IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 31, NO. 15, AUGUST 1, 2019 1293 Spoof Plasmonic-Based Band-Pass Filter With High Selectivity and Wide Rejection Bandwidth Rahul Kumar Jaiswal, Student Member, IEEE, Nidhi Pandit, Student Member, IEEE, and Nagendra Prasad Pathak , Member, IEEE Abstract—In this letter, we report the design theory and analysis of an end-coupled broadband spoof plasmonic based band-pass filter (BPF) at sub-wavelength scale enabled by multi-mode grooved stepped impedance resonators (GSIRs) at THz frequency regime. The proposed GSIRs are end-coupled to each other and externally coupled to the source and load through spoof surface plasmon polaritons (SPP)-based transmis- sion line. The designed filter has 3-dB bandwidth of 0.35 THz (from 0.17 THz to 0.52 THz) and fractional bandwidth (FBW) of 87.85%, respectively, with the shape factor of 1.35. The simulated transmission and reflection coefficients in the pass-band of proposed filter are better than ∼6 dB and 15 dB respectively. Also, 60 dB spurious rejection bandwidth of the designed filter in the upper stop-band is 1.3 THz (0.62–1.92 THz). For the validation of the proposed approach, similar design is implemented at microwave frequency due to the geometrical dependence feature of spoof SPP and its S-parameters response is presented. Such broadband BPF will pave a path to design and development of advanced integrated plasmonic circuits and systems for high-speed and low-loss THz wireless communication applications. Index Terms—Band-pass filter (BPF), broadband, dispersion, end-coupling, quasi transverse electromagnetic (QTEM) mode, spoof surface plasmon polaritons (SSPP). I. I NTRODUCTION T ERAHERTZ frequency regime has spurred a great atten- tion in wireless communication as the demand for wire- less bandwidth increases. As the lower frequency ranges has already become congested, occupancy of more users or equipments in these regimes may cause interference, crosstalk and mutual coupling problem [1]. Also, the energy efficient, low-loss, low-crosstalk and compact integrated circuits need to be developed towards the design of advanced THz transceivers. Band-pass filters (BPF) are one of the essential components of the wireless transceiver so design a broadband BPF is an important goal for RF circuit designers. In this direction, spoof surface plasmon polaritons (SSPP) based band-pass filtering structure can be a potential candidate. Natural SPP’s are known for the highly confined and localized EM surface modes at the sub-wavelength scale found at the vicinity of metal-dielectric interface and provide a route to development of miniaturized highly integrated optical circuits and devices [2], [3]. However, Manuscript received May 18, 2019; revised June 3, 2019; accepted June 28, 2019. Date of publication July 1, 2019; date of current version July 17, 2019. (Corresponding author: Nagendra Prasad Pathak.) The authors are with the Department of Electronics and Com- munication Engineering, IIT Roorkee, Roorkee 247667, India (e-mail: rjaiswal@ec.iitr.ac.in; npandit@ec.iitr.ac.in; nagppfec@iitr.ac.in). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2019.2926094 Fig. 1. Schematic of the proposed end-coupled broadband spoof plasmonic band-pass filter, (i) Layout of the SSPP based grooved stepped impedance resonator, (ii) Layout of the proposed design of band-pass filter. such confinement of EM modes is not found at THz and microwave frequencies due to the absence of SPP waves. Since these frequencies have numerous applications in navi- gation, radars, sensors and characterization of various material properties, properties like SPP’s can be beneficial to achieve these applications by enabling the concept of plasmonics into these lower frequency regimes. Spoof SPP mimic SPP-like characteristics even on perfect conductor where the real SPP cannot exist [3]. These spoof SPP are also called as designer SPP as one can alter the dispersion and cutoff characteristics of these structure according to their design specifications just by changing the geometry and texture of sub-wavelength grooves. In the last few years, a number of works have been reported in order to achieve sub-wavelength confinement in plasmonic waveguide and circuits like filters, tunable filters, Si-MOS amplifier and excitation of the antennas [4]–[18]. Like in [13], an H-shape unit cell was used to design filter at microwave frequency with 20 dB rejection in the upper stop-band. In [14], and [15], filters have been designed at THz frequency with U-shaped grooves and dumbbell grooves respectively. In [10], authors reported a band-pass filter using T-shaped resonator which support multi-mode response. Multi-mode resonators are known for the miniaturization because a single resonator can support multiple resonant frequencies and create transmis- sion zeroes (TZ) near the pass-band to obtain sharp selectiv- ity [19]. However, it has very small bandwidth of 176 MHz. In this letter, a quasi-elliptic spoof plasmonic BPF has been designed at THz frequency, as shown in Fig. 1, with a high selectivity, wide pass-band and high spurious rejec- tion bandwidth characteristics in which end-coupled multi- mode GSIRs are excited by SSPP transmission lines at its input/output port. The design and performance of the filter have been analyzed in detail. For validating the design concept, experimental characterization has been provided at microwave frequency. 1041-1135 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.