Beam deflectors, which are able to change or control the propagation direction of the beam in free space, are important optical components in integrated optical circuit and optical communication systems. However, with the development of miniaturization of the optical systems, conventional reflector-based mechanical beam deflectors confront a huge challenge due to their large size and incompatible to the device integration. Recently, metasurfaces, also known as two-dimensional metamaterials, have attracted significant attentions due to their ultrathin thicknesses and perfect con-trolling of amplitude, phase and polarization of the beams. On account of full 2- phase control, metasurfaces are widely used in lensing, holograms, wave plates and other applications. The original metasurfaces are mainly designed using metallic resonant structures. However, metallic metasurfaces always have large ohmic losses, which are similar to the plasmonic structures. To overcome the loss issue, metasurfaces using dielectrics, such as silicon and titanium dioxide (TiO2), appear and are widely employed in the novel optical devices’ design. Here we propose and design an all-dielectric flat metasurface beam deflector which is composed of a single layer array of TiO2 nanoantennas resting on a fused-silica substrate. The TiO2 nanoantennas are considered as birefringent elements and the Jones transfer matrix can be used to model electrometric response of each TiO2 nanoantenna. Based on the phase discontinuity principle, we design the beam deflectors that operate at the wavelengths of 450 nm, 532 nm, and 633 nm, respectively. For the circularly polarized in-cident light, the polarization conversion efficiencies of the designed beam deflectors are all higher than 90% at the oper-ation wavelength. Numerical simulations based on the finite-difference time-domain (FDTD) algorithm show that de-flecting behaviors of the proposed devices with deflection angles of 15°, 30° and 45° are all in excellent agreement with our theoretical predictions. The simulated optical transmissions of the designed deflectors are 88.2%, 86.8% and 71.3% for 15°, 30° and 45° at wavelength of 450nm; 86.7%, 86.4%, 69.7% for 15°, 30° and 45° at wavelength of 532nm and 89.3%, 80.6%, 62.0% for 15°, 30° and 45° at wavelength of 633nm, respectively. Compared with other thin-film plasmonic beam deflectors using metallic nanoslits, the transmission efficiencies of the metasurface beam deflectors are much higher. The all-dielectric metasurface beam deflector may have potential applications for manipulation of the light propagation in the high-integration optical systems.

The seminal study reported in 2011 demonstrated that arbitrary abrupt phase of scattering wave in 2π range can be realized by spatially tailoring the geometry of nanoantennas with deep-subwavelength sizes both in horizontal and vertical directions. Quite different from the traditional optics, the abrupt phase is generated from the resonance of the nanoantenna, rather than the accumulation of propagation in space or dielectric materials. Thereupon, metasurfaces composed of such nanoantennas can break the thickness limitation of traditional optical devices, with the advantage of flexible phase distribution arrangement, leading to a bright prospect in highly integrated nano-optical system. A lot of works have been reported that metasurfaces are ability of flexibly manipulating the wavefront of scattering, leading to applications of ultrathin flat metalenses, beam shaper, quarter-wave plates, optical holography, optical vortices generation, anomalous light bend etc. Although the metasurface is regarded as the alternative for the next generation optical device, how to improve the efficiency for the transmission light is still a challenge. Two approaches are generally used. One is to set the operation mode as reflection, i.e. the light source and the target light are on the same plane respect to the metasurface. Nanoantennas with high reflective coefficient are easier to be designed in comparison with high trans-mission coefficient, especially in metallic metasurfaces. The other way is to replace the host material as dielectric. Due to the low loss, ratio of the transmitted port of incoming light is weighted. The cost, however, is the increased profile. In popular, all the metasurfaces mentioned above are discrete, i.e. the neighbor nanoantennas are unconnected in physical configuration, yielding a phase profile of discontinuous. In this paper, we verify that structure of phase continuity can enhance the manipulation efficiency by suppressing high-order diffractions of nanoantennas. The sine-shaped metallic meanderline fabricated by focused ion beam technology converts circularly polarized (CP) light to its opposite handed-ness and sends it into different propagation directions depending on the polarization states in near-infrared and visible frequency regions. The beam splitting behavior is well characterized by a simple geometry relation, following the rule concluded from other works on the wavefront manipulation of metasurface with phase discontinuity. Importantly, the meanderline is demonstrated to be more efficient in realizing same functions due to the suppressed high order diffrac-tions resulted from the absence of interruption in phase profile. The theoretical efficiency reaches 67%. Particularly, po-tential improvements are feasible by changing or optimizing shape of the meanderline, offering high flexibility in appli-cations for optical imaging, communications and other phase-relative techniques. Additionally, since the continuous phase provided by the meanderline can improve the sampling efficiency of the phase function, it is helpful in realizing high quality hologram.

Hiding a detector is not a nascent topic yet, which has many fascinating and important applications in both civil and military areas. In antenna fields, microwave engineers often use an absorbing or frequency-selective layer to construct a radome in order to reduce the radar-echo of inner antenna. However, the absorbed or redirected incident wave is out of the operating band of the antenna, thus having no consequential help on the reduction of antenna’s in-band echo. As a result, it is a challenge to reduce the antenna’s in-band echo, which has attracted much attention during the last 50 years. In this paper, we designed and demonstrated a low-echo metamaterial (MM) shelter with in-band electromagnetic narrow window. The unit-cell of the MM shelter is composed of two basic elements: a metallic square-loop with two small gaps in each side, and an electric-resonant-ring (ERR) embedded into a sub-wavelength aperture. Eight chip-resistors are respectively inserted into the gaps of square-loop to provide necessary lossy source. When the electromagnetic wave impinges the square-loop side, most of the energy would be absorbed by these chip-resistors, provided that the oscillating frequency does not coincide with the resonant frequencies of the ERR. At the resonant frequency of the ERR, the interplay of the transmission and absorption may greatly increase the transmitted energy through the aperture decorated with ERR. By introducing a selective re-emission mechanism of the energy captured by an absorbing layer, we observe that at least half of energy can pass through the narrow window located into the absorption band, and the broad low-echo feature is not influenced. The performance of the MM shelter and its ability for shading electromagnetic receiver is demonstrated by carrying out a numerical experiment, including three main portions: a dipole array acting as a signal receiver, a remote point source with tunable amplitude utilized to model the far-field excitation of a signal source, and an inserted MM shelter. This device is believed to be suitable for the in-band scattering reduction of antenna which is especially designed to work in the receiving mode. We expect that, the concept and design reported here will influence the future design of electromagnetic absorber and radome, generating a new research hot topic in electromagnetic invisibility field. Our design may be applied in wireless local area network to cancel additional multi-paths, or signal degradation because it can effectively absorb useless signals without significantly attenuating mobile phone signals.

Circular dichroism (CD) is helpful in providing useful knowledge about the structure of biological macromolecules. However, CD generated by intrinsic chiral materials is always too weak so it is inefficient to probe the micro-structure of biological molecules. To enhance the CD signal, chiral metamaterials composed of artificial meta-atoms with controllable chirality, have been developed in the past few years. Besides chiral metamaterials, recent theoretical and experimental results have shown that planar metasurfaces can also lead to CD at oblique incidence. In principle, both absorption and scattering of circularly polarized light contribute to CD. Nevertheless, scattering of incident left and right circularly polarized light can be an important contribution to the CD of the molecules, whose dimensions are greater than 1/20 of the incident wavelength. Therefore, abnormal scattering will induce a considerable CD.

The ability to manipulate the polarization of electromagnetic waves is sought-after for numerous applications. Traditional polarization rotation devices utilizing natural occurring birefringence or total internal reflection effects are bulky. As an alternative solution, metamaterial-based converters exhibiting strong anisotropy or chiral can be extremely compact and thus flourished in the last decade. Nevertheless, metamaterial-based schemes suffer from the narrow bandwidth due to their highly dispersive meta-molecules.

Manipulation of terahertz wave by metasurfaces has shown tremendous potentials in developing compact and functional terahertz optical devices. However, there are still some obstacles that limit the practical applications of these meta-devices, such as low working efficiency and narrow operating bandwidth. Here, we propose complementary bilayer metasurfaces for enhanced terahertz wave amplitude and phase manipulation. The metasurfaces are composed of one layer of metal cut-wire arrays and one layer of their complementary aperture arrays separated by a dielectric spacer. The complementary apertures in the metasurfaces give rise to extraordinary optical transmission. When metal cut-wires are positioned near the apertures, the structures can manipulate the cross polarization conversion and phase dispersion of terahertz wave through the near-field coupling between transverse magnetic resonances in the metal apertures and elec-tric resonances in the metal cut-wires. Particularly, when the thickness of the dielectric spacer is 8 μm and the rotation angle between the cut-wire and the aperture is 45°, the metasurfaces demonstrate a phase delay of 180° between two or-thogonal axes with the same transmission amplitude between 0.70 and 1.0 THz, enabling a 45° broadband polarization conversion. A transmission peak at 0.25 THz can be observed for the co-polarized light. This peak corresponds to the extraordinary optical transmission effect in the metal apertures. A small peak for cross-polarized light at 0.25 THz cor-responds to a weak excitation of the dipole resonance in the metal cut-wires. Numerical simulated surface current dis-tributions in these two layers show opposite directions, indicating that a magnetic dipole can be formed within the cir-culating currents between the aperture and the cut-wire. The strong coupling between these two layers leads to a trans-mission peak at 0.80 THz. Furthermore, the phase dispersion of the transmitted light is modified by this coupling effect and a phase delay of 180° between 0.70 and 1.0 THz is achieved. When the metal cut-wires are rotated with respect to the apertures, the phase delays maintain 180° in a broadband with a small shift of the frequency. When thickness of the die-lectric spacer increases, the resonance frequency of the metal aperture decreases, while the frequency of the coupled magnetic dipole resonance increases. When the thickness of the dielectric spacer is smaller, a larger transmission peak for the cross-polarized light is achieved. This indicates that a thinner dielectric spacer would provide a stronger coupling between the aperture and the cut-wire. Meanwhile, with a thinner dielectric spacer, a broader bandwidth for the phase delay of 180° can be realized. Such complementary coupled bilayer metasurfaces offer a new method to control the am-plitude and phase dispersion of terahertz wave and promise great potential for applications in terahertz meta-devices.

Three dimensional (3D) metamaterials are artificial micro-structures periodically arranged in three-dimen-sional space. The lattice constant of metamaterial is much smaller than the wavelength of the incident EM wave, so the metamaterial can be treated as an artificial effective medium for the propagating of the EM waves. By controlling the geometrical parameters of its lattice, the metamaterials can be developed with properties never found in nature, which can be utilized to manipulate the propagation of EM waves, such as blocking, absorbing, capturing, or bending waves. With these magic properties, 3D metamaterials get bright prospects for applications in stealth technology, communication technology, optical imaging and sensing technology.

Thermal energy has been proposed to have ever greater potential for human beings if the heat carriers, phonons can be controlled in micron-scale as easy as its counterpart, electrons in solid. However, it is a challenge to control phonons due to its relatively short wavelength, which is in the order of a few nanometers to a few tens of nanometers. Alternatively, in macroscopical scale, functional thermal materials are used to control thermal energy. The transformation of macroscopical thermal diffusion equation is proposed to obtain the asymmetrical thermal conductivity in real space. This new type of thermal functional materials helps to control heat flow and to realize thermal cloak and thermal camouflage. In this review, we summarize the recent advances in constructing thermal functional materials (also called thermal metamaterials). In Sec Ⅰ, we discussed the history of functional materials and the principles of constructing thermal functional materials , special focus was given to the thermal cloak, followed by the realization of thermal cloak in Sec Ⅱ.Thermal camouflage, based on the realization of thermal cloak, was discussed in Sec Ⅲ, which is proposed to have great potentials in military usage. We stressed both the principle and practical based challenges in thermal cloak and thermal camouflage in Sec Ⅳ, in which outlooks were also given.

Metamaterials, with artificially engineered periodic structure, has attracted a great deal of research attention, for the ability of manipulating the path of propagation of light or electromagnetic wave, sound or acoustic / elastic wave, heat or thermal wave and the possibility of cloaking objects from a certain incoming physical radiation, which has brought the invisibility or stealth, a tantalizing concept for mankind over several centuries in the science fiction to a technological reality. As a relative new member of the metamaterials family, thermal metamaterial has also gained much attention from the very beginning, and has been intensively investigated in recent years, for the promising ability of controlling the conduction of heat or the distribution of temperature. Thermal metamaterials and their applications in thermal management are significant in the design of electronic devices and systems, such as supercomputer, solid-state laser and solid-state lighting, high power microwave devices, thermoelectric energy harvesters and thermal imagers, where the thermal design plays a key role in performance and device reliability.

Pentamode metamaterials (PMs) are one of the artificial periodic materials for which the six eigenvalues of the effective elasticity tensor only take one non-zero but five zero. Hence，PMs are also called “metafluids” by making the bulk modulus B extremely large compared to the shear modulus G. PMs with anisotropic elastic tensor have potential applications for acoustic cloaking, noise insulation, and other special acoustic devices. So it is concerned by scientists. In the review, pentamode materials and their recent progress are introduced. It includes the concept of PMs, acoustic and elastic properties of Bragg scattering PMs, locally resonant PMs, fabrications and measurement methods.

Perception of color with our eyes is one of the major sources of information that we gain from our surroundings. The color of an object depends on which portion of light (range of wavelengths) reaches our eyes. In nature, structural colors are often caused by the interaction of light with dielectric structures whose dimensions are on the order of visible-light wavelengths. For example, in beetles, the color is originated from the microstructure of the skin which is acting as scattering center; while in some butterflies, the colorful patterns are routed from the reflection from the top of the wings. Different optical interactions, including multilayer interference, light scattering and photonic crystal effect, give rise to selective transmission or reflection of particular light wavelengths, which leads to the generation of structural colors. With the consumption of dyes and pigments, recycling of colored discarded materials has been a very difficult issue because of the hardships in relation to the dissociation of diverse chemical compounds present in the colorant agents. Plasmonic colors therefore draw attention as they enable generation of vivid colors only by geometrical arrangement of metals which not only eases the recycling but also enhances the chemical stability of the colors. Plasmonic colors are structural colors that originate from the interaction between light and metallic nanostructures. Rapid development in nanofabrication and characterization of plasmonic structures provides an efficient way to control light properties at subwavelength scale, which can generate plasmonic structural colors. The engineering of plasmonic colors is a promising, rapidly emerging research field that could have a large technological impact. Artificial surfaces, in particular, on which the colors are generated via a resonant interaction between light and subwavelength metallic nanostructures, have emerged as nanomaterials or metamaterials for the realization of structural colors. Here we introduce several representative plasmonic nanostructures which can generate visible structural colors, including nanogratings, perforated metallic films, metal-insulator-metal resonators, dynamically tunable color generators and perfect absorbers. We highlight the properties of plasmonic colors and discuss the intrinsic plasmonic resonance mechanisms. Plasmonic structural colors have features of sub-diffraction localization, high-fidelity color rendering and rapid responses of external changes, which are believed to offer a promising future in the applications including ultra-high resolution color display, spectral filtering and sensing, holography, three-dimensional stereoscopic imaging and real-time colors controlling with extremely com-pact device architectures.

Since it was firstly illustrated by the pronounced prism experiment of Isaac Newton, the chromatic dispersions in light matter interaction have been extensively explored. It is generally thought that the dispersion of materials introduces a significant wavelength dependence of the group velocity, leading to undesired signal distortion in communications system, chromatic aberration in imaging system and limited bandwidth of optical devices. However, if dispersions can be properly controlled, they will play a significant role in many applications. For example, dispersion management will suppress the nonlinearities of fiber dense wavelength division multiplexing (DWDM) system and the soliton propagation. Chromatic aberration can be corrected approximately by using materials that exhibit complementary dispersion. Nevertheless, because the dispersion of natural materials is determined by the electronic and molecular energy levels, traditional dispersion management technologies are cumbersome and cannot be required in integrated optics.

Full-color 3D Meta-holography with extended viewing angle is realized by a single layer of nanostructured meta- surface.

Lasing and anti-lasing can get along well with each other in a single cavity.

Data capacity is rapidly reaching its limit in modern op-tical communications. Optical vortex with spiral wave-front has been explored to enhance the data capacity for its unbounded quantum states of orbit angular momen-tum (OAM). However, traditional devices used to gener-ate OAM carrying beams suffer from bulky size. Surface plasmon polaritons (SPPs) have created an appealing platform to design various optical components with small footprints. Ultrathin plasmonic metasurfaces with phase abruptions exhibit the ability to break the traditional refraction and reflection law, which makes it possible for compact OAM generators.

Metamaterials (MMs) composed of periodic resonant subwavelength structures exhibit exotic electromagnetic properties that do not exist in nature, and open an avenue for electromagnetic waves (EMWs) manipulation. Dispersion is an inherent property of MMs. By engineering the electromagnetic resonances of MMs, extraordinary dispersion can be achieved thereby one can break the traditional physical laws and manipulate the EWMs at will. Subsequently, a serial of applications emerge including super-resolution imaging/lithography, electromagnetic absorber/radiator and planar optical devices. In this review, we summarize several typical approaches, theories and relevant applications of dispersion engineering of MMs.

Plasmonic structural colors originate from the interaction between light and metallic nanostructures. Rapid development in nanofabrication and characterization of plasmonic structures provides an efficient way to control light properties at subwavelength scale, which can generate plasmonic structural colors. Here we introduce several representative plasmonic nanostructures which can generate visible structural colors, including nanogratings, perforated metallic films, metal-insulator-metal resonators, dynamically tunable color generators and perfect absorbers. We highlight the properties of plasmonic colors and discuss the intrinsic plasmonic resonance mechanisms. Plasmonic structural colors have the advantages such as ultra-small patterns and rapid response of external change, which are believed to offer a promising future in high-resolution color displays and real-time colors controlling.

Pentamode Metamaterials (PMs) with anisotropic elastic tensor have potential applications for acoustic cloaking, so it is attracted a lot of research interest. In the review, pentamode materials and their recent progress are introduced. It includes the concept of PMs, the acoustic and elastic properties of Bragg scattering PMs and Local resonant type of PMs. The fabrications and measurement methods are also introduced. PMs perturbed structures have advantages of anisotropic elastic tensor and 3D complete acoustic bandgap, therefore they pro-vide a way for low-frequency acoustic cloaking.

As a new member of the metamaterial family, thermal metamaterial has gained much attention from the very beginning, and has been intensively investigated in recent years. Based on the key technology of thermal metamaterials, the basic theory of coordinate transformation as well as the novel properties of thermal metamaterials is introduced, and the recent progresses of thermal metamaterials, such as thermal cloaking/stealth, thermal protection, thermal management, thermal Information and other aspects of applications have also been reviewed in this paper. Based on the research status and development trend of thermal metamaterials, we systematically classify and sort out the contents and characteristics of relevant research in recent years, and make some prospects about the future applications of thermal metamaterials in the fields of cloaking, thermal management, information and so on.

Thermal energy has been proposed to have ever greater potential for human beings if the heat carriers, i.e. phonons can be controlled in micron scale as easy as its counterpart, electrons in solid. Alternatively, in macroscopical scale, functional thermal materials are used to control thermal energy. The transformation of macroscopical thermal diffusion equation is proposed to obtain the asymmetrical thermal conductivity in real space. This new type of thermal functional materials helps to control heat flow and to realize thermal cloak and thermal camouflage.

Three dimensional (3D) metamaterials are periodic artificial microstructures arranged in three-dimensional space. They are in scale of subwavelength, thus getting extraordinary physical properties which are not found in nature. The recent developments on basic theory and manufacturing process of the 3D metamaterials are discussed in this letter. Manufacturing processes of the 3D metamaterials are sorted out, including printing circuit board and assembly, machining technology, 3D printing technology, micro/nano-manufacturing technology. As the representative devices of 3D metamaterials, the electromagnetic cloaks, lens antennas, absorbers and flexible metamaterials are discussed to show how the 3D metamaterials control the electromagnetic-wave. Devices of left-handed metamaterials, graded refractive index metamaterials, intelligent metamaterials are also discussed. Finally, the development trends of 3D metamaterials in future are discussed and predicted according to the present problems of 3D metamaterials.