Enhanced Absorption with Core/Shell Silicon Carbide/Graphene Nanowires for Tunable Mid-Infrared Nanophotonics

Abstract

Abstract Body: Surface plasmon polariton (SPP), which result from the strong coupling of bound electromagnetic (EM) waves and collective charge oscillations, enable subwavelength manipulation of light and matter interaction. Conventional noble metals-based SPP excited in the visible and near-Infrared spectra ranges suffer from large energy losses and cannot be dynamically tuned. Thanks to its 2D nature, graphene has emerged as a promising alternative plasmonic material with well- confined SPPs in the mid-infrared (MIR) and terahertz (THz) ranges and remarkable tunability. On the other hand, polar dielectric materials support low optical losses and sub-wavelength mode in the MIR and THz regions via surface phonon polariton (SPhP)mode' stimulation within the Reststrahlen band, a narrow spectral range between transverse optical (TO) and longitudinal optical (LO) frequencies. Due to the excellent polaritonic responses in both graphene and silicon carbide in the MIR and THz regions, devices combining the two materials are inferred to greatly advance photonics and THz technologies. We have recently theoretically demonstrated the strong confinement and large propagation figures of merit (FOM) for hybrid surface plasmon and surface phonon polariton in the epitaxial graphene on silicon carbide. The ability to confine such MIR light into small volume through excitation of SPP and SPhP has many implications for optoelectronic technologies such as the MIR photodetection[2], sensing[3], and solar cells[4]. Epitaxial graphene(EG) grown on silicon carbide(SiC) on silicon(Si) has been investigated as a suitable platform for graphene growth on semiconductors to meet the prospective technologies [5]. Despite this platform being very advantageous, experimental and theoretical works demonstrating the potentials properties/applications of EG on SiC on Si are still missing. Here we combine electromagnetic simulations based on finite elements method (FEM) with attenuated total reflectance Fourier transformed infrared (ATR-FTIR) spectroscopy to indicate improved MIR absorption, E field enhancement, and tunability for graphene-coated SiC nanowires (NWs). As anticipated, low absorption was confirmed from both simulations and measurements on bare SiC NWs, further supported by the calculated weak electric field due to the thin size of SiC NWs being incapable of absorbing sufficient light. On the other hand, enhanced absorption for the SiC NWs was measured by ATR-FTIR after a conformal graphitization of NWs[6]. Our previous study of this material system revealed a low-density oxide layer formed between graphene and SiC NWs[6]. This oxide layer between EG and SiC NW has been implemented in the FEM model, leading to field enhancement of incident EM field and strongly confined in the thin oxide. This substantial absorption and field enhancement originates from the coupling between the incident MIR photon and the plasmon/ phonon in graphene and SiC, respectively, with the oxide layer acting as a coupling medium. Furthermore, we demonstrate the modes' tunability whereby tuning the graphene's Fermi energy, the modes' resonance frequencies can be shifted to ~ 100 cm-1. Lastly, we realize substantial field enhancement, which is achieved by carefully tuning the graphene's Fermi energy and adjusting the spectra position of mode 1(mode on the left of LO frequency) closer to TO frequency. These results hold promise for various photonic applications such as perfect absorber to improve solar cell, MIR photodetectors, and other tunable photonics. [1]P. Rufangura, et al., J. Phys. Mater., vol. 3, no. 3, p. 032005, 2020. [2]T. Low and P. Avouris, ACS nano, vol. 8, no. 2, pp. 1086-1101, 2014. [3]A. G. Brolo, Nat. Photon., vol. 6, no. 11, p. 709, 2012. [4]H. A. Atwater and A. Polman, Nat. Mater., vol. 9, no. 3, p. 205, 2010. [5] N. Mishra et al., Phys. Status Solidi (a), vol. 213, no. 9, pp. 2277-2289, 2016. [6]N. Mishra et al., J. Appl. Phys., vol. 126, no. 6, p. 065304, 2019.