1536-125X (c) 2018 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNANO.2018.2822277, IEEE Transactions on Nanotechnology Abstract—A graphene-based slot cavity is proposed and investigated numerically by using two-dimensional finite- difference time-domain method. The structure can be used as a tunable mid-infrared multi-channel band-pass filter, mode separator, multi/demultiplexer, and logic gates. Passbands of the designed filters can be tuned by varying the length and/or the chemical potential of the graphene slot cavity. The filter can be promoted to a mode separator by using another graphene output waveguide in the middle of the slot cavity filter. The even and odd order resonance modes can be separated and transmitted to the outputs. The proposed mode separator can also be used to design a wavelength multi/demultiplexer with minimum number of the slot cavities. We also proposed and simulated an ultra-compact nano-scale logic gates of XOR, OR, and NOT with graphene slot cavity resonators, especially in the mid-infrared wavelengths with an extinction ratio of 19.85 dB, which can be utilized for design of the nanoscale photonic integrated circuits (PICs). The gates behaviors are based on the suppression or enhancement of the resonance modes in the slot cavity due to the place of the input and output waveguide ports. These components are efficient and can be utilized in the design of mid-infrared nanoscale devices and PICs. Index Terms— Graphene Slot Cavity, Surface Plasmon Polariton, Mode Separator, Multi/Demultiplexer, Logic Gates I. INTRODUCTION cent developments in optical computing and processing have demanded requirements to high speed, low loss, and compact devices [1]. Surface plasmon polaritons (SPPs) can be a noticeable selection to provide optical devices [2]. Up to now, different kinds of plasmonic devices and systems based on noble metals and photonic crystals have been designed and proposed [3-6]. High loss and lack of enough tunability have become obstacles for plasmonic structures to be employed vastly [7]. Recently, graphene, a two dimensional with hexagonal arrangement of carbon atoms, has attracted a great attention due to its prominent optical features which provides low loss, tunable performance, and ultra-compact devices and systems compared to other materials such as metals [8-12]. Various kinds of graphene based devices, such as Antennas [13], waveguides [14, 15], modulator [16], logic gates [17], absorbers [18], and filters [19-23] have been proposed and analyzed numerically and some fabricated. Graphene ribbons The authors, Somayyeh Asgari and Nosrat Granpayeh are with Center of Excellence in Electromagnetics, Faculty of Electrical Engineering, K.N. Toosi University of Technology, Tehran 1631714191, Iran support SPPs in terms of the edge and the waveguide modes [24, 25], and the symmetric and the anti-symmetric modes corresponding respectively to the symmetric and anti- symmetric electromagnetic field distribution across the gap between the parallel graphene sheets. Also, some other plasmonic structures such as power splitters [26], switches [27], and directional couplers [28], with three parallel layer graphene system have been analyzed. In this paper, a structure composed of two parallel input/output graphene layers, coupled through a slot cavity is proposed and analyzed numerically using the two-dimensional finite- difference time-domain (2D-FDTD) method. A mid-infrared band-pass filter is achieved which the resonance wavelengths can be tuned by varying the length of the cavity and the chemical potential of the graphene slot cavity utilizing an external bias voltage. A mode selector, a four channel wavelength multi/demultiplexer, and some novel logic gates are also designed and proposed. FDTD simulation results are in good agreement with the theoretical calculation ones. The advantages of these proposed structures are their simplicity, subwavelength dimensions, and feasibility of fabrication process to be utilized in nano scale mid-infrared devices and photonic integrated circuits for optical processing. Also, compared to metals [8-12], graphene based devices have some advantages such as small size, low loss, long propagation length, and tunability. The slot cavity filter structure which is composed of two horizontal and one perpendicular graphene layers is very simple compared to previous resonators such as planar and curved structure ones [13, 16, 20, 21, 23, 29]. So, the fabrication process could be easier. Moreover, graphene/SiO2 mid-infrared slot cavity logic gates are compatible with CMOS logic gates [30, 31]. In addition, some other useful novel ultra- compact structures such as mode separator, multi/demultiplexer, and logic gates could be designed based on the cavity. The slot cavity filter could be a useful functional segment in future nano devices and circuits. The remaining of the paper is organized as follows: In Section 2, the method of analysis and simulation are introduced. In Section 3, the results are presented and discussed. The paper is concluded in Section 4. (Emails: s_asgari@ee.kntu.ac.ir; granpayeh@kntu.ac.ir). Applications of Tunable Nano-Scale Mid-Infrared Graphene-Based Slot Cavity in Nano photonic Integrated Circuits Somayyeh Asgari and Nosrat Granpayeh, Senior Member, IEEE R