Introduction
Localized surface plasmon resonance (LSPR) in strongly coupled subwavelength metal nanoparticles has been studied extensively due to the potential applications in plasmonic sensing and surface-enhanced Raman spectroscopy (SERS). Recent studies have shown that “elevating” the nanoparticles results in even larger field enhancements, due to less screening of fields by the substrate. Here, we present dark-field measurements of circular Au dimer antennas on top of SiO2 pillars. Placing the Au nanodisks on 240 nm tall pillars results in an enhanced plasmonic response compared to the case without pillars. At 480 nm pillar height, a secondary mode at lower wavelengths appears, which is believed to arise from scattering by the pillars themselves.
Methods
Nanodisk dimer arrays are defined using electron-beam lithography with well-defined gaps down to 15 nm. This is then followed by Au deposition and lift-off. Finally, nanopillars are defined using reactive-ion etching (RIE) with the nanoparticles functioning as an etch mask. A schematic of the fabrication process is given in figure 1a and an SEM image of a single dimer is shown in figure 1b. A high-density array of nanopillar dimers is shown in figure 1c. The FDTD simulations are done with Lumerical.
Optical scattering spectra are measured with an optical dark-field microscope (Nikon Ti-U, NA = 0.9-1.0) in transmission mode. Light scattered by the sample is collected by a dark-field objective (Nikon, NA = 0.7, 60x) and sent to a high-sensitivity spectrometer (Andor, SR-303i) equipped with an EMCCD (Andor Newton) detector. A schematic of the setup is given in figure 2a. All measured spectra are averages of 10 nanopillar dimers separated by 5 um, with dark-field images of arrays with pillar heights of 240 nm and 480 nm given in figure 2b and 2c, respectively.
Results and Discussion
Figure 3a shows measured scattering spectra of 120 nm diameter nanopillars with a height of 240 nm. A clear enhancement of the plasmonic response is observed for the antenna on pillars, attributed to reduced substrate interaction. Simulations of the response reproduce this trend, see figure 3b.
Figure 3c and 3d show measured and simulated scattering spectra, respectively, of 150 nm diameter antennas with 240 nm and 480 nm tall pillars. The measured spectrum for the 240 nm pillars is dominated by a single peak at 775 nm, which is the expected coupled LSPR mode of the dimer. However, the 480 nm spectrum displays a strong tail at wavelengths below 600 nm in addition to the resonance at 840 nm, which manifests itself also in the white colour of the antennas in the dark-field image in figure 2c. Such a tail can also be observed for the 240 nm, but with a much lower amplitude. The measured spectra are qualitatively reproduced by the simulations, which show a very similar scattering behaviour at low wavelengths.
As the height of the pillars is increased, the scattering efficiency at lower wavelengths increases. This is assumed to be due to an increased scattering of the lower wavelength light on the taller SiO2 pillars. This hypothesis is further confirmed by the fact that an independent FEM simulation (Comsol), which only considers the scattering from the Au nanodisks, shows no tail (see figure 3e).
Conclusion
The findings presented here show that placing nanoantennas on nanopillars enhances their plasmonic response. However, the pillars create additional scattering which has to be taking into account.
Photonic & plasmonic nanomaterials , Optical properties of nanostructures
