A review of anomalous refractive and reflective metasurfaces

Siqi Liu,Zhenyu Ma,Jian Pei,Qingbin Jiao,Lin Yang, Wei Zhang, Hui Li, Yuhang Li, Yubo Zou, and Xin Tan

ABSTRACT Abnormal refraction and reflectio refers to the phenomenon in which light does not follow its traditional laws of propagation and instead is subject to refraction and reflectio at abnormal angles that satisfy a generalization of Snell’s law.Metasurfaces can realize this phenomenon through appropriate selection of materials and structural design,and they have a wide range of potential applications in the military,communications,scientific and biomedical fields This paper summarizes the current state of research on abnormal refractive and reflectiv metasurfaces and their application scenarios.It discusses types of abnormal refractive and reflectiv metasurfaces based on their tuning modes(active and passive),their applications in different wavelength bands,and their future development.The technical obstacles that arise with existing metasurface technology are summarized,and prospects for future development and applications of abnormal refractive and reflectiv metasurfaces are discussed.©2022 Author(s).All article content,except where otherwise noted,is licensed under a Creative Commons Attribution(CC BY)license(http://creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/10.0010119

KEYWORDS Anomalous refraction and reflection,Generalized Snell’s law,Metasurface material,Metalens

I.INTRODUCTION

Anomalous refraction and reflection as the term implies,is a phenomenon in which,as a result of appropriate regulation of the wave direction of incident light,anomalous-angle or even negativeangle refraction occurs,and transmission and reflectio do not satisfy Snell’s law.1-3The main modulation methods currently available for the production of anomalous reflectio and refraction are glass fibe arrays,three-dimensional metamaterials,and metasurface modulation.The glass fibe array method is based on the principle of an optical waveguide and can achieve a negative refractive function of visible light and realize anomalous refraction within the scope of classical optics.However,this method is complicated and only single modulation is possible,and so it does not provide a basis for further development.4With the use of threedimensional metamaterials,flexibl beam control can be achieved,but the complicated three-dimensional structures required to obtain complex optical functions increase the device size,posing obstacles to flattenin and miniaturization.5-9On the other hand,metasurfaces incorporating a subwavelength structure are able not only to realize abnormal reflectio and refraction,but also to alter the angle of reflectio and state of polarization of outgoing light to enable such functions as electromagnetic wave front shaping.Because of these capabilities,metasurfaces have become an important topic of current research.10-12

In 1621,the Dutch scientist Snell firs provided a mathematical description of the phenomenon of light refraction at the interface between two media in what is now called Snell’s law.3On the basis of this law,a phase-matching theory was later derived according to which the electromagnetic wave vector components in the direction parallel to the interface are constants.Snell’s law strictly constrains the angles of reflectio and refraction.When the angle of incidence is known,together with the refractive indices of the media on each side of the interface,the angles of reflectio and refraction can easily be calculated.Snell’s law imposes limitations on the manipulation of light by conventional optical devices,owing to the uniform refractive index distributions of the materials from which such devices are constructed.Therefore,attempts have been made to use artificia metasurfaces to achieve arbitrary manipulation of light.An abrupt phase shift can be applied to an incident electromagnetic wave by a careful design of the metasurface microstructure.Anomalous refraction and reflectio can be obtained according to Fermat’s principle,and a generalization of Snell’s law can be derived to describe this situation.If we assume that an abrupt phase shiftΦ(x,y)is generated at a point(x,y)on the interface,then this generalized Snell’s law can be expressed as

whereθi,θr,andθtare the angles of incidence,reflection and refraction,respectively,whileniandntare the refractive indices of the materials on the incident and refracted sides of the interface,respectively.Equation(1)lead to a modificatio of the expressions for phase matching,which become

On the basis of these expressions,it is found that,in theory,the introduction of an abrupt phase shiftΦby the metasurface structure will enable a wider range of manipulations of light.By controlling the magnitude of the phase shift,it should be possible to change the amplitude and phase of light,as well as its direction of emission,thereby producing a variety of anomalous refractive and reflectiv phenomena.A number of different methods are currently being used to design and fabricate metasurfaces capable of producing anomalous refraction and reflection

After more than a hundred years of development since Wood’s discovery of anomalously reflecte beams from subwavelength metal gratings in 1902,metasurfaces are now able to achieve modulation of electromagnetic waves in most spectral ranges.The design and application of anomalous refractive and reflectiv metasurfaces have been realized from the microwave and terahertz(THz)bands,through the infrared(IR)part of the spectrum,to visible light.Compared with conventional optical components and metamaterial components,and the conventional optical lenses used in the cumulative phase method,13metasurface structures are smaller.They do not need to be bent,and it is only necessary to employ a specially designed surface microstructure.Therefore,for example,metalenses constructed using anomalous refractive metasurfaces can facilitate the use of greatly simplifie optical paths.14-16Because of the planar geometry of metasurface,they take up less space than other components used for similar purposes,they are easier to manufacture,and they have a wider range of applications.Anomalous refractive and reflectiv metasurfaces now have practical uses in the military,scientific and biomedical field for such purposes as optical cloaking,wavefront shaping,and information transmission.17-21

This paper summarizes the current state of development of anomalous refractive and reflectiv metasurfaces,focusing on two core issues,namely,tuning mode selection and the applications in different wavelength bands.It firs describes the classificatio of anomalous refractive and reflectiv metasurface materials,then the progress that has been achieved in the application of anomalous refractive and reflectiv metasurface in different wavelength bands,and finall outlines the likely future development of these metasurfaces.

II.TYPES OF ANOMALOUS REFRACTIVE AND REFLECTIVE METASURFACES

Anomalous refractive and reflectiv metasurfaces,as functional components,can be classifie as passive or active metasurfaces,depending on their tuning capabilities.Passive metasurfaces achieves their function through a fixe structure with a specifi design and cannot be modulated.Active metasurfaces,on the other hand,combine metasurface structures with different sensitive units.Active tuning of such metasurfaces for incident electromagnetic waves can be achieved through the application of temperature,electricity,or light to expand their function or increase their bandwidth.In this section,the tuning modes of passively and actively tuned metasurfaces are compared and analyzed.

A.Passive metasurface tuning mode

Metasurface structures use optical antenna units to modulate light waves.In passive metasurfaces,metal antennas are usually used.An initial approach used a V-shaped Au antenna22to add phase modulation to an incident light wave and thereby produce anomalous refraction and reflectio of light.In a development of this method,as shown in Fig.1(a),a double-layer metasurface structure was realized by combining a C-shaped Au antenna with an Si layer23and fabricating a grating on the back side,which significantl improved the efficienc of optical modulation in the THz band.It has even proved possible to integrate three metallic functional layers in a cascade of Au and dielectric layers to obtain a transmission-reflectio dual-function structure.24

FIG.1.Opticalmetasurfaces based on different tuning modes.(a)Cascaded multifunctionalpassive metasurface based on an Au antenna.23(b)Composite passive metasurface based on F4B dielectric and metal.25(c)All-dielectric passive metasurface based on TiO2.27(d)Electrically controlled active metasurface based on graphene.30(e)Temperature-controlled active metasurface based on GST.39

However,in high-frequency bands,the efficienc of a metasurface will be greatly affected by metallic dissipation at visible wavelengths,and therefore metasurface structures have been investigated in which only dielectric materials are used.For example,Xuet al.25have designed a composite metasurface formed from a combination of an F4B dielectric plate with a split ring resonator(SRR)and a crossbar metal patch,as shown in Fig.1(b).This has a variety of potential applications in the microwave band.It is capable of generating orbital angular momentum and vortex light scattering in the frequency range of 7-14 GHz,in addition to anomalous refraction and reflection and it can achieve up to 10 channels of beam modulation by encoding multiple wave vectors.It has been found that by exploiting the Pancharatnam-Berry(P-B)phase,a TiO2dielectric antenna integrated on an SiO2substrate can achieve high transmission efficiencie of 66%-83%in the 405,532,and 660 nm bands.26By using similar materials for optical nanoantennas,as shown in Fig.1(c),an anomalous refractive and reflectiv metasurface has achieved 71.3%-88.2%transmission efficienc in three bands in the 450-633 nm range.27

Passive metasurfaces are designed with subwavelength antennas,and various types of tuning units can be formed by using different shapes and arrangements to achieve beam modulation for target light waves.At present,passive anomalous refractive and reflectiv metasurfaces are mainly prepared using metallic or all-dielectric materials.Metallic materials have reached a mature stage of development and are widely used,but they produce extremely high dissipation in the case of high-frequency light.All-dielectric materials can overcome this shortcoming of metallic materials for near-IR and visible light,and they improve the transmission efficienc and working performance of metasurfaces,and they have therefore become the focus of research on metasurface materials.

B.Active metasurface tuning mode

Passive metasurfaces face many obstacles to practical application owing to the limitations imposed by their fixe structures.By contrast,actively modulated metasurfaces can be dynamically controlled by bringing in an external excitation source,and the materials used in their construction are selected according to their modulation modes.At present,there are three main metasurface driving methods,namely,electrical,optical,and thermal,which are summarized in this subsection.

Electrical modulation can be achieved in different ways.At microwave wavelengths,with a combination of a bottom metal layer and a PIN diode,it is possible to change the center frequency of the modulation and switch between transmission with the PIN off and reflectio with it on.28In the microwave region,by combining a varactor diode and a metasurface microstructure,it is possible to realize anomalous refraction and reflectio over a wide bandwidth through voltage regulation,and,by adding a horn-like structure,an incident wave can be transformed into a surface wave,thereby enabling dynamic functional switching with a wide bandwidth.29In the THz band,with the configuratio shown in Fig.1(d),in which a graphene-based metasurface is separated from an Au electrode by a thin SiO2layer,it is possible to adjust the operating bandwidth of the metasurface by modulating the Fermi energy level of the graphene.30In another approach,Leeet al.31used liquid crystals as electrically sensitive materials integrated with a V-shaped metasurface,which was fabricated by a focused ion beam technique,thereby providing an optoelectronic switch with reflectio and refraction switching in the near-IR region.

In the case of light modulation,a photosensitive material is combined with a metasurface to achieve modulation.Photosensitive semiconductor materials are widely used in this modulation method.For example,by combining a GaAs substrate with structured Cu cells to form an SRR array and using an electromagnetic drive for the switching of a light-controlled metasurface,it is possible to achieve electromagnetic wave transmission and cutoff in the THz band.32As another example,an optically modulated metasurface capable of light wave splitting has been fabricated using an Si substrate and Al structural units combined into an SRR array.33

Thermal modulation takes advantage of the different crystal phase structures of thermally sensitive materials at different temperatures,which can produce changes in conductor/insulator properties.“On”and“off”functions have been studied from the THz to the visible bands.Liet al.34used an alternating combination of VO2and Cr separated by polyimide layers to form a metal-insulator-metal(MIM)resonator structure in the THz band.This bifunctional metasurface is able to act as a metalens,deflectin a beam when the VO2is in the insulating state,and transforming into an absorber when it is in the metallic state.In another application of the thermal phase-change material VO2,a sandwich structure with Si and Au was fabricated in the far-IR band to accomplish the dual functions of absorption and beam control.35A VO2-based metasurface has also been used in a multifunctional design at visible wavelengths.36Another thermal phase-change material,GST-225 integrated with an Si substrate,can also be used to realize switchable reflectio and absorption by a metasurface in the THz band,controlled by temperature changes.37Liet al.38investigated a temperature-controlled metasurface in the near-IR in which GST-225 formed a transition layer between an Au structure and an SiO2substrate,and they verifie that this metasurface could be changed from an overcoupling state to a critical coupling state,thereby providing a basis for active tuning of an anomalous refractive and reflectiv metasurface.Figure 1(e)shows a metasurface construction for the visible band in which a W metasurface structural unit sits above a GST-225 layer,followed by a Wmetal layer that itself is on top of a SiO2substrate.39With this structure,it is possible to realize variable tuning of the transmission peak at 500-740 THz.

As mentioned above,anomalous refractive and reflectiv metasurfaces can be constructed from different materials,depending on the choice of modulation method and mechanism.There has been an evolution from the initially adopted passive modulation approach to an active one.In the case of passive tuning,metal or all-dielectric materials with high reflectivit and low scattering coefficient have usually been selected.For active tuning of metasurfaces,depending on the tuning method adopted,appropriate sensitive materials are selected with the aim of achieving a rapid tuning response.In addition,in contrast to low-frequency bands,in high-frequency bands,when selecting the metasurface material,it is also necessary to consider the structural scale and the difficult of the fabrication process.Thus,in addition to the metasurface material itself,the structural design of the metasurface is also of critical importance.

III.APPLICATIONS OF ANOMALOUS REFRACTIVE AND REFLECTIVE METASURFACES IN DIFFERENT WAVELENGTH BANDS

The design of anomalous refractive and reflectiv metasurfaces is strongly dependent on the wavelength range of the incident electromagnetic waves.Different wavelengths require different choices of materials and different structural details.Metasurfaces can be classifie into three groups,depending on the incident wavelength range to which they are to be applied:(1)the microwave band,(2)the THz band,and(3)the IR and visible bands.Various types of metasurface structure have been designed for application in these different wavelength ranges.

A.Applications in the microwave band

The microwave band ranges from 300 MHz to 300 GHz,with wavelengths ranging from 1 mm to 1 m,and it is characterized by high transmittance.Anomalous refractive and reflectiv metasurfaces in this band could be applied to military cloaking,40-42communications,43,44and focused heating.45According to the number of wavelengths that are modulated,microwave metasurfaces can be divided into two types:single-frequency-acting and multifrequency-acting.

A single-frequency-acting metasurface is a metasurface that achieves the same modulation effect for a single band of electromagnetic waves.As shown in Fig.2(a),negative-refractiveindex metasurface based on SRRs is able to perform narrowbandwidth negative-refractive-index regulation in the microwave range of 13.6-14.8 GHz,46but it has a narrow scope of application.Linet al.,47using a design based on the P-B phase,have made an important breakthrough in achieving a super-bandwidth reflecte light anomaly in the range of 7.7-19.9 GHz for circularly polarized light,which solves the narrow-bandwidth problem with regard to applications.Their metasurface is able to modulate incident linear or elliptically polarized light and split a beam into two polarized beams with left and right rotation.It can be used as a low-loss polarization device.However,the frequencies considered in current research on microwave refractive metasurfaces are mostly below 40 GHz,and fewer metasurfaces have been designed for the Wband(75-110 GHz)commonly used in current automotive radar and 5G communications.As shown in Fig.2(b),to fil this gap in Wband research,Huygens’principle was used to design a microwave metasurface for applications above 40 GHz ignored by existing microwave metasurface designs.48This high-efficienc transmissive Huygens metasurface in the W-band achieves negative refraction at 70-95 GHz.

Multifrequency-acting metasurfaces are able to produce different modulation effects for two or even more electromagnetic waves incident at the same time and are thus able to perform fil tering or beam splitting.Compared with single-frequency-acting metasurfaces,their information capacities and functionalities are greatly increased,thus reducing manufacturing costs and device size.The currently adopted approaches to achieving multifrequency modulation use either(1)passively tuned metasurfaces where the metasurface structure is designed directly to achieve simultaneous modulation of light at different frequencies or(2)actively tuned metasurfaces where the metasurface microstructure is adjusted by application of external control to change the tuning frequency or function.Among the latter,with the use of the generalized Snell’s law,it is possible to design a three-dimensional single-crystal structure49to generate different reflectio angles when two beams of orthogonally polarized light are incident at 18 and 32 GHz,and,in this way,filterin of electromagnetic waves at different wavelengths can be achieved with 98%reflectiv efficiency On the basis of the P-B phase,Yanget al.24designed the cascaded metasurface shown in Fig.2(c)for anomalous reflectiv and refractive modulation of left-handed polarized light in the 10.8-11.6 GHz band and of right-handed polarized light in the 6.0-6.3 GHz band,achieving good spectral characteristics.Through interference tuning of circularly polarized light,using a metasurface based on a double SRR with a concentric structure,absorption of left-rotation polarized waves and total reflectio of right-rotation polarized waves were achieved,with a tuning efficienc of around 95%.50A Janus metasurface has been designed based on anomalous refraction and reflection with a three-layer structure that expanded the available degrees of freedom,resulting in a six-channel function with an efficienc of about 75%in the microwave band.51In this structure,two surfaces generate two asymmetric anomalous reflectio channels each at wavelengthsλ1andλ2,with two anomalous refraction channels being generated at a wavelengthλ3.This metasurface is capable of achieving completely independent anomalous refraction and reflectio functions at different frequencies,polarizations,or angles,thus significantl enhancing functional diversity,albeit with considerable operational difficulties A conformal metasurface with a moderate curvature has been constructed with a structure using a rotating Jerusalem cross element for tuning,based on the P-B phase,which can phasecompensate any linearly polarized or circularly polarized light and anomalously reflec it back to a biplane wave.52Its reflectio effi ciency reaches 82%in the 8-18 GHz band and it can serve as a foundation for the further development of conformal metasurfaces providing anomalous refraction and reflectio functions.

FIG.2.Design of anomalous refractive and reflectiv metasurfaces in the microwave band.(a)Abnormalrefractive and reflectiv metasurface based on an SRR array design.46(b)Huygens-type anomalous refractive and reflectiv metasurface in the W-band.48(c)Multifunctionalabnormalrefractive and reflectiv metasurface based on the P-B phase.24(d)Design of focusing metalens formed from a double-layer gradient metasurface.54

Although multifunctional integration is possible for passive metasurfaces through methods such as same-layer metasurface design and multilayer anisotropic coding,once such a surface has been fabricated,it cannot be changed in response to actual conditions and needs.As shown in Sec.III,active tuning of a metasurface can be achieved by combining it with an active component,such as by adding a PIN diode to the middle of the metasurface.28In this case,by powering up and powering down the metasurface,it can be switched between refractive and reflectiv behavior in the 2.07-2.10 GHz band.Adopting a similar approach,Maoet al.53extended the application spectrum to achieve active tuning of a refractive and reflectiv metasurface in the 4.7-5.9 GHz range.

The properties of anomalous refractive and reflectiv metasurfaces can be exploited for applications to optical wave beam modulation.For microwaves,as shown in Fig.2(d),80%transmission efficienc was achieved using a wide-angle phase-gradient metasurface with a double-layer structure,which overcame the limitation of previous metasurface lenses that could only receive vertical incident light and achieved incident focusing below an angle of 40○,thereby extending the range of application of metasurface lenses.54As an antenna for directional reception of microwave signals,55an innovative metasurface lens has been designed that converts spherical electromagnetic waves into planar electromagnetic waves,improving the antenna gain and operating bandwidth.

In summary,the trend of development of anomalous refractive and reflectiv metasurfaces in the microwave band is toward high frequencies,ultrawide bandwidth,and multifunctionality.These metasurfaces can act as plane lenses to focus for microwaves,with applications in radar detection,5Gcommunication,and other fields

B.Applications in the THz band

The THz band ranges from 0.3 to 30 THz and is used in 6G communications,security inspections,56,57biomedicine,58and laminar microscopy.59,60To meet the requirements of these applications,active modulation of anomalous refractive and reflectiv metasurface in the THz band is essential.Unlike the materials required for active modulation in the microwave band,the THz band requires a combination of metasurface and phase-change dielectric materials to achieve device functionality.Currently,thermal phasechange materials and graphene are the dominant materials,together with semiconductors,61liquid crystals,62-64and superconductors,65which are also capable of achieving tunable functionality.

In the device shown in Fig.3(a),VO2is used as a thermal phasechange material combined with the metasurface.66At temperatures of 300 and 400 K,VO2undergoes transitions between insulating and metallic states,and by placing different types of metasurface at each end of the phase-change material,both refractive and reflec tive metasurfaces are obtained in the range 0.44-1.4 THz.However,this integration of two types of metasurfaces is at the cost of greater thickness.To solve this problem,as shown in Fig.3(b),incorporation of graphene into the metasurface enables anomalous refraction and and reflectio in the range 0.75-2 THz,67with active tuning being achieved by changing the Fermi energy without changing the microstructure.With this approach,the metasurface could be thin enough to allow its integration into portable devices.As shown in Fig.3(c),Luoet al.68also used graphene in combination with a metasurface to produce anomalous refraction in the 4.26-4.69 THz band and were again able to achieve active tuning.

There have been a number of investigations of the possible use of anomalous refractive and reflectiv metasurfaces in a variety of applications.For example,it has been demonstrated experimentally that a metasurface-based lens in the THz band can produce different holograms in different planes with 85%transmission.23The double-layer structure of this metalens,consisting of C-shaped antennas and a metal grating,does not require strict alignment,reducing the cost required for device mounting and maintenance.This design can also be used as a basis for future miniaturization and integration of such THz devices.Another tunable metasurface in the THz band has two different modes,with the reflectio mode providing polarization conversion of incident light,while the transmission mode achieves beam focusing.66This design can provide a foundation for the development of multifunctional metasurface devices.

FIG.3.Design ofanomalous refractive and reflectiv metasurfaces in the THz band.(a)Application of a thermalphase-change refractive and reflectiv metasurface based on VO2.66(b)and(c)Active anomalous refractive and reflectiv metasurfaces based on graphene.67,68(d)Vortex beam generation based on a reflectiv metasurface.69

According to the basic principle of phase tuning of a metasurface,an appropriate design of the metasurface structure will enable tuning of the reflectio and transmission processes of a light wavefront in such a way that orbital angular momentum(OAM)is generated.Encoding of information in OAM of light can theoretically increase information capacity indefinitely Therefore,in the THz band,a reflectiv metasurface was used to tune circularly polarized light,generating two types of vortex light in the 0.3-0.45 THz band in a simple and effective means of OAMgeneration,69as shown in Fig.3(d).Shi and Zhang70used a multilayer graphene structure designed as a reflectiv metasurface that can form vortex light in thel=±1-l=±3 mode.By tuning the Fermi energy,it was possible to tune the OAMbeam in the range of 1.8-2.8 THz.Another approach to realize a tunable OAM generator is by combining a transmissive metasurface with the thermal phase-change material VO2,71which enables the tuning frequency to be manipulated by controlling the state of the VO2through temperature.A diffraction-free OAM beam can thereby be generated,with tuning being achieved without any need to change the physical structure.A three-layer metallic structure has been used to fabricate a transmissive metasurface that providesl=±1,l=±2,andl=±3 modes of vortex light in the 0.3-0.9 THz range.72Using an SRR and a structured phase-gradient metasurface will allow simultaneously modulation of the transmission and reflectio of vertical incident bicircularly polarized light at THz wavelengths to produce two sets of vortex light in reflectio and transmission,73thereby realizing the generation of vortex beams in transmission and reflectio modes and overcoming the limitations of previous metasurfaces,which only provide either transmission or reflectio functions,and expanding the electromagnetic tuning space.

Thus,active tuning of metasurfaces in the THz band has been achieved through the integration of thermal phase-change materials or graphene,thereby providing tuning of anomalous refraction and reflection The use of thermal phase-change materials enables fast reversible tuning,but this is limited to two-state discrete tuning.With graphene,continuous electronically controlled tuning is possible,but the tuning range is small compared with that obtained with thermal control.Materials such as GST74,75and GaAs61can also be applied to the tuning of THz and IR-THz bands to achieve multifunctional integration.The next development goal in this area is the ability to achieve continuous multipoint modulation in the THz band with a large tuning range of anomalous refraction and reflection Another important direction of research in the THz band concerns the development of metasurface lenses and the generation of light with OAM light using anomalous refractive metasurfaces,with a wide range of potential applications in the field of quantum information,optical communication,etc.70,76

FIG.4.Design ofanomalous refraction and reflectio metasurfaces in the IR and visible bands.(a)Metasurface designed on the basis ofthe P-B phase.78(b)Metasurface with a high-contrastdesign.80(c)Metasurface with a Huygens design.81(d)Adjustable metalens structure incorporating an abnormalrefraction and reflectio metasurface and MEMS technology.82

C.Applications in the IR and visible bands

The IR and visible wavelength bands range from 450 to 1 mm.In this wavelength range,high-frequency electromagnetic waves are strongly dissipated in metals,77which poses a major obstacle to the design of metasurfaces.However,all-dielectric metasurfaces with low Ohmic loss and high transmittance offer a solution to this problem and are key to the generation of anomalous refraction and reflection All-dielectric metasurfaces can be broadly classifie into three categories:P-B phase modulation metasurfaces,high-contrast metasurfaces,and Huygens metasurfaces.

In the all-dielectric design shown in Fig.4(a),a P-B phasebased graphene metasurface produces anomalous reflectio of circularly polarized light in the IR region.78In an example of the second approach,a topological algorithm was used to design a high-contrast metasurface incorporating a silicon-based wavelength grating.79Anomalous refraction was achieved in the visible and near-IR regions,with an efficienc reaching 60%-90%.Taking advantage of the high refractive index of SiGe in the visible region,80the phase gradient grating shown in Fig.4(b)was able to achieve negative refraction in the range from 400 to 470 nm.The Huygens metasurface design scheme shown in Fig.4(c),which was proposed by Tianet al.,81uses disk-shaped nanoparticles,and it was able to achieve optical negative refraction at 1.32μm and a transmittance of about 70%in the near-IR band.Compared with the firs two types of all-dielectric metasurface,Huygens metasurfaces are easier to prepare,and the experimental results cited here demonstrate that they have great promise in the fiel of ultrathin optical lenses and optical chips.

A variety of applications have been proposed for anomalous refractive and reflectiv metasurfaces in the IR and visible bands.Column and aspheric lenses have been designed using all-dielectric Huygens metasurfaces.81Transmission efficiencie of 73%and 68%,respectively,have been achieved at 1.32μm,and the low aspect ratios of these lenses make them easy to fabricate and enable their application in compact precision devices such as those for hyperspectral imaging and in high-energy lasers.Figure 4(d)shows a metasurface lens combined with microelectromechanical systems(MEMS)technology.82It uses a variable-focal-length conformal lens mechanism to achieve a focal shift at 1.55μm incident wavelength.Aietaet al.83designed a metasurface that could be considered as a three-wavelength achromatic lens and,unlike previous achromatic elements,was not thick,complex,and costly.They were able to overcome the chromatic aberration produced by previous refractive and diffractive lenses.The excellent performance of this lens allows its application in a near-IR compact spectrometer.

The above three types of all-dielectric metasurfaces use different principles to realize anomalous refraction and reflectio in the IR and visible bands.Compared with metallic materials,metasurfaces made of all-dielectric materials have high transmission efficienc and can realize high-band light focusing and achromatic planes,making them suitable for a variety of applications.However,technical limitations mean that most such metasurfaces currently suffer from narrow bandwidth and relatively low efficiency Also,because of manufacturing difficulties many studies have only conducted theoretical simulations without experimental testing,which means that it is impossible to verify the results of these simulations.Therefore,key problems remain with regard to how the transmission spectra of these refractive and reflectiv metasurfaces can be broadened,how their transmission efficiencie can be improved,and how their fabrication can be made easier and cheaper.

IV.OUTLOOK FOR ANOMALOUS REFRACTIVE AND REFLECTIVE METASURFACES

A.Prospects for multifunctional anomalous refractive and reflective metasurfaces

Today,the development of anomalous refractive and reflectiv metasurfaces is moving toward multifunctionality,integration,and conformality.For passive refractive metasurfaces,a double-sided nanostructure design will enable multifunctional,wide-bandwidth and multi-angle electromagnetic wave tuning.By designing different metastructures on the two sides of such a metasurface and using an information-encoding design for the nanostructure,multifunctional modulation under different polarization conditions,wavelengths,and angles of incidence can greatly expand both functionality and degrees of freedom.For active anomalous refractive metasurfaces,the use of active or nonlinear materials can allow dynamical control of surface morphology and functionality.The functionality and operability of anomalous refractive and reflectiv metasurfaces can be enhanced by the use of a cascade of metasurfaces together with a phase-change material through which functional switching can be realized by the application of an external excitation.

However,there remains much scope for the further development of multifunctional anomalous refractive and reflectiv metasurfaces.For example,multiple-degree-of-freedom coupling is still a very complex process when considering the design of metasurfaces with a two-sided microstructure.Other important areas of potential research include the exploration of new materials suitable for use in dynamic multifunctional metasurfaces,the refinemen of optical antenna fabrication processes,and methods for the alignment of cascaded metasurfaces.Further efforts are also needed to improve the efficienc of these devices,the signal-to-noise ratios of the images obtained,and the dynamic tuning rate.

B.Prospects for the application of anomalous refractive and reflective metasurfaces

Anomalous refractive and reflectiv metasurfaces have a wide variety of exciting possible applications.They are capable of generating vortex beams by wavefront shaping,which is a critical aspect of encrypted communication,and there is a current trend to integrate metasurfaces into multichannel vortex beam schemes.Through encoding of nanostructured arrays of metasurfaces,a multiplexing technique is used to split the information contained in different incident signals,which are resolved in different channels of the metasurface and then outputted.Such a design is able to significantly increase information capacity and is likely to become widely adopted.

In the fiel of holography,there are important roles for these metasurfaces in interferometry,stereo imaging,medical detection,and image datafication Reversing the design of an anomalous refractive and reflectiv metasurface and encoding polarization or phase information can create more degrees of freedom and expand information capacity,thereby overcoming the disadvantages of diffraction multilevel interference,small field of view,and narrow bandwidth that arise in the conventional case,as well as greatly reducing noise in images.

There are also various directions for the development of anomalous refractive and reflectiv metasurfaces in the fiel of electromagnetic wave shielding and military stealth.On the one hand,by enabling absorption and anomalous reflectio of incident electromagnetic waves,these metasurfaces can significantl reduce reflecte radar waves and enhance stealth capability.On the other hand,flexibl metasurfaces have become an interesting topic of research,in particular with regard to methods for enhancing their robustness.Their flexibilit will broaden the range of applications of anomalous refractive and reflectiv metasurface by enabling their attachment to uneven surfaces.

V.CONCLUSION AND OUTLOOK

This paper has reviewed the progress of research on anomalous refractive and reflectiv metasurfaces.The driving modes of these metasurfaces have been described,they have been classifie in terms of wavelength bands,methods for their structural design have been compared,and scenarios for their application in different bands have been described.Finally,the prospects for application of anomalous refractive and reflectiv metasurfaces have been presented,and directions for future research have been outlined.

However,despite the encouraging developments in metasurface technology,many problems and obstacles remain.For example,challenges remain regarding how precise regulation of electromagnetic waves on a metasurface can be ensured to achieve multifrequency co-regulation.The search for new dielectric materials to enable different ways of tuning and new functions of these metasurfaces is another important task.In the visible region,narrow bandwidth and low conversion efficienc impose limitations on regulation of metasurface functionality.In the context of experimental studies and practical applications,the problem arises that most of the current fabrication processes for metasurfaces are very cumbersome and do not lend themselves to mass production.Indeed,some theoretical metasurface models can only be simulated,and experimental test is unavailable.This presents an obstacle to the productization of metasurfaces.If the above problems can be solved,this will allow simplificatio of the design and fabrication processes and the realization of more powerful functions,leading to a wider range of applications for these metasurfaces in the military,communications,scientific and biomedical fields

ACKNOWLEDGMENTS

This work was supported by the Chinese Academy of Sciences Strategic Pioneering Science and Technology Special Project(XDA28050200),the Jilin Province Science and Technology Development Program in China(20200403062SF,20200401141GX,20210201023GX,20210201140GX,and 20210203059SF),the Chinese Academy of Sciences Research Instrumentation Development Project(YJKYYQ20200048),and the Science and Technology Innovation Platform of Jilin Province(20210502016ZP).

AUTHOR DECLARATIONS

Conflict of Interest

The authors have no conflict to disclose.

DATA AVAILABILITY

Data sharing is not applicable to this article as no new data were created or analyzed in this study.