Comparison of frequency responses of cloaking devices under nonmonochromatic illumination

Efthymios Kallos, Christos Argyropoulos, Yang Hao, and Andrea Alù

Phys. Rev. B 84, 045102 – Published 5 July 2011

Abstract

Plasmonic coatings have been proposed as a robust method to suppress the scattering signature of conventional dielectric objects, typically by surrounding a given object of moderate size with an isotropic, homogeneous, dispersive material that has a low or negative permittivity. In contrast to transformation-based cloaking devices, where highly anisotropic inhomogeneous materials achieve invisibility by electromagnetically isolating a certain region of space, plasmonic cloaks operate under the principle of scattering cancellation and usually allow field energy to enter the device’s core. So far, plasmonic devices have been mostly examined using single-frequency, plane-wave excitations. In this work, the performance of such plasmonic cloaks when illuminated by more realistic, broadband nonmonochromatic pulses is investigated and compared with other cloaking mechanisms. The two-dimensional total-field scattered-field method is used within the finite-difference time-domain dispersive numerical technique in order to simulate time domain effects when temporally Gaussian pulses are launched toward cylindrical moderately-sized dielectric objects which are covered with appropriate plasmonic coatings. The results are compared to the performance of transformation-based cloaks, finding that the plasmonic cloaks may suppress scattering more effectively over a wider frequency range.

Received 22 November 2010

DOI:https://doi.org/10.1103/PhysRevB.84.045102

Authors & Affiliations

Efthymios Kallos^1,2,*, Christos Argyropoulos^1,3, Yang Hao¹, and Andrea Alù³

¹School of Electronic Engineering and Computer Science, Queen Mary University of London, London E1 4NS, United Kingdom
²Department of Materials Science, University of Patras, Patras 26500, Greece
³Department of Electrical and Computer Engineering, University of Texas at Austin, Austin, Texas 78712, USA

^*ekallos@upatras.gr

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Vol. 84, Iss. 4 — 15 July 2011

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Images

Figure 1
FDTD simulated electric-field amplitude distribution $E_{z}$ , after steady state is reached, for infinite TE plane-wave illumination ( $0.5$ $μ$ m by $0.5$ $μ$ m domain, $λ / 200$ resolution, $600$ THz frequency). Propagation direction is bottom to top. (a) Bare dielectric object with $ɛ_{1} = 3$ and $R_{1} = λ / 8.4$ . (b) The dielectric object of (a) covered with a plasmonic coating with $ɛ_{2} = - 14.28$ and $R_{2} = 1.08 R_{1}$ . (c) A dielectric object with $R_{1} = λ / 8.0$ and $ɛ_{1} = 4$ , surrounded by four antiphase satellites with permittivities $ɛ_{2} = - 102$ and radii $R_{2} = 0.19 R_{1}$ , centered at $1.2 R_{1}$ away from the center of the object. The dashed white lines in (b) indicate locations where the fields are recorded.Reuse & Permissions
Figure 2
Field diagnostics for the device of Fig. 1b, with and without a plasmonic coating tuned to operate at $600$ THz. (a) Spatially averaged scattering coefficient extracted by recording the steady-state fields in the total field region around the device [circular dashed white line in Fig. 1b]. Successive simulations are performed illuminating the device with monochromatic planes waves at seven different frequencies. (b) Frequency dependence of the total scattering width extracted by recording the time-dependent fields in the scattered field region [rectangular white dashed line in Fig. 1b]. (c) Scattered-field amplitude as a function of frequency as recorded at a single point inside the scattered-field region in the forward direction [top side in Fig. 1b]. In (c), the amplitude is normalized to the amplitude of the total incoming field in free space. In (b) and (c) the device is illuminated by a $400$ -THz-wide broadband pulse.Reuse & Permissions
Figure 3
Normalized scattered-field amplitude as a function of frequency when multiple $80$ -THz-wide narrowband pulses centered at various frequencies impinge on the dielectric object of Fig. 1b, with (dashed lines) and without (solid lines) a plasmonic cover. The scattering cancellation frequency is adjusted to be (a) at $400$ THz, (b) at $600$ THz, or (c) at $800$ THz. The geometric size of the object and the coating remain constant for all simulation runs.Reuse & Permissions
Figure 4
Scattering behavior as a function of frequency when the dielectric object of Fig. 1a is successively covered with four different cloaks to test their performance. (a) The scattered-field energy recorded at a single point in the forward direction. The scattered energy is normalized to the total field energy in free space. (b) The scattering width for each device.Reuse & Permissions
Figure 5
Scattered field energy as a function of frequency when a moderately sized dielectric object of diameter $λ / 3.14$ and $ɛ_{1} = 3$ , when the object is covered either by an optimized plasmonic coating, or by the ideal cloak. The scattered energy is normalized to the total field energy in free space.Reuse & Permissions

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