Superresolution Moiré Mapping of Particle Plasmon Modes

The spontaneous emission rate of a fluorophore provides a direct probe of the photonic local density of states at the fluorophore position. Here we exploit this capability to map the plasmonic modes of gold nanoparticles by imaging the fluorescence intensity in combined regular arrays of identical gold and fluorophore-doped polymer nanoparticles. By varying the distance between gold and polymer particles across the array, the fluorophore emission generates an optical Moiré pattern corresponding to a magnified spatial map of the plasmonic mode, which can be directly imaged with an optical microscope. Our results are corroborated by supplementary theoretical model calculations.

Figure 1

Principles of fluorescence Moiré mapping of gold nanoparticles. (a) Normalized plasmonic extinction (black curve) and photoluminescence spectra (red curve) of gold and SU8/PtTFPP nanoparticle arrays, respectively. (b) Scanning electron micrographs of (upper row) arrays of gold and SU8/PtTFPP nanoparticles and (lower row) a combined array of both nanoparticle types. (c) Photoluminescence spectra acquired from heterogeneous arrays of gold and SU8/PtTFPP nanoparticles. The relative mutual position of the nanoparticles within the array is shown in the insets for one particle pair. (d) Schematic sketch of distance variations of both types of nanoparticles within the combined arrays used for Moiré imaging, due to two different grating constants $G$ and $G+dG$.

Figure 2

Fluorescence Moiré mapping of a gold nanoparticle. (a) Image of a square gold nanoparticle. Unpolarized excitation at wavelengths of $365\pm 20\mathrm{nm}$ and detection through a high-pass filter with a cutoff wavelength of 515 nm were applied using a $20\mathrm{x}$, $\mathrm{\text{numerical aperture}}=0.5$ microscope objective. The scanning electron micrographs in the insets illustrate the variable distance between gold and SU8/PtTFPP nanoparticles, according to their position within the array. (b) Cross cut along the dashed line in (a) revealing a ratio of Moiré image size and geometrical particle size (inset) of 210.

Figure 3

Polarization and wavelength dependence of the plasmon mode of a square gold nanoparticle. Polarization directions and wavelength ranges are given to the left and on top, respectively. (a) Experimental images for (upper row) horizontal and (lower row) vertical polarization and spectral ranges (left to right) $650\pm 10\mathrm{nm}$, $670\pm 10\mathrm{nm}$ and $\ge 720\mathrm{nm}$. (b) BEM simulations for the polarization directions indicated to the left and for discrete wavelengths of (from left to right) 650, 670, and 720 nm.

Figure 4

Polarization and wavelength dependence of the plasmon mode of a gold nanorod. (a) Experimental images as in Fig. 3, but for a gold nanorod with 300 nm length, 100 nm width, and 50 nm height. Corresponding BEM simulations (compare Fig. 3).