IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 5, NO. 5, SEPTEMBER 2015 725 Subwavelength Graphene-Based Plasmonic THz Switches and Logic Gates Morteza Yarahmadi, Graduate Student Member, IEEE, Mohammad K. Moravvej-Farshi, Senior Member, IEEE, and Leila Yousefi, Member, IEEE Abstract—In this paper, we report on the design procedure for developing subwavelength graphene-based plasmonic wave- guide, performing as a THz switch or an AND/OR logic gate. The propagation length of the surface plasmons (SPs), stimulated by a 6 THz TM polarized incident wave along this waveguide with a top graphene layer whose chemical potential is held at meV (ON state) is more than 35 times larger than that in the waveguide with eV (OFF state). Numerical results, obtained from full wave simulations using a finite element method, also show that the modulation depth density obtained for the straight plasmonic switching waveguide, whose length is just about 20% of the incident wavelength, is larger than those reported to date. Moreover, we also designed a logic AND gate composed of a straight waveguide, a Y-branch switch, and a logic OR gate composed of two face to face Y-branches, whose total lengths are 37%, 45%, and 53% of the incident wavelength, respectively. Simulations show that the maximum ON/OFF ratios for these subwavelength plasmonic waveguides that occur between their ‘1 1’ and ‘0 0’ logical states are 41.37, 39.87, and 40.76 dB, respectively. These numerical data also show that the modulation depth densities obtained for these devices are also greater than those reported to date. The proposed graphene-based plasmonic switches and gates offer potential building blocks for the future digital plasmonic circuits operating around 6 THz. Index Terms—Graphene, plasmonic logic gate, plasmonic switch. I. INTRODUCTION T ERAHERTZ waves can be used in a wide range of ap- plications in communications, sensing, medical imaging, spectroscopy, and ultra-fast computing [1]–[4]. In order to achieve high quality imaging, device integration, or effective nonlinear interaction in the THz frequency range, one needs to confine the electromagnetic field. A promising approach to achieve this goal is the use of plasmonics [5], [6]. In spite of being in photonics regime, metals do not support surface plasmons at THz frequencies. However, it has been recently demonstrated that graphene, as a 2-D metallic material pos- sessing extraordinary electronic and photonic properties, can be Manuscript received January 02, 2015; revised March 30, 2015; accepted July 20, 2015. Date of publication August 03, 2015; date of current version August 31, 2015. M. Yarahmadi and M. K. Moravvej-Farshi are with Faculty of Electrical and Computer Engineering, Advanced Devices Simulation Lab, Tarbiat Modares University, Tehran 1411713116, Iran (e-mail: m.yarahmadi@modares.ac.ir; farshi_k@modares.ac.ir; moravej@ieee.org). L. Yousefi is with the School of Electrical and Computer Engineering, Uni- versity of Tehran, Tehran, 14174, Iran (e-mail: lyousefi@ut.ac.ir). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TTHZ.2015.2459674 exploited for THz plasmonic applications [7]–[12]. In addition to advantages such as having a 2-D structure, low loss, unique response to THz radiation, graphene illustrates much larger tunability compared to a conventional 2-D electron gas (2DEG) [12]. Therefore, graphene can be utilized to develop ultra-com- pact, high-performance and actively-tunable devices [7], [8]. Recently, a deep sub-wavelength THz plasmonic waveguide has been proposed, by means of graphene–metal structure [12]. This type of sub-wavelength structure, unlike the conventional or metamaterial waveguides is not bulky, and strongly confines electromagnetic fields as desired. Following this work, we have shown that this type of the waveguide can be used to develop THz plasmonic switches with desired characteristics [13], [14]. Electronic device integration faces two major obstacles- in- terconnect delays and losses and electron velocity limited data transmission rates [15]. These can be overcome by use of appro- priate high frequency devices. Moreover, integration of ordinary diffraction-limited high frequency components into nano-scaled electronic chips faces a new constraint [16]–[18]. On the con- trary, surface plasmonic devices with subwavelength dimen- sions can smoothly be integrated into nano-scaled electronic chips [18]. Yet, a metal-based plasmonic system experiences relatively short propagation length, due to the metal inherent loss. A total promising solution that can overcome this deterio- rating obstacle is graphene-based nano-plasmonics [9], [19]. In this paper, we propose a functional, ultra-compact, and low power consumption graphene-based plasmonic waveguides acting as THz switches and logic gates with subwavelength di- mensions, suitable for data transmission in modern telecom- munication systems. The modulation mechanism is similar to the electro-optical modulation mechanism, except for the op- tical carrier signal that has been replaced by a plasmonic wave. Here, the modulating signal is the voltage that is applied to the graphene layer. The advantages that these proposed devices offer lie with their subwavelength dimensions, capability for on-chip integration, and low power consumptions. The rest of this paper is organized as follows. In Section II, after introducing the building blocks of a graphene-based plas- monic switch and giving a brief overview of its basic operating principle, we show how it can be optimized. In Section III, by taking advantage of the optimized parameters, we have designed a logical AND gate with subwavelength dimensions that can op- erate with very large ON/OFF ratio around the center frequency of 6 THz. In Section IV, we show that using the same prin- ciples, a subwavelength Y-branch plasmonic switch with high ON/OFF ratio can also be designed. Section V is dedicated to design of a plasmonic OR gate with subwavelength dimensions. 2156-342X © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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