Abstract

Comprehensive reflectivity mapping of the angular dispersion of nanostructured arrays comprising of inverted pyramidal pits is demonstrated. By comparing equivalently structured dielectric and metallic arrays, diffraction and plasmonic features are readily distinguished. While the diffraction features match expected theory, localised plasmons are also observed with severely flattened energy dispersions. Using pit arrays with identical pitch, but graded pit dimensions, energy scaling of the localised plasmon is observed. These localised plasmons are found to match a simple model which confines surface plasmons onto the pit sidewalls thus allowing an intuitive picture of the plasmons to be developed. This model agrees well with a 2D finite-difference time-domain simulation which shows the same dependence on pit dimensions. We believe these tuneable plasmons are responsible for the surface-enhancement of the Raman scattering (SERS) of an attached layer of benzenethiol molecules. Such SERS substrates have a wide range of applications both in security, chemical identification, environmental monitoring and healthcare.

© 2006 Optical Society of America

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References

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    [CrossRef] [PubMed]
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Appl. Phys. Lett. (1)

J. J. Baumberg, N. M. B. Perney, M. C. Netti, M. D. B. Charlton, M. Zoorob, and G. J. Parker, "Visible-wavelength Super-refraction in Photonic Crystal Superprisms," Appl. Phys. Lett. 85, 354-356 (2004).
[CrossRef]

ElectroChemistry Comm. (1)

Mamdouh E. Abdelsalam, Philip N. Bartlett, Jeremy J. Baumberg, Tim A. Kelf, Suzanne Cintra and Andrea E. Russell, "Electrochemical SERS at a structured gold surface," ElectroChemistry Comm. 7, 740 (2005)
[CrossRef]

J. Phys. Chem. B (1)

C. L. Haynes, R. P. Van Duyne, "Plasmon-Sampled Surface-Enhanced Raman Excitation Spectroscopy," J. Phys. Chem. B, 107, 7426-7433 (2003).
[CrossRef]

J. Phys. Chem. B. (1)

Z.Q. Tian, B. Ren, D.Y. Wu, "Surface-enhanced raman scattering: from noble to transition metals and from rough surfaces to ordered nanostructures," J. Phys. Chem. B., 106, 9463-9483 (2002).
[CrossRef]

Langmuir. (1)

C. A. Szafranski, W. Tanner, P. E. Laibinis, and R. L. Garrell, "Surface-enhanced Raman spectroscopy of aromatic thiols and disulfides on gold electrodes," Langmuir. 14, 3570-3579 (1998).
[CrossRef]

NanoLett (1)

J.J. Baumberg, T. A. Kelf, Y Sugawara, S Pelfrey, M Adelsalam, PN Bartlett, AE Russell, "Angle-Resolved Surface-Enhanced Raman Scattering on Metal Nanostructured Plasmonic Crystals," NanoLett, 11, 2262-2267 (2005).
[CrossRef]

Nature (1)

William L. Barnes, Alain Dereux and Thomas W. Ebbesen, "Surface plasmon subwavelength optics," Nature 424, 824-830 (2003)
[CrossRef] [PubMed]

Opt. Express (1)

Phys. Rev. B (2)

W.-C. Tan, T.W.P., J. R. Sambles, and N. P. Wanstall, "Flat surface-plasmon-polariton bands and resonant optical absorption on short-pitch metal gratings," Phys. Rev. B, 59, 12661-12666, (1999)
[CrossRef]

W. L. Barnes, T. W. Preist, S. C. Kitson, and J. R. Sambles, "Physical origin of photonic energy gaps in the propagation of surface plasmons on gratings," Phys. Rev. B, 54, 6227-6244 (1996).
[CrossRef]

Phys. Rev. Lett. (4)

S. Coyle, M.C. Netti, J.J. Baumberg, M.A Ghanem, P.R. Birkin, P.N. Bartlett, D.M. Whittaker, "Confined Plasmons in Metallic Nanocavities," Phys. Rev. Lett. 87, 176801 (2001)
[CrossRef] [PubMed]

T.A. Kelf, Y. Sugawara, J.J. Baumberg, M. Abdelsalam and P.N. Bartlett, "Plasmonic bandgaps and Trapped Plasmons on Nanostructured Metal Surfaces," Phys. Rev. Lett. 95, 116802 (2005)
[CrossRef] [PubMed]

H. Gersen, T. J. Karle, R. J. P. Engelen, W. Bogaerts, J. P. Korterik, N. F. van Hulst, T. F. Krauss, and L. Kuipers, "Real-Space Observation of Ultraslow Light in Photonic Crystal Waveguides," Phys. Rev. Lett. 94, 073903 (2005)
[CrossRef] [PubMed]

T.V. Teperik, V.V. Popov, F.J.Garcia de Abajo, J.J. Baumberg, T. A. Kelf and Y.Sugawara "Enhancement of surface plasmon-polariton resonances on nanoporous metal surface," submitted to Phys. Rev. Lett. (2005)

Sens. Actuators A (1)

K. Sato, M. Shikida, T. Yamashiro, M. Tsunekawa, and S. Ito, "Roughening of single-crystal silicon: surface etched by KOH water solution," Sens. Actuators A 73, 122-30 (1999)
[CrossRef]

Supplementary Material (3)

» Media 1: MOV (1503 KB)     
» Media 2: MOV (2573 KB)     
» Media 3: MOV (1089 KB)     

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Figures (8)

Fig. 1.
Fig. 1.

(a) 2D cross section through the pits, the pitch (Λ) is 2μm. The aperture size (r) and therefore the depth ( d ) are graded across the sample. (b) Schematic representation of the sample with ϕ orientation of pits and incident angle θ of the laser. (c) and (d) are top and cross section SEM of the sample before metallization.

Fig. 2.
Fig. 2.

Reflection spectra at 50° incidence and 3° azimuthal angle for a square array of silicon pits which are uncoated (black) and gold-coated (yellow). (a) TM incident, TM analysed, (b) TM incident, TE analysed. Dotted lines are diffraction features, arrows mark plasmons.

Fig. 3.
Fig. 3.

(a) Angular dependent experimental reflectivity on Au-coated pit array (colour coded: blue = 100%, white = 0%) for TM/TE at ϕ=0° [Media 1], (b) combined plot of (a,c), (c) Grating theory modes up to 5th order. Animated data of (a) as a function of azimuthal angle ϕ is available as a multimedia file (1.5Mb)

Fig. 4.
Fig. 4.

Experimental angular dispersion for (a) uncoated and [Media 2] (b) Au-coated pit arrays, together with theoretical diffraction modes. The dynamic range in (a) is 10 times less than in (b). Animated data of (a) as a function of azimuthal angle is available as a multimedia file (2.5Mb)

Fig. 5.
Fig. 5.

(a) Reflection spectrum vs pit depth at θ=0° for (a) experiment and (b) FDTD 2D simulations at θ=0°. Faint diffraction features common to all spectra are present at 894nm, 708nm, 635nm and 556nm. (c) Extracted plasmon dip energies vs. pit depth. (d) Intensity distribution from FDTD simulation at λ=785nm for d =1250nm. (e) Schematic field (black line) and intensity distribution (filled red) for an m=2 plasmon mode confined in the pit for actual (upper) and un-folded (lower) depiction.

Fig. 6.
Fig. 6.

(a) Angular dispersion of reflectivity on Au-coated pit array for ϕ=45° and d=920nm, with (b) superimposed diffraction curves [Media 3]. (c) Strong coupling observed in angular dispersion of samples with increasing pit depth. Animation of (b) as a function of decreasing pit depth is available as a multimedia file (1Mb)

Fig. 7.
Fig. 7.

Raman scattering from a monolayer of benzenethiol on a pit arrays with depth=1μm, for 3mW of 633nm laser, integration time is 10s. (a) Spectra scanned across pit array, shifted vertically for clarity, (b) spectra across pit array, for 1572cm-1 line, background subtracted.

Fig. 8.
Fig. 8.

(a)Raman scattering of a monolayer of aminothiophenol as a function of pit depth, for 785nm excitation wavelength. (b) SERS signal as a function of pit depth for the 1080 cm-1 line.

Equations (2)

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2 a = ( m + 1 2 ) λ SPP
ħω = πħc n SPP cos α d ( m + 1 2 )

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