Plasmonic Invisibility

  • Edwards B, Alu A, Silveirinha M, Engheta N. "Experimental Verification of Plasmonic Cloaking at Microwave Frequencies" Physical Review Letters. (103) 2009. (pdf)

Introduction

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When a wave is incident on an object it will reflect and scatter, and based on this scattering we know that the object is there. For objects that are less than a few wavelengths in size, the scattering can be expressed as the first few terms of a harmonic expansion with the majority of the energy going into the first term. Put more simply, while different small objects may scatter a wave with different intensities, the scattered wave always has the same basic shape. For electromagnetics, that shape is a dipole radiation pattern caused by the dipole moment of the scatterer. In a very similar manner to a diver attempting to reach neutral weight by combining his buoyant body with very dense lead, in this project we attempted to reach an electromagnetic neutrality by balancing the generally positive permittivity of a test object with the negative permittivity of a metamaterial cloak. When the plasmonic cloak is properly designed, this dipole radiation can be completely canceled out to leave only the lower power higher-order scattering modes.

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Plasmonic materials are readily found in nature at optical frequencies in the form of gold, silver, and many other metals. However, there are no naturally available plasmonic materials in the RF and microwave regime. Therefore we needed to build our own using a metamaterial technique first discussed by Walter Rotman in the early 60's. I'll attempt to explain it without going into much mathematics. When electromagnetic energy is incident on a slab of plasmonic material, it will penetrate it evanescently, similar to electron tunneling. A small portion of the energy will appear on the other side and much of it will be reflected. Similarly, when electromagnetic is incident on a waveguide for which its frequency is below cut-off, some of the field will penetrate this section, continuing on the other side, while much of it will be reflected. Therefore, a waveguide below cutoff and a bulk plasmonic material can interact with an electromagnetic wave in a very similar fashion.

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This is apparent in the figure above, which shows a transverse electric (TE) wave incident on a plasmonic and metamaterial slab. In both cases the wave is incident and transmitted into vacuum. The plasmonic material has a relative permittivity less than zero, while the metamaterial is made of a material with a permittivity greater than one and has evenly spaced metal fins. However, despite the radically different geometries and materials, the effect on the field is almost identical.

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Therefore, in this work we took a plastic cylinder and illuminated it with a TE wave in a parallel plate wave guide, simulating an infinite 2D geometry. We differenced the total and incident fields both in experiment and simulation to determine the scattered field. We did the same for a plastic cylinder wrapped in metal fins and a high permittivity dielectric (acetone). By comparing the relative scattered power between the cloaked and uncloaked object, we could determine the efficacy of our design. We found that at the design frequency in theory, simulation and experiment there was a dramatic dip in relative scattered power for the cloaked structure.  Put another way, for that one frequency, the dielectric cylinder becomes nearly invisible.

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