204 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 10, NO. 2, MARCH 2020 High-Q Terahertz Absorber With Stable Angular Response Amir Ebrahimi , Member, IEEE, Rajour Tanyi Ako , Wendy S. L. Lee, Member, IEEE, Madhu Bhaskaran, Member, IEEE, Sharath Sriram, Member, IEEE, and Withawat Withayachumnankul , Senior Member, IEEE Abstract—This article presents a narrow-band terahertz ab- sorber using miniaturized unit cells. The absorber is made of three metallic layers separated from each other using low-loss cyclic olefin copolymer as dielectric spacers. A high quality factor (Q) is obtained using a first-order resonating structure with additional capacitive loading formed by the top two metallic layers. A circuit model is developed for the analysis and design of the proposed ab- sorber. The designed absorber shows 1% fractional bandwidth for more than 90% absorption around a center frequency of 0.5 THz under normal incidence angle. This is equivalent to a quality factor of Q = 31, calculated at 50% absorbance. The structure shows a highly stable absorbance over a wide range of oblique incidence angles. The designed absorber has been fabricated using a microfabrication process. The measured results of the fabricated prototype are in good agreement with the simulations. Index Terms—Circuit analog absorbers, miniaturized elements, narrow-band absorber, terahertz absorber. I. INTRODUCTION A BSORBERS are fundamental components in many imag- ing, spectroscopy, and communications systems from microwave to terahertz and optical frequencies [1]. Important characteristics in absorber designs are the bandwidth, reflec- tivity, thickness, and stability of the absorbance over oblique incidence angles [2]. A simple absorber configuration is the Salisbury screen formed by a resistive sheet with a quarter- wavelength distance above a conducting plane [3]. The achiev- able absorbance bandwidth based on the Salisbury screen is limited to around 26%. Therefore, there have been a lot of efforts Manuscript received April 1, 2019; revised October 22, 2019; accepted December 30, 2019. Date of publication January 8, 2020; date of current version March 3, 2020. This work was supported in part by RMIT Micro Nano Research Facility (MNRF) in the Victorian Node of the Australian National Fabrication Facility (ANFF) and in part by Australian Research Council (ARC) Discovery Project DP170101922. (Amir Ebrahimi and Rajour Tanyi Ako are co-first authors.) (Corresponding author: Amir Ebrahimi.) A. Ebrahimi is with the School of Engineering, RMIT University, Melbourne, VIC 3001, Australia (e-mail: amir.ebrahimi@rmit.edu.au). R. T. Ako, M. Bhaskaran, and S. Sriram are with the Functional Materials and Microsystems Research Group, School of Engineering, RMIT University, Melbourne, VIC 3001, Australia (e-mail: rajour.tanyi.ako@rmit.student.edu.au; madhu.bhaskaran@rmit.edu.au; sharath.sriram@rmit.edu.au). W. S. L. Lee and W. Withayachumnankul are with the Terahertz Engineering Laboratory, School of Electrical and Electronic Engineering, The University of Adelaide, Adelaide, SA 5005, Australia (e-mail: wendy.lee@adelaide.edu.au; withawat@adelaide.edu.au). Color versions of one or more of the figures in this article are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TTHZ.2020.2964812 to design absorbers with ultra-wideband responses [4]–[19]. Broadband terahertz absorbers are implemented in [7] and [8] using single or multiple stacked layers of metallic resonator arrays. In order to broaden the absorption band, resonators of various sizes in an array were designed in [9] and [10] to achieve multiple resonances. Doped silicon (Si) gratings were realized in [11] and [12] for inducing multi-order diffraction and widening the absorption band. Three-dimensional (3-D) structures such as multilayer metal-dielectric pyramids and mi- cropyramids on a PDMS base were implemented for broadband absorbers [13]–[15]. Plasmonic structures were implemented in [17] and [18]. In addition, there are plenty of designs in the lit- erature implementing multiband absorbers for various frequency ranges from the microwave up to optical domain [20]–[31]. However, designing a high quality factor (Q) absorber with a stable frequency response under oblique incidence angles is still challenging [32]–[35]. So far, there is no report on the implemen- tation of such absorbers at terahertz frequencies. Such a high-Q absorber can be used for implementing microbolometers for uncooled real-time terahertz imaging systems [36]. Moreover, a very high sensitivity of the narrow-band resonance produced by this absorber to dielectric coatings could be useful for sensing applications at the terahertz regime [37]. Here, we design a narrowband terahertz absorber based on a miniaturized unit cell. The proposed narrowband absorber consists of three metallic layers separated from each other using cyclic olefin copolymer (COC) dielectric spacers. The minia- turized elements have shown a great performance in improving the stability of the frequency-selective surfaces (FSSs) under oblique incidence angles [38]–[43]. Their response remains consistent, when excited by a wave with nonplanar phase front owing to a less severe phase imbalance on the two edges of each unit cell [44]. A high-Q resonance is realized by improving the capacitive loading in the unit cells using the stacked metallic lay- ers. A lumped element circuit model is developed for analyzing the electromagnetic behavior of the structure and for providing a design guide for the response optimization. This article is organized as follows. Section II describes the proposed narrow-band absorber and presents a design procedure based on an equivalent circuit model. Section III explains the microfabrication steps for implementation of the narrowband absorber. Section IV verifies our design of the narrowband ab- sorber through measurement, and, finally, Section V concludes this article. 2156-342X © 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: RMIT University Library. Downloaded on May 03,2020 at 06:06:29 UTC from IEEE Xplore. Restrictions apply.