Towards Polarization Routing of Magnetic and Electric Dipolar Emission with Dielectric Metasurfaces

Abstract

We investigate the polarization properties of emission associated with the magnetic dipole and electric dipole transitions of europium(III) coupled to an anisotropic dielectric metasurface with polarization-engineered electric and magnetic photonic local density of states. The metasurface consists of a square array of Mie-resonant elliptical a-Si:H dimers situated on an SiO$_2$ substrate and embedded in a PMMA film containing Eu(TTA)$_3$. Based on reciprocity principle, it was designed to achieve maximum electric (magnetic) field enhancement in the dimer gap at 610 nm (590 nm) for $x$-polarized ($y$-polarized) normally incident light in order to selectively enhance the electric dipole (magnetic dipole) emission into the $x$-polarized ($y$-polarized) emission channel, respectively. Momentum-resolved spectroscopy and back-focal plane imaging of emission of the fabricated light-emitting metasurface clearly reveal the intended polarization-dependent emission behaviour, with the $x$-polarized ($y$-polarized) emission showing a reduced (enhanced) ratio of the magnetic-/electric dipole emission intensity, correspondingly where the magnetic dipole emission is enhanced with a magnetic field enhancement from the nanostructures. The demonstrated polarization-dependent interaction of a designed nanostructure with the electric- and magnetic dipolar transitions of trivalent lanthanide ions opens an avenue towards routing of emission of different multipolar orders into different polarization channels.

Figures (6)

• Figure 1: Concept of polarization routing metasurfaces. In the image, we see the a-Si:H dimer array metasurface encapsulated in a layer of PMMA$:$Eu(TTA)_3 where the red dots represent the Eu(TTA)_3 emitters are excited by unpolarized UV light and start to fluoresce. In the top left corner the energy diagram of Eu(TTA)_3 is shown, the upward purple arrow shows the UV excitation, the white dashed arrows indicate non radiate decay process, and the red and orange downward arrows indicate the ED and MD transitions, respectively. In the for ground the working principle of the dimers is shown, where EDs oriented along the dimer axis $(x)$ are enhanced by electric field enhancement, at the ED transition wavelength 610nm, resulting in $x$-polarized emission. While simultaneously MDs oriented along the dimer axis are enhanced by magnetic field enhancement, at the MD transition wavelength 590nm, resulting in $y$-polarized emission.
• Figure 2: a) Schematic of the dimer array unit cell showing elliptical dimers with parameters $D_x = 120nm$, $D_y = 180nm$, $D_z = 80nm$, and $g = 40nm$, covered with a $h = 150nm$ thick PMMA layer. b, c) Simulated linear momentum resolved transmission spectra of the metasurface for (b) $x$- (c) $y$-polarized incident light. The green dashed line indicates 590nm and the black dashed line indicates 610nm. d) Calculated near-field intensity profiles in the $x-y$ plane through the center of the nanoresonators. The top row shows the magnetic, the bottom row the electric field intensity enhancement. The polarization direction is indicated by the white arrow in each color map. The left half are maps at $\lambda = 590nm$ and the right half are maps at $\lambda = 610nm$ separated by the gray dashed line. The green and black dashed boxes highlight the cases of most pronounced electric and magnetic hotspot formation in the feedgap, respectively.
• Figure 3: a) Oblique-view (52°) scanning-electron micrograph (SEM) of the fabricated metasurface. The inset shows an SEM of a focused-ion-beam (FIB) cross section of the sample. (b, c) Momentum-resolved linear-optical transmission spectra of the metasurface shown in (b) for $x$- and (c) $y$-polarized incident light. Here the data is normalized to the maximum transmittance. The green dashed line and black dashed line correspond to the MD (590nm) and ED (610nm) transition wavelength, respectively.
• Figure 4: Momentum-resolved fluorescence spectra of the hybrid metasurface for (a) $x$- and (b) $y$- polarized detection. (c, d) Fluorescence spectra averaged around the $\Gamma$ point ($(k_{x}/k_0 = [-0.035,+0.035]$) for the hybrid metasurface and the same PMMA$:$Eu(TTA)$_3$ film on the bare substrate, labeled as ms and sub in the legend, respectively. Here (c, d)) shows the $x$-polarized ($y$-polarized) emission. The insets in (c, d) show a the data between $\lambda = 570nm$ and 610nm magnified. (e, f) Fluorescence enhancement spectra calculated by dividing the metasurface spectra by the substrate spectra for $x$- and $y$-polarized detection, respectively. The green and black vertical dashed lines incidence the MD and ED transition locations, respectively.
• Figure 5: Back-focal plane images of the fluorescence from the dimer metasurface for (a, d) $x$- and (b, e) $y$-polarized detection. (c, f) shows the intensity difference $\Delta \text{I} =\text{I}_y - \text{I}_x$, where $\text{I}_x$ and $\text{I}_y$ denote the $x$-polarized and $y$-polarized intensities, respectively. The dashed lines show the first-order solution of the grating equation at the respective wavelength for (a-c) $\lambda = 590nm$ and (d-f) $\lambda = 610nm$. The black circle in the center indicates a $\text{NA}=0.1$.