Dielectric super-absorbing (
>
50
%
) metasurfaces, born of necessity to break the 50% absorption limit of an ultrathin film, offer an efficient way to manipulate light. However, in previous works, super absorption in dielectric systems was predominately realized via making two modes reach the degenerate critical coupling condition, which restricted the two modes to be orthogonal. Here, we demonstrate that in nonorthogonal-mode systems, which represent a broader range of metasurfaces, super absorption can be achieved by breaking parity-time (PT) symmetry. As a proof of concept, super absorption (100% in simulation and 71% in experiment) at near-infrared frequencies is achieved in a Si-Ge-Si metasurface with two nonorthogonal modes. Engineering PT symmetry enriches the field of non-Hermitian flat photonics, opening-up new possibilities in optical sensing, thermal emission, photovoltaic, and photodetecting devices.
The development of ultrathin, flexible, large-scale, high-temperature-tolerant infrared camouflage devices, which are immune to the external environment, has emerged as an important unsolved challenge. This paper proposes an infrared camouflage device based on the Lambertian surface. The proposed device simultaneously exhibits low emissivity (≈0.1), low specular reflectance (≈0.05), and high temperature (290°C) tolerance over a broad infrared range (0.75-25 μm). Furthermore, the proposed device is ultrathin (≈50 μm), highly flexible, scalable, and can be fabricated at a low cost. The experimental results show that while camouflaging a target (at 65°C), the proposed Lambertian surface can reduce the peak value of the target-background contrast by 68.4% (indoor case) and 76.0% (outdoor case) compared to the conventional low-e (low-emissivity) smooth surface. The calculated detection range of the proposed low-e Lambertian surface is 60% less than that of both the low-e smooth surface and the blackbody. This work proposes a novel method to simultaneously control the radiation and the reflection, thereby introducing a new design paradigm for modern camouflage technology and energy harvesting applications.
Tailoring the absorption/emission through nanostructures from broadband to narrowband has attracted increasing attention due to the merits of high efficiency and compactness. However, most of the reported narrowband emitters require either complex fabrication process or thick coating of the suitable materials (~mm). In this paper, by exciting Berreman mode through a combination of Au mirrors and ultra‐thin layers of low‐loss uniaxial/biaxial anisotropic 2D van der Waals polar crystal hBN/α‐MoO3, a narrowband absorption/emission peak is realized in the mid‐infrared region. The measured quality‐factor of Berreman mode for hBN/Au and α‐MoO3/Au structure can reach up to 134 and 164, respectively. The deep subwavelength thickness (~nm) and bilayer architecture simplify the fabrication process. Furthermore, taking advantage of the in‐plane birefringence originating from α‐MoO3, the polarization of the absorbed/emitted radiation can be controlled through this planar configuration. Such ultrathin narrowband absorbers/emitters provide new opportunities for the advancements in thermal sources, infrared sensors, and thermophotovoltaic power generation systems.
In this article we present a three-dimensional loop Yagi-Uda array for efficient, polarization independent and directional absorption of THz radiation over a narrow frequency range (f0 = 0.657 THz & Q factor = 7.5). Unit cell of the array consists of three vertically stacked gold micro rings separated from each other by 30 µm thick SU-8 layers. The proposed array also exhibits a filtering response in its transmittance spectrum. The characteristics are explained by plasmon hybridization method. The transmission, reflection and absorption spectra of the structure are measured and they show a good agreement with corresponding simulated results.
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