Vol.:(0123456789) 1 3 Applied Physics B ( 2019) 125:189 https://doi.org/10.1007/s00340-019-7305-x Improving the performance of nanostructure multifunctional graphene plasmonic logic gates utilizing coupled‑mode theory Tannaz Sadeghi 1  · Saeed Golmohammadi 1  · Ali Farmani 2  · Hamed Baghban 1 Received: 12 June 2019 / Accepted: 9 September 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Nanostructure ring resonators are suitable devices for photonic integrated circuits. A nano-scale multifunctional logic device based on plasmon-induced transparency (PIT) is presented here. This device consists of a pair of hexagonal ring resonators coupled with two parallel metal–insulator–metal (MIM) waveguides. According to the coupled-mode theory, the appropriate detuning between the resonances wavelengths of two resonators acts as the key factor to achieve the PIT phenomenon. For this purpose, the PIT phenomenon for several metals utilized in MIM waveguides is studied. Also, graphene has been employed as the replacement for the metal under the hexagonal ring resonator and its parallel waveguides. However, the interaction of light with graphene as a 2D material is weak, by varying the dimensions of waveguides, rings, and their distances, and also, incident light wavelength and graphene chemical potential, we have achieved the desired couplings in the structure. Finite- diference-time-domain (FDTD) simulations confrm that “1” and “0” logic states which represent the high and low levels of the optical power can be achieved at the through and drop ports by changing the refractive index. It has been demonstrated that the proposed structure implements the function of logical operations including XOR and XNOR, simultaneously. 1 Introduction All-optical logic gates have signifcant efects on the devel- opment of high-tech technologies such as optical networks, optical computers, and optical signal processing. In these devices, photons are used for transferring information instead of electrons [1]. Therefore, data transmission rate [2] and optical signal processing speed will be increased, efec- tively [3]. All-optical logic gates can be divided into primary AND, OR, NOT [4], and complex XNOR, NOR, NAND [5], logic gates. Generally, there are two main methods to describe all-optical logic gates: one is based on the nonlin- ear optical efects, while the other approach is based on the linear optical efects such as multi-beam interferences [6]. Free electrons that oscillate collectively at the surface of metals [7] and are known as surface plasmons are the main factor of light confnement in areas smaller than the incident wavelength [8]. It is based on the propagation of surface plasmon polaritons (SPPs) [9], which are electromagnetic waves propagating along the surface interface between a metal (or a semiconductor) and a dielectric [10]. Noble metals [11] such as silver and gold support surface waves in the infrared and visible ranges and for that reason, they are accessible materials for constructing optical metamate- rials [12]. Surface plasmons have served as a basis for the development of nanophotonics devices [13] and combine photonics and electronics felds at the nano-scale. A variety of plasmonic device based on SPP waveguides (SPPWG). Plasmonic splitters [14], switches [15], optical routers [16], and lasers [17] have been considered as prom- ising candidates to break the difraction limit of light since the optical signals could be guided and confned well at sub- wavelength scale [18]. However, the traditional plasmonic devices have a low-quality factor and are not suitable for nano-scale multifunctional logic gates. A plasmonic mode can be either superradiant (radiative mode) or subradiant (dark mode) depending on how strong an incident light from free space can be coupled into the plasmonic mode [19]. The radiative mode has a large scat- tering cross section and a low quality factor due to the radia- tion coupling. On the contrary, the dark mode normally has a signifcantly larger quality factor which is limited only by the loss of the metal and this leads to the analogy of the * Ali Farmani farmani.a@lu.ac.ir 1 School of Engineering Emerging Technology, Tabriz University, Tabriz, Iran 2 Department of Electrical Engineering, Lorestan University, Khoram Abbad, Iran