Investigations On Photonic Spin Hall Effect And Two-channel Vector Beam Generation Based On Elliptic Metal Nanoslits

As an analogy of the electron spin Hall effect,the photonic spin Hall effect(PSHE)refers to the lateral separations of left and right circularly polarized lights due to the transverse shifts of refracted or reflected lights perpendicular to the incident plane when the incident lightpropagates in the medium with refractive index gradient.Over the past decades,the PSHE has manifested fundamental significance and important applications in the rapidly developing frontierareas such as nanophotonicmanipulations,topological photonics and quantum optics.Concurrently,the observations and measurements of this effect itself have also made important advances,among which the most conspicuous ones inlude the weak measurement,split amplification by multiple reflections of light in glass rod,and strong spin Hall effect in metasurfaces.By using the spin-orbit interations(SOIs)and the geometric phase,the multiple semicircular metal nano-slits focus the light spot to nano-scale size and break the spatial symmetry in the circularly polarized light field distributions,making the spin splitting effect more obvious;thereby this scheme has become one of the important methods for PSHE measurement.It would be an interesting topic to break the symmetry of the light field with nano-slit of a non-circularly symmetric closed-curves and exploring observation of the PSHE,and it would be of great significance to the application of spin Hall effect.The design of metasurface is one of the typical applications of PSHE and SOIs,while they are inhomogeneous and anisotropic planar metamaterials composed of artificial subwavelength structures.Through selecting and arranging of the nanostructure units,the phases,polarizations and amplitudes of the light fields can be arbitrarily modulated,demonstrating the unprecedent ability to manipulate the light field at nanoscale.In particular,the generation of vector beams,in which the metasurfaces are used to manipulate the left and right circularly polarized vortex beams,has shown broad prospects of important applications in versatile fields such as optical encryption,classical and quantum communication and compact device integrations.However,in the current generation of focused multi-channel vector beams(VBs),the metasurfaces using the pure geometric phase for multi-channel focusing and for vector vortex controlling will lead to the unindependent manipulation of the VBs and to the possible noise background induced by the divergent hyperbolic phase.While the metasurfaces using the spatial multiplexing of area divisions could cause possible deteriorations of the vector beams.Therefore,it is an important issue to explore the metasurface designs that use propagation phase for focusing and use the interleaved nanostructures without spatial divisions to generate dual-channel vector beams.In this thesis,firstly,a metal elliptical nano-slit which breaks the circular symmetry but has the central rotational symmetry is used to study the PSHE,while the polarization separation method is introduced to separate the incident spin component and the converted spin component in the transmitted light field.The PSHE in the light field and the shifts of the characteristic spots are observed and analyzed.It is discovered that the inversed slope of the elliptical slit at the two side of the principal axes provides a geometric phase gradient to yield the opposite shifts of the characteristic spots in centrosymmetry,with a double shift achieved between the spots.Regarding the mechanism of this phenomenon,the flip of the spin angular momentum(SAM)in the the converted spin component gives rise to an extrinsic orbital angular momentum corresponding to the shifts of the wavelet profiles of slit elements in the same rotational direction to satisfy the conservation law.The analytical calculation,the simulation of finite-difference time-domain,and experimental demonstrations were performed.On this basis,a semi-elliptical nano slit illuminated an incident vortex beam carrying angular momentum is used to study the simultaneous generation of PSHE and photonic orbit Hall effect(POHE).It is found that both the AOM and SAM correspond to the characteristic spots in fields of incident spin component(ISC)and converted spin component(CSC),and the magnitudes and the directions of the spot shifts depends on the topological charges and SAM of the illuminating light.Particularly,the dependence of optical field distribution and the spot shifts on the PB phase,the spin-angular momentum conversion,and the semi-ellipse geometry are analyzed.Then,the metasurface with two sets of interleaved nanoslits arranged on elliptic curves is designed to generate independently controlled dual-channel focused vector beams(FVBs).Each set of nanoslits was composed of the in-phase and the out-of-phase groups of nanoslits to introduce the constructive interference and destructive interference of the output light field of the nanoslits,focusing the converted spin component and eliminating the incident spin component at the focal point.The two sets of nanoslits for the channels at the two focal points were interleaved on the same ellipses,and by setting their parameters independently,the FVBs in the two channels are generated under illumination of linearly polarized light,while their orders and polarization states of FVBs were controlled independently.The generation of the FVBs with the designed metasurfaces was demonstrated by the finite-difference timedomain(FDTD)simulations and by the experimental verifications.Specifically,this paper is arranged as follows:1.Chapter 1 gives a summary of the background and progress in the research fields related to the content of this thesis.First,the introduction to the basic concepts,properties and applications of surface plasmon polaritons(SPPs)is given;the interesting phenomena of spin-orbit interactions and origin of geometric phase of light are introduced;the discovery and advances of the photonic spin Hall effect,the related spin-split and beam shift,and the concept of the optical orbital Hall effect are described;a simple concepts for describing the polarization states of light are summarized;the recent progress in the manipulations of vector beams is depicted.2.In chapter 2,a closed elliptic metal curved nano-slit is proposed to replace the usually truncated curved nano-slit,and the phenomenon of the PSHE in the optical field produced the slit is studied.Using the characteristic spots in the light field generated by the noncircularly symmetric elliptical slit,and by introducing a method of separating the incident spin component and the converted spin component included in the outgoing light field,the PSHE exhibited by the shifts of the spots in the converted spin component field are clearly observed and measured.The theoretical calculation and finite-difference timedomain(FDTD)simulation of the light field generated by the slit element and the entire elliptical slit are carried out.The dependence of the characteristic spot shifts and the the PSHE on ellipse parameters is obtained.The experimental results are consistent with the simulations.3.In chapter 3,a vortex beam with IOAM is used to illuminate the semi-elliptical metal nano-slit,and an extension is made to include the optical orbit Hall effect.Firstly,the Huygens-Fresnel principle for plasmonic light waves is used to theoretically analyze the light field excited by the nano-slit,and the numerical calculations of theoretical integrals and FDTD simulations are performed to obtain the transmitted light field generated by the nano slit.With the polarization separation method,the PSHE and the POHE in the light field generated by the semi-elliptical nano-slit under the vortex light illumination are analyzed,and they are resulted from the geometric phase and spin-orbit conversion.The distributions and the shifts of characteristic spots in the incident spin component and the converted spin component fields are analyzed and discussed as well.The semielliptical nano-slits makes it easier to observe the PSHE and the orbital Hall effect.4.In chapter 4,a multiplexed metasurface formed by two sets of interleaved nano-slits arranged on an elliptic curve is used to generate a dual-channel focused VBs that can be independently controlled.Each set of nanoslits is composed of two sets of nanoslits whose initial orientations are perpendicular to each other,the optical paths to the focal points are an integer multiple of the wavelength and an odd multiple of the halfwavelength,and the constructive interference and destructive interference between the nanoslits are used to achieve the focusing of the converted spin component and the elimination of the incident spin component at the focal point By independently setting the orientation angle of each set of nano slits,the independent control of the orders and polarization distributions of the VBs in the two channels are realized under linearly polarized light illumination.The focused VBs generated by the designed metasurface were simulated and optimized by using the FDTD method,and the experiment performances were conducted.This research is of significance to deeper understanding,and to the related applications,of the photonic spin Hall effect.It would be great important to such subjects as generating multi-channel VBs,broadening the application range of metasurfaces,and the miniaturizing VBs generator devices.