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A novel plasmonic lens (PL) with simple nano-structure is proposed to realize the subwavelength focusing. The proposed PL is composed of the gold film with only five dielectric-filled nanoslits. The exit surface of the gold film is processed into quadric shape that can be parabolic, elliptical or hyperbolic cylinders. The film is fabricated to form five uniformly spaced nanoslits with different widths and depths. All five slits are symmetrically arranged with respect to the center of lens and filled with a dielectric medium (i.e., SiO2). Under the illumination of TM polarized beams, the surface plasmon polaritons (SPPs) are excited at the entrance surface of the PL, then pass through the SiO2-filled slits while acquiring specific phase retardations, and are finally coupled to the light waves in the free space. Each light wave originating from the slit can be regarded as an individual point source, and the constructive interference of light waves from slits gives rise to the beam focusing at the focal plane of the PL. We investigate the phase modulation mechanism of the PL and find that the focusing performance relies on the shape of exit surface, filling medium and geometric parameters of the slits. A suitable phase modulation can be achieved by adjusting the structure parameters of the PL with a specific exit surface shape. Three kinds of quadratic cylindrical PLs, i.e., parabolic, elliptical and hyperbolic cylindrical ones with continuous or stepped exit surface are designed to realize the focusing of TM polarized subwavelength beams in visible spectrum. The finite difference time domain (FDTD) method is employed to compute the light field and to investigate the focusing characteristics of the proposed PL. The performance measurements include the focal length, depth of focus (DOF) and full-width half-maximum (FWHM). The simulation results confirm that the proposed PL with a 2-m-diameter aperture can achieve the subwavelength focusing at a focal length of micron scale. The attainable smallest FWHM of the focal spot is 0.4050 (0 denoting the wavelength of the incident light) which is well beyond the diffraction limit. It is also worth mentioning that the step-like cylindrical PL can yield a sharper focal spot than the continuous cylindrical PL. For example, the FWHM of focal spot produced by the stepped elliptical cylindrical PL is about 92% of that produced by the continuous elliptical cylindrical PL. The proposed PL has the advantages of simple and compact structure with much smaller lateral dimension and easy integration with other photonic devices. Our study helps design the easy-to-fabricate PLs and facilitates applications of plasmonic devices in the fields such as optical micro manipulation, super-resolution imaging, optical storage and biochemical sensing.
[1] Kawata S 2001 Near-Field Optics and Surface Plasmon Polaritons (Vol.81) (Berlin Heidelberg: Springer) p19
[2] Juan M L, Righini M Quidant R 2011 Nat Photonics 5 349
[3] MAIER S A 2006 Plasmonics:Fundamentals and Applications (New York: Springer) p21
[4] Chen J, Wang C, Lu G, Li W, Xiao J, Gong Q 2012 Opt. Express 20 17734
[5] Takeda1 M, Kimura1 N, Inoue T, Aizawa K 2015 Jpn. J. Appl. Phys. 54 09MG02
[6] Zhao X N, Zhang X P, Cao P F, Cheng L, Duan J X, Cheng L B, Kong W J, Yang L L 2013 Optik 124 6740
[7] Lan L, Jiang W, Ma Y 2013 Appl. Phys.Lett. 102 231119
[8] Venugopalan P, Zhang Q M, Li X P, Kuipers L, Gu M 2014 Opt. Lett. 39 5744
[9] Wang J, Zhou W 2010 Plasmonics 5 325
[10] Guo K, Liu J L, Liu S T 2014 Opt Commun 331 124
[11] Liu Y, Fu Y Q, Zhou X L 2010 Plasmonics 5 117
[12] Hao F H, Wang R Wang J 2010 Plasmonics 5 45
[13] Okuda S, Kimur N, Takeda M, Inoue T, Aizawa K 2014 Opt. Rev. 21 560
[14] Liu Y X, Xu Hua, Stief F, Zhitenev N, Yu M 2011 Opt. Express 19 20233
[15] Wu G, Chen J J Zhang R, Xiao J H, Gong Q H 2013 Opt. Lett. 38 3776
[16] Duan X F Zhou G R, Huang Y Q, Shang Y F, Ren X M 2015 Opt. Express 23 2639
[17] Cheng L, Cao P F, Li Y, Kong W J, Zhao X N, Zhang X P 2012 Plasmonics 7 175
[18] Sun Z J, Kim H K 2004 Appl. Phys. Lett. 85 642
[19] Yu Y T, Zappe H 2011 Opt. Express 19 9434
[20] Xu T, Wang C T, Du C L, Luo X G 2008 Opt. Express 16 4753
[21] Johnson R B, Christy R W 1972 Phys. Rev. B 6 4370
[22] Palik E D 1985 Handbook of optical constants of solids (New York: Academic Press) pp723-729
[23] Barnes W L 2006 J. Opt. A-Pure Appl. Opt. 8 S87
[24] Chen J N, Xu Q F, Wang G 2011 Chinese. Phys. B 20 114211
[25] Zhan Q, Leger J 2002 Opt. Express 10 324
[26] Li Y, Wolf E 1981 Opt. Commun. 39 211
[27] Feng D 2014 J. Opt. Soc. Am. A 31 2071
[28] Shi H F, Du C L, Luo X G 2007 Appl. Phys. Lett. 91 093111
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[1] Kawata S 2001 Near-Field Optics and Surface Plasmon Polaritons (Vol.81) (Berlin Heidelberg: Springer) p19
[2] Juan M L, Righini M Quidant R 2011 Nat Photonics 5 349
[3] MAIER S A 2006 Plasmonics:Fundamentals and Applications (New York: Springer) p21
[4] Chen J, Wang C, Lu G, Li W, Xiao J, Gong Q 2012 Opt. Express 20 17734
[5] Takeda1 M, Kimura1 N, Inoue T, Aizawa K 2015 Jpn. J. Appl. Phys. 54 09MG02
[6] Zhao X N, Zhang X P, Cao P F, Cheng L, Duan J X, Cheng L B, Kong W J, Yang L L 2013 Optik 124 6740
[7] Lan L, Jiang W, Ma Y 2013 Appl. Phys.Lett. 102 231119
[8] Venugopalan P, Zhang Q M, Li X P, Kuipers L, Gu M 2014 Opt. Lett. 39 5744
[9] Wang J, Zhou W 2010 Plasmonics 5 325
[10] Guo K, Liu J L, Liu S T 2014 Opt Commun 331 124
[11] Liu Y, Fu Y Q, Zhou X L 2010 Plasmonics 5 117
[12] Hao F H, Wang R Wang J 2010 Plasmonics 5 45
[13] Okuda S, Kimur N, Takeda M, Inoue T, Aizawa K 2014 Opt. Rev. 21 560
[14] Liu Y X, Xu Hua, Stief F, Zhitenev N, Yu M 2011 Opt. Express 19 20233
[15] Wu G, Chen J J Zhang R, Xiao J H, Gong Q H 2013 Opt. Lett. 38 3776
[16] Duan X F Zhou G R, Huang Y Q, Shang Y F, Ren X M 2015 Opt. Express 23 2639
[17] Cheng L, Cao P F, Li Y, Kong W J, Zhao X N, Zhang X P 2012 Plasmonics 7 175
[18] Sun Z J, Kim H K 2004 Appl. Phys. Lett. 85 642
[19] Yu Y T, Zappe H 2011 Opt. Express 19 9434
[20] Xu T, Wang C T, Du C L, Luo X G 2008 Opt. Express 16 4753
[21] Johnson R B, Christy R W 1972 Phys. Rev. B 6 4370
[22] Palik E D 1985 Handbook of optical constants of solids (New York: Academic Press) pp723-729
[23] Barnes W L 2006 J. Opt. A-Pure Appl. Opt. 8 S87
[24] Chen J N, Xu Q F, Wang G 2011 Chinese. Phys. B 20 114211
[25] Zhan Q, Leger J 2002 Opt. Express 10 324
[26] Li Y, Wolf E 1981 Opt. Commun. 39 211
[27] Feng D 2014 J. Opt. Soc. Am. A 31 2071
[28] Shi H F, Du C L, Luo X G 2007 Appl. Phys. Lett. 91 093111
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