Diffractive Focusing Of Structured Light Based On Ultrathin Spiral Surface

As an important optical device for the optical field modulation,the optical lens has been widely used in various optical systems.However,due to the large size of traditional lenses,it is difficult to integrate into micro-nano optical systems,so the artificial ultra-thin material came into being,which is called metamaterial.Metamaterial is usually realized by the local response of several subwavelength structures.However,those local phase modulation approaches inevitably introduce wavefront aberration to the required global phase profile,also cause difficulties in units of subwavelength structures fabrication,the application of metamaterial is greatly limited.Therefore,based on the recently reported sharp-edged diffractive focusing effect of waves,this paper proposes a new technique for manipulating light fields.The underlying mechanism is on the base of a spatial truncation of incident light and redistribution of the diffractive wave vectors in the Fourier domain,thereby forming a global phase domain to manipulate the light field globally.In this paper,an ultrathin spiral surface structure is investigated,and new techniques of manipulating the light field provide theoretical support for the structure,which can manipulate the wavefront of the entire incident light field under global domain conditions.The design of the structure does not depend on the mode of the incident light field,so it can well modulate different modes of light,including scalar light fields,vortex light fields,and vector light fields;At the same time,the design of the structure does not depend on the wavelength of the incident light,so its working on broadband;Since the structure can be made from many different materials,it is easier to manufacture at low cost and to couple compact optical systems.The specific research contents are as follows:First,the diffraction focusing of a scalar light field by an ultrathin spiral surface is studied.Specifically,starting from the paraxial Helmholtz equation,this paper derives the analytical solution of the diffraction field of the scalar light field after passing through the structure.The spiral phase with the same topological charge as the ultrathin spiral surface structure is induced as the diffracted field propagates.This ultrathin spiral surface structure was subsequently prepared,and the diffraction focusing effect and the topological properties of the ultrathin spiral surface structure were verified experimentally.Second,the diffraction focusing of the vortex optical field by the ultrathin spiral surface is studied.Specifically,the analytical solution of the diffraction field of the vortex optical field after passing through the ultrathin spiral surface is derived.Based on the topological properties of the structure,the de-vortex(disappearance of vortex phase and vortex singularities)effect of the vortex optical field and the ultrathin spiral surface structure under topological charge-number matching conditions is studied.At the same time,due to the diffraction focusing effect of the ultrathin spiral surface structure,it is found that the focused spot size after de-vortex can approach the diffraction limit.Under the condition that the topological charges do not match,the manipulation of the topological charges of the vortex optical field can be realized at the micro-nano scale.Third,the diffraction focusing of the vector light field by the ultrathin spiral surface is investigated.Specifically,the analytical solution of the diffraction field of the vector light field after passing through the ultrathin spiral surface is derived.Based on the manipulation of the topological charge of the light field by the structure,the depolarization effect of the vector light field under the condition of matching the topological charge,including the high-order vector light field and the vector vortex light,was studied.Similarly,due to the diffraction focusing effect of the ultrathin spiral surface structure,the depolarized diffraction field produces a hot spot on the optical axis,and the focused spot size can also approach the diffraction limit.Under the condition of mismatch of topological charge and number,the twisting of the polarization state of the vector light field can be realized at the micro-nano scale.