The Study Of Designing And Associated Physical Properties Of High-efficiency Retroreflectors

Nowadays metamaterials are the building blocks for designing new photonic devices,which have always been one of the frontier researches in the field of optics.In particular,phase-gradient metasurface has provided unprecedented potentials for wavefront manipulation.The core concept in the phase-gradient metasurface is to introduce a spatially local abrupt phase shift on the interface,so that the phenomena of light reflection and refraction occurring at the interface are governed by the generalized Snell's law.The abrupt phase shift provides a new degree of freedom for manipulating the propagation of light.In the past decades,although various new photonic devices have been proposed,such as metalenses,polarization conversion,anomalous wavefront control and so on.There are still many problems such as low efficiency and narrow working angle response in using the metasurface to realize retroreflection of light,that is,returning the incident light back to its source direction.In this thesis,we focus on the study of designing and associated physical properties of high-efficiency retroreflectors.Based on the concept of abrupt phase shift,we use metal sub-wavelength grating structures to construct retroreflective devices in the operating band from infrared to visible light.The designed device has the feature of simple structure,high conversion efficiency,multi-channels and asymmetric reflection.The specific contents are as follows:1.Mid-infrared retroreflector based on subwavelength metal grating which has the properties of large-angle and high-efficiency.Based on the anomalous diffraction law of the metagrating,a two-channel mid-infrared planar metagrating device with simple structure and easy fabrication is designed and studied in this thesis.In order to reduce the complexity of the system and the loss caused by the large number of cells,a period only contains 2 cells in the device.Numerical calculations and simulation results show that the metagrating can achieve nearly 100%retroreflective efficiency,it also can achieve near perfect retroreflective efficiency even at large angles.In addition,the working angle of retroreflection can be designed by adjusting the geometric structure,which can theoretically cover from 0 to 90 degrees.2.High-efficiency retroreflector in visible light band and its physical characteristics.Based on the findings of mid-infrared retroreflector,we shifted studies to visible frequency range.We designed and studies two cases:dielectric/metal hybrid metagrating and purely metallic metagrating.Their performances of wavefront manipulation were revealed by studying their diffraction properties.It was found that all devices can be used to achieve high-efficiency two-channel retroreflection.By changing the width and depth of metallic grooves and the period of the supercell,retroreflective angle and conversion efficiency can be adjusted accordingly.3.Angularly asymmetric metal metagrating retroreflector.Based on the asymmetric diffraction characteristics in lossy phase-gradient metasurfaces that we revealed previously,we constructed a phase-gradient metagrating of metallic structure at visible frequency range.Due to the intrinsic loss of the metal itself,the device can achieve angularly asymmetric retroreflection.For instance,when the incident angle is-30 degrees,a high efficiency retroreflection can be predicted;when the incident angle is 30 degrees,all electromagnetic wave of incident light is absorbed completely,resulting in the retroreflection disappeared.Numerical calculations confirm our findings.Moreover,we studied and demonstrated that the unit cell number m of the supercell in phase-gradient metagrating plays a significant role in the conversion efficiency of retroreflection and degree of angular asymmetry.The relationship between them and the behind mechanism were illustrated in this thesis.