additionally,it is found that the vortex order plays a pivotal role in steering the focused light fields and related optical forces and spin torques.Likewise,in the light of the present experimental setup,we initially demonstrate the generation and application of high-order vector-vortex light fields.The proposed pathway and acquired results not only provide a novel degree of freedom for the ultrafast control of light fields,but also have potential applications in integrated spectroscopic analysis,ultrafast optical tweezers and spanners,and high-speed optical measurements.