Research On The Realization Of Mid-infrared Achromatic Optical Devices Based On The Dielectric Metasurface

The metasurface is composed of a periodic sub-wavelength unit element,which has extraordinary electromagnetic properties not found in natural materials.Thus,it has caused widespread concerns.By adjusting the sub-wavelength unit element,we can achieve strange dispersion characteristics,thereby breaking the limits of traditional laws and realizing various achromatic devices.Based on the dielectric metasurface,this paper proposes achromatic devices for different polarization states in the mid-infrared range,that is,achromatic metalenses and deflectors.What's more,we also theoretically analyze the performance of the corresponding devices.The research contents of this dissertation are as follows:(1)Based on the dielectric metasurface,an achromatic principle for the circular polarization is proposed,and two types of achromatic devices are designed and investigated,that is,a metalens and a deflector.The unit element is a high refractive index material silicon(Si)rectangular brick on the low-refractive-index material calcium fluoride(Ca F2)substrate.Under normal incidence,the studies found that optimizing the size parameters(including length,width,and height)of the silicon rectangular brick can produce multiple waveguide resonances and achieve multiple compensation phases.We use the geometric phase and the propagation phase to satisfy the focusing(deflection)phase and the compensation phase,respectively,so as to ensure that focus light and suppress chromatic aberration at the same time.Due to the large potential application value of the mid-infrared band,such as for communication and molecular detection,we chose the mid-infrared range of 3.7-4.5 ?m as the working bandwidth.Simulation results show that the achromatic devices can focus almost all the wavelengths within the bandwidth to the same point or deflect to the same angle,which proves that it can suppress the chromatic aberration effect well.(2)Based on the dielectric metasurface,a polarization-independent achromatic principle is proposed,and two types of achromatic lenses are realized: hyperbolic phase metalens and axilen lens.They use three different prototypes as the basic unit elements.These three prototypes are: cylinder,ring and coaxial structure.By changing the respective radii of the three prototypes,different propagation phases can be obtained.The propagation phase at the selected center wavelength of 4.2 ?m is used as the focusing phase,and the propagation phase difference within the bandwidth of 3.7-4.7 ?m is used as the compensation phase to eliminate chromatic aberration.Since the three prototype structures are all center-symmetric with polarization-independence,the designed devices based on this structure are also polarization-independent.However,symmetrical structures cannot use geometric phases.The principle proposed in(1)is not available here.The simulation results show that the designed metalenses approximately have the same focal points within the selected bandwidth,and its deviation error is much smaller than the chromatic lens.