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Engineering the optical response of plasmonic nanoantennas

Holger Fischer and Olivier J. F. Martin

Open Access Open Access

Abstract

The optical properties of plasmonic dipole and bowtie nanoantennas are investigated in detail using the Green’s tensor technique. The influence of the geometrical parameters (antenna length, gap dimension and bow angle) on the antenna field enhancement and spectral response is discussed. Dipole and bowtie antennas confine the field in a volume well below the diffraction limit, defined by the gap dimensions. The dipole antenna produces a stronger field enhancement than the bowtie antenna for all investigated antenna geometries. This enhancement can reach three orders of magnitude for the smallest examined gap. Whereas the dipole antenna is monomode in the considered spectral range, the bowtie antenna exhibits multiple resonances. Furthermore, the sensitivity of the antennas to index changes of the environment and of the substrate is investigated in detail for biosensing applications; the bowtie antennas show slightly higher sensitivity than the dipole antenna.

©2008 Optical Society of America

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Supplementary Material (2)

Media 1: MOV (436 KB)     
Media 2: MOV (1516 KB)     

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Figures (12)
Fig. 1.
Fig. 1. Geometry of the investigated system: (a) Bowtie and (b) dipole antennas. The illumination is shown in panel (b).

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Fig. 2.
Fig. 2. Relative field intensity spectra in the gap for (a) bowtie and (b) dipole antennas. The dotted line in panel (b) indicates the relative field intensity at the extremity of the dipole antenna. (l=230nm; g=30nm)

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Fig. 3.
Fig. 3. Near-field intensity distribution 20nm above a dipole antenna (l=230nm) as a function of the illumination wavelength λ. The corresponding spectrum (field intensity in the gap) is shown in the inset. (file size: 0.4MB) [Media 1]

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Fig. 4.
Fig. 4. Near-field intensity distributions 20nm above bowtie antennas (l=230nm) as a function of the illumination wavelength λ. Four different bow angles are considered (from top left to bottom right): α 1=28°, α 1=53°, α 1=90° and α 4=127°. The corresponding spectra (field intensity in the gap) are shown in the insets. (file size: 1.5MB) [Media 2]

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Fig. 5.
Fig. 5. (a) xz-map and (b) yz-map of the relative field intensity in a plane through the middle of a dipole antenna at the resonance wavelength.

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Fig. 6.
Fig. 6. Relative intensity enhancement in the gap as a function of the antenna length l between l=110nm and l=270nm in 20nm increments. (a) Dipole and (b) bowtie geometry (α=90°). The antenna gap is kept constant (g=30nm).

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Fig. 7.
Fig. 7. (a) Resonance position shift for dipole (+) and bowtie (α=90°, *) antennas as a function of the antenna length. (b) Field enhancement as a function of the antenna length for both antennas.

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Fig. 8.
Fig. 8. (a) The three main resonances of a bowtie antenna (l=210nm; α=90°) and (b) their spectral position as a function of the antenna length.

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Fig. 9.
Fig. 9. Relative intensity spectra in the gap as a function of the illumination wavelength and gap width for (a) a dipole and (b) a bowtie (α=90°) antenna (l=230nm). (c) Spectral position and (d) relative field enhancement in the gap of the corresponding intensity maximum as a function of the gap width. For the bowtie antenna the three main resonances are again treated separately (see Fig. 8). The dot in panel (c) indicates the spectral position of the maximum for the corresponding monopole antenna.

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Fig. 10.
Fig. 10. Relative field intensity enhancement in the gap for (a) a dipole and (b) a bowtie antenna (l=110nm, g=30nm, α=90°) as a function of the illumination wavelength. Different refractive indexes ns are used for the substrate material.

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Fig. 11.
Fig. 11. Relative field intensity enhancement in the gap for (a) a dipole and (b) a bowtie antenna (l=110nm, g=30nm, α=90°) as a function of the illumination wavelength. Different refractive indexes nenv are used for the cover material, the substrate index is ns =1,5.

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Fig. 12.
Fig. 12. Sensitivity of the (a) dipole and (b) bowtie antenna (l=110nm, g=30nm, α=90°) as a function of the environment index nenv .

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