Broadband absorption of sunlight is key for solar cell technologies, so metasurface-based structure has emerged as a promising technique for their efficiency improvement. [15][16][17] It is widely used in photothermal energy generation, [18,19] thermal emitters, [20] and detectors [21] to combine the metasurface-based PAs like a blackbody with the broadband absorption of sunlight. To realize the broadband absorption of sunlight, carbon-based surfaces, [22][23][24] Si-based surfaces, [25][26][27][28] and thin-film metallic structures [29,30] are demonstrated with low-surface absorptivity over the whole solar spectrum. For the carbon-based surfaces, they generally have broadband absorption, nonpolarized selection, and wide-angle insensitivity for solar spectrum. However, most carbon-based absorbers have large thickness of tens to hundreds of micrometers which is a challenge for device integration. Silicon-based surfaces have good performance photovoltaic characteristics. However, some Sibased light-trapping schemes [16,27,28] indicated that limitation of Lambertian expression [31] brings the absorption of silicon cells only in the range from 400 to 1100 nm. In addition, it still has challenge to use patterned metal materials that are unstable at the high temperature to achieve solar spectral absorption. It is promising to design the ultrathin absorbers that have broadband solar absorption while save enabling material saving and shorter deposition times. [32] Due to the high-temperature stability and metal-like optical properties in the visible and nearinfrared spectral regions, titanium nitride (TiN) is an ideal candidate for solar absorbing materials. [33,34] In some reports, [35][36][37] it has shown that Mie resonance by combining ultrathin TiN gratings with different refractive index materials achieves perfect absorption in the near-infrared spectral region. However, inverse design toward this kind of Mie resonance is rarely reported, which may provide novel approach to the study of TiN structure with multimode coupling for broadband absorption.In order to obtain a TiN structure with excellent absorption characteristics, it is necessary to introduce inverse design. As shown in Figure 1a, in the traditional design process, most researches obtain the initial structure through a priori method, including but not limited patterned structure (topological structure), multilayer structure, and stage structure, Broadband absorption of sunlight plays a crucial role in applying solar energy. However, despite being a decade-old technology, there are only a handful of simple metasurfaces designed by conventional methods. This work theoretically combines inverse design with broadband absorption of sunlight to optimize a metasurface that exhibits triple coupling mode resonance for maximizing solar spectral absorption. The metasurface consists of dual-layer titanium nitride (TiN) cylinder grating arrays, TiN dielectric layers, and silicon nitride layers. The simulation results reveal the high absorptivity of 93% in the range o...

Infrared (IR) stealth with thermal management is highly desirable in military applications and astronomy. However, developing selective IR emitters with properties suitable for IR stealth and thermal management is challenging. In this study, we present the theoretical framework for a selective emitter based on an inverse-designed metasurface for IR stealth with thermal management. The emitter comprises an inverse-designed gold grating, a Ge2Sb2Te5 (GST) dielectric layer, and a gold reflective layer. The hat-like function, which describes an ideal thermal selective emitter, is involved in the inverse design algorithm. The emitter exhibits high performance in IR stealth with thermal management, with the low emissivity (ɛ3–5 µm =0.17; ɛ8–14 µm =0.16) for dual-band atmospheric transmission windows and high emissivity (ɛ5–8 µm =0.85) for non-atmospheric windows. Moreover, the proposed selective emitter can realize tunable control of thermal radiation in the wavelength range of 3–14 µm by changing the crystallization fraction of GST. In addition, the polarization-insensitive structure supports strong selective emission at large angles (60°). Thus, the selective emitter has potential for IR stealth, thermal imaging, and mid-infrared multifunctional equipment.

The nanostructure composed of nanomaterials and subwavelength units offers flexible design freedom and outstanding advantages over conventional devices. In this paper, a multifunctional nanostructure with phase-change material (PCM) is proposed to achieve tunable infrared detection, radiation cooling and infrared (IR)-laser compatible camouflage. The structure is very simple and is modified from the classic metal–dielectric–metal (MIM) multilayer film structure. We innovatively composed the top layer of metals with slits, and introduced a non-volatile PCM Ge2Sb2Te5 (GST) for selective absorption/radiation regulation. According to the simulation results, wide-angle and polarization-insensitive dual-band infrared detection is realized in the four-layer structure. The transformation from infrared detection to infrared stealth is realized in the five-layer structure, and laser stealth is realized in the atmospheric window by electromagnetic absorption. Moreover, better radiation cooling is realized in the non-atmospheric window. The proposed device can achieve more than a 50% laser absorption rate at 10.6 μm while ensuring an average infrared emissivity below 20%. Compared with previous works, our proposed multifunctional nanostructures can realize multiple applications with a compact structure only by changing the temperature. Such ultra-thin, integratable and multifunctional nanostructures have great application prospects extending to various fields such as electromagnetic shielding, optical communication and sensing.

Second-order topological insulators (SOTIs) have recently attracted much attention due to their capability to support lower-dimensional topological states, namely, the corner states. Here, we demonstrate that properly designed supercell metasurfaces can support photonic corner states, meanwhile further serving as an ideal platform for the implementations of topological polaritons and dynamically reconfigurable corner states by assembling two-dimensional materials. Such metasurfaces consist of an array of finite-sized SOTIs mimicking the two-dimensional Su–Schrieffer–Heeger model. We reveal that the topological transition happens in unit cells without the bandgap, and nondegenerate multipolar corner states emerge in the supercell metasurface due to the inter- and intrasupercell coupling effects. Especially since these corner states are above the light line of the metasurface, we realize the collective stimulation of the two dipolar corner states and their superposition state via far-field excitation. By stacking monolayer hexagonal boron nitride film onto the metasurface, we further achieve the topological phonon polaritons through the strong coupling between the corner state and the phonon, which is confirmed by the Rabi splitting as well as anticrossing behavior emerging in the transmission spectra. Furthermore, we reveal the robustness of the corner state and strong coupling by introducing defects into the metasurface. Finally, tunable corner state and strong coupling with on-demand control are realized by assembling monolayer graphene onto the metasurface. Our theoretical study proposes a unique hybrid-material platform for topological polaritonics and reconfigurable topological photonics, which can promote large-area topological applications in practice.

Infrared camouflage is an effective technique to avoid many kinds of target detection by detectors in the infrared band. For a high-temperature environment, thermal management of selective emission is crucial to dissipate heat in the mid-infrared non-atmospheric window (5–8 μm). However, it still remains challenges for balancing infrared camouflage and thermal management. Here, we experimentally demonstrate a multilayer film structure (MFS) for infrared camouflage with thermal management. Combining the ideal emission spectrum and genetic algorithm (GA), the inverse-design MFS containing 7 layers of five materials (SiO2, Ge, ZnS, Pt and Au) has been designed. Based on the hierarchical metamaterial, the optimized MFS has high performance of infrared camouflage to against the lidar detection in the near-infrared band. The experimental results reveal the high compatible efficiency among thermal camouflage (ε 3–5μm = 0.21, ε 8–14μm = 0.16), laser stealth (ε 1.06μm = 0.64, ε 1.55μm = 0.90, ε 10.6μm = 0.76) and thermal management (ε 5–8μm = 0.54). Therefore, the proposed MFSs are attractive as basic building block of selective emitter, for the application of advanced photonics such as radiative cooling, infrared camouflage, and thermal emission.