Waveguide Metacouplers for In-Plane Polarimetry

Anders Pors and Sergey I. Bozhevolnyi

Phys. Rev. Applied 5, 064015 – Published 27 June 2016

Abstract

The state of polarization (SOP) is an inherent property of the vectorial nature of light and a crucial parameter in a wide range of remote sensing applications. Nevertheless, the SOP is rather cumbersome to probe experimentally, as conventional detectors respond only to the intensity of the light, hence losing the phase information between orthogonal vector components. In this work, we propose a type of polarimeter that is compact and well suited for in-plane optical circuitry while allowing for immediate determination of the SOP through simultaneous retrieval of the associated Stokes parameters. The polarimeter is based on plasmonic phase-gradient birefringent metasurfaces that facilitate normal incident light to launch in-plane photonic-waveguide modes propagating in six predefined directions with the coupling efficiencies providing a direct measure of the incident SOP. The functionality and accuracy of the polarimeter, which essentially is an all-polarization-sensitive waveguide metacoupler, is confirmed through full-wave simulations at the operation wavelength of $1.55 μ m$ .

Received 8 March 2016

DOI:https://doi.org/10.1103/PhysRevApplied.5.064015

Physics Subject Headings (PhySH)

Integrated optics Light-matter interaction Metamaterials Plasmonics

Atomic, Molecular & Optical

Authors & Affiliations

Anders Pors^* and Sergey I. Bozhevolnyi

SDU Nano Optics, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark

^*alp@iti.sdu.dk

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Vol. 5, Iss. 6 — June 2016

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Images

Figure 1
Waveguide configuration. (a) Sketch of the waveguide configuration that is assumed spatial invariant along the $x$ and $z$ directions. (b) The electric field of the TE1 mode for propagation along the $z$ direction and a PMMA thickness of 400 nm. (c) The real part of the effective index $N_{eff} = β / k_{0}$ , where $β$ is the propagation constant of the mode and $k_{0}$ is the vacuum wave number, and (d) the propagation length for modes sustained by the configuration in (a) as a function of the PMMA thickness. The ${SiO}_{2}$ thickness is fixed at 70 nm, and the wavelength is $λ = 1550 nm$ .
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Figure 2
Design of metacouplers. (a) Sketch of the unit cell of a GSP-based metacoupler. (b) Calculated reflection coefficient as a function of nanobrick widths ( $L_{x}$ , $L_{y}$ ) for $x$ -polarized normal incident light and geometrical parameters $Λ = 470 nm$ , $t_{s} = 70 nm$ , $t_{PMMA} = 400 nm$ , $t = 50 nm$ , and wavelength $λ = 800 nm$ . The color map shows the reflection amplitude, whereas the solid lines represent contour lines of the reflection phase for both $x$ - and $y$ -polarized light.
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Figure 3
Performance of the individual metacouplers. (a),(d),(g) Top view of the supercell of coupler 1, 2, and 3, respectively. (b),(c),(e),(f),(h),(i) Color map of the intensity in the center of the PMMA layer for couplers consisting of $3 \times 3$ supercells when the incident light is a Gaussian beam with beam radius $3 μ m$ . Note that the scale bar is chosen to better highlight weak intensity features, while the numbers (in percent) displayed in the panels correspond to coupling efficiencies through the areas marked by gray lines. (b),(c) Coupler 1 for incident polarization states $| x ⟩$ and $| y ⟩$ ; (e),(f) coupler 2 for incident polarization states $| a ⟩$ and $| b ⟩$ ; (h),(i) coupler 3 for incident polarization states $| r ⟩$ and $| l ⟩$ .
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Figure 4
Performance of the combined metacoupler. (a) Top view of the combined waveguide metacoupler. (b)–(g) Color map of the intensity in the center of the PMMA layer for the metacoupler in (a) when the incident light is a Gaussian beam with the beam radius of $6 μ m$ . The polarization state $| u ⟩$ of the beam is displayed in the upper-right corner. Note that the scale bar is chosen to better highlight weak intensity features, while (b) shows the numbering of the six ports marked with gray lines. (h) Circles and asterisks indicate retrieved and exact polarization states, respectively, of the incident beam for 42 different SOPs, plotted in the ( $s_{1}$ , $s_{2}$ , $s_{3}$ ) space together with the Poincaré sphere. Colors are used as an aid to differentiate between the different circle-asterisk pairs.
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