All-Silicon Polarization-Insensitive Metamaterial Absorber in the Terahertz Range

All-silicon terahertz absorbers have attracted considerable interest. We present a design and numerical study of an all-silicon polarization-insensitive terahertz metamaterial absorber. The meta-atoms of the metamaterial absorber are square silicon rings which can be viewed as gratings. By properly optimizing the structure of the meta-atom, we achieve a broadband absorptivity that is above 90% ranging from 0.77 THz to 2.53 THz, with a relative bandwidth of 106.7%. Impedance matching reduces the reflection of the terahertz waves and the (0, ±1)-order diffraction induce the strong absorption. The absorption of this absorber is insensitive to the polarization of the terahertz wave and has a large incident angle tolerance of up to 60 degrees. The all-silicon metamaterial absorber proposed here provides an effective way to obtain broadband absorption in the terahertz regime. Metamaterial absorbers have outstanding applications in terahertz communication and imaging.

Metamaterial Absorbers with Archimedean Tiling Structures: Toward Response and Absorption of Multiband Electromagnetic Waves

With the continuous development of electromagnetic wave-absorbing materials, the design of artificial structures for electromagnetic absorbers based on the concept of metamaterials is becoming more abundant. However, in the design process, it is difficult to further broaden the effective absorption band due to the limitation that the traditional single-size structure responds to electromagnetic waves only in specific frequency bands. Therefore, in this paper, based on the moth-eye bionic hexagonal structure absorber with antireflection performance, an Archimedean tiling structure is designed to optimize it, and through the introduction of a variety of primitives with large differences in dimensions, a multifrequency band-response mechanism is achieved to enhance the multireflection mechanism, which can effectively broaden the absorption band and improve the wave absorption performance. Ultimately, the moth-eye bionic structure absorber optimized by (3.4.6.4) can achieve an effective absorption of 10.26 GHz at a thickness of 2 mm. This work presents a new idea for the design work of electromagnetic wave-absorbing metamaterials, which has a broad application prospect in the aerospace, electronic information countermeasures, communication, and detection industries.

A Metastructure Based on Amorphous Carbon for High Efficiency and Selective Solar Absorption

Efficient solar thermal conversion is crucial for renewable clean energy technologies such as solar thermal power generation, solar thermophotovoltaic and seawater desalination. To maximize solar energy conversion efficiency, a solar selective absorber with tailored absorption properties designed for solar applications is indispensable. In this study, we propose a broadband selective absorber based on amorphous carbon (a-C) metamaterials that achieves high absorption in the ultraviolet (UV), visible (Vis) and near-infrared (NIR) spectral ranges. Additionally, through metal doping, the optical properties of carbon matrix materials can be modulated. We introduce Ti@a-C thin film into the nanostructure to enhance light absorption across most of the solar spectrum, particularly in the NIR wavelength band, which is essential for improving energy utilization. The impressive solar absorptivity and photothermal conversion efficiency reach 97.8% and 95.6%, respectively. Notably, these superior performances are well-maintained even at large incident angles with different polarized states. These findings open new avenues for the application of a-C matrix materials, especially in fields related to solar energy harvesting.

Ultra-Thin and Broadband P-Band Metamaterial Absorber Based on Carbonyl Iron Powder Composites

The field of P-band (0.3-1 GHz) absorption has witnessed rapid development in metamaterial absorbers due to their exceptional designability and the absence of restrictions imposed by the one-fourth wavelength rule. In this study, we combined carbonyl iron powder (CIP) composites with a periodic structure composed of metal capacitive patterns and employed a genetic algorithm (GA) to optimize the electromagnetic parameters of the CIP substrate. By selecting the appropriate shape and material for the units of pattern based on transmission line theory, as well as regulating relevant structural parameters, we successfully designed an ultra-thin broadband metamaterial absorber for the P-band. Experimental results demonstrate that within the range of 0.3-0.85 GHz, the reflection loss of our absorber remains below -5 dB, with a maximum value of -9.54 dB occurring at 0.45 GHz. Remarkably, this absorber possesses a thickness equivalent to only 1/293 of its working wavelength. Then, we conducted analyses on electric field distribution, magnetic field distribution, and energy loss density. Our findings suggest that high-performance absorption in metamaterials can be attributed to λ/4 resonant or coupling effects between structural units or diffraction phenomena. This absorber offers several advantages, including broad low-frequency absorption capability, ultra-thin profile, and convenient fabrication process, thus providing valuable theoretical insights for designing metamaterial structures.

Pneumatic Structural Deformation to Enhance Resonance Behavior for Broadband and Adaptive Radar Stealth

Ideal radar absorbing materials (RAMs) require instantaneous, programmable, and spontaneous adaptability to cope with a complex electromagnetic (EM) environment across the full working frequency. Despite various material systems and adaptive mechanisms having been demonstrated, it remains a formidable challenge to integrate these benefits simultaneously. Here, we present a pneumatic matrix that couples morphable MXene/elastomer conductors with dielectric spacers, which leverages controllable airflow to reconfigure the spatial structure between a flat sheet and a hemispherical crown while maintaining resistance stability via wrinkle folding and unfolding. The interdimensional reconfigurations drastically induce multiple resonance behavior, enabling the matrix remarkable frequency tunability (144.5%), ultrawide bandwidth (15 GHz), weak angular dependence (45° incidence), ultrafast responsiveness (∼30 ms), and excellent reproducibility (1000 cycles). With multichannel fluidic and conceptual automated control systems, the final pneumatic device demonstrates a multiplexed, programmable, and autonomous transformable mode that builds a promising platform for smart radar cloaking.

Enhanced Broadband Light Harvesting in Ultrathin Absorbers Enabled by Epitaxial Stabilization of Silver Thin Film Mirrors

Silver thin film mirrors are attractive candidates for use as specular back reflectors to enhance broadband light absorption via strong optical interference in ultrathin film semiconductor photoabsorbers. However, deposition of metal-oxide absorbers often requires exposure to high temperature in an oxygen atmosphere, conditions that cause thermal etching and degrade the specular reflectance of silver films. Here, we overcome this challenge and demonstrate that epitaxial growth of silver mitigates thermal etching under the high-temperature oxygen-containing environments that cause polycrystalline films to degrade. The degree of thermal etching resistance is related to the epitaxial film structure, where high-quality films completely prevent thermal etching, allowing for direct deposition of metal-oxide thin film photoabsorbers at elevated temperatures without any degradation of the optical properties of the silver layer. As a proof of concept for device applications, a metal-oxide photoanode for photoelectrochemical water splitting is fabricated by directly growing epitaxial SnO2 and Ti-doped α-Fe2O3 (hematite) thin films onto stabilized silver reflectors by pulsed laser deposition. The photoanode displays enhanced broadband light absorption due to strong interference effects enabled by the highly reflective silver film and demonstrates stable operation in a photoelectrochemical cell under conditions of water photo-oxidation in alkaline electrolyte.

Rapid Solar Heating of Antimicrobial Ag and Cu2O Nanostructured Plasmonic Textile for Clean Water Production

Solar steam generation is an attractive method to produce clean water, and high steam generation rates have been achieved using nanostructured light absorbers. However, since it usually takes minutes to reach the temperature for steady-state steam generation under solar illumination, a material that responds quickly to intermittent sunlight is strongly desired. Here, we report an unprecedented heating rate in an ultralight freestanding textile consisting of interconnected Ag and Cu₂O nanoparticles. The textile demonstrated high solar absorption with low reflectance and transmittance, which were rationalized using our multiphysics simulations. A commercial polystyrene foam wrapped with this broadband light-absorbing textile showed the fastest response to sunlight together with a good steam generation rate compared to reported inorganic nanostructured steam generators. Furthermore, the textile exhibited antibacterial property, which might lower the risk of the vapor-induced transfer of bacteria during long-term intermittent use and the cost of subsequent water sanitization.

Dynamic Modulation of THz Absorption Frequency, Bandwidth, and Amplitude via Strontium Titanate and Graphene

A multi-functional broadband absorber based on graphene and strontium titanate (STO) film was designed. Additionally, the frequency, bandwidth, and amplitude of the absorber could be tuned by adjusting temperature and Fermi level of the graphene. By using the finite-difference time-domain (FDTD) method, the numerical calculation result shows that, when keeping the device temperature at 230 K and setting graphene Fermi level to be 1 eV, three absorption peaks at 1.72 THz, 2.08 THz, and 2.59 THz were realized and combined into a broadband absorption from 1.68 to 2.74 THz. As the STO temperature was increased from 230 K to 310 K, the center frequency moved from 2.2 THz to 2.45 THz; correspondingly, the broadband absorption range was widened from 1.06 THz to 1.24 THz. When the temperature was fixed at 230 K and the graphene Fermi level was tuned from 1 eV to 0.7 eV, the absorption bandwidth decreased from 1.06 THz to 0.64 THz. While the Fermi level was tuned continually to be 0.01 eV, only a single absorption peak with an absorption rate of 0.29 existed. The broadband absorption and tuning mechanism of the absorber were analyzed using impedance matching theory. Furthermore, we also studied the effect of incident angle and polarization direction on the properties of the absorber. The multi-functional tunable absorber provides potential applications for the design of more efficient terahertz functional devices in the future.

Formamidinium Perovskitizers and Aromatic Spacers Synergistically Building Bilayer Dion-Jacobson Perovskite Photoelectric Bulk Crystals

Two-dimensional (2D) multilayer Dion-Jacobson (DJ) phase organic inorganic hybrid perovskites (OIHPs) have attracted extensive research attention due to the high stability and excellent charge-transport properties in the optoelectronic field. However, the synthesis of 2D multilayer DJ OIHPs is still very challenging. Until now, only few multilayer DJ perovskites have been reported and most of them are based on volatile methylamine (MA) cations. Compared with MA-based OIHPs, the OIHPs constructed with formamidinium (FA) as perovskitizers not only improve the stability but also extend the light absorption range. Meanwhile, the introducing aromatic diamines as spacers could promote the electron-hole separation in such DJ hybrids. However, the DJ OIHP bulk single crystal constructed by using the advantages of FA as perovskitizers and aromatic diamines as spacers is still blank. Herein, we integrate the properties of organic cations and inorganic skeletons at a molecular-scale to construct a broadband-responsive 2D bilayer DJ perovskite (3AMPY)(FA)Pb₂I₇ [3AMPY = 3-(aminomethyl)pyridinium], which shows a fascinating detectivity from X-ray (5.23 × 10⁴ μC Gy_air^-1 cm^-2 at 200 V bias) and visible light (6 × 10¹² jones at 637 nm) to the near-infrared region (2.6 × 10⁹ jones at 780 nm). After an in-depth analysis of structure and optical properties, we found that the distortion degree of Pb-I-Pb bond angles between adjacent PbI₆ octahedra plays a crucial role on optical properties; on the other hand, the interlayer spacer cations (3AMPY) and intralayer perovskitizers (FA) mutual participate in the contribution of the conduction band, making (3AMPY)(FA)Pb₂I₇ have a narrow optical band gap of 1.54 eV. Such a 2D perovskite material with a wide spectra response will be the preferred choice for photodetection under complex conditions.

Ultra-Broadband Refractory All-Metal Metamaterial Selective Absorber for Solar Thermal Energy Conversion

A full-spectrum near-unity solar absorber has attracted substantial attention in recent years, and exhibited broad application prospects in solar thermal energy conversion. In this paper, an all-metal titanium (Ti) pyramid structured metamaterial absorber (MMA) is proposed to achieve broadband absorption from the near-infrared to ultraviolet, exhibiting efficient solar-selective absorption. The simulation results show that the average absorption rate in the wavelength range of 200-2620 nm reached more than 98.68%, and the solar irradiation absorption efficiency in the entire solar spectrum reached 98.27%. The photothermal conversion efficiency (PTCE) reached 95.88% in the entire solar spectrum at a temperature of 700 °C. In addition, the strong and broadband absorption of the MMA are due to the strong absorption of local surface plasmon polariton (LSPP), coupled results of multiple plasmons and the strong loss of the refractory titanium material itself. Additionally, the analysis of the results show that the MMA has wide-angle incidence and polarization insensitivity, and has a great processing accuracy tolerance. This broadband MMA paves the way for selective high-temperature photothermal conversion devices for solar energy harvesting and seawater desalination applications.

Design of Multimodal Absorption in the Mid-IR: A Metal Dielectric Metal Approach

Specific control on the mid-infrared (mid-IR) emission properties is attracting increasing attention for thermal camouflage and passive cooling applications. Metal-dielectric-metal (MDM) structures are well known to support strong magnetic polariton resonances in the optical and near-infrared range. We extend the current understanding of such an MDM structure by specifically designing Au disc arrays on top of ZnS-Au-Si substrates and pushing their resonances to the mid-IR regime. Therefore, we combine fabrication via lift-off photolithography with the finite element method and an inductance-capacitance model. With this combination of techniques, we demonstrate that the magnetic polariton resonance of the first order strongly depends on the individual disc diameter. Furthermore, the fabrication of multiple discs within one unit cell allows a linear combination of the fundamental resonances to conceive broadband absorptance. Quite importantly, even in mixed resonator cases, the absorptance spectra can be fully described by a superposition of the individual disc properties. Our contribution provides rational guidance to deterministically design mid-IR emitting materials with specific narrow- or broadband properties.

Extending Absorption Edge through the Hybrid Resonator-Based Absorber with Wideband and Near-Perfect Absorption in Visible Region

Metamaterial absorber with the unexpected capability for harvesting electromagnetic energy has been regarded as a potential route for various applications, including chemical/biological sensing, cloaking and photovoltaic applications. In this study, we presented the simple absorber design made with Al/SiO₂/Al sandwich structures through the involvement of hybrid dual-resonators that could allow the wideband light absorption covered from 450 nm to 600 nm with average absorptivity above 95%. Examinations of excited electric field, magnetic field and total magnitude of electric field in three-dimensional space at resonances were performed to clarify the origin of resonant behaviors. In addition, an equivalent inductance-capacitance circuit model was proposed that could qualitatively explore the geometry-dependent absorption characteristics by modulating the constitutive parameters of hybrid resonators. As a result, the designed light absorber might enable to be practically applied for various optical-management and photovoltaic applications, and even offered the tunability for other desired frequency regions.

Ultra-Broadband High-Efficiency Solar Absorber Based on Double-Size Cross-Shaped Refractory Metals

In this paper, a theoretical simulation based on a finite-difference time-domain method (FDTD) shows that the solar absorber can reach ultra-broadband and high-efficiency by refractory metals titanium (Ti) and titanium nitride (TiN). In the absorption spectrum of double-size cross-shaped absorber, the absorption bandwidth of more than 90% is 1182 nm (415.648–1597.39 nm). Through the analysis of the field distribution, we know the physical mechanism is the combined action of propagating plasmon resonance and local surface plasmon resonance. After that, the paper has a discussion about the influence of different structure parameters, polarization angle and angle of incident light on the absorptivity of the absorber. At last, the absorption spectrum of the absorber under the standard spectrum of solar radiance Air Mass 1.5 (AM1.5) is studied. The absorber we proposed can be used in solar energy absorber, thermal photovoltaics, hot-electron devices and so on.

Solar-to-Steam Generation via Porous Black Membranes with Tailored Pore Structures

Solar-to-steam generation is a powerful, intense, and efficient method to harvest solar energy. Many efforts have been devoted to the development of a durable, affordable, and easy-to-manufacture solar steam device. In this study, we use a versatile polydimethylsiloxane material to fabricate an open porous black membrane with different pore structures using a simple salt water etching process and vapor deposition polymerization of pyrrole into a matrix. The porous black membrane absorbed radiation from a broad section of the light spectrum from near-infrared to ultraviolet and retained its initial pore structures and floating ability. We found that our black membrane with a tailored pore structure and surface exhibits excellent solar-to-steam generation efficiency of up to 72% at five sun irradiation. Also, a series of analyses including density functional theory calculation was used to prove the outstanding efficiency of solar-to-steam generation.

DNA Multilayers with Mono-, Hetero-, and Mixed-Type Plasmonic Nanoparticles for Broadband Absorption and Energy Storage

DNA incorporated with functional materials has led to development of hybrids with different functionalities. Among the functional materials, metal nanoparticles such as Au, Ag, and Cu (also known as plasmonic nanoparticles [PNPs]), which can exhibit surface plasmon resonance, are good candidates to fabricate useful optoelectronic devices and sensors. Here, we constructed PNP-assorted DNA (PNP-DNA) layers with mono-, hetero-, and mixed-type PNPs formed by successive spin-coating to obtain the required number of layers. Further, structural analysis of PNP-DNA was performed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The optical evaluation was carried out by Raman, UV-visible, and photoluminescence (PL) spectroscopies followed by measurement of capacitance. Cross-sectional SEM images of DNA single, DNA triple, and PNP-DNA triple layers indicated their thicknesses (i.e., 90, 280, and 395 nm, respectively), while the base pair distance of double helixes (∼0.4 nm) for the PNP-DNA multilayers was measured by XRD. The presence of Ag, Au, and Cu PNPs was confirmed by existence of spin-orbit coupling in the corresponding XPS spectra. The addition of PNPs in DNA multilayers caused significant enhancement in the intensities of Raman bands (especially in the range of 1200-1850 cm^-1) due to Raman resonance. UV-vis absorption and PL demonstrated stacking-order-dependent and layer-dependent light absorption and energy transfer (observed as quenching of fluorescence between PNPs and DNA), respectively. We observed n-type semiconducting behavior with a relatively higher dielectric constant for a PNP-assorted DNA single layer at a low frequency of 5 kHz. The dielectric constants of all samples decreased exponentially with increased frequency. Upon addition of PNPs, enhancement in the dielectric constant as well as capacitance was noted. Consequently, the simple fabrication method used in this study can be adopted to construct various nanomaterial-assorted DNA multilayers whose specific functionalities may be controlled with high efficiency.

van der Waals Transition-Metal Oxide for Vis-MIR Broadband Photodetection via Intercalation Strategy

Defects engineering can broaden the absorption band of wide band gap van der Waals (vdW) materials to the visible or near-IR regime at the expense of material stability and photoresponse speed. Herein, we introduce an atomic intercalation method that brings the wide band gap vdW α-MoO₃ for vis-MIR broadband optoelectronic conversion. We confirm experimentally that intercalation significantly enhances photoabsorption and electrical conductivity buts effects negligible change to the lattice structure as compared with ion intercalation. Charge transfer from the Sn atom to the lattices induces an optoelectrical change. As a result, the Sn-intercalated α-MoO₃ shows room temperature, air stable, broadband photodetection ability from 405 nm to 10 μm, with photoresponsivity better than 9.0 A W^-1 in 405-1500 nm, ∼0.4 A W^-1 at 3700 nm, and 0.16 A W^-1 at 10 μm, response time of ∼0.1 s, and peak D* of 7.3 × 10⁷ cm Hz^0.5 W^-1 at 520 nm. We further reveal that photothermal effect dominates in our detection range by real-time photothermal-electrical measurement, and the materials show a high temperature coefficient of resistance value of -1.658% K^-1 at 300 K. These results provide feasible route for designing broadband absorption materials for photoelectrical, photothermal, or thermal-electrical application.

Triplet-Triplet Annihilation Upconversion in Broadly Absorbing Layered Film Systems for Sub-Bandgap Photocatalysis

Upconversion (UC) of sub-bandgap photons extends the effective light absorption range of photovoltaic and photocatalytic devices, allowing them to reach higher conversion efficiencies. Recent advances in polymer host materials make it possible to translate triplet-triplet annihilation (TTA)-UC, the UC mechanism most suitable for this purpose, to solid films that can be integrated into devices. The promise of these films is currently limited by the narrow light absorption of TTA-UC sensitizer chromophores, but incorporating multiple sensitizers into layered film systems presents a promising strategy for producing UC materials with broadened light absorption. This strategy is herein applied for photocatalytic air purification, demonstrating its use in a real-world application for the first time. We superimpose optimized red-to-blue and green-to-blue UC films within dual-layer systems and develop a new photocatalyst compatible with their fluorescence emission. By integrating the dual-layer UC film systems with films of this photocatalyst, we produce the first devices that use TTA-UC to harness both red and green sub-bandgap photons for hydroxyl radical generation and photocatalytic degradation of gaseous acetaldehyde, a model volatile organic compound (VOC). Under white light-emitting diode excitation, the dual-layer film systems' broadened light absorption enhances their devices' photocatalytic degradation efficiency, enabling them to degrade twice as much acetaldehyde as their single-sensitizer counterparts. We show that as a result of the different absorption profiles of the two sensitizers, the film order significantly impacts UC fluorescence and VOC degradation. By probing the influence of the excitation light source, excitation geometry, and chromophore spectral overlap on the film systems' UC performance, we propose a framework for the design of multilayer TTA-UC film systems suitable for integration with a variety of photovoltaic and photocatalytic devices.

Black Diatom Colloids toward Efficient Photothermal Converters for Solar-to-Steam Generation

Steam generation from solar power using converters has attracted significant research attention in recent years as an alternative form of energy conversion from solar energy. Rationally designed photothermal converters are essential to increase the efficiency of steam generation. Here, we propose a novel colloidal type of photothermal converter based on a frustule skeleton, which is a naturally designed colloid containing through-pore structures. Several coating processes were used to provide broadband absorption, magnetic, and water-floating properties without deteriorating pore structures, through vapor deposition polymerization of polypyrrole, weak base treatment, and additional vapor deposition polymerization of polystyrene. The prepared colloidal photothermal converter showed superior efficiency for steam generation under sunlight irradiation.

Space-Confined Seeded Growth of Black Silver Nanostructures for Solar Steam Generation

Plasmonic metal nanostructures have attracted considerable attention for solar energy harvesting due to their capability in photothermal conversion. However, the narrow resonant band of the conventional plasmonic nanoparticles greatly limits their application as only a small fraction of the solar energy can be utilized. Herein, a unique confined seeded growth strategy is developed to synthesize black silver nanostructures with broadband absorption in the visible and near-infrared spectrum. Through this novel strategy, assemblages of silver nanoparticles with widely distributed interparticle distances are generated in rod-shaped tubular spaces, leading to strong random plasmonic coupling and accordingly broadband absorption for significantly improved utilization of solar energy. With excellent efficiency in converting solar energy to heat, the resulting black Ag nanostructures can be made into thin films floating at the air/water interface for efficient generation of clean water steam through localized interfacial heating.

Graphene Based Controllable Broadband Terahertz Metamaterial Absorber with Transmission Band

A graphene-based controllable broadband terahertz metamaterial absorber with transmission band is presented in this paper. It consists of a graphene-SiO₂-frequency selective surface (FSS) sandwich structure. The sinusoidal graphene layer supports continuous plasmonic resonances, forming a broad electric-tuning absorbing band. Bandpass FSS constructs a transmission window outside the absorbing band. The simulation results indicate that the absorption from 0.5 THz to 1 THz can be tuned continuously from 0.4 to 0.9 with angle and polarization independence. A transparent window peaking at 1.65 THz maintains high transmittance over 0.7. The metamaterial absorber has potential applications for detection, stealth, filtering, and electromagnetic compatibility.

Ultrathin Semiconductor Superabsorbers from the Visible to the Near-Infrared

The design of ultrathin semiconducting materials that achieve broadband absorption is a long-sought-after goal of crucial importance for optoelectronic applications. To date, attempts to tackle this problem consisted either of the use of strong-but narrowband-or broader-but moderate-light-trapping mechanisms. Here, a strategy that achieves broadband optimal absorption in arbitrarily thin semiconductor materials for all energies above their bandgap is presented. This stems from the strong interplay between Brewster modes, sustained by judiciously nanostructured thin semiconductors on metal films, and photonic crystal modes. Broadband near-unity absorption in Ge ultrathin films is demonstrated, which extends from the visible to the Ge bandgap in the near-infrared and is robust against angle of incidence variation. The strategy follows an easy and scalable fabrication route enabled by soft nanoimprinting lithography, a technique that allows seamless integration in many optoelectronic fabrication procedures.

Three-Dimensional Resistive Metamaterial Absorber Loaded with Metallic Resonators for the Enhancement of Lower-Frequency Absorption

Resistive patch array incorporating with metallic backplane provided an effective way to achieve broadband metamaterial absorbers (MAs) in microwave frequency, and the outstanding construction contributed more flexible and diversified broadband absorption. In this paper, we attempted to load metallic resonators (MRs) to three-dimensional resistive MA to further enhance the lower-frequency absorption performance. Simulation showed that the partial absorption peak was separated to the lower frequency, while the rest of broadband absorption was unaffected. Meanwhile, after combining multi-unit of the proposed MAs, the stair-stepping broadband absorption was also achieved. Finally, three samples were fabricated. The agreements between simulations and experimental results demonstrated that resistive MA loaded with MRs provided an effective way for further enhancement of lower-frequency absorption with almost no change of the absorbing structure and lightweight characteristic. Thus, it was worthy to expect a wide range of applications to emerge inspired from the proposed attempt.

Theoretical Analysis and Design of Ultrathin Broadband Optically Transparent Microwave Metamaterial Absorbers

Optically Transparent Microwave Metamaterial Absorber (OTMMA) is of significant use in both civil and military field. In this paper, equivalent circuit model is adopted as springboard to navigate the design of OTMMA. The physical model and absorption mechanisms of ideal lightweight ultrathin OTMMA are comprehensively researched. Both the theoretical value of equivalent resistance and the quantitative relation between the equivalent inductance and equivalent capacitance are derived for design. Frequency-dependent characteristics of theoretical equivalent resistance are also investigated. Based on these theoretical works, an effective and controllable design approach is proposed. To validate the approach, a wideband OTMMA is designed, fabricated, analyzed and tested. The results reveal that high absorption more than 90% can be achieved in the whole 6~18 GHz band. The fabricated OTMMA also has an optical transparency up to 78% at 600 nm and is much thinner and lighter than its counterparts.

Broadband Light Absorption and Efficient Charge Separation Using a Light Scattering Layer with Mixed Cavities for High-Performance Perovskite Photovoltaic Cells with Stability

CH₃ NH₃ PbI₃ is one of the promising light sensitizers for perovskite photovoltaic cells, but a thick layer is required to enhance light absorption in the long-wavelength regime ranging from PbI₂ absorption edge (500 nm) to its optical band-gap edge (780 nm) in visible light. Meanwhile, the thick perovskite layer suppresses visible-light absorption in the short wavelengths below 500 nm and charge extraction capability of electron-hole pairs produced upon light absorption. Herein, we find that a new light scattering layer with the mixed cavities of sizes in 100 and 200 nm between transparent fluorine-doped tin oxide and mesoporous titanium dioxide electron transport layer enables full absorption of short-wavelength photons (λ < 500 nm) to the perovskite along with enhanced absorption of long-wavelength photons (500 nm < λ < 780 nm). Moreover, the light-driven electric field is proven to allow efficient charge extraction upon light absorption, thereby leading to the increased photocurrent density as well as the fill factor prompted by the slow recombination rate. Additionally, the photocurrent density of the cell with a light scattering layer of mixed cavities is stabilized due to suppressed charge accumulation. Consequently, this work provides a new route to realize broadband light harvesting of visible light for high-performance perovskite photovoltaic cells.

Cobalt Phosphide Double-Shelled Nanocages: Broadband Light-Harvesting Nanostructures for Efficient Photothermal Therapy and Self-Powered Photoelectrochemical Biosensing

Ultra-broadband light-absorbing materials are highly desired for effective solar-energy harvesting. Herein, novel cobalt phosphide double-shelled nanocages (CoP-NCs) are synthesized. Uniquely, these CoP-NCs are able to nonselectively absorb light spanning the full solar spectrum, benefiting from its electronic properties and hollow nanostructure. They promise a wide range of applications involving solar energy utilization. As proof-of-concept demonstrations, CoP-NCs are employed here as effective photothermal agents to ablate cancer cells by utilizing their ability of near-infrared heat conversion, and as photoactive material for self-powered photoelectrochemical sensing by taking advantage of their ability of photon-to-electricity conversion.

Lithography-Free Broadband Ultrathin-Film Absorbers with Gap-Plasmon Resonance for Organic Photovoltaics

Strategies to confine electromagnetic field within ultrathin film emerge as essential technologies for applications from thin-film solar cells to imaging and sensing devices. We demonstrate a lithography-free, low-cost, large-scale method to realize broadband ultrathi-film metal-dielectric-metal (MDM) absorbers, by exploiting gap-plasmon resonances for strongly confined electromagnetic field. A two-steps method, first organizing Au nanoparticles via thermal dewetting and then transferring the nanoparticles to a spacer-reflector substrate, is used to achieve broader absorption bandwidth by manipulating geometric shapes of the top metallic layer into hemiellipsoids. A fast-deposited nominal Au film, instead of a conventional slow one, is employed in the Ostwald ripening process to attain hemiellipsoidal nanoparticles. A polymer supported transferring step allows a wider range of dewetting temperature to manipulate the nanoparticles' shape. By incorporating circularity with ImageJ software, the geometries of hemiellipsoidal nanoparticles are quantitatively characterized. Controlling the top geometry of MDM structure from hemisphere to hemiellipsoid increases the average absorption at 500-900 nm from 23.1% to 43.5% in the ultrathin film and full width at half-maximum of 132-324 nm, which is consistently explained by finite-difference time-domain simulation. The structural advantages of our scheme are easily applicable to thin-film photovoltaic devices because metal electrodes can act as metal reflectors and semiconductor layers as dielectric spacers.

Broadband and tunable high-performance microwave absorption of an ultralight and highly compressible graphene foam

The broadband and tunable high-performance microwave absorption properties of an ultralight and highly compressible graphene foam (GF) are investigated. Simply via physical compression, the microwave absorption performance can be tuned. The qualified bandwidth coverage of 93.8% (60.5 GHz/64.5 GHz) is achieved for the GF under 90% compressive strain (1.0 mm thickness). This mainly because of the 3D conductive network.