Plasma Sources Science and Technology ACCEPTED MANUSCRIPT Tunable reflectionless absorption of electromagnetic waves in a plasma- metamaterial composite structure To cite this article before publication: Nolan Uchizono et al 2020 Plasma Sources Sci. Technol. in press https://doi.org/10.1088/1361- 6595/aba489 Manuscript version: Accepted Manuscript Accepted Manuscript is “the version of the article accepted for publication including all changes made as a result of the peer review process, and which may also include the addition to the article by IOP Publishing of a header, an article ID, a cover sheet and/or an ‘Accepted Manuscript’ watermark, but excluding any other editing, typesetting or other changes made by IOP Publishing and/or its licensors” This Accepted Manuscript is © 2020 IOP Publishing Ltd. 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Before using any content from this article, please refer to the Version of Record on IOPscience once published for full citation and copyright details, as permissions will likely be required. All third party content is fully copyright protected, unless specifically stated otherwise in the figure caption in the Version of Record. View the article online for updates and enhancements. This content was downloaded from IP address 131.179.158.10 on 10/07/2020 at 02:04 Page 1 of 11 AUTHOR SUBMITTED MANUSCRIPT - PSST-103817.R1 1 2 3 4 5 6 7 8 9 10 Tunable Reflectionless Absorption of 11 Electromagnetic Waves in a Plasma-Metamaterial 12 13 Composite Structure 14 15 16 Nolan M. Uchizono, Stephen A. Samples, and Richard E. Wirz 17 University of California, Los Angeles, Department of Mechanical and Aerospace 18 Engineering, Los Angeles, CA, 90043 19 20 E-mail: [email protected], [email protected] 21 22 June 2020 23 24 Abstract. We present the first experimental demonstration of a tunable reflectionless 25 absorption resonance in a metamaterial integrated with a plasma discharge. A 26 one-dimensional metamaterial structure excites transverse magnetic slow-wave modes 27 known as \spoof" surface plasmon polaritons. When interfaced with an argon 28 plasma discharge, the metamaterial-induced \spoof" plasmon mode is converted to a 29 30 plasmon polariton mode confined to the plasma/dielectric interface. The reflectionless 31 absorption band that manifests in the metamaterial's spectral response exhibits a 32 dependency on the plasma's electron density that agrees well with theory. 33 34 35 36 Keywords: plasma-metamaterials, surface plasmon polariton, tunable metamaterial, 37 38 plasma waves 39 40 Submitted to: Plasma Sources Sci. Technol. 41 42 43 44 45 1. Introduction 46 47 A metamaterial is an engineered structure designed to exhibit extraordinary properties, 48 49 such as zero or negative refractive index [1, 2]. Such unique properties have been 50 used to develop \cloaking" [3, 4], super lenses [5, 6, 7], and enhanced antennas [8, 9]. 51 Metamaterials are characterized by periodic features spaced at intervals much shorter 52 53 than the wavelength of a given excitation wave. The dependency of a metamaterial's 54 spectral response on its geometry, shape, orientation, and size limits its expression of 55 unique properties to a narrow frequency spectrum. Recent research efforts have focused 56 57 on overcoming the inherent bandwidth limitations by developing metamaterials with a 58 tunable frequency response [10]. Tunable metamaterials have improved functionality, 59 60 Accepted Manuscript AUTHOR SUBMITTED MANUSCRIPT - PSST-103817.R1 Page 2 of 11 1 2 3 Reflectionless Absorption in Plasma-Metamaterials 2 4 5 Spoof-SPP Coupling SPP 6 Plasma (εp < 0) 7 8 - - - -++ ++ (ε > 0) 9 Dielectric d 10 z 0 < v < c v = 0 Metamaterial 11 x g g 12 13 Figure 1: Qualitative description of spoof-SPP to SPP conversion in the plasma- 14 metamaterial structure. In the absence of the plasma (shown to the left), the device generates 15 16 spoof-SPPs, which are surface waves that have a longitudinal electric field. Spoof-SPPs 17 propagate along the surface of the metamaterial with a group velocity (vg) much lower than 18 the speed of light. In the presence of a plasma (shown to the right), these spoof-SPPs couple 19 to the free electrons in the plasma, producing surface plasmon polariton (SPP) modes along 20 the plasma/dielectric interface, which have a group velocity of zero. 21 22 23 24 enabling broader adoption for mainstream applications, such as communication systems 25 [11], terahertz-band technologies [12, 13], and sensing [14, 15]. 26 One promising approach to achieving tunable metamaterials is through the 27 integration of gas discharges into resonant structures to create plasma-metamaterial 28 29 composites [16, 17, 18, 19, 20]. The response of a plasma-metamaterial composite is 30 easily controlled by tuning gaseous discharge parameters, such as electron density or 31 neutral gas pressure. Bounded plasmas express many types of resonances in response to 32 33 electromagnetic excitation [21, 22, 23, 24]. Numerical simulations have predicted plasma 34 surface wave resonances induced by metamaterial structures, yet this phenomenon has 35 not been observed experimentally [25, 20]. In the presented work, we report the first 36 37 empirical observations of reflectionless absorption caused by coupling between plasma 38 and metamaterial resonances, thus demonstrating a new method of tuning the spectral 39 response of a metamaterial. 40 41 When considering the interface between plasmas, dielectrics, and metamaterials, it 42 is germane to discuss the concept of surface waves. A surface plasmon polariton (SPP) 43 describes a surface plasma density oscillation that is coupled to an electromagnetic wave. 44 45 SPPs occur at the interface of a conductor and dielectric at visible and near-infrared 46 frequencies (∼200-800 THz) in metals [26], and microwave frequencies (∼0.1-100 GHz) 47 in gaseous discharges [27, 28, 29], where they are occasionally referred to as \gaseous 48 49 plasmon polaritons" [30]. Pendry et al. demonstrated an excitation analogous to SPPs 50 that has been termed a \spoof-SPP", which can be excited at sub-terahertz frequencies 51 using a metamaterial [31]. SPPs and spoof-SPPs exhibit similar characteristics, such 52 53 as nonlinear dispersion and longitudinal electric field. Both embodiments of plasmonic 54 excitation exhibit a characteristic resonance defined as the frequency where wavenumber 55 asymptotically approaches infinity. However, spoof-SPP excitation manifests as a result 56 57 of interactions between incident electromagnetic waves and the metamaterial's periodic 58 structure, rather than the motion of surface charges as described by SPPs [31, 32]. The 59 presented work demonstrates how the analogous behavior between SPPs and spoof- 60 Accepted Manuscript Page 3 of 11 AUTHOR SUBMITTED MANUSCRIPT - PSST-103817.R1 1 2 3 Reflectionless Absorption in Plasma-Metamaterials 3 4 (a) (b) (c) 5 6 Vacuum Chamber 14 mm 10 mm 7 z 8 Anode Transmit x 9 Helmholtz 10 Coils 11 Network 12 Analyzer 5.5 mm 13 Probe 14 15 14 mm 16 Receive 17 18 Cathode 19 20 21 Figure 2: (a) Image of experimental operation of the metamaterial immersed in a 22 magnetized DC plasma discharge during Langmuir probing. (b) Diagram of the UCLA 23 Plasma-Metamaterial Interactions Facility. A thermionic cathode and copper anode produce 24 a magnetized plasma column. The uniform DC magnetic field is produced by a 10-inch 25 Helmholtz coil. (c) Dimensions of the corrugated microstrip metamaterial. 26 27 28 29 SPPs may be exploited to couple electromagnetic energy from a metamaterial to the 30 free electrons in a plasma. Significant effort has been invested into the research of 31 spoof-SPP excitation in metamaterials due to their practical application in microwave 32 33 to terahertz frequency electronics [33, 34, 35]. Hence, the novel spoof-SPP to SPP 34 coupling phenomena presented here opens new possibilities for useful application of 35 metamaterial tuning. A diagram that qualitatively illustrates the spoof-SPP to SPP 36 37 coupling in the presented experimental geometry is shown in figure 1. 38 To excite an SPP, a photon must have the same wavenumber and frequency as the 39 surface plasmon. Free-space photons cannot excite an SPP, and so a dispersive medium 40 must be used as a coupling mechanism. Wavenumber matching is typically achieved with 41 42 a prism or grating [26], yet metamaterials that can excite spoof-SPPs are also dispersive 43 media (often referred to as slow-wave structures), which reduce the group velocity of 44 incident waves. The presented experiment uses a planar metamaterial as a dispersion 45 46 medium to couple to a nearby DC plasma discharge. Since wavenumber matching is 47 a key condition for excitation of SPPs, the dispersion relations of our metamaterial 48 structure (defined later in the Results & Discussion section) are instrumental for 49 50 analyzing our presented results. 51 In the following sections, we discuss the methods and diagnostics used in the 52 experiment, the results that demonstrate tunable reflectionless abosorption in a 53 54 metamaterial, and our analysis that leads us to conclude that this behavior manifests 55 as a results of spoof-SPP to SPP resonant coupling.