Metal-insulator-metal based plasmonic structures incorporating nanoslit for infrared rectification

Infrared (IR) rectification is promising for high-performance IR detection at room temperature. We propose metal-insulator-metal (MIM)-based plasmonic structures incorporating a nanoslit for IR rectification. Gold and SiO2 were used as the metal and insulator layers, respectively. A high-aspect-ratio nanoslit was incorporated onto the top of the metal layer of an MIM structure. This slit works as a coupler for incident IR light, and a surface plasmon mode is induced in the slit. The coupled IR light is then guided into the middle insulator layer and waveguide modes are formed. Rectification can be achieved by applying a voltage between the top and bottom metal layers. Finite-difference timedomain calculations show that wavelength selective detection can be achieved by controlling the slit width or depth. However, these proposed structures are difficult to fabricate because a metal-based high-aspect-ratio nanoslit cannot be formed by conventional dry or wet etching. We have developed fabrication procedures using gold electroplating and chemical mechanical polishing (CMP). The former method uses a photoresist as a sacrificial layer for the narrow slit, and the top metal is formed by electroplating. The latter uses SiO2 as a sacrificial layer, and the top metal is formed by sputtering and CMP. Both methods can be used to fabricate an MIM structure with a nanoslit. It was found that the CMP method can achieve a higher aspect ratio. These proposed structures and fabrication techniques could contribute to the development of novel IR detectors using plasmonic rectification.