Abstract— The designs for Frequency Selective Micro Cavity (FSMC) structures in the terahertz band have been proposed, based on ferrite composite layers. A high-value complex- permittivity Ni0.5Co0.2Zn0.3Fe2O4 base layer has been previously reported and demonstrates low transmittance at 1 THz and above. The scope entails design of Anti-Reflective Coatings (ARC) for broad-band (0.5-2.5 THz) cavity absorbance of 90% and higher. The reported deliverables are as follows - (i) development of thick film coatings with tunable refractive index based on SU8 and micro-machined ferrite grains, (ii) analytical studies to determine ARC properties (single and bi-layer) for optimal performance, and (iii) multi-physics simulations. The results reported from (ii) and (iii) are well matched. Ongoing work involves setup development for THz reflectance measurements to help validate cavity performance and explore cavity applications. I. INTRODUCTION OLYMER-ferrite micrograin composites have been recently explored for novel and significant magnetic, optical and electrical properties [1]. Accordingly, multiple, applications have been explored such as electromagnetic shields [2], flexible piezo actuators [3] and as a cancer therapy agent [4]. In particular, spinel ferrites have been reported as an electromagnetic (0.5–14 GHz) wave-absorbers with high refractive index (~2.3) and low dielectric and magnetic losses [5]. Such ferrite materials are useful for the design of RF/THz frequency selective surfaces (FSS) [6]. Ongoing FSS design activities are focused on terahertz components such as microcavity resonators, plasmon detector, and cavity-coupled bolometers [7-9]. Spinel ferrites are a compound with a generic formula MeFe2O4 (Me is a transition metal 2+ ion). The ferrites are categorized as (Me1-xFex)[MexFe2-x]O4, where ‘x’ is an inversion factor. The () and [] indicate A site and B site of the spinel structure, respectively [10]. The A-site is occupied by (Fe 3+ Zn 2+ ) ions and B-site is occupied by [Fe 3+ Co 2+ Ni 2+ ] ions. The tunable composite films reported in this work are comprised of ferrite micrograins in polymer matrix with conforming refractive indices. This is applied for design of single and bi-layer anti-reflective coatings (ARC). In THz studies, thick SU8-ferrite micrograin films (19 μm -22 μm) are required. The objective is to combine low transmittance property of the base ferrite layer (high complex permittivity as the function of stoichiometry), and low reflectance of ARC films, to attain (i) maximum absorption for a specific EM frequency (for example, notch filter) or (ii) high absorption threshold over a broader band. This paper is structured as follows. Firstly, we report on the synthesis of SU8-ferrite micrograin composite thick films. The composite refractive index is dependent on concentration of Ni0.5Co0.2Zn0.3Fe2O4 micrograins. Next, we present material characterization studies followed by numerical simulations of THz cavity structures. Experimental work to match simulation results is currently in progress. Numerical simulations of the ferrite base layer, single and bi- layer ARC structures are performed using CST Microwave Studio 2015® multiphysics software in a frequency range from 0.5 to 2.5 THz. Ni0.5Co0.2Zn0.3Fe2O4 is synthesized by sol-gel combustion method and the resultant ferrite powder is milled using a planetary ball mill to get the ferrite micrograins that is reported in the previous work [6]. A base layer of Ni0.5Co0.2Zn0.3Fe2O4 is prepared by application of a hydraulic press. Based on literature [11], the w/w% concentrations of SU8/ferrite micrograins are used to obtain gel composite as shown in Table 1. The gel material is further processed to generate thick composite films with a tunable refractive index. Table 1 w/w% concentration of SU8 matrix/ferrite micrograins for a tunable refractive index (n). Composite SU8 matrix Ferrite micrograins n A1 76% 24% 1.84 B1 66% 34% 2.0 B2 85% 15% 1.7 THz time-domain spectroscopy (THz-TDS) analysis is carried out using an indigenously developed THz-TDS setup in a frequency range from 0.5 to 2.5 THz that is reported in the previous work [10]. In this analysis, a complex transmission coefficient of the sample in frequency domain is obtained by dividing the THz signal transmitted in the presence of the sample ( , ) to the signal transmitted without the sample ( , ) that is   () =  , ()  , () ⁄ , and a complex reflection coefficient,   () is obtained by the multiphysics software using the extracted optical parameters (n, k). Later, the absorbance (A) is determined by using the transmission and reflection coefficients that is expressed by Eq. (1) [12],  = 1 − |  ()|  − |  ()|  . (1) Fig. 1 (a) shows the ferrite base layer (thickness ~ 500 μm), while, Fig. 1(b) and Fig. 1(c) display single and bi-layer anti- reflection coating on the base layer, respectively. Single layer ARC is denoted by micro-composite A1, and bi- layer ARC is comprised of micro-composites; B1 and B2 (Table 1). Meenakshi Arya 1 , Arnab Pattanayak 2 , Mayuri N Gandhi 1 , Kousik Pradhan 1 , Ajinkya Punjal 3 , Shriganesh S Prabhu 3 , Venu Gopal Achanta 3,4 , and Siddhartha P Duttagupta 1 1 Indian Institute of Technology Bombay, Mumbai, 400076, India 2 Thapar Institute of Engineering and Technology, Punjab, 147004, India 3 Tata Institute of Fundamental Research, Mumbai, 400005, India 4 CSIR-National Physical Laboratory, Delhi, 110012, India Soft Ferrite Bi-layer Design of Frequency Selective Micro Cavity P 978-1-7281-9427-1/22/$31.00 ©2022 IEEE 2022 47th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz) | 978-1-7281-9427-1/22/$31.00 ©2022 IEEE | DOI: 10.1109/IRMMW-THz50927.2022.9895935 Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on November 13,2022 at 05:11:39 UTC from IEEE Xplore. Restrictions apply.