  Plasmonic structures interacting with light provide electromagnetic
resonances which result in a high degree of local field confinement enabling
the enhancement of light-matter interaction. Plasmonic structures typically
consist of metals which, however, suffer from very high ohmic losses and
heating. High-index dielectrics, on the other hand, can serve as an alternative
material due to their low-dissipative nature and strong scattering abilities.
We study the optical properties of a system composed of all-dielectric
nanoparticle arrays covered with a film of organic dye molecules (IR-792), and
compare these dielectric arrays with metallic nanoparticle arrays. We tune the
light-matter interaction by changing the concentration in the dye film and
report the system to be in the strong coupling regime. We observe a Rabi
splitting between the surface lattice resonances (SLRs) of the nanoparticle
arrays and the absorption line of the dye molecules of up to 253 meV and 293
meV, for the dielectric and metallic nanoparticles, respectively. The Rabi
splitting depends linearly on the square root of the dye molecule concentration
, and we further assess how the Rabi splitting depends on the film thickness
for a low dye molecule concentration. For thinner films of thicknesses up to
260 nm, we observe no visible Rabi splitting. However, a Rabi splitting evolves
at thicknesses from 540 nm to 990 nm. We perform finite-difference time-domain
simulations to analyze the near-field enhancements for the dielectric and
metallic nanoparticle arrays. The electric fields are enhanced by a factor of
1200 and 400, close to the particles for gold and amorphous silicon,
respectively, and the modes extend over half a micron around the particles for
both materials.
