Anomalous Diffraction In Phase-gradient Metagratings And The Design Of Novel Optical Devices

How to freely control the propagation of light or electromagnetic wave,and to reveal new mechanisms for the light-materials interactions,has always been one of the cutting-edge researches in optical community.In the past decades,optical metamaterials have provided new diagram for this purpose,and opened up new horizons for the study of new wave field manipulations.In particular,phase-gradient metasurface has provided unprecedented opportunities for wavefront manipulation.The core concept in the phase-gradient metasurface is to introduce a spatially local abrupt phase shift at the interface,so that the phenomenon of light reflection and refraction occurring on the interface,is governed by the generalized Snell's law.The abrupt phase shift provides a new degree of freedom for the control of light propagation.However,this approach suffers from fundamental limits on conversion efficiency.And in some cases,higher-order diffraction caused by the periodicity can be observed distinctly,while the working mechanism is still not fully understood,especially in refractive-type metagratings.In this thesis,we try to address these problems by designing and studying a phase-gradient metagrating(PGM)which is a kind of non-ultrathin phase-gradient metasurface with a thickness of about one operating wavelength.Due to relatively simple configuration compared with that of ultrathin phase-gradient metasurfaces,the PGM provides a good platform to uncover the rule of high-order diffraction effect in the phase-gradient metasurfaces and to reveal the underlined mechanism,through rigorous theoretical analysis.Here starting from the design of PGM,we theoretically and experimentally uncover a new diffraction rule and mechanism in PGM.More importantly,by proposing a series of new optical devices with PGM to realize numerous new optical phenomena,we demonstrate a new degree of freedom,i.e.,the unit cell number m in the supercell of the PGM,which plays a significant role in the anomalous diffraction in PGM.The thesis is organized as follows:1.Parity-dependent anomalous diffraction rules and physical mechanismsIn this chapter,we design and study a transmission-type PGM.It is found that the PGM has almost perfect abnormal transmission or reflection in a wide range of incident angles.Starting from the effect of multiple reflections,the complex diffraction mechanism in PGM is revealed,and a new kind of grating diffraction rule is discovered.This found diffraction rule can fully explain the higher-order diffraction phenomena observed in previous studies.In addition,we also show that the unit cell number m can be regarded as a new degree of freedom for manipulating the propagation of electromagnetic wave.By the parity of m,the higher-order outgoing wave can be reversed between the anomalous transmission channel and the anomalous reflection channel,and this phenomenon is very robust.The correctness of the new diffraction law is also verified experimentally.2.Loss-induced angular asymmetric diffraction and physical mechanismBased on the disclosed new grating diffraction law,this chapter focuses our studies on the PGM with loss,i.e.,non-Hermitian optical system.It is found that there exists the phenomenon of angular asymmetric absorption in the lossy PGM,and the degree of asymmetric absorption is relevant to the number of unit cells m.We presented a simple and intuitive semianalytical approach to explain the observed angular asymmetric absorptivity.Our theory clearly indicates that the phenomenon of m-dependent angular asymmetric absorptivity is very robust and can be observed in any grating with a phase gradient.The mechanism is that the phase gradient ensures an additional process of multiple total internal reflections inside the gratings,leading to more dissipation and an asymmetric diffraction response.3.The design of PGM-based novel optical devices and their performanceBased on the discovered diffraction laws and physics,a series of novel optical functional devices were designed.This chapter highlights two examples:(1)a novel type of dielectric metagrating based on local Fabry-Perot(FP)resonance for wavefront control,and(2)incident angle-selected transmission based on PGM.For the first one,we designed a transmission-type PGM formed by periodic sub-wavelength metallic slit arrays filled with identical dielectrics of different heights.It is found that when FP resonances occur locally inside the dielectric materials,in addition to the common phenomenon of complete transmission,the transmitted phase differences between two adjacent slits are exactly the same.The local FP resonances total phase shift across a supercell can fully cover the range of 0 to 2?,satisfying the abrupt phase shift of PGM.More studies reveal that the local FP resonances lead to a one-to-one correspondence between the transmitted phase difference and the permittivity of the filled dielectric material.For the second case,we introduce the concept of abrupt phase shift into the zero index metamaterials to design a device that allows light with specific incident angle transmitting it.In particular,it is found that in some cases,transmission occurs only when light is incident at a specific oblique angle,and there is a high efficiency of single-angle selective transmission characteristic.Due to the anomalous properties of zero-index metamaterials,the device can also allow efficient focusing of single-angle electromagnetic wave and multi-port single-angle beam emission.4.Multifunctional devices based on phase-gradient metagratingBased on the diffraction laws and asymmetric diffraction characteristics in PGM,two multifunctional phase-gradient metagrating devices were designed.One of them is multifunctional retroreflective device.Through numerical simulation,it is verified that the device can achieve high-efficiency three-channel retroreflective function,quasi-retroreflective function and specular reflection function.By changing the period length of the device,the retroreflection angle can be adjusted,as well as the incident angle range of quasi-retroreflection and specular reflection.The other is a device that can allow both the functions of multi-channel orbital angular momentum conversion with high efficiency and absorption.Through theoretical analysis and numerical simulation,it is found that for the incident vortex wave with topological chargel,the device can convert the topological charge of reflected vortex wave to a;for the incident vortex wave with topological charge-l,the high efficiency absorption can be achieved.We also obtain the relationship between l and?,and reveal the behind mechanism.