Angular-momentum selectivity and asymmetry in highly confined wave propagation along sheath-helical metasurface tubes

Yarden Mazor and Andrea Alù

Phys. Rev. B 99, 155425 – Published 25 April 2019

Abstract

Highly confined surface waves present unique opportunities to enhance light interactions with localized emitters or molecules. Hyperbolic dispersion in metasurfaces allows us to tailor and manipulate surface waves, enhancing the local density of states over broad bandwidths. So far, propagation on this platform was mainly studied in planar geometries, which facilitates the analysis but somehow limits the realm of possibilities. Here we show that “wrapping” hyperbolic metasurfaces into tubes may greatly enrich the wave propagation dynamics along their axis. This system shows strong interaction with fields and sources carrying optical angular momentum and pronounced field asymmetries, and opens pathways to valley-specific excitation and routing. In addition, we demonstrate that various parameter regimes enable strong spin/helicity momentum locking.

Received 26 November 2018
Revised 5 April 2019

DOI:https://doi.org/10.1103/PhysRevB.99.155425

Physics Subject Headings (PhySH)

Angular momentum of light Hyperbolic metamaterials Light-matter interaction Surface plasmons

Surfaces

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Authors & Affiliations

Yarden Mazor¹ and Andrea Alù^1,2,3,4

¹Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78712, USA
²Photonics Initiative, Advanced Science Research Center, City University of New York, New York, New York 10031, USA
³Physics Program, Graduate Center, City University of New York, New York, New York 10026, USA
⁴Department of Electrical Engineering, City College of New York, New York, New York 10031, USA

yardenm@utexas.edu
aalu@gc.cuny.edu

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Issue

Vol. 99, Iss. 15 — 15 April 2019

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Images

Figure 1
The geometry under analysis. (a) The principal axes $z^{'}, y^{'}$ over which the constitutive relations tensors in the reference planar surface are given. The $z$ axis indicates the direction of the cylinder axis, and the $φ$ axis is the azimuthal direction after the folding. (b) When rotated and folded they form a skewed reference system over the surface of the cylinder with angle $θ$ .
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Figure 2
(a) Surface wave dispersion curves for the folded cylindrical surface when only $X_{z^{'} z^{'}} \neq 0$ in Eq. (4) with $X_{z^{'} z^{'}} = - 4$ and $θ = π / 4$ allowing current to flow only in a direction making $45^{\circ}$ with the cylinder axis. Dashed black lines show the light lines, $k_{0} = ω / c$ . (b) Surface wave dispersion curves for a folded hyperbolic metasurface with $X_{y^{'} y^{'}} = 4, X_{z^{'} z^{'}} = - 4$ and $θ = π / 4$ . Dashed black lines shown the light lines. The mode $n = 0$ is not present, as it does not propagate. (c) $E_{z}$ sampled close to cylinder surface $(r = 1.02 a)$ , when exciting with two dephased electric dipoles. The cylinder properties correspond to the dispersion in panel (a). The black, perpendicular panels show the electric-field intensity, where we see the fast decay as the distance from the cylinder increases. The inset shows the modal content of the fields near the cylinder, showing a mixture of many modes. (d) Same as (c), but corresponding to the cylinder properties matching the dispersion in panel (b). Highly directional excitation is observed, and pronounced mode selectivity with a single mode propagating $(n = - 3)$ .
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Figure 3
(a) Excitation of surface waves over a planar surface with $Y_{y^{'} y^{'}} = - j 0.3 / η_{0}, Y_{z^{'} z^{'}} = j 16.133 / η_{0}$ and $Ω = 0.2$ , corresponding to Eq. (10). The modal fields are concentrated at the bottom surface. (b) Surface wave dispersion curves for a cylinder with electric surface admittance and magnetic surface impedance satisfying ${\underset{̲}{\underset{̲}{Z}}}_{m s}^{p} = η_{0}^{2} {\underset{̲}{\underset{̲}{Y}}}_{s}^{p}$ , corresponding to $X = X_{m} = - 4$ in Eq. (11). Dashed black lines are the light lines and $k_{0} = ω / c$ . Thin lines correspond to the family of modes concentrated mostly on the outer part of the cylinder, and thick lines correspond to the family of modes concentrated mostly on the inner part of the cylinder, as described by Eqs. (12) and (14). (c) Same as (b) but with $X = X_{m} = - 2$ .
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Figure 4
Dispersion curves for a folded metasurface, corresponding to $X_{m} = - 4 / X$ in Eq. (11). The surface parameters are $X = 4, X_{m} = - 1$ , and the folding angle is $θ = π / 4$ . Black dashed lines are the light lines, and $k_{0} = ω / c$ .
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