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We investigate the fundamental limit of radiative cooling of objects on the Earth's surfaces under general conditions including nonradiative heat transfer. We deduce the lowest steady-state temperature attainable and highest net radiative cooling power density available as a function of temperature. We present the exact spectral emissivity that can reach such limiting values, and show that the previously used 8–13 μm atmospheric window is highly inappropriate in low-temperature cases. The critical need for materials with simultaneously optimized optical and thermal properties is also identified. These results provide a reference against which radiative coolers can be benchmarked.

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that extreme anti-reflection (EAR) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20° and magnetic domain imaging visibility of 0.47. Especially, EAR-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The EAR technique is promising for optical investigations and applications of nanomagnetic systems. Magneto-optic Kerr effect microscopy is useful for dynamic magnetic studies, but is limited by the weak magneto-optical activity. Here, the authors show that extreme anti-reflection result in a Kerr amplitude as large as 20° and enables real-time detection of sub-wavelength magnetic domain reversals.

An ultrawideband electromagnetic metamaterial absorber is proposed that consists of double-layer metapatterns optimally designed by the genetic algorithm and printed using carbon paste. By setting the sheet resistance of the intermediate carbon metapattern to a half of that of the top one, it is possible to find an optimal intermediate metapattern that reflects and absorbs the EM wave simultaneously. By adding an absorption resonance via a constructive interference at the top metapattern induced by the reflection from the intermediate one, an ultrawideband absorption can be achieved without increasing the number of layers. Moreover, it is found that the metapatterns support the surface plasmon polaritons which can supply an additional absorption resonance as well as boost the absorption in a broad bandwidth. Based on the simulation, the 90 % absorption bandwidth is confirmed from 6.3 to 30.1 GHz of which the fractional bandwidth is 130.77 % for the normal incidence. The accuracy is verified via measurements well matched with the simulations. The proposed metamaterial absorber could not only break though the conventional concept that the number of layers should be increased to extend the bandwidth but also provide a powerful solution to realize a low-profile, lightweight, and low cost electromagnetic absorber.

The refractive index of natural transparent materials is limited to 2–3 throughout the visible wavelength range. Wider controllability of the refractive index is desired for novel optical applications such as nanoimaging and integrated photonics. We report that metamaterials consisting of period and symmetry-tunable self-assembled nanopatterns can provide a controllable refractive index medium for a broad wavelength range, including the visible region. Our approach exploits the independent control of permeability and permittivity with nanoscale objects smaller than the skin depth. The precise manipulation of the interobject distance in block copolymer nanopatterns via pattern shrinkage increased the effective refractive index up to 5.10. The effective refractive index remains above 3.0 over more than 1,000 nm wavelength bandwidth. Spatially graded and anisotropic refractive indices are also obtained with the design of transitional and rotational symmetry modification. Wider control of the refractive index is desired for new optical applications. Here the authors manipulate block copolymer self-assembled nanopatterns via shrinkage in order to control the refractive index. They achieve an index above 3 over 1,000 nm bandwidth.

Metamaterials can possess extraordinary properties not readily available in nature. While most of the early metamaterials had narrow frequency bandwidth of operation, many recent works have focused on how to implement exotic properties and functions over broad bandwidth for practical applications. Here, we provide two definitions of broadband operation in terms of effective material properties and device functionality, suitable for describing materials and devices, respectively, and overview existing broadband metamaterial designs in such two categories. Broadband metamaterials with nearly constant effective material properties are discussed in the materials part, and broadband absorbers, lens, and hologram devices based on metamaterials and metasurfaces are discussed in the devices part.

Large language models (LLMs) have gained significant prominence in language processing, demonstrating remarkable performance across a wide range of tasks. Recently, LLMs have been explored in various scientific fields beyond language-based tasks. However, their application in the design of nanophotonic devices remains less explored. Here, we investigate the capabilities of LLMs to address nanophotonic design problems without requiring domain-specific expertise of the user. Our findings show that an LLM with in-context learning enables nonexpert users to calculate optical responses of multilayer films via numerical simulations. Through conversational interaction and feedback between the LLM and the user, an optimal design of the multilayer films can be also produced for the user-provided target optical properties. Furthermore, we fine-tune the LLM using text-based representations of the structure and properties of optical metasurfaces. We demonstrate that the fine-tuned LLM can generate metasurface designs with target properties by reversing the input and output text. This research highlights the potential of LLMs to expedite the nanophotonic design process and to make it more accessible to a wider audience.

The refractive index is the fundamental property of all optical materials and dictates Snell’s law, propagation speed, wavelength, diffraction, energy density, absorption and emission of light in materials. Experimentally realized broadband refractive indices remain <40, even with intricately designed artificial media. Herein, we demonstrate a measured index >1,800 resulting from a mesoscopic crystal with a dielectric constant greater than three million. This gigantic enhancement effect originates from the space-filling curve concept from mathematics. The principle is inherently very broad band, the enhancement being nearly constant from zero up to the frequency of interest. This broadband giant-refractive-index medium promises not only enhanced resolution in imaging and raised fundamental absorption limits in solar energy devices, but also compact, power-efficient components for optical communication and increased performance in many other applications. The refractive index of natural materials only covers a limited range. Here, Chang et al . use the principle of space-filling curves to construct a mesoscopic crystal with a refractive index greater than 1000 at GHz frequencies. The concept is inherently broadband and scalable.

Smart windows, capable of tailoring light transmission, can significantly reduce energy consumption in building services. While mechano-responsive windows activated by strains are promising candidates, they face long-lasting challenges in which the space for the light scatterer’s operation has to be enlarged along with the window size, undermining the practicality. Recent attempts to tackle this challenge inevitably generate side effects with compromised performance in light modulation. Here, we introduce a cuttlefish-inspired design to enable the closing and opening of pores within the 3D porous structure by through-thickness compression, offering opacity and transparency upon release and compression. By changing the activation mode from the conventional in-plane to through-thickness direction, the space requirement is intrinsically decoupled from the lateral size of the scatterer. Central to our design is the asymmetry of pore orientation in the 3D porous structure. These inclined pores against the normal direction increase the opaqueness upon release and improve light modulation sensitivity to compression, enabling transmittance regulation upon compression by an infinitesimal displacement of 50 μ m. This work establishes a milestone for smart window technologies and will drive advancements in the development of opto-electric devices. Strain-responsive smart windows represent a green technology for transparency control but demand large space to activate. Here, a new material structure activated by compression is proposed, yielding strong optical regulation with minute deformation

Smart chromatic materials with optical transmittances that can be modified by light scattering upon external stimuli are attracting extensive interest because of their appealing applications in smart windows, privacy protection, electronic displays, etc. However, the development of these scatterers, which are mostly activated by electric fields, is hindered by their intrinsic energy consumption, slow responses, and poor stability. Recently, mechanoresponsive scatterers based on a strain-driven reconfiguration of the surface or internal structure have emerged, featuring fast responses and a simple composition/fabrication. Because there is no energy consumption to maintain the transparency/opacity, this novel scheme for scatterers holds great promise to break the existing bottleneck. This article presents recent advances in the development of mechanoresponsive scatterers and compares different structural design strategies. The scatterers are categorized into 2D, 3D, and other types according to the dimensions of their functioning structures. The fabrication methods, mechanisms, and relationships between the structural parameters and optical modulating performances are discussed for each category. Next, the potential applications of these scatterers are outlined. Finally, the advantages and disadvantages of the mainstream 2D and 3D categories are summarized, followed by a perspective on future research directions.

Microwave absorbers using conductive ink are generally fabricated by printing an array pattern on a substrate to generate electromagnetic fields. However, screen printing processes are difficult to vary the sheet resistance values for different regions of the pattern on the same layer, because the printing process deposits materials at the same height over the entire surface of substrate. In this study, a promising manufacturing process was suggested for engraved resistive double square loop arrays with ultra-wide bandwidth microwave. The developed manufacturing process consists of a micro-end-milling, inking, and planing processes. A 144-number of double square loop array was precisely machined on a polymethyl methacrylate workpiece with the micro-end-milling process. After engraving array structures, the machined surface was completely covered with the developed conductive carbon ink with a sheet resistance of 15 Ω/sq. It was cured at room temperature. Excluding the ink that filled the machined double square loop array, overflowed ink was removed with the planing process to achieve full filled and isolated resistive array patterns. The fabricated microwave absorber showed a small radar cross-section with reflectance less than − 10 dB in the frequency band range of 8.0–14.6 GHz.