Generation And Manipulation Of Orbital Angular Momentum Beam Based On Dielectric Metasurfaces

Since 1992 Allen et al.demonstrated that light with spiral phase e^il?carried the OAM,relevant studies on the OAM have got a rapid development.The angular momentum(AM)of the light includes two parts—OAM and spin angular momentum(SAM).The spin angular momentum corresponding to the circular polarization of light,and there are only desirable two values±?.For the OAM,which is corresponding to the spiral phase of the wave front with the value of l?,where l is an arbitrary integer.Due to the arbitrary of l,infinitely values of OAM can be chosen.Due to the unique properties of the OAM beam,it has shown important application value in various research fields,such as high-capacity optical communication,particle manipulation,microscopic imaging and high-dimensional quantum entanglement.Traditional way for obtaining and controlling OAM beam by using increment optical elements such as spiral phase plate,spatial light modulator and grating suffer from the bulky size,the complexity of the structure and difficult to integrate.Recently,benefiting from the progress of nano fabrication technology,the optical metasurface with planar structure has obtained unprecedented development.Optical metasurface is a two-dimensional artificial microstructure array,which can manipulate properties of light in the sub-wavelength scale,providing an important solution for miniaturization and integration of OAM beam generation and control systems.In this paper,focusing on the dielectric metasurface,the generation and manipulation of the OAM beam are studied.The research in this thesis covers the following aspects:Firstly,theoretically analyzed the characters of tightly focused OAM beams and design metasurface for the generation of these beams.The characteristics of tight focusing for different polarized OAM beams are derived by using vector diffraction theory,and the analytical expressions of each electric field component in cartesian coordinate system are obtained.Then,the influence of parameters of rectangular nanoantenna on the polarization conversion efficiency is studied.Selecting appropriate parameters by optimizing,tightly focused OAM beam for circular polarized light can be generated.In addition,the influence of the radius of the nanofins on the phase delay is analyzed.Suitable circular nanofins with high transmittance and phase coverage of0?2?were selected.Based on these structures,polarization-independent tightly focused OAM beams are obtained.Secondly,generating the OAM beam using multi-parameter manipulation.The interaction between the Fabry-Pérot(FP)of nanofins and the geometric phase of rectangular nanorods is analyzed theoretically,and a new structure for simultaneously amplitude phase modulation is proposed with a polarization conversion efficiency of over 90%.By employing the designed structures,OAM beams with different amplitudes are generated.In addition,the principle of dual-wavelength independent manipulation is studied,and a model is established based on the different dispersion between dynamic phase and geometric phase.In this way,we proposed a metasurface which can realize OAM beams with different topological charges when incident with different wavelengths.Thirdly,a metasurface is proposed to realize the asymmetric spin-to-orbital conversion.Based on the birefringence characteristics of rectangular nanoantenna,the constraints of independent manipulation of the spin photons are derived by employing transfer matrix method.When the nanoantennas behave as the half-wave plate,when the orthogonal circularly polarized light is incident,they will obtain the same dynamic phase and opposite geometric phase.Based on this principle,the metasurface is designed and the asymmetric spin-to-orbital conversion is verified theoretically and experimentally.Finally,a metasurface is proposed to realize optical angular momentum multiplexing and demultiplexing with high efficiency.Photon spin hall effect and off-axis technique are used to realize multiplexing and demultiplexing of SAM and OAM.A theoretical model of 10 OAM channels is investigated.The multiplexing-/demultiplexing efficiency of all channels exceeds 8%,and the maximum crosstalk between channels is-12.8 d B.Finally,the performance of the designed metasurface in the whole O-band is evaluated,and the scheme of integrating the designed system with the WDM system is presented.