Infrared (IR) absorbers based on a metal–insulator–metal (MIM) have been widely investigated due to their high absorption performance and simple structure. However, MIM absorbers based on ultrathin spacers suffer from low field enhancement. In this study, we propose a new MIM absorber structure to overcome this drawback. The proposed absorber utilizes a reactive impedance surface (RIS) to boost field enhancement without an ultrathin spacer and maintains near-perfect absorption by impedance matching with the vacuum. The RIS is a metallic patch array on a grounded dielectric substrate that can change its surface impedance, unlike conventional metallic reflectors. The final circular nanodisk array mounted on the optimum RIS offers an electric field enhancement factor of 180 with nearly perfect absorption of 98% at 230 THz. The proposed absorber exhibits robust performance even with a change in polarization of the incident wave. The RIS-integrated MIM absorber can be used to enhance the sensitivity of a local surface plasmon resonance (LSPR) sensor and surface-enhanced IR spectroscopy.
High field enhancement and near-perfect absorption in nanoantennas were realized by using in-plane (between nanoantenna arms), out-of-plane (between nanoantenna and reflector), and array coupling (between nanoantennas in an array); however, it was challenging to satisfy both conditions at the same time. In this paper, we show that a bowtie nanoantenna array integrated with an artificial impedance surface can simultaneously satisfy both high field enhancement and perfect absorption. The artificial impedance surface is implemented as a metallic patch array on a grounded 50 nm-thick SiO 2 substrate with reactive impedance surface (RIS) or high impedance surface (HIS) characteristic. Through the proposed design methodology, we designed a bowtie nanoantenna array on an optimum RIS patch array and achieved a high field enhancement factor (E/E 0) of 228 and a nearly perfect absorption rate of 98% at 230 THz. This novel design outperforms the previously reported nanoantenna structures and the same bowtie nanoantenna array designed using a conventional grounded SiO 2. We also show that the HIS-integrated bowtie antenna array cannot realize both goals at the same time because the highly reflective HIS cannot guarantee perfect absorption. The proposed RIS-combined nanoantenna array with high field enhancement and near-perfect absorption can be used for efficient infrared (IR) and optical detectors, sensors, and energy harvesting devices.
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