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Wideband perfect absorbers are widely demanded for various applications, including efficient photodetection, radiation cooling. However, achieving perfect absorption across an extensive range of wavelengths on engraved structured substrate remains a challenge due to the complex light responses. It presents a hierarchical conical metasurface that demonstrates distinct perfect absorption from visible to far‐infrared range (0.4–16 µm), which is composed of surface‐engraved high aspect ratio nanogratings on microcones. The perfect absorption in the typical reststrahlen band of 4H‐SiC primarily relies on a light‐trapping‐enhanced surface phonon polaritons (SPhPs) mechanism, where the microcones effectively confine and enhance the SPhPs excited by the nanogratings through light trapping. Outside the reststrahlen band, the dominant mechanism for light antireflection is the light trapping of microcones assisted by nanogratings, which act as an equivalent antireflection layer. The hierarchical cones exhibit exceptional broadband performance, achieving an average absorptance of 98% over the spectrum of 0.4–16 µm with wide‐angle feasibility, and are fabricated using one‐step ultrafast‐laser ablation with ring‐shaped vector beams. Notably, the hierarchical cones can be applied to various materials, showing significant potential for use in photodetectors. [ABSTRACT FROM AUTHOR]

A temperature‐tunable terahertz (THz) perfect absorber (PA) composed of a periodic array of deep subwavelength micro‐cross‐shaped (MCS) structures of the strontium titanate (STO) resonator is proposed and investigated theoretically, which can be applicable for the temperature sensing. Simulation results indicate that the absorbance of 99.8% at 0.221 THz can be achieved when the designed PA is under the room temperature of T = 300 K, which is in good agreement with the calculation done by the coupling mode theory (CMT). The simulated distributions of electric and magnetic fields in the unit‐cell structure of the designed PA reveal that the observed perfect absorption is mainly attributed to the Mie resonance of the all‐dielectric MCS structure STO. In addition, this designed PA is polarization‐insensitive and wide‐angle absorption for both transverse magnetic (TM) and transverse electric (TE) waves. The resonance absorption properties of the designed PA can be controlled by changing the geometrical parameters of STO resonator structure. The designed PA can be served as a temperature sensor, which has a sensitivity of about 0.37 GHz K−1 since the electrical property of the STO is dependent on the variation of surrounding temperature. Furthermore, the perfect absorption can also be achieved by the PAs using the square, circular, and ring STO structures. The proposed design concepts of the STO‐based tunable PAs can find potential THz applications in sensing, detecting, imaging, and so on. [ABSTRACT FROM AUTHOR]

Terahertz metamaterial absorbers (TMAs) can efficiently absorb electromagnetic wave in the range of 0.1–10 THz based on the tunable electromagnetic properties of artificially designed unit cells. TMAs can achieve perfect absorption, as well as broad band absorption. Specifically, TMAs can break the thickness limit of a device at a quarter wavelength, realizing ultra‐thin devices so as to facilitate integration with other systems. Accordingly, TMAs have high application value in communication, imaging, detection, security inspection, and other fields because of their characteristics of small size, perfect absorption, less restriction of natural materials, and tunable electromagnetic parameters. This review covers the fundamental principles and recent advances of TMAs, from the essential structural features of TMAs and working principle of various types of TMAs to the design and applications of corresponding structures, especially the TMAs that can be tuned by various approaches. The development trend and challenges of TMAs are briefly discussed too. It is envisioned that this review can help R&D of the future TMAs for real applications. [ABSTRACT FROM AUTHOR]

A very large dynamic optical reflection modulation from a simple unpatterned layered stack of phase‐change material ultra‐thin films is experimentally demonstrated. Specifically, this work demonstrates that properly designed systems comprising deeply subwavelength GeSbTe (GST) films, a dielectric spacer, and a metallic mirror produce a dynamic modulation of light in the near‐infrared from very strong reflection (up to R≈80%) to perfect absorption (A>99.995%) by simply controlling the crystalline state of the phase‐change material. While the amplitude of modulation experimentally reaches an optical contrast higher than 104, intermediate levels of reflection in between extreme values can also be actively encoded, corresponding to partial crystallization of the GST layer. Several layered system designs are further explored and guidelines are provided to tailor the efficient wavelength range, the angle of operation, and the degree of crystallization leading to perfect absorption. [ABSTRACT FROM AUTHOR]

Metasurfaces allow for the spatiotemporal variation of amplitude, phase, and polarization of optical wavefronts. Implementation of active tunability of metasurfaces promises compact flat optics capable of reconfigurable wavefront shaping. Phase‐change materials (PCMs) are a prominent material class enabling reconfigurable metasurfaces due to their large refractive index change upon structural transition. However, commonly employed laser‐induced switching of PCMs limits the achievable feature sizes and restricts device miniaturization. Thermal scanning‐probe‐induced local switching of the PCM germanium telluride is proposed to realize near‐infrared metasurfaces with feature sizes far below what is achievable with diffraction‐limited optical switching. The design is based on a planar multilayer and does not require fabrication of protruding resonators as commonly applied in the literature. Instead, it is numerically demonstrated that a broad‐band tuning of perfect absorption can be realized by the localized tip‐induced crystallization of the PCM. The spectral response of the metasurface is explained using resonance mode analysis and numerical simulations. To facilitate experimental realization, a theoretical description of the tip‐induced crystallization employing multiphysics simulations is provided to demonstrate the great potential for fabricating compact reconfigurable metasurfaces. The concept can be applied not only for plasmonic sensing and spatial frequency filtering, but also be transferred to all‐dielectric metasurfaces. [ABSTRACT FROM AUTHOR]

Broadband perfect absorbers are of great importance for various applications such as stealth technologies, thermal photovoltaic cells, optical communication, photodetection, and photovoltaic devices. However, they are often made by lithographic techniques with high cost and complexity, which hinders their practical applications. The design of broadband and ultrahigh absorber at low cost and ease in fabrication is still a big challenge in the field of optics. Here, an all‐solution‐processed plasmonic perfect absorber is proposed and demonstrated by incorporating Mxene Ti3C2Tx (Tx = F, O or OH) and gold nanoparticles (NPs) in metal–insulator–metal configuration. The absorption of the designed plasmonic absorber can reach up to 99% with polarization‐ and angle‐independence in a broad range of wavelengths from ultraviolet to near‐infrared region. This ultrahigh and broadband absorbance is attributed to the synergy of plasmon resonance, magnetic resonance, and cavity effect between gold NPs layer and Mxene film, which is further corroborated by simulations of the finite element method for both random and patterned orientation of the gold nanoparticles. The excellent optical property together with the easy fabrication method paves the way for preparing photon‐absorbing nanostructures in photocatalysis and solar photovoltaics. [ABSTRACT FROM AUTHOR]

A thin screen exhibiting dynamically switchable transmission/absorption functionalities is highly desired in practice. However, a trilayer transmissive metasurface with active elements controlled in a uniform manner cannot exhibit independently controlled transmission and absorption properties due to intriguing interplays between the scattering and absorbing properties in systems exhibiting inversion symmetries. This motivates to employ the coupled‐mode theory to establish a generic phase diagram for such transmissive metasurfaces with active elements loaded in different layers tuned independently and to guide researchers design tunable metadevices with completely decoupled transmission/absorption responses. Based on such a phase diagram, a microwave metasurface is designed/fabricated with PIN diodes incorporated, and it is experimentally demonstrated that its functionality can switch from perfect transparency to perfect absorption, controlled by external voltages applied across the diodes. In addition to finding immediate applications in practice (e.g., smart radomes), the results of this study also provide a new type of tunable meta‐atom for building metasurfaces with flexible wave‐front control abilities. [ABSTRACT FROM AUTHOR]

A metamaterial perfect absorber based on layered, doped and undoped semiconductors is experimentally and theoretically investigated. Design rules are given to control the multispectral, narrow, strong absorption features (>98% absorption) in the mid‐IR spectrum. The proposed sub‐wavelength grating structures support localized surface plasmons and photonic resonances associated to the quarter wavelength optical thickness of the undoped spacer layer. The resonances hybridize depending on the geometric setup of the metamaterial and the material properties of the heavily doped semiconductor. The angular response of the resonances is studied to determine the acceptance angle. While the absorption is best at near‐normal incidence, the spectral position of the absorption bands is weakly dependent on the angular incidence up to angles of 45°. Complementary to the polarization‐dependent grating design of the metamaterial, an alternative based on a crossed grating nanopattern is also investigated for polarization insensitivity. Furthermore, it is demonstrated that the metamaterial perfect absorber provides a tailored thermal emission spectrum. Based on the heavily doped InAsSb/GaSb metamaterial platform, integrable and miniaturized perfect absorbers and thermal sources can be built. [ABSTRACT FROM AUTHOR]

Spatial overlap between the electromagnetic fields and the analytes is a key factor for strong light-matter interaction leading to high sensitivity for label-free refractive index sensing. Usually, the overlap and therefore the sensitivity are limited by either the localized near field of plasmonic antennas or the decayed resonant mode outside the cavity applied to monitor the refractive index variation. In this paper, by constructing a metal microstructure array-dielectric-metal (MDM) structure, a novel metamaterial absorber integrated microfluidic (MAIM) sensor is proposed and demonstrated in terahertz (THz) range, where the dielectric layer of the MDM structure is hollow and acts as the microfluidic channel. Tuning the electromagnetic parameters of metamaterial absorber, greatly confined electromagnetic fields can be obtained in the channel resulting in significantly enhanced interaction between the analytes and the THz wave. A high sensitivity of 3.5 THz/RIU is predicted. The experimental results of devices working around 1 THz agree with the simulation ones well. The proposed idea to integrate metamaterial and microfluid with a large light-matter interaction can be extended to other frequency regions and has promising applications in matter detection and biosensing. [ABSTRACT FROM AUTHOR]

Graphene, as a popular 2D semiconductor, with a carrier concentration that can be extensively tailored by external stimulants, such as electric bias and photoexcitation, is a promising candidate for active optoelectronics. An electrically tunable perfect terahertz absorber is presented by integrating a metallic grating into classical graphene Salisbury screen. The measurement shows that perfect absorption can be achieved for even lightly‐doped graphene and a modulation depth up to 25% is realized. Numerical simulation and analytical model based on the transmission line theory are developed to validate the design, showing a good agreement with the measured results. In addition, dependence of the terahertz response on physical dimensions and polarization of the incident terahertz field is also experimentally explored. It indicates that the hybrid terahertz device is able to function as tunable perfect absorber or tunable polarizer with an extinction ratio up to 23 dB. These results demonstrate the feasibility of using low carrier mobility chemical vapor deposition (CVD) grown graphene for tunable perfect terahertz absorption, which is suitable for potential applications, such as terahertz switching, and time domain and spatial domain modulation. [ABSTRACT FROM AUTHOR]

The design, characterization, and experimental demonstrations are presented for triple‐band, near‐perfect, composite metamaterials that absorb in the terahertz regime. Three absorption peaks are observed at 0.537  THz, 0.948 THz, and 1.59 THz with the respective absorption coefficients of 0.957, 0.988, and 0.96. The effect of spacer thickness on absorption is analyzed with interference theory, and the conditions for unity absorption are obtained via comparison of amplitude and phase. The effect of the length (l) of four outer parts of Jerusalem cross on the absorption is analyzed via transmission line theory. Furthermore, the absorption effects of capacitance and inductance caused by the changing l are examined. The calculated multi‐band transmission line model corresponded well to simulations. The effect of the outside length on the absorber performance is further explained by electric field distributions, which is experimentally confirmed. Overall, the consistent conclusions provide design guidance for metamaterial absorbers, or the realization of tunable absorbers, and provide a further understanding of the mechanisms. A triple‐band THz absorber based on composite metamaterials consisting of rectangular, circle, and cross unit cell is presented. Three absorption peaks are observed at 0.537 THz, 0.948 THz, and 1.59 THz with the respective absorption of 0.957, 0.988, and 0.96. The physical mechanism of the perfect absorption is elucidated by the interference theory combining with transmission line model. [ABSTRACT FROM AUTHOR]

Here, a new class of transflective oxide metaparticles that are able to capture solar radiation in a selective manner based on their size and morphology is demonstrated. The metaparticles possess an electronic polarization response "polarizonics" and show a specular reflective color at the visible frequencies. As a specific highlight, a resonating ultrathin reflective while transparent metacomposite, i.e., a transflective coating is designed. Further, a perfect solar absorber is devised whose function is morphology controlled and supplemented by the transflective coating via the selective interference concept. The multidisciplinary approach presented here enables design of materials with customized functionalities, bridging the sustainable nanochemistry to nanoengineering and proposes a new way for solving challenges in energy and solar harvesting. The simple, green Leidenfrost nanosynthesis is utilized to produce self‐organized oxide metaparticles as building blocks of a novel transflective coating that is transparent and shows polarizonic specular reflection at the visible frequencies. The transflective coating atop a metal mirror allows the design of the thinnest oxide perfect solar absorber ever with an amplifying intensity of ≈100% at the visible frequencies. [ABSTRACT FROM AUTHOR]

The development of novel approaches that control absorption and emission operating in the long wavelength infrared (LWIR) spectral region is of fundamental importance for many applications, such as remote temperature sensing, thermal imaging, radiation cooling, environmental monitoring, and night vision. A high performance plasmonic metasurface–based absorber for the LWIR spectral region is presented. In the design, a pyroelectric thin film, poly(vinylidene fluoride‐trifluoroethylene) (P(VDF‐TrFE)) copolymer, is introduced as spacer, that offers the device not only multiple selective high absorption bands but also promising potential for application in optoelectronics. The angle‐resolved optical responses show that the absorption effect is sensitive to the incident angles and can be controlled by the periodicity, indicating that the design can function as optical devices with directional and frequency‐selective absorption/emission characteristics. By employing near‐field optical microscopy, both the near‐field amplitude and phase optical responses of the absorber are investigated at resonant wavelength, thereby providing direct experimental evidence to verify the nature of the absorption effect. To further demonstrate the versatility of the design, a particular metasurface patterned by the building blocks of the absorber is fabricated. 2D hyperspectral images show that such a patterned structure exhibits both frequency and spatially selective absorption. A high performance plasmonic metasurface–based absorber for the long wavelength infrared spectral region, is presented which possesses multiple selective high absorption bands. Patterned by the building blocks of the absorber, a spatial and frequency varying absorption metasurface is also experimentally demonstrated. The design has various applications including thermal infrared light sources, thermal management, uncooled infrared photodetectors, and hyperspectral thermal imaging. [ABSTRACT FROM AUTHOR]