URSI AP-RASC 2019, New Delhi, India, 09 - 15 March 2019 A Broadband FSS Bandstop Filter in Terahertz Region Sambuddha Sarkar (1) , Tanmoy Chakrabarti (2) , Sambit Kumar Ghosh (3) and Somak Bhattacharyya (4)* (1), (2) Department of Electronics and Communication Engineering, Academy of Technology, Adisaptagram, Hooghly-712121, India (3), (4) Department of Electronics Engineering, Indian Institute of Technology (BHU), Varanasi-221005, India 1 sambuddha96@gmail.com, 2 tanmoychakrabarti1997@gmail.com, 3 sambitkrghosh.rs.ece17@iitbhu.ac.in, 4 somakbhattacharyya.ece@iitbhu.ac.in Abstract A wideband bandstop filter using frequency selective surface (FSS) operating in the terahertz (THz) frequency range has been reported. The simulation results show that it has a 3-dB bandwidth of 22.9 THz over the resonating frequency of 24.1 THz. The transmission stopband lesser than 30dB has been achieved along with a fractional bandwidth of 95%. The structure has been analyzed under the variations of several geometrical parameters. The structure is found to behave as wideband stopband filter till 40 o incident angle. The structure offers wide stopband till 15 o polarization angle. Being ultra-thin (~λ/12) and simple in nature, the filter can be very easy to realize in practice. The structure finds its application in spectroscopy, imaging etc. 1. Introduction Terahertz (THz) technology has rapidly become an attractive research area due to its potential for applications related to imaging and sensing, space science, security applications etc [1-3]. Blocking of signals over a particular frequency range is extremely important for these applications [4]. Frequency selective surfaces (FSSs) have been extensively used in filter designs due to the design of the surface by using repetitive design over substrate to transmit or reflect electromagnetic waves at a particular frequency range over the last few years [5]. Realization of filters using FSS is particularly advantageous as it does not require any physical electrical connections and is significantly useful in the field of wireless communication [5]. The frequency response of such a filter depends upon the geometry of the design, shape, size and dimension. The design geometry of an FSS can be of many types like solid interior or plate type, center connected elements, loop type structures, and the combination of the above three together [6]. FSS based filters can be applied in radomes, dichoric subreflectors, lenses, RFID, modification in radar cross section of a device, GSM, GPS, ISM, selective shielding of frequencies in military and airport communication, isolation of unwanted and harmful radiation in hospitals, schools and domestic environment [7]. Recently, stopband FSS filters have been realized in terahertz domain, although they suffer from the narrow bandwidth responses [8-9]. In this manuscript, we present a wideband bandstop frequency selective surface (FSS) filter of bandwidth 22.9 THz over the resonating bandwidth of 24.1 THz to realize a fractional bandwidth close to 95%. This is probably the maximum bandwidth ever achieved at a resonating frequency of 24.1 THz with a transmission minima lower than 30 dB. The structure is made of periodic metallic patch made of gold imprinted over silicon dioxide. The structure has been studied under the variations of several geometrical parameters and the optimized geometrical dimensions are considered. The structure has been studied under oblique incidences, where the wide stopband is realized upto 40 o incident angles. The structure is also studied under polarization angle variations and the structure offers wide stopband response till 15 o polarization angle owing to its two-fold symmetry. 2. Design of FSS unit cell Figure 1. (a) Top view and (b) side view of the unit cell of the broadband bandstop FSS filter. The resonant frequency in the unit cell is controlled by the inductance derived by the metallic strips while the two symmetrical slots give rise to the equivalent capacitive effect. Hence the geometrical dimensional variations of the slots and metallic patches will lead to different sets of capacitances and inductances [10]. The bandwidth of the