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Unidirectional Surface Plasmon Polariton Coupler in the Visible Using Metasurfaces Fei Ding1,2, Nathaniel Kinsey1, Jingjing Liu1, Zhuoxian Wang1, Vladimir M. Shalaev1, and Alexander V. Kildishev1 1
Birck Nanotechnology Center, School of Electrical &Computer Engineering, Purdue University, West Lafayette, IN 47907, USA 2
Centre for Optical and Electromagnetic Research, State Key Laboratory of Modern Optical Instrumentations, Zhejiang University, Hangzhou 310058, China [email protected]
Abstract: We have theoretically investigated a metasurface as a unidirectional surface plasmon polariton (SPP) coupler. The structure can work over a broad bandwidth in the visible region. OCIS codes: (160.3918) Metamaterials; (250.5403) Plasmonics
1. Introduction Recently, the two dimensional (2D) version of metamaterials, called metasurfaces, have been used to demonstrate many exotic phenomena, such as anomalous reflection/refraction [1,2], coupling between propagating waves and surface waves , and planar lenses . Here, we theoretically propose a metasurface-based unidirectional SPP coupler built on the metal-dielectric-metal geometry. Our unidirectional SPP coupler can work over a broad spectrum, distinct from other asymmetric configurations [5-7]. Additionally, we report the decoupling efficiency of the device is around 63.5% at λ = 676 nm. 2. Simulation Results To design the SPP coupler, we use the reflection-phase gradient of the metasurface to compensate the momentum mismatch between the propagating wave (PW) and the SPP . As the refection-phase gradient is unidirectional, we can route the SPP to a specific direction. In our design, four nano-antennas with different sizes compose a unit cell of the metasurface. This structure realizes a 2π phase change with a periodicity equal to the SPP wavelength. In the simulation, we consider a device composed of three unit cells along the x direction (total length is 1.98 μm Fig.1a). A Gaussian beam (w0 = 0.99 μm) is normally incident on the metasurface with a TM polarization (the electric field is along x direction Fig.1b). Fig.1c shows that the structure efficiently couples the incident light into SPPs which propagate to the left. Right-propagating SPPs are greatly suppressed.
Fig. 1. (a) Schematic of the coupler. (b) Incident Gaussian beam at λ = 676 nm. (c) Amplitude of the electric field Ez.
CLEO:2014 © 2014 OSA
Fig. 2. Amplitude of electric field Ez at different wavelengths.
As the response of the metasurface is broadband, the unidirectional SPP coupling can be observed over a large wavelength range, as shown in Fig.2. The decoupler is realized based on the reciprocal principle, as shown in Fig. 3, with the incident light on the right and the decoupled light on the left. The efficiencies of coupling and decoupling are approximately 31% and 64%, respectively; lower than the 60% and 75% coupling and decoupling efficiencies reported in reference . This is ascribed to the different working wavelengths (800 nm in ) and the higher absorption of gold in the metal-dielectric-metal geometry.
Fig. 3. Electric field Ex scattered by the metasurface at λ = 676 nm. The distance between the coupler and decoupler is 5.2 μm.
3. Conclusion We have proposed and theoretically investigated a unidirectional SPP coupler based on a metasurface with reflection-phase gradient for normal incidence. The device is distinct from previous works because it does rely on a gap-plasmon mode in each antenna, which can therefore control a wider range of phases. As a result, the device length is only 1.98 μm, i.e. about 3λ for operation at λ = 676 nm. Acknowledgement: This work was supported by U.S. Army Research Office grant 63133-PH (W911NF-13-1-0226) "Flat photonics with metasurfaces". References 
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