Three-Dimensional Metamaterial Absorber for Gas Molecular Sensing

Metamaterials have attracted considerable attention due to their capabilities to manipulate light. Recently we applied metamaterial light absorber for improving the sensitivity of IR spectroscopy. Owing to its plasmonically resonant interaction with incident light wave, and molecules, unwanted background and noises in IR spectroscopy were suppressed, and molecular signals are enhanced and clearly observed at the bottom of the suppressed background. This technique has already been applied for self-assembled monolayer of 16-mercaptohexadecanoic acid molecules on the surface of metamaterial absorber and atto-molar (10^-18 mol) level molecular sensitivity was realized [1]. For liquid samples, to introduce target molecules into the hot spot region of the metamaterial absorber, we proposed a metamaterial absorptive device that incorporates with nanofluidics and demonstrated an ultra-high sensitivity of IR absorption detection [2, 3, 4]. In this paper, we introduced metamaterial absorber with three-dimensional vertical-oriented metal-insulator-metal (v-MIM) structure for ultra-sensitive infrared spectroscopic gas molecule sensing [5]. We designed the v-MIM structure with a nano-gap of 25 nm channel which enabled the delivery of small molecules into hot-spot region. We developed the fabrication technique to realize v-MIMs on the surface of Si substrate using electron-beam lithography and reactive ion etching techniques. Owing to small footprint of v-MIM comparing conventional lateral MIM structure, the density of hot spots was dramatically increased resulting in strong signal enhancement and efficient suppression of background light.<br/>This metamaterial was applied to carbon dioxide and butane detection designing to exhibit a resonance at 4033 cm⁻¹ and 2945cm^-1 which spectral overlap with the C=O and –CH₂ vibration mode, respectively. The mutual coupling of these two resonant modes creates a Fano resonance, and their distinct peaks are clearly observed in the corresponding transmission dips. In addition, owing to its small footprint, v-MIM structure allows the detection of a 20 ppm concentration with suppressed background and high selectivity in the mid-infrared region.<br/><br/>[1] A. Ishikawa and T. Tanaka, Scientific Reports 5, 12570 (2015).<br/>[2] T. Le and T. Tanaka, ACS Nano 11, 9780 (2017).<br/>[3] T. Le, A. Morita, K. Mawatari, T. Kitamori, and T. Tanaka, ACS Photonics 5, 3179 (2018).<br/>[4] T. H. H. Le, A. Morita, and T. Tanaka, Nanoscale Horiz. 5, 1016 (2020).<br/>[5] D.-S. Su, D. P. Tsai, T.-J. Yen, and T. Tanaka, ACS Sensors 4, 2900 (2019).