nanomaterials Article Active Terahertz Modulator and Slow Light Metamaterial Devices with Hybrid Graphene–Superconductor Photonic Integrated Circuits Samane Kalhor 1 , Stephen J. Kindness 2 , Robert Wallis 2 , Harvey E. Beere 2 , Majid Ghanaatshoar 3 , Riccardo Degl’Innocenti 4 , Michael J. Kelly 2,5 , Stephan Hofmann 5 , Hannah J. Joyce 5 , David A. Ritchie 2 and Kaveh Delfanazari 1,2,5, *   Citation: Kalhor, S.; Kindness, S.J.; Wallis, R.; Beere, H.E.; Ghanaatshoar, M.; Degl’Innocenti, R.; Kelly, M.J.; Hofmann, S.; Joyce, H.J.; Ritchie, D.A.; et al. Active Terahertz Modulator and Slow Light Metamaterial Devices with Hybrid Graphene–Superconductor Photonic Integrated Circuits. Nanomaterials 2021, 11, 2999. https:// doi.org/10.3390/nano11112999 Academic Editor: Werner Blau Received: 13 October 2021 Accepted: 1 November 2021 Published: 8 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 James Watt School of Engineering, University of Glasgow, Glasgow G12 8QQ, UK; 2658952k@student.gla.ac.uk 2 Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK; sjk80@cam.ac.uk (S.J.K.); rw497@cam.ac.uk (R.W.); heb1000@hermes.cam.ac.uk (H.E.B.); mjk1@cam.ac.uk (M.J.K.); dar11@cam.ac.uk (D.A.R.) 3 Laser and Plasma Research Institute, Shahid Beheshti University, Tehran 19839 69411, Iran; m-ghanaat@sbu.ac.ir 4 Department of Engineering, University of Lancaster Bailrigg, Lancaster LA1 4YW, UK; r.deglinnocenti@lancaster.ac.uk 5 Engineering Department, University of Cambridge, Cambridge CB3 0FA, UK; sh315@cam.ac.uk (S.H.); hannah.joyce@eng.cam.ac.uk (H.J.J.) * Correspondence: kaveh.delfanazari@glasgow.ac.uk Abstract: Metamaterial photonic integrated circuits with arrays of hybrid graphene–superconductor coupled split-ring resonators (SRR) capable of modulating and slowing down terahertz (THz) light are introduced and proposed. The hybrid device’s optical responses, such as electromagnetic- induced transparency (EIT) and group delay, can be modulated in several ways. First, it is modulated electrically by changing the conductivity and carrier concentrations in graphene. Alternatively, the optical response can be modified by acting on the device temperature sensitivity by switching Nb from a lossy normal phase to a low-loss quantum mechanical phase below the transition temperature (T c ) of Nb. Maximum modulation depths of 57.3% and 97.61% are achieved for EIT and group delay at the THz transmission window, respectively. A comparison is carried out between the Nb-graphene- Nb coupled SRR-based devices with those of Au-graphene-Au SRRs, and significant enhancements of the THz transmission, group delay, and EIT responses are observed when Nb is in the quantum mechanical phase. Such hybrid devices with their reasonably large and tunable slow light bandwidth pave the way for the realization of active optoelectronic modulators, filters, phase shifters, and slow light devices for applications in chip-scale future communication and computation systems. Keywords: hybrid photonic integrated circuits; graphene; superconductors; terahertz photonics; terahertz electronics; electromagnetic induced transparency; slow light devices 1. Introduction Metallic superconductors are macroscopic quantum systems and gain their electro- magnetic properties from pairs of electrons, Cooper pairs [1]. Due to their intrinsic low-loss and plasmonic properties, they are excellent platforms for applications, especially in cryo- genic nano-electronics and nano-photonics [2–6]. Graphene is a thin layer of carbon atoms arranged in a hexagonal network. It is a two-dimensional (2D) material, the thinnest example of a material [7]. The combination of 2D materials and superconductors offers novel electronic and photonic properties that may not be found in either of these materials independently [8,9]. For example, it is possible to measure the superconducting gap in graphene when it is placed in close and clean proximity to a host superconducting material, Nanomaterials 2021, 11, 2999. https://doi.org/10.3390/nano11112999 https://www.mdpi.com/journal/nanomaterials