Nonperturbative Visualization of Nanoscale Plasmonic Field Distributions via Photon Localization Microscopy

We demonstrate the nonperturbative use of diffraction-limited optics and photon localization microscopy to visualize the controlled nanoscale shifts of zeptoliter mode volumes within plasmonic nanostructures. Unlike tip- or coating-based methods for mapping near fields, these measurements do not affect the electromagnetic properties of the structure being investigated. We quantify the local field manipulation capabilities of asymmetric bowtie antennas, in agreement with theoretical calculations. The photon-limited localization accuracy of nanoscale mode positions is determined for many of the measured devices to be within a 95% confidence interval of $+/-2.5\mathrm{nm}$. This accuracy also enables us to characterize the effects of nm-scale fabrication irregularities on local plasmonic mode distributions.

Figure 1

(a),(b) Calculated images of the spatially and spectrally distinct near-field $|E{|}^2$ distributions surrounding an ABnC when excited at (a) the higher-energy blue mode, and (b) the lower-energy red mode. Scanning electron microscope (SEM) images of a representative (c) ABnC and (d) associated disk-dimer fiduciary mark. TPPL images of the structures in (c),(d) are shown in (e). (f) Two-dimensional Gaussian fits of the TPPL images in (e). Standard deviations of these fits are (${\sigma }_x=151.5\mathrm{nm}$, ${\sigma }_y=154.3\mathrm{nm}$) for the upper spot and (${\sigma }_x=136.3\mathrm{nm}$, ${\sigma }_y=129\mathrm{nm}$) for the lower spot in (f).

Figure 2

The measured mode positions for an ABnC with approximate Cs symmetry are overlaid in (a) on its SEM image. The centroid of the blue mode image is marked by the left $X$ and the center position of the red mode image is marked by the right $X$. The center positions are known with accuracies given in the text. Theoretically modeled positions of the $|E_{\mathrm{Au}}{|}^4$ centroids are shown in (b) for the blue mode and (c) for red mode excitation. The color values in (b–c) and (e–f) correspond to near field $|E{|}^2$ values on a log scale. Center positions from a more asymmetric ABnC (offsets of the constituent triangles are in both $x$- and $y$-directions) are shown in (d). Calculated $|E_{\mathrm{Au}}{|}^4$ centroid positions for the ABnC in (d) are plotted in (e) and (f) for blue and red mode excitation, respectively.

Figure 3

TPPL image centroid shifts plotted as a function of normalized offset of the vertical-bowtie component in the $+x$ direction. Experimentally measured values are marked by circles with error bars and FDTD-calculated values are shown by the dotted line with circles. The error bars correspond to the one-sigma accuracy of the experimentally determined shifts along the $x$ axis.