Gap surface plasmon polaritons enhancedby a plasmonic lensHyun Chul Kim1,2 and Xing Cheng1,*

1Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843-3128, USA2Current address: DRAM Process Architecture Team, Memory Division, Samsung Electronics Co., Ltd,

San #16 Banwol-Dong, Hwasung-City, Gyeonggi-Do, 445-701, South Korea*Corresponding author: chengx@ece.tamu.edu

Received May 31, 2011; revised July 9, 2011; accepted July 13, 2011;posted July 13, 2011 (Doc. ID 148360); published August 8, 2011

We numerically investigate the optical field enhancement based on gap surface plasmon polaritons (GSPPs) that areenhanced by propagating surface waves launched by a circular slit at a metal–dielectric interface. The optical fieldenhancement originates not only from multiple scattering and coupling of GSPPs in the spacer region between twometal layers but also from propagating surface plasmon polaritons (SPPs) launched by a circular plasmonic lens. Wefind that the combination of the GSPPs and the propagating SPPs launched by the plasmonic lens can achieveextremely strong field confinement, and we find that the surface-enhanced Raman scattering (SERS) enhancementfactor can be up to 1015 at the tip of the equilateral triangular nanostructures. The structure proposed here isexpected to find promising applications where strong field enhancement is desired, such as optical sensing withthe SERS effect. © 2011 Optical Society of AmericaOCIS codes: 240.6680, 240.6695.

Surface-enhanced Raman scattering (SERS) has beenwidely used as a technique for molecule detection andidentification. The SERS effect strongly increases Ramansignals from molecules that have been attached tonanometer-sized metallic structures or aggregates of col-loidal particles of metals [1,2]. Among many attractiveSERS-active substrates, adjacent nanoparticle pairs,known as dimers, can give rise very large field enhance-ments in the junction regions. It was found that the SERSenhancement factor (EF) increases rapidly as the gapsize of the junction decreases, and it is only from gapson the order of 1 to 2 nm that can provide exceptionallylarge SERS EF of the order of 1011 [3]. Most recently, thegap surface plasmon polaritons (GSPPs) have attractedconsiderable attention [4–6]. The narrow gap of the di-electric layer between two metal layers can providestrong electromagnetic field enhancement in GSPPs. Inour previous study, we demonstrated numerically thatlocalized surface plasmon polaritons (LSPPs) supportedby GSPPs can provide very high field enhancement(>1011) at the edge of a nanostructure that can be easilyfabricated [7]. To further improve the sensitivity of theSERS technique, the efficiency of Raman scatteringneeds further enhancement. For single-molecule detec-tion by the Raman technique, it is estimated that theSERS EF needs to reach 1014–1015 [1,8].In traditional metallic nanostructures, such as optical

antennas, the enhancement of field intensity is achievedby channeling incident light into very tight regions. Thesize of the metallic nanostructures is usually in the rangeof tens to hundreds of nanometers because of the con-straints imposed by the resonant condition. The smalloverall size of those metallic nanostructures may placean upper limit on the maximum achievable field enhance-ment. In recent years, plasmonic lenses have beenproposed and demonstrated as devices for in-planefocusing of surface plasmon polaritons (SPPs) [9–12].Because of the dispersion relation of SPPs, discontinu-ities on metal surfaces, such as slits, grooves, nanoholes,and particles, can launch propagating SPPs by providing

the additional momentum to the incident light [13–15]. Byguiding the propagating SPPs using geometric design,plasmonic lenses can focus the SPPs to achieve high in-tensity at the focal point. A typical plasmonic lens has adiameter of a few to tens of micrometers. Such a largesurface area allows it to receive more incident light thanmetallic nanostructures. In this work, we propose an ap-proach to achieve very high field enhancement by a two-stage focusing of the incident light. By placing a GSPPnanostructure at the focal spot of a plasmonic lens, theprefocused SPPs can be further squeezed into the tip ofthe GSPP nanostructure, resulting in an exceptionallyhigh field enhancement that is suitable for single-molecule detection by the SERS effect.

The schematic diagrams of the cascade focusing struc-ture are shown in Fig. 1, where an isolated equilateraltriangular structure is formed on a thin SiO2 spacer layerand a thick silver layer. The top triangular structure, theSiO2 spacer, and the thick silver layer form the GSPPstructure, while the single annular subwavelength slitmilled into the thick sliver layer forms the plasmonic lensstructure [Fig. 1(a)]. The incident wavelength is 785 nm,which is commonly used in Raman spectrometers. Thelength of the isolated equilateral triangular nanostructureis fixed at L ¼ 110 nm, where the maximum SERS EF

Fig. 1. Schematic diagrams of the proposed nanostructures:(a) cross-sectional view in the x–z plane and (b) top-down viewin the x–y plane (structures are not drawn to the scale).

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was estimated in our previous study [7]. The thicknessesof the isolated equilateral triangular nanostructure, theSiO2 spacer layer (t), and the silver layer (h) are fixedat 40, 40, and 300 nm, respectively. The slit width ofthe plasmonic lens used as a launcher of propagatingSPPs is fixed at 200 nm. The dielectric constants at theincident wavelength are εAg ¼ −26:7þ 1:48i, εSiO2

¼2:10, and εAir ¼ 1:00 [16]. The light used to excite the pro-pagating SPPs in the plasmonic lens is a circularly polar-ized wave with normal incidence from the bottom of thequartz substrate. The optical field confinement proper-ties of the proposed structure are simulated and analyzedby TEMPEST, a Maxwell equation solver based on thethree-dimensional finite-difference time-domain method.The mesh size used in the simulation is λ=150, which isabout 5:2nm for 785 nm incident wavelength. The strongSERS EFs are generally composed of both the electro-magnetic enhancement and the chemical enhancement.It is generally believed that the majority of the enhance-ment of the Raman signal intensity is attributed to theelectromagnetic field strength on the surface of themetallic substrate [17]. In this work, the SERS EF dueto electromagnetic contribution is calculated as jELocj4.When the SPPs arrive at a slit in the thick bottom silver

layer, most of them are scattered by the slit edge, leakinto the slit, and radiate new bulk waves and surfacewaves [18]. These new bulk waves and surface waves ex-cite new SPPs on the slit walls. These new SPPs propa-gate along the metal–dielectric slit and arrive at an exitslit on the other side. All SPPs excited from the circularslit are directed to the center of the plasmonic lens.Finally, as shown in Fig. 1(b), these SPPs can couple intothe GSPPs in the center region. In the traveling pathof the SPPs, we can regard the structure as a metal–dielectric–air (MDA) waveguide and a metal–insulator–metal (MIM) waveguide in which a thin dielectric layer(t) is sandwiched between metal and air or between twometal layers. The SPP propagation constant (kspp) for theMDA waveguide is given in [19], and for the MIM wave-guide it is given in [20].Figures 2(a) and 2(b) plot the real part of the effective

index and the SPP propagation length as a function ofSiO2 thickness. For the MDA waveguide, the real partsof the effective indices increase as the SiO2 thicknessincreases, while the SPP propagation lengths decrease.Inversely, for the MIM waveguide, the real parts of theeffective refractive indices decrease as the SiO2 spacerthickness increases, while the SPP propagation lengths

increase. For instance, at 785 nm and tSiO2¼ 40nm, the

real part of the effective refractive index and its SPPpropagation length are n0eff ¼ 1:08 and Lspp ¼ 22:9 μmfor the MDA waveguide and n0eff ¼ 2:22 and Lspp ¼2:86 μm for the MIM waveguide.

Figures 3(a)–3(d) show the distribution of the field in-tensity on the x–y plane at the top region of the dielectricSiO2 and the x–z plane at the center region. Very strongfield enhancement is obtained at the edge of the triangu-lar structure. Figures 3(e) and 3(f) show a comparison ofthe SERS EFs with and without the triangular structureat the center of the plasmonic lens. It is clearly seen thatthe SERS EF is greatly increased at the gap region of thetriangular nanostructure. In particular, the maximumSERS EF at the tip of the triangular structure is estimatedto be about 6:65 × 1015, which is about 7 orders of mag-nitude higher than that of a plasmonic lens only. The sizeof the tip is equal to the grid size used in simulation. Theperiodicity of the fringe patterns is expected to be half ofthe propagating SPP wavelength. For 785 nm incidentlight on the MDA waveguide, the SPP wavelength is727 nm and the interference periodicity should be364 nm. This agrees well with the simulated periodicityof 360 nm shown in Fig. 3(e).

To evaluate the effect of the geometrical parameters ofthe plasmonic lens on the SERS EF, simulations arefurther performed by varying the radius (r), slit width(s), and metal thickness (h) of the thick silver layer. Ineach simulation, other device parameters are kept thesame as those in Fig. 3. As shown in Fig. 4(a), the SERS

Fig. 2. (Color online) Calculated effective refractive index neffand SPP propagation length Lspp as functions of dielectric SiO2thickness, for (a) the MDA waveguide and (b) the MIM wave-guide. The wavelength of the incident light is 785nm.

Fig. 3. (Color online) Simulated intensities in the (a) and(b) x–y plane and (b) and (d) the x–z plane: (a) and (c) showthe intensity distribution without the triangular nanostructure,and (b) and (d) show the intensity distribution with thetriangular nanostructure. (e) and (f) show the comparison ofSERS EFs with and without the triangular nanostructure atthe locations indicated by the dashed line and arrow.

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EFs at the center of the circle increase with periodicvariation as the diameter of the circle is increased. Theperiodic variation of the SERS EFs is observed due toconstructive and destructive interferences. Limited bythe memory size of the workstation used in this simula-tion, the maximum radius of the circle studied in this workis 1:9 μm. It is possible to achieve even higher SERS EFswith larger radii until the radius of the circle approachesthe propagation length of the SPPs. Figure 4(b) shows thedependence of the SERS EF on slit width. The periodicvariation of the SERS EFs at narrower widths is observed.As the silt width increases, the SERS EF approaches tothat of a disk. The effect of the thickness of the thick metallayer is shown in Fig. 4(c). An overall decreasing trend inthe SERS EFs is observed when the metal thicknessincreases. This can be attributed to the increased SPPpropagation loss in thicker slits.In summary, we introduced and investigated synergis-

tic interaction between the GSPPs and the propagatingSPPs launched by a plasmonic lens. Excited by annularslits, the propagating SPPs concentrate the electromag-netic fields at the center of the circle. At the same time,the focused electromagnetic fields are further enhancedby a subwavelength metal gap that can support GSPPs.By utilizing GSPPs enhanced by propagating SPPs, weattained SERS EFs of up to 1015 in the proposed nanos-tructure. This novel structure can be utilized as anexceptionally sensitive SERS-active substrate for sin-gle-molecule detection and identification. Because ofits strong field enhancement capability, the proposed na-nostructure can also find many other important applica-tions in nanophotonics and optoelectronics.

References

1. S. Nie and S. R. Emory, Science 275, 1102 (1997).

2. K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan,R. R. Dasari, and M. S. Feld, Phys. Rev. Lett. 78, 1667(1997).

3. E. Hao and G. C. Schatz, J. Chem. Phys. 120, 357 (2004).4. K. H. Su, Q. H. Wei, and X. Zhang, Appl. Phys. Lett. 88,

063118 (2006).5. J. Jung, T. Sondergaard, and S. I. Bozhevolnyi, Phys. Rev. B

79, 035401 (2009).6. Y. Chu and K. B. Crozier, Opt. Lett. 34, 244 (2009).7. H. C. Kim and X. Cheng, Opt. Express 17, 17234 (2009).8. X.-M. Qian and S. M. Nie, Chem. Soc. Rev. 37, 912 (2008).9. L. Yin, V. K. Vlasko-Vlasov, J. Pearson, J. M. Hiller, J. Hua,

U. Welp, D. E. Brown, and C. W. Kimball, Nano Lett. 5,1399 (2005).

10. Z. Liu, J. M. Steele, W. Srituravanich, Y. Pikus, C. Sun, andX. Zhang, Nano Lett. 5, 1726 (2005).

11. G. M. Lerman, A. Yanai, and U. Levy, Nano Lett. 9, 2139(2009).

12. J. M. Steele, Z. Liu, Y. Wang, and X. Zhang, Opt. Express 14,5664 (2006).

13. P. Lalanne, J. P. Hugonin, and J. C. Rodier, J. Opt. Soc. Am.A 23, 1608 (2006).

14. F. Lopez-Tejeira, S. G. Rodrigo, L. Martin-Moreno, F. J.Garcia-Vidal, E. Devaux, T. W. Ebbesen, J. R. Krenn, I. P.Radko, S. I. Bozhevolnyi, M. U. Gonzalez, J. C. Weeber,and A. Dereux, Nat. Phys. 3, 324 (2007).

15. J. Renger, S. Grafstrom, and L. M. Eng, Phys. Rev. B 76,045431 (2007).

16. E. D. Palik and G. Ghosh,Handbook of Optical Constants ofSolids (Academic, 1998).

17. E. C. L. Ru and P. G. Etchegoin, Principles of Surface-Enhanced Raman Spectroscopy and Related PlasmonicEffects (Elsevier, 2009).

18. H. C. Kim, H. Ko, and M. Cheng, Opt. Express 17, 3078(2009).

19. B. Wang and G. P. Wang, Appl. Phys. Lett. 88, 013114(2006).

20. S. I. Bozhevolnyi and T. Sodergaard, Opt. Express 15,10869 (2007).

Fig. 4. Effect of the geometrical parameters of the plasmonic lens on SERS EFs: (a) the radius, (b) the slit width, and (c) thethickness of the plasmonic lens.

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