Directional Excitation of Surface Plasmon Polaritons by Circularly Polarized Vortex Beams
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Directional Excitation of Surface Plasmon Polaritons by Circularly Polarized Vortex Beams Qing Tu 1 & Jianxun Liu 1 & Shaolin Ke 2 & Bing Wang 1 & Peixiang Lu 1,2 Received: 23 July 2019 / Accepted: 3 November 2019 # Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract We investigate the excitation of surface plasmon polaritons (SPPs) using a metallic nanoaperture array illuminated by circularly polarized Laguerre-Gaussian (LG) vortex beams. The direction of SPP excitation is tunable by changing the circular polarization and topological charge of LG beams. The left- or right-handed circular polarization determines SPP propagation on either side of the nanoaperture array. Furthermore, varying the topological charge of LG beam will result in beam splitting of SPPs. We also utilize a composite nanoaperture array with different periods to achieve unidirectional excitation of SPPs. The study provides an interesting approach to control the excitation direction of SPPs and may find great applications in SPP generators and optical switches. Keywords Surface plasmon polaritons . Directional excitation . Laguerre-Gaussian beam
Introduction Surface plasmon polaritons (SPPs) are excited due to the coupling of incident light and collective oscillations of electrons at the interface of metal and dielectric, the field of which can be tightly confined and strongly enhanced on the metal surface [1, 2]. SPPs have been investigated intensively in plasmonic circuits [3], second harmonic generation [4–9], photolithography [10, 11], photodetectors [12–14], and solar cells [15, 16]. Since the wavevector of SPPs is larger than that of incident light in free space, they cannot be directly coupled to each other. Traditional approaches to excite SPPs like the Kretschmann prism [2] usually brings about bulk size of optical elements. Subwavelength scatters [17, 18] may benefit to developing compact SPP generators, but the excitation efficiency is relatively low. Metallic subwavelength grating [19–22] could be a proper candidate to reach both acceptable element size and excitation efficiency.
* Bing Wang [email protected] 1
Wuhan National Laboratory for Optoelectronics and School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China
2
Hubei Key Laboratory of Optical Information and Pattern Recognition, Wuhan Institute of Technology, Wuhan 430205, China
Exciting SPP efficiently in a well-defined direction evokes many considerations in recent years. A diversity of optical configurations such as metallic slits [23–29], ridges [30–32], and nanoantennas [33–35] have been proposed to achieve unidirectional excitation of SPPs. The metasurfaces have also been employed to improve the efficiency of SPP excitation [36, 37]. Apart from utilizing asymmetric structures and incident angles, the excitation direction is also tunable by changing the polarization of incident waves [38, 39]. Since polarized incident waves possess certain spin angular momentum (SAM), the SPP excitation can be contributed to t
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