Position Resolved Plasmon Resonance Shift Induced by a Dielectric Tip

We experimentally investigate the spectral change of the localized surface plasmon resonance (LSPR) of lithographically fabricated metal nanoparticles induced by a dielectric tip. The experimental setup incorporates a white light supercontinuum source for dark-field illumination of the particles. The fast aquisition times of few tens of ms make high resolution maps with thousands of tip-position dependent particle spectra possible. The fiber tip is laterally scanned across the particle and kept at constant height above the sample surface by shear force feedback. The lateral resolution is limited only by the size of the etched fiber tip of ≈ 80 nm.

In the figure, the dipolar plasmon mode of a gold nanrod with dimensions of 120 × 100 nm2 and a height of 30 nm is excited with light polarized along the long particle axis (x-axis). In this image, the height measured by the scanned fiber tip is superimposed with the (simultaneously measured) color-coded LSPR wavelength. The LSPR shift is primarily along the polarization direction and highest when the tip is close to the edge of the nanoparticle. Besides the resonance shift on each position of the tip, the spectral width and amplitude of the LSPR peak in the scattered light spectra show distinct, position dependent features. We compare the experimental results to numerical simulations using the MNPBEM toolbox [1] to relate the observed features to the optical near fields of the particle.

This near field technique can map the influence of a small perturbation of the particle environment induced by the dielectric tip. The obtained local sensitivity maps are of interest for e.g. the design of plasmonic molecular sensors.

[1] U. Hohenester and A. Trügler, “MNPBEM – a matlab toolbox for the simulation of plasmonic nanoparticles,” Computer Physics Communications, vol. 183, pp. 370–381, Feb. 2012

markus.krug@uni-graz.at