Localized Surface Plasmons in Nanostructured Monolayer Black Phosphorus Zizhuo Liu and Koray Aydin* Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States * S Supporting Information ABSTRACT: Plasmonic materials provide electric-field local- ization and light confinement at subwavelength scales due to strong light-matter interaction around resonance frequencies. Graphene has been recently studied as an atomically thin plasmonic material for infrared and terahertz wavelengths. Here, we theoretically investigate localized surface plasmon resonances (LSPR) in a monolayer, nanostructured black phosphorus (BP). Using finite-difference time-domain simu- lations, we demonstrate LSPRs at mid-infrared and far-infrared wavelength regime in BP nanoribbon and nanopatch arrays. Because of strong anisotropic in-plane properties of black phosphorus emerging from its puckered crystal structure, black phosphorus nanostructures provide polarization dependent, anisotropic plasmonic response. Electromagnetic simulations reveal that monolayer black phosphorus nanostructures can strongly confine infrared radiation in an atomically thin material. Black phosphorus can find use as a highly anisotropic plasmonic devices. KEYWORDS: Black phosphorus, plasmonics, LSPR, 2D materials, anisotropy Atomically thin two-dimensional (2D) materials have received burgeoning amount of interest in recent years due to exciting physical properties. They exhibit interesting electronic, optical, mechanical and thermal properties due to their mono to few- layer thicknesses. 1-4 The 2D materials offer a new class of platform to achieve novel electronic and photonic properties in ultracompact sizes. Although such materials have inherently monolayer to few-layer thicknesses, it has been showed that 2D materials can interact strongly with the incident light. In particular, graphene emerged as a monolayer plasmonic platform. 5-8 Theoretical and experimental studies confirmed that both propagating 9,10 and localized 11-13 surface plasmon modes can be excited in an atomically thin, nanostructured graphene. Until the discovery of the graphene as a plasmonic material, noble metals such as silver and gold have been the material of choice both for fundamental and applied research. However, 2D plasmonic materials provide a unique opportunity by confining plasmons in an extremely thin optical material and also represent a great challenge for light-matter interactions due to their inherent atomically thin thickness. With such highly localized electric fields in 2D plasmonics, it is possible to build and integrate devices in smaller sizes that exhibit novel electronic and optical properties compared with traditional bulk plasmonic materials. By patterning periodic nanostructures in 2D materials, resonances can be observed optically by measuring the extinction spectra. 14,15 Black phosphorus (BP) is a recently emerging 2D layered material that can be exfoliated to few layers and mono- layer. 16-23 In a monolayer BP, the phosphorus atoms form a hexagonal lattice with a puckered structure resulting in in-plane anisotropic properties. Black phosphorus has been recently investigated for potential applications including field effect transistors, 24-27 heterojunction p-n diode, 28 photovoltaic devices, 29 and photodetectors. 30 The Raman scattering spectra and photoluminescence measurements from BP revealed its highly anisotropic properties. 31 A recent theoretical paper by Low et al. reports collective plasmonic excitations in BP. 32 It has been shown that mono to few-layer BP supports anisotropic plasmonic dispersion due to different effective mass along different crystal directions. Here, we propose and numerically demonstrate that localized surface plasmons can be excited in nanostructured monolayer black phosphorus. We present the study of the electromagnetic response of periodically patterned black phosphorus sheet. We consider the parameters varied in a wide range and the different absorption spectrum is compared due to the inherent anisotropy in BP. In particular, we analyze the confinement of the electromagnetic field in both BP nanoribbon and nanopatch arrays. We demonstrate the anisotropic behavior by replacing BP nanoribbons in x-direction (armchair direction) and y-direction (zigzag direction). The performance of the structures may allow for the realization of new plasmonic devices which will take advantage of the directional dependence of its plasmon properties. Received: December 17, 2015 Revised: April 8, 2016 Published: May 6, 2016 Letter pubs.acs.org/NanoLett © 2016 American Chemical Society 3457 DOI: 10.1021/acs.nanolett.5b05166 Nano Lett. 2016, 16, 3457-3462