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Plasmonic lens (PL) is a nano-optical device, with which a tight focusing spot in a subwavelength-scale can be achieved by exciting and controlling surface plasmon polaritons (SPPs), thus the diffraction limit can be broken for attaining the shorter effective wavelength of the SPPs. The key issue in studying the PL is to achieve a tight focusing point and focus-control effectively. Optimal plasmonic focusing can be achieved by utilizing the radially polarized light and the rotational symmetric structures of the PL. Radially polarized light is a cylindrical vector beam whose local polarization of electric field is always parallel to the radial direction. As a radially polarized light is used as the incident light in a PL, the SPPs can be excited in all directions, so as to increase the efficiency of focussing. The focussing efficiency can be further increased, and the characteristics of the focus, such as spot size, shape, and strength etc., can be manipulated through appropriate designs of the PL structures. In this work, under an illumination of a radially polarized light, a new type of plasmonic lens is proposed to achieve a long depth of focus (DOF), a long focal length, and a sub-wavelength-scale tight focussing spot. This kind of plasmonic lens consists of a T-shape micro-hole, concentric rings, and multi-level step-like structures. The focussing properties of such plasmonic lenses are analyzed in terms of the finite element method (FEM). Simulation results show that SPPs can be excited efficiently in such structures and the tight-focusing is realized via the multiple-beam interference between the light radiating from the concentric rings and the transmitted light from the center hole. The T-shape micro-hole and step-like concentric ring structures can provide control for the phase modulation and the propagation direction of the SPPs along the bottom of the groove, thus leading to a compressed focal spot, a longer focal length, an increased depth of focus, and to improving the focussing properties. In an optimized PL design, a focal spot of ～2.5λ0 DOF, ～0.388λ0 FWHM, and ～3.22λ0 focal length is achieved under the illumination of a radially polarized light (λ0=632.8 nm). The PL structure is compact, and can be easily integrated with other nano-devices. The PL proposed above has potential applications in nano-scale photonic integration, near-field imaging and sensing, nano-photolithography, and in other related areas.
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Keywords:
- Plasmonic lens /
- radially polarized light /
- tight focusing /
- long focal length
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[1] Chen J, Li Z, Zhang X, Xiao J, Gong Q 2013 Sci. Rep. 3 1451
[2] Raether H 1988 Surface plasmons on smooth surfaces (Berlin Heidelberg: Springer)
[3] Ghaemi H F, Thio T, Grupp D E, Ebbesen T W, Lezec H J 1998 Phys. Rev. B 58 6779
[4] Martin-Moreno L, Garcia-Vidal F, Lezec H, Pellerin K, Thio T, Pendry J, Ebbesen T 2001 Phys. Rev. Lett. 86 1114
[5] Lezec H J, Degiron A, Devaux E, Linke R, Martin-Moreno L, Garcia-Vidal F, Ebbesen T W 2002 Science 297 820
[6] Zheng G G, Xu L H, Pei S X, Chen Y Y 2014 Chin. Phys. B 23 034213
[7] Chen J, Wang C, Lu G, Li W, Xiao J, Gong Q 2012 Opt. Express. 20 17734
[8] Wang J, Fu Y Q 2013 Chin. Phys. B 22 090206
[9] Zhang M, Wang J, Tian Q 2013 Opt. Express. 21 9414
[10] Chen W, Abeysinghe D C, Nelson R L, Zhan Q 2009 Nano Lett. 9 4320
[11] Yi J M, Cuche A, Devaux E, Genet C, Ebbesen T W 2014 ACS Photonics 1 365
[12] Peng R, Li X, Zhao Z, Wang C, Hong M, Luo X 2014 Plasmonics 9 55
[13] Chen J 2013 Plasmonics 8 931
[14] Song W T, Lin F, Fang Z Y, Zhu X 2010 Acta Phys. Sin. 59 6921 (in Chinese) [宋文涛, 林峰, 方哲宇, 朱星 2010 物理学报 59 6921]
[15] Ebbesen T W, Lezec H, Ghaemi H, Thio T, Wolff P 1998 Nature 391 667
[16] Genet C, Ebbesen T W 2007 Nature 445 39
[17] Goh X M, Lin L, Roberts A 2011 J. Opt. Soc. Am. B 28 547
[18] Liu Z, Lee H, Xiong Y, Sun C, Zhang X 2007 Science 315 1686
[19] Smolyaninov I I, Hung Y J, Davis C C 2007 Science 315 1699
[20] Kim S, Jin J, Kim Y J, Park I Y, Kim Y, Kim S W 2008 Nature 453 757
[21] Lee B, Kim S, Kim H, Lim Y 2010 Prog. Quantum Electron. 34 47
[22] Li L, Li T, Wang S, Zhu S, Zhang X 2011 Nano Lett. 11 4357
[23] Jia B, Shi H, Li J, Fu Y, Du C, Gu M 2009 Appl. Phys. Lett. 94 151912
[24] Min C J, Shen Z, Shen J F, Zhang Y Q, Fang H, Yuan G H, Du L, Zhu S, Lei T, Yuan X C 2013 Nat. Commun. 4 2891
[25] Zhao W Q, Tang F, Qiu L R, Liu D L 2013 Acta Phys. Sin. 62 054201 (in Chinese) [赵维谦, 唐芳, 邱丽荣, 刘大礼 2013 物理学报 62 054201]
[26] Wang Z, Gao C Q, Xin J T 2012 Acta Phys. Sin. 61 124209 (in Chinese) [王铮, 高春清, 辛璟焘 2012 物理学报 61 124209]
[27] Wang H F, Shi L P, Lukyanchuk B, Sheppard C, Chong C T 2008 Nature Photonics 2 501
[28] Jackson J D 1999 Classical electrodynamics (3rd ed.) (New York: Wiley)
[29] Vial A, Grimault A S, Macías D, Barchiesi D, de La Chapelle M L 2005 Phys. Rev. B 71 085416
[30] Rakic A D, Djurišic A B, Elazar J M, Majewski M L 1998 Appl. Opt. 37 5271
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