| With the rapid development of optical technology,traditional optical components are becoming increasingly large and difficult to integrate.In response,researchers have developed a new two-dimensional artificial composite materials—metasurface,which can effectively regulate the amplitude,polarization state and phase of electromagnetic waves.A vortex beam is a special beam with a spiral phase and a circular amplitude distribution.The ability to produce a vortex phase is based on the spiral phase factor exp(ilθ),where θrepresents the azimuthal angle and l is the topological charge value,which can theoretically be taken to any value.There are a number of ways to generate OAM using conventional vortex beam control devices,but there are limitations to their development.With the development of integration technology,the trend towards smaller,flatter optics is becoming a major trend.Due to the divergent nature of vortex light,overcoming this property is necessary for better utilisation of vortex light in communication systems.A vortex metalens is a device that converges a vortex beam.For vortex metalenses,the possibility of chromatic aberration in the process of achieving convergence of vortex light is taken into account.This is why it is so important for the focus of the vortex metalens and its achromatization.In this paper,it is proposed an idea to design a vortex metalens using propagation phase,and a double-layer structure is also designed to achieve achromatic aberration on the vortex metalens using dual phase,which provides a design idea for vortex metalens achromatic aberration and is beneficial for applications in communication systems as well as radar detection.The main research contents include:(1)The vortex metalens with three polarisation multiplexes has been designed using propagation phase modulation at 1550 nm in the near infrared band.Vortex metalenses are produced with different focal lengths or topological charges based on the same hypersurface under both X-polarised and Y-polarised incidence conditions.The first design is a vortex metalens with the same focal length and topological charge,with a focal length of 3 μm and a topological charge of 3.The design demonstrated that the vortex metalens design is feasible.The second design is a vortex metalens with the same focal length and different topological charges,with focal length of 3 μm and topological charges of 1 and 2respectively.The third design is a vortex metalens with different focal lengths of the same topological charge,with focal lengths 3 μm and 4 μm respectively and a topological charge of 1.As a result of simulations,the focusing efficiency of all three vortex metalenses is above 84%,which is of interest in the direction of optical communication and radar detection.(2)The design of a double-layer vortex metalens achromatic based on the phase change material Ge2Sb2Te5(GST-225)is proposed in the mid-infrared optical band.A two-layer structure has been designed in the 4 ～ 5 μm continuous wavelength band of the mid-infrared light,where the lower layer is designed by propagation phase modulation using a polarization-independent combination structure,and the upper layer is designed by geometric phase modulation using a rectangular structure.Two types of vortex metalenses were designed,one at a single wavelength of 5 μm and the other with a topological charge of 1,maintaining a focal length of 20 μm in the 4 ～ 5 μm continuous waveband.In order to improve the performance of the achromatic difference of the designed structure,the GST-225 material with adjustable crystallization rate was introduced to change the refractive index of the material by adjusting the crystallization rate to reduce the chromatic aberration of the vortex metalens in the 4 ～ 5 μm continuous band.After simulation calculations,the achromatic function is basically realized in the studied waveband,which provides a new design idea for the achromatic of the vortex metalens. |
