Surface-Enhanced Raman and Fluorescence Spectroscopy with an All-Dielectric Metasurface

Plasmonic substrates play a crucial role in the confinement and manipulation of localized electromagnetic fields at the nanoscale. The large electromagnetic field enhancement at metal/dielectric interfaces is widely exploited in surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS) spectroscopies. Despite the advantage of near-field enhancement, unfortunately, in metals, the large absorption at optical frequencies induces local heating of the analyte fluid with possible damage of the biological material. In addition, in SEF plasmonic substrates, spacer layers are necessary to minimize undesired fluorescence quenching due to nonradiative decay, which strongly depends on the distance between molecules and metallic substrates. Therefore, the possibility of managing surface electromagnetic states mimicking surface-plasmon resonances in terms of spatial localization, high-field intensity, and dispersion characteristics, while avoiding metallic losses is of great interest. However, dielectric nanoantennas can currently provide limited possibilities in the visible range of optical frequencies. We present the realization of all-dielectric metasurfaces made of nanostructured transparent silicon nitride supporting bound states in the continuum (BICs). We show that this special kind of Fano resonances can be effectively used in standard microscopy for practical applications. We achieved concurrent enhancements of ~103 fold of fluorescence emission and Raman scattering farfield intensities of molecules dispersed on these metasurfaces. In addition, we demonstrate that the gain of conventional SERS signals can be increased by more than one order of magnitude by resonant matching of the localized surface plasmon resonance with the BIC field. Our results can find significant applications for enhanced sensing, Raman imaging, and nonlinear processes.