Progress In Electromagnetics Research M, Vol. 107, 13–23, 2022 A Triple Band Highly Sensitive Refractive Index Sensor Using Terahertz Metamaterial Perfect Absorber Sagnik Banerjee 1 , Purba Dutta 1 , Amitkumar V. Jha 1 , Prabhat R. Tripathi 2 , Avireni Srinivasulu 3 , Bhargav Appasani 1, * , and Cristian Ravariu 4 Abstract—This research introduces a novel design of a metamaterial absorber having the range in terahertz, capable of sensing changes in the refractive index of the encircling medium. The layout includes adjoining rectangular patches in the form of a plus symbol along with four circular patch resonators (CPRs) on the pinnacle of a Gallium Arsenide (GaAs) substrate. The proposed design comes up with three consecutive absorption peaks, with an absorptivity of 99.0%, 99.75%, and 98.0% at three different resonant frequencies of 2.36 THz, 2.675 THz, and 2.97 THz, respectively, and a Full Width Half Maximum (FWHM) of 0.08, 0.04, and 0.05. This structure’s quality factor (Q-factor) at the three resonant frequencies is 29.5, 66.8, and 59.4 together with 6.75, 17.5, and 30 as figure of merit (FoM), respectively. The proposed design offers a sensitivity of 0.54 THz/RIU, 0.7 THz/RIU, and 1.5 THz/RIU in those three absorption bands. To support the selection of design parameters, parametric assessment was done. The designed sensor can find its applications in terahertz sensing. 1. INTRODUCTION Metamaterials, composed of periodic sub-wavelength or deep-sub-wavelength structures, are artificially architectured composite structures that exhibit distinctive properties like permeability, refractive index, negative permittivity, optical magnetism, etc. Due to their exclusive properties, realized by physical means rather than chemical composition, these structures have attracted many researchers [1]. One of the subclasses of the structures, which absorbs the electromagnetic radiation incident upon them, is called metamaterial absorber. Due to its various applications, it has become a vital area of research [2]. A unique design of multiband metamaterial absorber operating in the 5–12 GHz frequency range has been reported in [3]. Another triple band metamaterial absorber based on hexagonal Split Ring Resonators (SRRs) was reported in [4] for detecting fuel contamination. A double band chiral metamaterial absorber was proposed in [5] to decide the standard of car lubricants in the MHz frequency regime. Another metamaterial absorber was proposed in [6] for X-band applications. These absorbers operate in the GHz and MHz frequency regimes. Metamaterial absorbers in the terahertz regime can be created with proper scaling and by proper choice of design materials [7, 8]. Terahertz metamaterial absorbers (TMA) can also function as extremely high-quality sensors [9, 10]. A silicon nitride-based bio-sensor was designed to estimate the thickness of thin films having a sensitivity of 4.05 × 10 −2 GHz/nm [11]. Similarly, sensors were developed for measuring temperature in which the absorption spectra shift with the variations in the temperature [12]. Bio-sensing applications have been achieved using TMAs that provided frequency shifts as 275 GHz [13] and 3.6 GHz [14]. TMAs are also Received 7 October 2021, Accepted 22 December 2021, Scheduled 30 December 2021 * Corresponding author: Bhargav Appasani (bhargav.appasanifet@kiit.ac.in). 1 School of Electronics Engineering, Kalinga Institute of Industrial Technology, Bhubaneswar, India. 2 Department of Electronics and Electrical Engineering, Birla Institute of Technology, Mesra, India. 3 School of Engineering & Technology, K. R. Mangalam University, Gurugram-122103, India. 4 Department of Electronic Devices, Circuits and Architectures, Polytechnic University of Bucharest, Bucharest, Romania.