Ultra-broadband metamaterial absorber from ultraviolet to long-wave infrared based on CMOS-compatible materials

NIR-ViS-UV broadband absorption in ultrathin electrochemically-grown, graded index nanoporous platinum films

Nanoporous platinum broadband absorber has attracted interest in thermosensorics and IR photodetection due to its unique properties. In this work we report the physical mechanism underlying broadband absorption in electrochemically-grown, nanoporous Pt films by analyzing NIR-ViS-UV spectral ellipsometry data of nanoporous Pt films in dependence on the Pt film thickness (27, 35, 38, 48 nm). For the two thinner films a single layer model with a graded optical index Pt surface layer was used. For the two thicker films a two-layer optical model with a constant optical index Pt substrate layer and a graded optical index Pt surface layer was used. The graded optical index of the Pt surface layer reduces surface reflectivity and the constant optical index Pt substrate layer supports multiple reflections in the Pt film. Finally, we relate the thickness dependent optical index with the nanostructure of the nanoporous Pt film, which can be controlled in the electrochemical growth process. We observed that while in the transverse plane the multilayer exhibits graded refractive index, in the top horizontal planes the multilayer assembly exhibits discontinuous refractive index values due to the distribution of platinum crystal islands in air, which allows a metamaterial behavior of the whole system.

Broadband Solar Absorber and Thermal Emitter Based on Single-Layer Molybdenum Disulfide

In recent years, solar energy has become popular because of its clean and renewable properties. Meanwhile, two-dimensional materials have become a new favorite in scientific research due to their unique physicochemical properties. Among them, monolayer molybdenum disulfide (MoS2), as an outstanding representative of transition metal sulfides, is a hot research topic after graphene. Therefore, we have conducted an in-depth theoretical study and design simulation using the finite-difference method in time domain (FDTD) for a solar absorber based on the two-dimensional material MoS2. In this paper, a broadband solar absorber and thermal emitter based on a single layer of molybdenum disulfide is designed. It is shown that the broadband absorption of the absorber is mainly due to the propagating plasma resonance on the metal surface of the patterned layer and the localized surface plasma resonance excited in the adjacent patterned air cavity. The research results show that the designed structure boasts an exceptional broadband performance, achieving an ultra-wide spectral range spanning 2040 nm, with an overall absorption efficiency exceeding 90%. Notably, it maintains an average absorption rate of 94.61% across its spectrum, and in a narrow bandwidth centered at 303 nm, it demonstrates a near-unity absorption rate, surpassing 99%, underscoring its remarkable absorptive capabilities. The weighted average absorption rate of the whole wavelength range (280 nm-2500 nm) at AM1.5 is above 95.03%, and even at the extreme temperature of up to 1500 K, its heat radiation efficiency is high. Furthermore, the solar absorber in question exhibits polarization insensitivity, ensuring its performance is not influenced by the orientation of incident light. These advantages can enable our absorber to be widely used in solar thermal photovoltaics and other fields and provide new ideas for broadband absorbers based on two-dimensional materials.

Gradient index effect assisted anisotropic broadband absorption in α-MoO3 metamaterial

As an excellent natural hyperbolic material (HM), α-M o O ₃ has a larger hyperbolic bandwidth and longer polariton lifetime than other HMs, which makes it an ideal candidate for broadband absorbers. In this work, we theoretically and numerically investigated the spectral absorption of an α-M o O ₃ metamaterial using the gradient index effect. The results show that the absorber has an average spectral absorbance of 99.99% at 12.5-18 µm at transverse electric polarization. When the incident light is transverse magnetic polarization, the broadband absorption region of the absorber is blueshifted, and a similar strong absorption is achieved at 10.6-12.2 µm. By simplifying the geometric model of the absorber using equivalent medium theory, we find that the broadband absorption is caused by the refractive index matching of the metamaterial to the surrounding medium. The electric field and power dissipation density distributions of the metamaterial were calculated to clarify the location of the absorption. Moreover, the influence of geometric parameters of pyramid structure on broadband absorption performance was discussed. Finally, we investigated the effect of polarization angle on the spectral absorption of the α-M o O ₃ metamaterial. This research contributes to developing broadband absorbers and related devices based on anisotropic materials, especially in solar thermal utilization and radiation cooling.

Broadband Antireflective Hybrid Micro/Nanostructure on Zinc Sulfide Fabricated by Optimal Bessel Femtosecond Laser

Enhancing the infrared window transmittance of zinc sulfide (ZnS) is important to improve the performance of infrared detector systems. In this work, a new hybrid micro/nanostructure was fabricated by an optimal Bessel femtosecond laser on ZnS substrate. The surface morphologies and profiles of ASS ablated by a 20× microscope objective Bessel beam are described, indicating that the nanoripples on the micropore were formed by the SPP interference and the SPP scattering in a particular direction. Further, the maximum average transmittance of ASS increased by 9.7% and 12.3% in the wavelength ranges of 5~12 μm and 8~12 μm, respectively. Finally, the antireflective mechanism of the hybrid micro/nanostructure is explored using the novel electromagnetic field model based on the FDTD method, and we attribute the stable antireflective performance of ASS in broadband to the interface effective dielectric effect and LLFE.

Compact multispectral photodetectors based on nanodisk arrays atop optical cavity substrates

It is challenging for the multi-spectral photodetector to have a compact structure, high spectral resolution, and high detection efficiency. This paper reports on a new approach for compact multi-spectral visible light detecting based on the hexagonal lattice silver nanodisk arrays atop optical cavity substrates. Through numerical calculations and optimizations of experiments, we verified that the narrow band responsivity of the photodetector was caused by coupling the surface plasmonic resonances and cavity mode. The multi-spectral photodetector exhibited that the minimum FWHM and the maximum responsivity of was achieved to be 80 nm and 91.5 mA·W^-1, respectively. Besides, we also analyzed the influence of the proposed structure on the energy wastage by numerical comparison. The proposed way for multi-spectral photodetector is promising to be an excellent design for the narrow band spectral detection. The design can also be easily integrated with CMOS devices and applied to other spectral regimes for different applications.

Multi-mode plasmonic resonance broadband LWIR metamaterial absorber based on lossy metal ring

Broadband perfect infrared wave absorption of unpolarized light over a wide range of angles in an ultrathin film is critical for applications such as thermal emitters and imaging. Although many efforts have been made in infrared broadband absorption, it is still challenging to cover the perfect absorption of broadband in the long-wave infrared band. We propose a long-wave infrared broadband, polarization, and incident angle insensitivity metamaterial absorber based on the supercell with four rings of two sizes. Broadband absorption covering the long-wave infrared band is realized by combining four PSPRs and LSPRs absorption peaks excited by the supercell structure. The absorptivity of our absorber exceeds 90% in the wavelength range of 7.76∼14µm, and the average absorptivity reaches 93.8%. The absorber maintains more than 80% absorptivity as the incident angle of unpolarized light reaches 60°, which may have promising applications for thermal emitters, infrared imaging, thermal detection.

Design of Polarization-Independent Reflective Metalens in the Ultraviolet-Visible Wavelength Region

Flat lens or metalens, as one of the most important application branches of metasurfaces, has recently been attracting significant research interest. Various reflective and transmissive metalenses have been demonstrated in the terathertz, infrared and visible wavelength range. However, metalens operating in the ultraviolet (UV) wavelength range is rare. Moreover, the development of reflective UV metalens, the important counterpart of transmissive ones, falls far behind. In this work, with thorough investigation of material properties, we propose a reflective metalens based on silicon dioxide (SiO₂) and aluminum (Al) that operates in the vacuum ultraviolet (VUV) to visible wavelength region. Four reflective metalenses were designed and optimized for wavelengths of 193, 441, 532 and 633 nm, and prominent focusing capability was observed, especially for the VUV wavelength of 193 nm. Dispersion characteristics of the metalenses were also studied within ±50 nm of the design wavelength, and negative dispersion was found for all cases. In addition, the SiO₂ + Al platform can be, in principle, extended to the mid-infrared (IR) wavelength range. The reflective VUV metalens proposed in this work is expected to propel miniaturization and integration of UV optics.