Research On Perfect Vortex Beam Regulation Based On Optical Metasurfaces

The vortex beams carrying orbital angular momentum(OAM)play an important role in the applications like Bose-Einstein condensation,particle rotation and manipulation,optical super-resolution imaging and quantum information coding,and have become a hotspot of the information optics research.The topological charge value is an important parameter for the vortex beam.For examples,topological charge is not only proportional to the orbit angular momentum(OAM)in particle manipulation,but also represents the ability of channel multiplexing and information coding in OAM optical communication.Therefore,the research about the large topological charge vortex beam is necessary.However,the radius of the bright ring of the vortex beam obtained by the traditional method will increase with the topological charge.It makes the large topological charge vortex beams confront huge difficulty under the situations like coupling and transmission in optical fiber.Therefore,it is necessary to develop some methods for generating a vortex beam whose bright ring radius does not depend on the topological charge.The perfect vortex light field is a kind of intensity distribution and beam radius that can be maintained regardless of the topological charge,and has a certain peak intensity radius.Each point in the light field has a clear orbital angular momentum density,and the relative Vortex beams that remain constant at the azimuth.It has important application value in the fields of particle manipulation,optical fiber communication and quantum optics,and has become a research focus in the field of light field regulation in recent years.Traditionally,the perfect vortex beams are produced by high-order Bessel Gauss beams with Fourier transformations,which generally require a series of traditional optical components such as axis prisms,spiral phase plates,convex lenses,etc.It inevitably leads to problems such as the large size and structural complexity of the systems involved.In addition,perfect vortex beams produce from conventional methods possess only one space structure pattern with a circular bright ring.Also,the position of the perfect vortex light is fixed to the focal plane of the convex lens.A single spatial energy distribution makes conventional perfect vortex beams unsuitable for some applications such as optical microfluidic sorting and acceleration,which require beams with a diverse structure.The fixed axial generation position is also not conducive to perfect vortex light for optical traction and propulsion in optical micro-manipulation,and is also difficult to couple to the fiber end-face in fiber optic communication applications.These deficiencies make the perfect vortex beams lacking in modulating freedom,which limits their applications in some extent.In recent years,optical metasurface components have attracted widespread attention for their compact and ultra-thin construction and their ease of optical integration compared to conventional optical components.Metasurfaces can manipulate light waves through the interaction of ultra-thin nanostructures with light to produce mutated phases in the ejection field,providing key techniques for building a variety of ultra-thin optics and advancing planar photonics,as well as new ideas for the generation and regulation of perfectly vortex beams.Therefore,it is of great importance to carry out research on the perfect vortex beam and its modulation based on the optical metasurface.In this paper,the spatial regulation of perfect vortex beams based on phase changes induced by nanostructures of metasurfaces is investigated.By designing different metasurface structures,the regulation of the ellipticity and focal plane position of the perfect vortex light is realized separately,which provides a new degree of regulation freedom in the spatial structure of the perfect vortex beam and fills a gap in the current research field.The main works of this paper include:1.To solve the single energy distribution problem of the traditional perfect vortex light field,a reflective geometric phase-type metasurface for generating elliptic perfect vortex beams has been designed and prepared.Based on the principle of geometric phase modulation,the study of the plasmonic metasurfaces design for the NIR broadband was carried out based on the phase distribution of the elliptical perfect vortex light.And a corresponding optical test system was built to experimentally verify the designed metasurfaces.With further optimization,this metasurface is expected to be used as an integrated device for optical micro-manipulation and optical communication systems.2.Since the position of perfect vortex relies on the Fourier transform plane,a set of perfect vortex dielectric metasurface combination with rotating focusing is designed to solve the problem that the focal plane position of perfect vortex beam is not adjustable in the practice.Based on the principle of transmission phase,a cascade metasurface combination can adjust the focal plane position of perfect vortex by changing the relative rotation angle.A model of the cascade metasurfaces is constructed to match and optimize the transmission phase design.Finally,it is transformed into the unit geometry information of optical metasurface.The metasurface model is constructed by electromagnetic simulation method,and the simulation results are compared with the theoretical results.The metasurface rotation focusing method does not require complex lens combination and mechanism,greatly simplifies the system structure and is easy to integrate.