Spectroscopic Mode Mapping of Resonant Plasmon Nanoantennas

We present spatially resolved spectral mode mapping of resonant plasmon gap antennas using two-photon luminescence microspectroscopy. The obtained maps are in good agreement with 3D calculations of the antenna modes. The evolution of the modal field with wavelength, both in the gap and along the two coupled gold nanowires forming the antenna, is directly visualized. At resonance, the luminescence for the gap area is enhanced at least 80 times and a comparison with the antenna extremities shows a dynamical charge redistribution due to the near-field coupling between the two arms.

Figure 1

TPL scans recorded on three different gold antennas and their respective scanning electron micrographs. (a) 500 nm bar, (b) coupled 500 nm antennas, and (c) $1\mu \mathrm{m}$ bar. All measurements were performed at 730 nm under the same incident laser power.

Figure 2

Distribution of (a) the local field $|E|$ and (b) $|E{|}^4$ computed in the half-plane of the three types of antennas at their respective resonance wavelength. (c) is obtained by convoluting the $|E{|}^4$ maps with a 200 nm waist 2D Gaussian profile and integrating over the third dimension.

Figure 3

Evolution with the incident wavelength of (a) the TPL map and (b) the computed convoluted $|E{|}^4$ distribution over a single gap antenna (scale bar 500 nm).

Figure 4

Evolution with the incident wavelength of the TPL signal (blue squares) (a) for the extremities of a 500 nm bar, (b) the gap of a coupled antenna, and (c) the extremities of a $1\mu \mathrm{m}$ bar. (d) The spectral evolution of the ratio between the signal in the antenna gap and at its extremities. The red circles account for the evolution of the corresponding calculated $|E{|}^4$. For comparison, all spectra have been normalized to unity (continuous lines are guides to the eyes).