Breaking the Symmetry of Forward-Backward Light Emission with Localized and Collective Magnetoelectric Resonances in Arrays of Pyramid-Shaped Aluminum Nanoparticles

We propose aluminum nanopyramids (ANPs) as magnetoelectric optical antennas to tailor the forward versus backward luminescence spectrum. We present light extinction and emission experiments for an ANP array wherein magnetoelectric localized resonances couple to in-plane diffracted orders. This coupling leads to spectrally sharp collective resonances. Luminescent molecules drive both localized and collective resonances, and we experimentally demonstrate an unconventional forward versus backward luminescence spectrum. Through analytical calculations, we show that the magnetic, magnetoelectric, and quadrupolar moments of ANPs—which lie at the origin of the observed effects—are enhanced by their tapering and height. Full-wave simulations show that localized and delocalized magnetic surface waves, with an excitation strength depending on the plane wave direction, direct the forward versus backward emitted intensity.

Figure 1

(a) Superpolarizability tensor ${\alpha }^S$ of an aluminum pyramid with $t=84\mathrm{nm}$, $h=150\mathrm{nm}$ and $b=144\mathrm{nm}$, at the electric dipolar resonance wavelength. (b) Schematic of the structure, and legend of Figs. (c),(d). (c) and (d) display the most significant elements of ${\alpha }^S$ as a function of the tapering and the height of the pyramids, respectively.

Figure 2

(a) Measured extinction, and (b) photoluminescence enhancement (PLE) towards the top (black line) and bottom (red line) of the pyramids. The inset in (a) displays a scanning electron micrograph of the fabricated structures prior to the deposition of the luminescent layer. The dashed line in (b) indicates the Rayleigh anomaly. The PLE data are noisier at the shortest and longest wavelengths because the absolute emission intensity is weaker at these wavelengths (see the Supplemental Material [50]).

Figure 3

Electric ($\mathbf{E}$) and magnetic ($\mathbf{H}$) fields at the localized surface plasmon resonance wavelength in simulations (700 nm) [50]. The color scale—equal for all plots in Figs. 3 and 4—indicates the local field intensity enhancement with respect to the incident field shown at the top right corner of each panel. The blue arrows represent the scattered field. Both colors and arrows pertain to the same field, except in (e) and (j), where we show the $\mathbf{H}$-field intensity enhancement and the scattered $\mathbf{E}$ field. Panels (a)–(e) correspond to top illumination, while panels (f)–(j) correspond to bottom illumination. The three horizontal dashed lines in panels (d),(e),(i),(j) indicate, from top to bottom, the planes at which panels (a)–(c) are evaluated for top illumination, and (f)–(h) for bottom illumination.

Figure 4

Details are the same as in Fig. 3, but at the surface lattice resonance wavelength (590 nm) [50].