Ring-Shaped Plasmonic Logic Gates Daniela Dragoman 1,2 & Elena Vlădescu 1 Received: 10 March 2018 /Accepted: 28 May 2018 # Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract Ring-shaped one-, two-, and three-bit plasmonic logic gate configurations and circuits have been proposed, which, besides being compact, are also versatile and can be easily cascaded, the output logic values being controlled by both the geometry of the multi- port rings and the phase of the incident beams. This latter degree of freedom, not fully exploited up to now in plasmonic circuits, offers a high degree of flexibility of logic gate configurations. Keywords Plasmonics . Optical computing Introduction Plasmonics has become a field of intense research due to its wide array of applications, from integrated optical circuits, to solar cells and spectroscopy, to name a few [1, 2]. In particular, many logic gate configurations based on surface plasmon polariton waves propagating along metallic nanowires, ridge, or slot waveguides, have been proposed [3–11]. These logic circuits could constitute the first steps toward a fast and reli- able computer, if problems such as plasmonic excitation and/ or waveguide alignment are solved. This paper proposes multi-port ring-shaped slot waveguide configurations and circuits that can implement several logic gates and, in addition, provide a compact design and ease of cascading in order to implement more complicated computing algorithms. We specifically study the three- and four-port con- figurations, which could operate as different reversible and irreversible logic gates depending not only on the amplitude but also on the phase of input signals. The ease of manipulat- ing this last degree of freedom increases the functionality of optical/plasmonic logic gates. Moreover, we show that in- creasing the port number is not always beneficial, increasing also the complexity of the response and imposing thus tighter tolerances. Instead, cascading ring-shaped slot waveguides with a smaller number of ports could simplify the design of logic gates. It should be pointed out that all plasmonic logic gate configurations studied in this paper are based on interfer- ometric effects, and as such could be implemented not only with slot waveguide configurations, but also with other plas- mon waveguide types [12], as well as common dielectric waveguides. The interest in plasmonic waveguides as op- posed to dielectric waveguides for any type of integrated op- tical circuits, in particular for logic gate implementation, is motivated by the fact that, due to the different waveguiding mechanism, the field confinement factor is highly increased, which favors a significant reduction of both waveguide core dimensions (to subwavelength ranges) and optical losses at bends, and thus offers the possibility to miniaturize integrated optical circuits in order to reach integration densities not achievable by non-plasmonic circuits. The field confinement factor is especially large in plasmonic slot waveguides [13], in which guiding is assured by coupling the surface plasmon polariton modes that form at each parallel metal/dielectric in- terface, coupling that occurs for thin dielectric layers. These bound modes are characterized by significant field enhance- ment in the dielectric waveguide core and the possibility to access/excite slot plasmons from the dielectric side by diffrac- tion gratings, as demonstrated in the experimental works re- ported in [6, 14], for instance. The propagation of plasmons in the slot waveguide gates studied in this paper is modeled using the transmission line analogy [15–17], which offers a fast and quite accurate com- putational method as long as only one mode can propagate throughout the waveguides. As such, the width of the slot waveguides must be chosen appropriately, and the studied * Daniela Dragoman danieladragoman@yahoo.com 1 Physics Faculty, University of Bucharest, P.O. Box MG-11, 077125 Bucharest, Romania 2 Academy of Romanian Scientists, Splaiul Independentei 54, 050094 Bucharest, Romania Plasmonics https://doi.org/10.1007/s11468-018-0779-2