Omnidirectional Absorption Research Articles

Graphene Microflower by Photothermal Marangoni-Induced Fluid Instability for Omnidirectional Broadband Photothermal Conversion.

2-D carbon-based materials are well-known for their broadband absorption properties for efficient solar energy conversion. However, their high reflectivity poses a challenge for achieving efficient omnidirectional light absorption. Inspired by the multilevel structures of the flower, a Graphene Microflower (GM) material with gradient refractive index surface was fabricated on polymer substrates using the UV-intense laser-induced phase explosion technique under the synergistic design of the photothermal Marangoni effect and the fluid instability principle. The refractive index gradient reduces light reflection and absorbs at least 96% of light at incident angles of 0-60° across the entire solar wavelength range (200-2500 nm). Over 90% absorption even at 75° angle of incidence. The light absorption is enhanced by the multiple interferometric phase cancelation and localized surface plasmon resonance, resulting in a steady-state temperature 60 °C higher than ambient conditions under one solar irradiation. The max rate of temperature rise can reach up to 62 °C s-1. The device is then integrated at the hot end of the temperature difference generator at high altitude to ensure continuous and efficient power generation, producing a steady-state power of 196 mW.

Robust Ultrablack Coating for Omnidirectional Broadband Light Absorption

AbstractDeveloping robust ultrahigh light absorption coatings has been a significant challenge, limited by the absorption‐adhesion tradeoff. Herein, a robust carbon‐based coating with omnidirectional broadband high absorption is designed and fabricated using a one‐step spray method. A novel rapidly cross‐linking resin is utilized as the binder to enhance adhesion. Beyond leveraging the intrinsic optical properties of carbon absorbents, the construction of a micro‐nano multi‐scale trapping structure is crucial for achieving high absorption efficiency in the coating. Innovatively, an inorganic matting agent is introduced to further diminish reflection by scattering incident light. The resulting ultrablack coating demonstrates good adhesion and an exceptionally high integral light absorption of 99.34% within the wavelength range of 1500–1800 nm, marking the highest reported value for binder‐incorporated spray coatings to date. This robust coating also exhibits outstanding thermal stability and resistance to vacuum thermal cycling and vibration. Furthermore, the application of the ultrablack coating via spraying is scalable and can be readily adapted to the surfaces or interiors of complex structural components made from diverse materials. The coating shows remarkable potential for solar energy harvesting and target stealth. This study presents a simple and efficient approach for developing robust ultrablack coatings suitable for widespread practical applications.

Compact multilayer selective absorbers based on amorphous carbon for solar-thermal conversion

In addressing the energy crisis and climate change, environmentally sustainable materials and structures with heightened absorption and enhanced photothermal conversion efficiency are urgently demanded. In this study, a series of ultrabroadband, omnidirectional, and near-perfect solar radiation absorbers based on amorphous carbon (a-C) were fabricated by magnetron sputtering. These absorbers, featuring 4, 6, and 8-layer compact multilayer thin film structures, exhibited measured absorption of 96.8 %, 96.5 %, and 96.6 % in the range of 300–2500 nm, respectively. The high absorption efficiency delves into the synergistic effects of intrinsic absorption of a-C and enhanced absorption through thin film interference. Through microstructure analysis, the reduced absorption compared to design originates from the optical thickness mismatch caused by the unstable growth rate of a-C. In addition, the absorbers show very slight variations in absorption within 40° and maintain a high absorption of more than 80 % in the incident angle of 60°. For samples with different air annealing temperatures from 100 to 250 °C, the microstructure, morphology, and optical properties were systematically investigated. The appearance of oxygen channels due to surface defects or voids leads to the oxidation reaction of the diffused Ti atoms at high temperature. Notably, the proposed absorber can be fabricated by a lithography-free method, paving a way for large-area application of a-C.

Ultrablack surface with omnidirectional high absorption.

Black surface plays an important role in solar-thermal conversion systems and space optical systems. Despite the significant efforts in developing a black surface with strong broadband absorption, realizing scalable omnidirectional high absorption ultrablack surfaces remains challenging. Here, we report an ultrablack surface based on a structural black paint (SBP), realized by femtosecond (fs) laser fabrication of high aspect ratio microstructure mold followed by mold transferring on black paint coating. The SBP exhibits extremely low hemispheric reflectance (R < 1.2%) in the solar spectrum at a normal incident angle; even at an 80° incident angle, the SBP also has good anti-reflection performance (R < 9%). Based on such properties, we further show the enhanced solar-thermal performance over commercial black paints. Our approach holds promises for scalable and cost-effective manufacturing of ultrablack surface, with potential applications in solar-thermal conversion efficiency improvement and space optical system stray light suppression.

Template‐Made Ultrahigh, Broadband, and Omnidirectional Light Absorption Surface for Enhanced Solar Energy Harvesting and Utilization

AbstractThe practical application of solar energy harvesting urgently calls for the facile preparation of high‐performance ultrablack surfaces with broadband and omnidirectional light absorption performance; however, some roadblocks still need to meet these requirements simultaneously. Herein, cone‐array ultrablack (CAUB) surfaces are prepared by combining feasible template replication and blade coating techniques. The elaborate micro‐sized cone structure and high absorptive materials (polydimethylsiloxane@carbon black, PDMS@CB) offer the CAUB extremely high, broadband, and omnidirectional light absorption over the solar spectrum with an average absorbance of 99.35%. Under one sun irradiation, the CAUB can efficiently absorb solar energy, reaching a surface temperature as high as 75 °C. A solar‐thermal‐electric generator (STEG) made from the CAUB film demonstrates real‐time solar‐heat‐electric conversion, achieving a high output power density of 330 µW cm−2. The CAUB film possesses excellent water repellency, flexibility, chemical solvent resistance, and potential for large‐scale production, satisfying the application requirements of solar energy harvesting and utilization.

Polarization-independent Spatial Kramers-Kronig medium for omnidirectional reflectionless absorption of unpolarized light

Spatial Kramers-Kronig (KK) media offer a possible route to obtain omnidirectional light absorption within a thin layer of material. However, the experimental realizations are typically limited to a specific polarization, i.e., either transverse electric (TE) or transverse magnetic (TM), hence lacking specific implementations for the absorption of unpolarized light. In this work, we propose theoretically and demonstrate experimentally a polarization-independent KK medium which performs omnidirectional reflectionless absorption for both TE and TM polarized waves. Our design makes use of a special matryoshka metamaterial, whose electric and magnetic responses can be independently controlled with minimized crosstalk. To extend the absorption spectrum, the inner truncation boundary of the KK medium is set at a position far away from the spatial Lorentz resonance, where the constitutive parameter of the metamaterial remains unitary over a broad frequency band. A mini anechoic chamber, 6.83-wavelength in diameter, is constructed using the designed annulus-shaped KK medium. The measured fields for both TE and TM polarizations confirm the polarization-independent omnidirectional and nearly reflectionless absorption in a broadband frequency range.

3D-honeycomb architectures evaporator toward mutual reinforcement of evaporation and catalysis for efficient freshwater harvesting

Solar-driven freshwater harvesting has emerged as an effective and sustainable technology to mitigate global water scarcity due to its high energy conversion efficiency. Nevertheless, achieving mutual reinforcement of photothermal and photocatalysis processes is in high demand because the contamination of harmful and highly concentrated volatile organic compounds (VOCs) will inevitably evaporate and condense alongside water. Herein, an integrated 3D-honeycomb fabric (NPPF) evaporator decorated with polydopamine (PDA), polypyrrole (PPy), and hydrotalcite (LDH) nanoflowers was designed innovatively by synergistically cooperating photothermal evaporation and advanced catalytic oxidation techniques. With periodically concave arrays, it can create the maximum level of light-trapping through multiple scattering and omnidirectional light absorption, thereby increasing its photothermal catalytic degradation ability. Meanwhile, introducing the LDH nanoflowers boosted the photothermal evaporation capacity of the NPPF evaporator. Therefore, it demonstrated high solar absorption efficiency of ∼98.02% and an evaporation rate of ∼2.02 kg m−2 h−1 under one sun. Furthermore, it can achieve superb degradation activity, especially for high-concentration VOCs, with efficiency of ∼93.99%, surpassing most of the other evaporators reported previously. Besides, the outdoor experiment demonstrated its practicability. More importantly, it can remove tetracycline and phenol with efficiency of ∼98.85% and ∼ 92.06%. The advantages make this evaporator a promising representative for cooperative freshwater production.

Ultra-thin and omnidirectional reflectionless absorbing surface working at dual-frequency

ABSTRACT Materials with characteristics of omnidirectional reflectionless absorption are of great importance in electromagnetic shielding and anechoic chamber. Recently emerged absorbing surfaces mostly work at a given angle, and absorbers working at a broad frequency band usually suffer from a 3-D structure. Thus, the fabrication of the ultra-thin and omnidirectional reflectionless absorber working at dual-frequency remains a technical challenge. In this work, we demonstrated the absorbing performance of an ultra-thin slab under different incident angles of the electric fields, and an approximate solution was derived to prove the realizability of the dual-frequency and omnidirectional reflectionless absorption. For illustration, a miniaturized dual-frequency anechoic chamber with a 343.8 mm diameter was constructed, and the full-wave simulations validated the results. The proposed design method is simple and robust, which provides a new clue for solving the practicability difficulty faced by existing anechoic chambers and microwave imaging systems.

Cupric oxide photonic microcavities and their omnidirectional high-efficiency light-trapping characteristics

Cupric Oxide's black appearance hides interesting topographic structures and optical properties. This work examines the structure of thermally-grown cupric oxide and describes optical effects exhibited by surface layers of cupric oxide. It also highlights connections of surface optical properties with the intricate 3D micro and nanostructures that are formed on the surface during the oxidation process. These structures exist over several orders of magnitude in length we show that the CuO surface has a random microcavity structure that leads not only to strong photon trapping – giving CuO its black appearance – but also confers omnidirectional light absorption characteristics. This can benefit a number of applications – both active and passive. Our work also shows that CuO surfaces can be restructured by irradiation with a laser beam. Furthermore, it is shown that CuO surfaces can emit strong black body cavity radiation due to their high emissivity, which could throw light on interesting physical phenomena.

Biomimetic Structural Design of Fabric for Low‐Cost, Scalable, and Highly Efficient Off‐Grid Solar‐Driven Water Purification

AbstractInterfacial solar‐vapor generation (ISVG) is an emerging technology for water purification. However, high cost, low evaporation rate, clogging issues, and limited solar utilization under natural condictions greatly hinder its practical application. Herein, inspired by the aligned microstructure of dragonfly wings, a 3D microarray structure composed of vertically aligned hierarchical and hydrophilic carbon fibers (CFs) is constructed using a scalable fiber manufacturing technology. The microarray structure of the high‐thermal‐conductivity CFs with nanocapillaries contributes to the fast mass (steam and salt ions) and heat transfer as well as high omnidirectional light absorption. More importantly, due to the strong multiscale capillary effect, the formed 3D water evaporation surface containing abundant micro‐meniscuses and nanoscale thin water layers in the CFs arrays effectively reduces the evaporation enthalpy and creates more water/air interfaces, leading to the significant increasing evaporation rate. As a result, a high evaporation rate of 2.21 kg m−2 h−1 under one‐sun irradiation can be achieved. Moreover, the off‐grid water treatment device assembled with multiple 3D CFs‐based customized spherical evaporators can obtain a high pure water collection of 10.71 kg m−2 per day without salt accumulation under real environmental conditions. This work demonstrates a high‐efficiency, cost‐effective, and scalable strategy for high‐performance ISVG.

Making Nanofiber Membrane Stand on End to Construct Vertically Interfacial Evaporators for Efficient Solar Evaporation, Omnidirectional Solar Absorption, and Ultrahigh-Salinity Brine Desalination.

Solar-driven interfacial desalination is widely considered to be a promising technology to address the global water crisis. This study proposes a novel electrospun nanofiber-based all-in-one vertically interfacial solar evaporator endowed with a high steam generation rate, steady omnidirectional evaporation, and enduring ultrahigh-salinity brine desalination. In particular, the electrospun nanofiber is collected into the tubular structure, followed by spraying with a dense crosslinked poly(vinyl alcohol) film, which renders them sufficiently strong for the preparation of a vertically array evaporator. The integrated evaporator made an individual capillary as a unit to form multiple thermal localization interfaces and steam dissipation channels, realizing zone heating of water. Thus a high steam generation rate exceeding 4.0kgm-2h-1 in pure water is demonstrated even under omnidirectional sunlight, and outperforms existing evaporators. Moreover, salt ions in the photothermal layer can be effectively transported to the water in capillaries and subsequently exchanged with the bulk water due to the strong action of capillary force, which ensures an ultrahigh desalination rate (≈12.5kgm-2h-1 under 3 sun) in 25wt% concentration brine over 300min. As such, this work provides a meaningful roadmap for the development of state-of-the-art solar-driven interfacial desalination.

Optical absorption driven by efficient refraction and light concentration for photovoltaic applications

Broadband absorption of the solar spectrum is a prerequisite to any photovoltaic technology. Subwavelength arrays are known to provide various mechanisms for broadband and omnidirectional absorption of the solar spectrum. In the current work we examine optical absorption in NP arrays decorated with SiO2 quasi-nanolenses (qNL arrays), which numerically exhibit 10–15 % increase in broadband absorption. Specifically, near-field scanning optical microscopy (NSOM) is employed to probe the underlying optical absorption mechanisms. Two NSOM methods are employed for the investigation: 1) far-field imaging using near-field excitation which allows the determination as to the origin of the reflected far-field photons, and 2) far-field excitation and near-field scanning to measure the lateral optical propagations in the arrays. We show that the enhanced optical absorption in qNL arrays is due to combination of three mechanisms: 1) each qNL serves as an effective anti-reflection coating, 2) the presence of qNLs increases the optical interactions between the adjacent dielectrics to conclude elevated levels of light trapping, and 3) the presence of qNL induces strong light concentration of the incoming radiation into the dielectric structures.

Inverse design of omnidirectional coherent absorbers for optical power beaming applications.

An efficient photovoltaic power converter is a critical element in laser power beaming systems for maximizing the end-to-end power transfer efficiency while minimizing beam reflections from the receiver for safety considerations. We designed a multilayer absorber that can efficiently trap monochromatic light from broad incident angles. The proposed design is built on the concept of a one-way coherent absorber with inverse-designed aperiodic multilayer front- and back-reflectors that enable maximal optical absorption in a thin-film photovoltaic material for broad angles. We argue that the broad bandwidth is achieved through an optimization search process that automatically engineers the modal content of the cavity to create multiple overlapping resonant modes at the desired angle or frequency range. A realistic design is provided based on GaAs thin films with inverse-designed multilayer binary AlAs/AlGaAs mirrors. The proposed device can pave the way for efficient optical power beaming systems.

Design of near-ideal and omnidirectional selective solar absorber for high-temperature applications

Efficient absorption of solar radiation with minimal thermal losses is required for efficient solar-to-thermal energy conversion. For this purpose, spectrally selective and omnidirectional surfaces are required that have strong, ideally unity, absorption in the solar spectral window for all incident angles and low, ideally zero, emissivity in the spectral range of blackbody radiation at operating temperature. In this article, we propose theoretically and experimentally lithographic free broadband and omnidirectional solar absorber for high-temperature applications made in a dielectric-metal-dielectric-metal fashion. We have used chromium (Cr), titanium (Ti), and tungsten (W) as a metal layer in the designed absorber due to their high thermal stability at high temperatures and compared their performance. The power balance equation is used to calculate the steady-state temperature considering free and forced convection conditions. W-based absorber shows >90% absorption in the solar window below cutoff wavelength for blackbody working at 600 K with <10% emissivity in the thermal window. Finally, we fabricate the W-based sample and experimentally measure the temperature rise of 100 K under one sun condition. The designed absorber can be used for several solar-thermal applications including solar-steam to electricity generation, water purification, space heating, and solar-thermal energy storage.

Broadband absorption based on multi-layered ENZ film: from directional to omnidirectional absorption

We propose a flat multilayered structure fabricated from gradient epsilon-near-zero (ENZ) materials (SiO2/Si3N4/Al2O3/AlN/TiO2/α-Ta2O5) coated on aluminium (Al) that enables broadband directional control of infrared absorption and adopt the Particle Swarm Optimization (PSO) method to optimize its structural parameters. The results demonstrate that the absorbers can exhibit high absorption (> 0.6) at specific angles (60° to 80°) and near-unity omnidirectional (0° to 80°) absorption both in p-polarization with covering the entire long-wavelength infrared (LWIR) range (8 μm to 14 μm), respectively. The directional control of the absorption with high efficiency and wide bandwidth would be great potential in applications such as thermal imaging, infrared camouflaging, and radiative cooling.

Arrays of Fresnel Nanosystems for Enhanced Photovoltaic Performance.

Omnidirectional broadband absorption of the solar radiation is pivotal to solar energy harvesting and particularly to low-cost non-tracking photovoltaic (PV) technologies. The current work numerically examines the utilization of surface arrays composed of Fresnel nanosystems (Fresnel arrays), which are reminiscent of the known Fresnel lenses, for the realization of ultra-thin silicon PV cells. Specifically, the optical and electrical performances of PV cells integrated with Fresnel arrays are compared with those of a PV cell incorporated with an optimized surface array of nanopillars (NP array). It is shown that the broadband absorption of specifically tailored Fresnel arrays can provide an enhancement of ∼20% over that of an optimized NP array. The performed analysis suggests that broadband absorption in ultra-thin films decorated with Fresnel arrays is driven by two light trapping mechanisms. The first is light trapping governed by light concentration, induced by the arrays, into the underlying substrates, which increases the optical coupling between the impinging illumination and the substrates. The second mechanism is light trapping motivated by refraction, as the Fresnel arrays induce lateral irradiance in the underlying substrates, which increases the optical interaction length and hence the overall probability for optical absorption. Finally, PV cells incorporated with surface Fresnel arrays are numerically calculated, with short-circuit current densities (Jsc) which are ∼50% higher than that of a PV cell incorporated with an optimized NP array. Also, the effect of increased surface area, due to the presence of Fresnel arrays, and its effect on surface recombination and open-circuit voltage (Voc) are discussed.

Flexible, Ultrathin, and Lithographic‐Free Nanocoral Metamaterial Membrane for Omnidirectional UV–vis–NIR Broadband Absorption

UV–vis–NIR metamaterial absorbers are key components for devices used in photothermal applications, especially those related to solar energy. Demanded by modern applications using nonplanar devices, they should be flexible, thin, mass producible, easy for integration, and demonstrate strong omnidirectional light absorption over wide spectral ranges. However, there are few ways to achieve these simultaneously. Additionally, most fabrication methods are substrate dependent, limiting integrability, or involve lithography, which can be costly and difficult. Herein, a metamaterial absorber that meets all the requirements above is demonstrated, using a simple substrate‐independent lithographic‐free process. It is composed of 3D randomized gold@SU‐8 hybrid nanocoral structures of ≈2 μm thin on a supporting membrane, which can be different materials to meet various integration needs depending on applications. Benefiting from multilevel hybridizations of plasmon resonance reinforced by strong light‐trapping effect, &gt;90% optical energy ranging from 250 to 2500 nm is omnidirectionally absorbed. The perfect metamaterial absorber exhibits very stable photothermal conversion properties with great reproducibility and durability. The excellent performance, thinness, flexibility, and simple fabrication method grant the presented absorber great potential in modern photothermal applications, especially those requiring nonplanar thin‐film devices.

Enabling Perovskite Solar Cell Omnidirectional Light Utilizing Via Trapping Technology

AbstractPromising optoelectronic properties of perovskites make it a research hotspot for the scientific community. Previously many groups are working on optical optimization, interface modification, material screening, and energy level matching of perovskite solar cells (PSCs). Herein, PSCs compatible with nano‐structure and photonic crystal (NSPC) are designed theoretically by using the numerical simulation technology that gives better light trapping capability to the PSC devices. The physical model is composed of perovskite spheres based on TiO2 and PDMS with the nanostructure. The size parameters of the photovoltaic devices are optimized and the wide‐angle absorption characteristics of PSCs with NSPC structure are investigated. Simulation results verify the superior optical performance of the new PSCs with a maximum photocurrent of 25.77 mA cm−2, which is 2.17 mA cm−2 higher than that of the conventional flat structured PSCs (23.60 mA cm−2). The results of the wide‐angle study demonstrate that the novel PSCs have omnidirectional absorption of photons. To elucidate the light‐trapping mechanism of the novel PSCs, the electric and magnetic field distributions of different structural PSCs are analyzed. The physical model the authors establish provides a solid theoretical basis for the research and development of PSCs with ultra‐low surface reflection loss and ultra‐high performance.

Magnetically Assembled Ultrablack Surface with Omnidirectional and Broadband Light Absorption

Ultrablack surfaces with stable and omnidirectional light absorption over a wide spectral range are fundamentally crucial for applications concerning strict optical requirements from high-end optical to solar-heat conversion devices. Inspired by nature, we report a needle-like array structure (NAS) prepared by spraying and self-assembling the magnetic composite ink under an external magnetic field. With high structure regularity and small feature size, the NAS presents extremely low hemispherical reflectance (≤0.3%) over a wide spectral range of 300-2000 nm and stable omnidirectional absorption (incident angle insensitivity up to 70°), which could be one of the darkest surfaces ever reported. The exciting light absorption performance can be attributed to the synergistic effects of (1) structural absorption caused by multiple scattering between array units and (2) strong forward scattering and high light absorptivity of magnetic particles. The NAS exhibits outstanding photothermal conversion for solar harvesting, self-cleaning performance, good flexibility, and thermal-aging resistance, offering an appealing alternative to construct ultrablack surfaces for practical applications.

Strong and Omnidirectional Light Absorption from Ultraviolet to Near‐Infrared Using GST Metasurface (Laser Photonics Rev. 17(1)/2023)

Strong and Omnidirectional Light Absorption from Ultraviolet to Near‐Infrared Using GST Metasurface (Laser Photonics Rev. 17(1)/2023)