Fabricating Three-Dimensional Microstructures And Detecting Chiral Structures By Femtosecond Vortex Beam

Chirality is used for describing the symmetry properties of an object.A chiral object,which cannot be superposed with its mirror by rotations or translations,is quite ubiquitous in nature.Light,as an electromagnetic wave,can also be chiral by carrying angular momentum.Circularly polarized light,which carrys spin angular momentum,has been widely used for detecting chiral molecules in chemistry and biology.Recently,optical vortex carrying orbital angular momentum has attracted more and more interest from researchers.There are three hot topics:1.Transfering the orbital angular momentum(chirality)of optical vortex into materials by interactions between light and matters;2.Detecting the chirality of materials by orbital angular momentum instead of spin angular momentum;3.Fabricating three-dimensional microstructures by a single pulse of modulating optical vortices for improving the efficiency of two-photon polymerization method.In this paper,we study the three topics by combining femtosecond two-photon-polymerization technique and modulated optical vortex.The main contents include:1.Three-dimensional chiral microstructures are achieved in isotropic material by introducing optical vortex into two-photon-polymerization system.A large-area array of chiral microstructures can be precisely fabricated by interfering optical vortex and plane wave.The helical phase gradient twists the interfering pattern to form a three-dimensional spiral optical field.After focusing in the polymer,a chiral microstructure can be achieved.Our further investigation reveals that the spiral lobes and chirality are caused by the interfering patterns and helical phase wavefronts,respectively.Moreover,interfering two coaxial optical vortices generates a double-helical optical field for fabricating double-helical microstructures.2.Achiral slanted micropillars are manufactured by introducing optical vortex into femtosecond two-photon-polymerization system.By eliminating the orbital angular momentum in two opposite optical vortices,multispots-shaped optical field without chirality is realized.After focusing in the polymer,mirror-symmetric slanted micropillars are fabricated by a single pulse.Controlling the distance between adjacent micropillars,these slanted micropillars can realize self-assembly.The assembled micropillars array can be used for trapping SiO2 particles with an ultra-high trapping ratio(>90%).The trapped particles can also be used in imaging.3.Chiral microstructures fabricated by two-photon-polymerization technique are used in detecting chirality of object with orbital angular momentum.A large vortical dichroism can be induced on the chiral microstructures by optical vortices.The vortical dichroism spectroscopy,based on the reflectance of vortex beams with topological charge l from-75 to 75,is mirror-symmetric for two enantiomers,where the vortical dichroism values are the same in magnitude but opposite in sign.The strong vortical dichroism effect has a significant enhancement of 200 times compared with chiral molecules by an indirect method.The vortical dichroism spectrum can quantitatively detect the geometric characteristics of chiral structures.Our results prove that the dichroism domain of optical properties on chiral structures can be expanded by the vortical dichroism response for complementing traditional circular dichroism response.The universality of vortical dichroism can be proved by detecting the chirality of double-helical microstructures.4.Achiral microstructures fabricated by two-photon-polymerization technique are used for inducing a quasi-vortical dichrosim by illuminated with circularly polarized orbital angular momentum beams.The coupling of spin and orbital angular momentum is exhibited by measuring the reflectance of orbital angular momentum beams with opposite circular polarization states,which realizes the difference of reflectance on the edge of disc-shaped microstructures.The sign of induced vortical dichroism depends on circularly polarized state and its magnitude relates to the diameter of microstructure.By combining femtosecond two-photon-polymerization and modulated optical vortex,we find that three-dimensional microstructures can be fabricated with a single pulse and a strong vortical dichroism can be achieved on chiral microstructures.These works pave the way for improving laser manufacturing efficient by modulated optical field and detecting chirality of structures.