NANO LETTERS
Subwavelength Focusing and Guiding of Surface Plasmons
2005 Vol. 5, No. 7 1399-1402
Leilei Yin,†,‡,| Vitali K. Vlasko-Vlasov,† John Pearson,† Jon M. Hiller,† Jiong Hua,†,‡ Ulrich Welp,*,† Dennis E. Brown,‡ and Clyde W. Kimball‡ Materials Science DiVision, Argonne National Laboratory, Argonne, Illinois 60439, and Physics Department, Northern Illinois UniVersity, DeKalb, Illinois 60115 Received April 19, 2005; Revised Manuscript Received May 20, 2005
ABSTRACT The constructive interference of surface plasmon polaritons (SPP) launched by nanometric holes allows us to focus SPP into a spot of high near-field intensity having subwavelength width. Near-field scanning optical microscopy is used to map the local SPP intensity. The resulting SPP patterns and their polarization dependence are accurately described in model calculations based on a dipolar model for the SPP emission at each hole. Furthermore, we show that the high SPP intensity in the focal spot can be launched and propagated on a Ag strip guide with a 250 × 50 nm2 cross section, thus overcoming the diffraction limit of conventional optics. The combination of focusing arrays and nanowaveguides may serve as a basic element in planar nano-photonic circuits.
The drive toward highly integrated optical devices and circuits for use in high-speed communication technologies and in future all-optical photonic chips has generated considerable interest in the unique properties of surface plasmon polaritons (SPPs).1-3 SPPs are electromagnetic waves that are confined to the interface between materials with dielectric constants of opposite sign.4 Most commonly, this situation is realized in noble metal films in contact with a dielectric substrate or with air. In the directions perpendicular to the interface, the SPP intensity decays exponentially within less than 100 nm, whereas the SPP can propagate along the interface over distances approaching 1 mm.2 The propagation distance is limited by intrinsic material losses as measured by the frequency-dependent imaginary part of the dielectric constant, by radiation losses, and by sample defects such as surface roughness. The dispersion of the surface plasmons depends strongly on the nature of the adjacent dielectric, a property that forms the basis for biochemical sensing applications5-7 as well as for SPP-based modulators and switches.8-12 The confinement of the SPP field to the interface opens the possibility of overcoming the diffraction limit encountered in classical optics and realizing planar nanoscale, highly integrated optical devices. Furthermore, the lightning-rod effect and resonant buildup13-17 can cause strongly enhanced local electric field strengths at corners or edges of metallic elements, thus enhancing nonlinear optical effects, which are at the heart of active optical elements. * Corresponding author. E-mail:
[email protected]. † Argonne National Laboratory. ‡ Northern Illinois University. | Present address: Department of Mechanical & Industrial Engineering, University of Illinois, Urbana, Illinois 61801. 10.1021/nl050723m CCC: $30.25 Published on Web 06/08/2005
© 2005 American Chemical Society
Here we show that the constructive interference of SPPs launched by nanometric holes that are arranged on a quarter circle allows an SPP to be focused into an intense spot having a subwavelength width. The resulting SPP intensity patterns are accurately described in calculations based on dipolar SPP sources at each hole. Furthermore, we show that the high SPP intensity in the focal spot can be launched and propagated on a 250-nm-wide metal strip guide. The combination of a focusing array and nano-wave guide may serve a basic element in planar nano-photonic circuits. The distribution of the local SPP intensity was imaged using a specially designed aperture near-field scanning optical microscope (NSOM) that is based on the scanner and electronics of the ThermoMicroscope Aurora II system18 shown schematically in Figure 1. A laser beam (λin ) 532 nm, 0.5-mm beam diameter) whose polarization and angle of incidence can be precisely controlled impinges onto the sample from the substrate side. The transmitted light is collected on the air side by an aluminum-coated pulled fiber tip with 50-80-nm nominal aperture size and measured with a photomultiplier (Hamamatsu HC120-1). Results for two types of silver films are described here: (i) 75 nm of silver was sputtered at a high rate of about 20 Å/s onto a glass substrate with an intervening adhesion layer of 2 nm of Cr and (ii) 50 nm of Ag on glass with an intervening Cr layer of 100 nm thickness. The high deposition rate ensures small Ag grains and smooth surfaces with an average roughness of 1.5-nm rms as determined from AFM images. The samples were patterned using focused ion beam machining.19 Figure 2a shows the SPP pattern at perpendicular incidence around a 200-nm hole machined into a 75-nm Ag/2-nm Cr/ glass sample. This pattern is characterized by a lobe-shaped
Figure 1. Schematic of the near-field scanning optical microscope.
intensity distribution aligned with the incident polarization direction, taken as the x direction, and by a series of periodic circular fringes. Lobe structures, although without the fringes, have also been observed when SPPs were created by the electromagnetic field emitted from an NSOM tip.20,21 The fringes in Figure 2a arise from the interference of the SPP field launched at the hole with the light directly transmitted through the film and constitute a holographic image of the x component, Ex, of the SPP field.18 The fringe spacing directly gives the SPP wavelength, λSPP ≈ 506 nm, a value that is in good agreement with the theoretical value λspp ) λinx(Re(m)+1)/Re(m) ) 509 nm for a flat Ag/air interface with the Ag dielectric constant m ) -11.93 + i0.41 at 532 nm.22 The angular and radial dependences of this SPP pattern are well described by a dipolar form Ex(r, θ) ) E0 cos2(θ)exp(-r/δ) cos(ωt - 2πr/λSPP)/xr, as is shown by the data in Figure 2b and c. Here, δ is the
propagation distance. The value of δ ≈ 14 µm, as determined from Figure 2c, is clearly smaller than the theoretical value of δ ) λin((Re(m) + 1)/Re(m))3/2(Re(m))2/(πIm(m)) ≈ 50 µm. This we attribute to enhanced scattering caused by surface roughness. The above expression for Ex is an approximation that is valid for distances large compared to the wavelength; at short distances, the SPP field is given in terms of Bessel functions.23 On the basis of the above results for the single hole, we constructed a hole array in which the coherent superposition of pointlike SPP sources allowed us to focusing the SPP power. Figure 3a shows an SEM image of a 50-nm Ag/100nm Cr/glass sample containing 19 200-nm through holes arranged on a quarter circle with a 5-µm radius. The centerto-center hole spacing is 0.44 µm. The thick Cr layer blocks light transmitted directly through the sample; therefore, the near-field optical image (Figure 3c) represents the timeaveraged SPP intensity, and the holographic fringes do not appear. The constructive interference of the SPP waves emanating from different holes gives rise to an intense SPP focal spot of subwavelength width located at the center of the circle. Longitudinal and transverse intensity profiles (Figure 3b) through the focal spot reveal full widths at halfmaximum (fwhm) of 1400 and 380 nm, respectively. The focusing and the shape of the focal spot are very well described in a model based on the coherent superposition of dipolar fields from individual holes. Previously, we have shown18,23 that the contrast in our near-field images is mostly a representation of the in-plane electric field intensity. By using the above expression for Ex along with the corresponding Ey, the calculated intensity gives a good description of the data as shown by the fits in Figure 3b and by a comparison of the calculated image frames (Figure 3d and f) with the experimental image (Figure 3c and e). Because of the dipolar directionality of the single-hole SPP fields, focusing occurs for an incident polarization parallel to the symmetry axis of the structure, and this focusing can be turned off for transverse polarization, as shown in Figure 3e. All of the interference features seen in the experimental images are accurately accounted for in the model calculations. In particular, for transverse polarization the minimum of SPP intensity at the center of the circle and the faint maxima located λ/2 below and above the symmetry axis are
Figure 2. (a) Holographic image of the horizontal component of the SPP field around a 200-nm hole in the 75-nm Ag film. Incident light at 532 nm is polarized horizontally. The width of the frame is 20 µm. (b) cos2(θ) is the angular dependence of the SPP field around the hole. (c) Line profile of the SPP field along the x axis and fit of the envelope according to E0 exp(-|x|/δ)/x|x|. 1400
Nano Lett., Vol. 5, No. 7, 2005
Figure 3. (a) SEM image of the 50-nm Ag film containing 19 200-nm holes arranged on a quarter circle with a 5-µm radius; scale bar, 2 µm. (b) Intensity profiles along and transverse to the focal spot together with fits based on the dipolar model. The values of the fwhm for both orientations are indicated. (c) NSOM image taken at 532-nm incident wavelength and horizontal polarization. The focus of SPP intensity at the center of the circle is clearly seen. (d) Image of the SPP intensity calculated with the dipolar model. (e) NSOM image taken for vertical polarization. The intensity at the focal point is strongly reduced. (f) Calculated image. Note that the contrast is enhanced as compared to that in frame d to reveal the fine features. Frames c-e have the same intensity scale.
correctly reproduced. Note that the calculated intensity for the transverse polarization (Figure 3f) is shown in enhanced contrast in order to reveal more clearly the interference features. Recently, it has been observed that plane SPP waves propagating in a continuous metal film can be focused because of scattering by arrays of surface defects24 such as protrusions.24-26 In contrast, in Figure 3 the nano-holes serve as pointlike sources for SPP, whose constructive interference gives rise to focusing. The near-field intensity in the SPP focus spot shown in Figure 3c reaches about 75% of the intensity measured directly above the holes. The availability of subwavelength spots of high optical near-field intensity may serve as an initial stage in planar nano-photonic circuits. Figure 4a shows an SEM image of a structure in which the focusing array from Figure 3a is coupled to a metal nano-strip waveguide. Nano Lett., Vol. 5, No. 7, 2005
Figure 4. (a) SEM image of the focusing array coupled to a 250nm-wide Ag strip guide; light gray, Ag; dark gray, Cr; scale bar, 2 µm. (b) NSOM image of the SPP intensity showing focusing and guiding.
Metal strip waveguides have emerged as a promising alternative to dielectric guides because they allow the confinement of light energy on subwavelength scales and can support various propagating SPP modes.27-32 A 250nm-wide, 4-µm-long Ag strip (Figure 4a) is defined by locally removing the Ag top layer around the strip using FIB machining. The near-field image in Figure 4b shows that the focused SPPs propagate along the subwavelength metal guide, partially penetrating into the 100-nm-wide bifurcation at the end of the guide, thus overcoming the diffraction limit of conventional optics. The propagation distance, δ ≈ 2 µm, of the strip guide is significantly shorter than on the extended film. This observation is consistent with previous reports on the propagation of SPP in metal strip guides30 and has been attributed to scattering by edge imperfections, which becomes increasingly important in narrow waveguides. Additional losses may arise in our sample because of the thick Cr underlayer. The propagation distances can be greatly enhanced with improved fabrication processes and by using properly designed metal-dielectric hybrid structures.33-35 We note that, because of the particular geometry, the spot of high SPP intensity will always be located at the geometrical center of the circle (for x polarization), independent of the SPP wavelength. Therefore, the focusing and guiding structure in Figure 4 can be used for a broad range of 1401
frequencies. Furthermore, by introducing phase shifts between the SPP emitted from each hole, the focal spot can be moved in a controlled way, similar to beam steering in phased antenna arrays. Such phase shifts can be achieved, for example, by inclining the incident light beam along the y axis. Calculations within the dipolar model show that an inclination of 4° induces a spot shift of about 1.5 µm along the y axis. In summary, we have shown that subwavelength holes in thin metal films are versatile sources for the launching of surface plasmon polaritons. Using near-field scanning optical microscopy, we have imaged the evanescent electromagnetic fields around individual holes and in hole arrays. For an arcshaped hole array fabricated with focused ion beam machining into an Ag film, we show that SPP can be focused into an intense spot with subwavelength width. Calculations based on a dipolar model give a quantitative description of the images. Furthermore, we show that the high SPP intensity in the focal spot can be launched and propagated on a 250nm-wide metal strip waveguide. The combination of focusing array and nano-waveguide may serve as the initial stage in planar nano-photonic circuits. Beam steering is possible by introducing phase delays between the nano holes, and this feature can lead the way toward multiplexing the SPP intensity between various waveguides. Acknowledgment. This work was supported by the U.S. Department of Energy, Basic Energy Sciences, under contract no. W-31-109-ENG-38 and the U.S. Department of Education. FIB work was performed in the EMC at Argonne National Laboratory and the CMM at the University of Illinois, both supported by the U.S. Department of Energy, Office of Science, under grant no. DEFG02-91-ER45439. We thank S. Gray and S.-H. Chang for numerous discussions. References (1) Polman, A.; Atwater, H. A. Mater. Today 2005, 8, 56. (2) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature (London) 2003, 424, 824-830. (3) Zayats, A. V.; Smolyaninov, I. I. J. Opt. A: Pure Appl. Opt. 2003, 5, S16-S50. (4) Raether, H. Surface Plasmons on Smooth and Rough Surfaces and on Gratings; Springer: Berlin, 1988. (5) Yonzon, C. R.; Jeoung, E.; Zou, S.; Schatz, G. C.; Mrksich M.; Van Duyne, R. P. J. Am. Chem. Soc. 2004, 126, 12669-12676. (6) Raschke, G.; Kowarik, S.; Franzl, T.; So¨nnichsen, C.; Klar, T. A.; Feldmann, J.; Nichtl, A.; Kurzinger, K. Nano Lett. 2003, 3, 935938.
1402
(7) For a review, see Hutter, E.; Fendler, J. H. AdV. Mater. 2004, 16, 1685-1706. (8) Jung, C.; Yee, S.; Kuhn, K. J. LightwaVe Technol. 1994, 12, 18021806. (9) Hoekstra, H. J. W. M.; Lambeck, P. V.; Krijnen, G. J. M.; Ctyroky, J.; De Minicis, M.; Sibilia, C.; Conradi, O.; Helfert, S.; Pregla, R. J. LightwaVe Technol. 1998, 16, 1921-1927. (10) Nikolajsen, T.; Leosson, K.; Bozhevolnyi, S. I. Appl. Phys. Lett. 2004, 85, 5833-5835. (11) Krasavin, A. V.; MacDonald, K. F.; Zheludev, N. I.; Zayats, A. V. Appl. Phys. Lett. 2004, 85, 3369-3371. (12) Sasaki, K.; Nagamura, T. J. Appl. Phys. 1998, 83, 2894-2900. (13) Brolo, A. G.; Arctander, E.; Gordon, R.; Leathem, B.; Kavanagh, K. L. Nano Lett. 2004, 4, 2015-2018. (14) Stockman, M. I. Phys. ReV. Lett. 2004, 93, 137404. (15) Jin, E. X.; Xu, X. Appl. Phys. Lett. 2005, 86, 111106. (16) Drachev, V. P.; Bragg, W. D.; Podolskiy, V. A.; Safonov, V. P.; Kim, W.-T.; Ying, Z. C.; Armstrong, R. L.; Shalaev, V. M. J. Opt. Soc. Am. B 2001, 18, 1896-1903. (17) Genov, D. A.; Sarychev, A. K.; Shalaev, V. M.; Wei, A. Nano Lett. 2004, 4, 153-158. (18) Yin, L.; Vlasko-Vlasov, V. K.; Rydh, A.; Pearson, J.; Welp, U.; Chang, S.-H.; Gray, S. K.; Schatz, G. C.; Brown, D. E.; Kimball, C. W. Appl. Phys. Lett. 2004, 85, 467-469. (19) An FEI dual-beam DB-235 system was used at the Center for Microanalysis of Materials (CMM), University of Illinois at Urbana, and a Zeiss 1540XB dual beam was used at the Electron Microscopy Center (EMC), ANL. (20) So¨nnichsen, C.; Duch, A. C.; Steininger, G.; Koch, M.; von Plessen, G.; Feldmann, J. Appl. Phys. Lett. 2000, 76, 140-143. (21) Baida, F. I.; Van Labeke, D.; Bouhelier, A.; Huser, T.; Pohl, D. W. J. Opt. Soc. Am. A 2001, 18, 1552-1561. (22) Johnson, P. B.; Christy, R. W. Phys. ReV. B 1972, 6, 4370-4379. (23) Chang, S.-H.; Gray, S. K.; Schatz, G. C. Opt. Express 2005, 13, 3150-3165. (24) Smolyaninov, I. I.; Mazzoni, D. L.; Mait, J.; Davis, C. C. Phys. ReV. B 1997, 56, 1601-1610. (25) Yatsui, T.; Nomura, W.; Kourogi, M.; Ohtsu, M. Int. Quantum Electron. Conf. (IQEC) 2004, 867-869. (26) Drezet, A.; Stepanov, A. L.; Ditlbacher, H.; Hohenau, A.; Steinberger, B.; Aussenegg, F. R.; Krenn, J. R. Appl. Phys. Lett. 2005, 86, 074104. (27) Burke, J. J.; Stegeman, G. I.; Tamir, T. Phys. ReV. B 1986, 33, 51865201. (28) Berini, P. Phys. ReV. B 2000, 61, 10484-10503; Berini, P. Phys. ReV. B 2001, 63, 125417. (29) Charbonneau, R.; Berini, P.; Berolo, E.; Lisicka-Shrzek, E. Opt. Lett. 2000, 25, 844-846. (30) Lamprecht, B.; Krenn, J. R.; Schider, G.; Ditlbacher, H.; Salerno, M.; Felidj, N.; Leitner, A.; Aussenegg, F. R.; Weeber, J. C. Appl. Phys. Lett. 2001, 79, 51-53. (31) Yatsui, T.; Kourogi, M.; Ohtsu, M. Appl. Phys. Lett. 2001, 79, 45834585. (32) For a review, see Krenn, J. R.; Weeber, J.-C. Philos. Trans. R. Soc. London, Ser. A 2004, 362, 739-756. (33) Takahara, J.; Yamagishi, S.; Taki, H.; Morimoto, A.; Kobayashi, T. Opt. Lett. 1997, 22, 475-477. (34) Zia, R.; Selker M. D.; Catrysse, P. B.; Brongersma, M. L. J. Opt. Soc. Am. A 2004, 21, 2442-2446. (35) Lee, T. W.; Gray, S. K. Appl. Phys. Lett. 2005, 86, 141105.
NL050723M
Nano Lett., Vol. 5, No. 7, 2005
