Investigation Of The Tunable Metasurfaces Based On The Phase-change Materials

During the past decades,metasurfaces have drawn much attention due to their ability to manipulate light in the sub-wavelength scale.By spatially adjusting the geometrical parameters of metasurfaces,one can control the phase,amplitude,and polarization of light,thus artificially engineering the reflected or transmitted wavefront.Phase change materials have been induced in the field of metasurfaces due to the ability of tunable electromagnetic response,which has given a new impetus to the development of this field.Based on the phase change material Ge₂Sb₂Te₅(GST),two tunable optical devices are proposed in the infrared band:achromatic metalenses,achromatic deflectors and oriented communication metalenses.What's more,we also theoretically analyze the performance of the two devices.The research contents of this dissertation are listed as follows:(1)Based on the GST metasurfaces,a polarization-independent achromatic metalenses and deflectors that work over a broad band of wavelength are designed.The unit element is a GST cylinder on the low-refractive-index material calcium fluoride(CaF₂)substrate.We designed a 35?m focal length metalens with a wavelength of 8.5?m and GST crystallization rate of 0.6.The metalens consists of 46 GST cylinders of suitable structure.We at first showed that the focal length depends on both the incident wavelength and the GST crystalline fraction,therefore the chromatic aberration can be reduced via uniformly revising the crystalline fraction of GST cylinders.This method has been utilized to reduce the chromatic aberration for both light focusing and deflection over the wavelength from 8?m to 9?m.To make full use of the tunable optical property of GST,we have further designed a focusing metalens with eliminated chromatic aberration by individually engineering the crystalline fraction of GST cylinders with fixed geometry parameters.This method can work over a wider bandwidth from 8?m to 11?m,achieving a broadband metalens for achromatic aberration.(2)Based on the GST metasurfaces,a polarization-independent oriented communication metalens is designed.Our proposed metasurface can focus or deflect the beam to the place we need arbitrarily at 8.5?m with a fixed structure.The metasurface is composed of 72 GST cylinders standing on on a Calcium Fluoride(CaF₂)substrate.The switching of focus or deflection mode is achieved by individually engineering the crystalline fraction of each GST cylinder.Therefore,we only need to change the temperature of GST instead of changing the metasurface structure parameters to achieve oriented beam control.In addition,we also studied the diffraction pattern at 2m for oriented deflection.This method improves the degree of freedom of active modulation and provides a new route for tunable light communication devices.