Optical Investigation of Interplay of Mie and Diffraction Resonances in Planar Non-Closely Packed Hybrid Metal-dielectric Photonic Crystals

Summary: In this experimental work, optical properties of monolayers of organic (polystyrene) sphere have been investigated with two sets of samples. Samples which consisted of monolayer of spheres (i) assembled on dielectric substrate and (ii) gold coated glass substrate. Spheres size distribution was from ~550 nm to ~1060 nm to work in the VIS-NIR range of light spectrum.

System of monolayer-glass samples were studied in first order approximation as photonic crystal slabs supporting wave guided modes and guided resonances. Spectral position of resonances were controlled by varying the filling fraction of spheres in overall volume the ordered monolayer lattice. Centre-to-center distance remained preserved while filling fraction was varied by plasma etching of spheres in our samples. Energy losses via dielectric substrate are concluded to be the main reason which limits the capability of controlling the resonances of 2D monolayer of spheres through the geometry of the lattice. It has been demonstrated that the analogy of 2D colloidal lattice of spheres with photonic crystal slab can only qualitatively explain the optical response of the system. Strong departure of resonance positions were found between the spectral positions predicted by effective medium theory and experimental values when effective refractive index of monolayer was reduced below 1.28 in our samples. Dispersion of resonances has been studied by angle resolved spectroscopy with s- and p-linearly polarized light.

Resonances provided by hybrid photonic plasmonic structure in ML-gold-glass samples have been observed and studied. Hybrid structure has been understood at its different architectural levels, capable of supporting wave guided modes, surface plasmon polariton modes, hybrid modes, and strong Mie. However, there was no persuasive evidence for the existence of SPPs found or observed in the transmission spectrum. Periodicity introduced by the ordered arrangement of spheres was confirmed in the well-defined resonant structure in the transmission spectra of the samples. It has been shown that the hybrid photonic plasmonic structure due to the inclusion of metallic character in system provides an extra degree of freedom in order to tune the optical response of structure through manipulating the geometry of colloidal monolayer. A large improvement in light confinement and the quality of resonances has been observed in ML-Au-glass samples in comparison to the similar samples with glass substrate. Using Mie plot software, Mie resonances has been traced, and strong Mie resonances are confirmed in ML-Au-glass sample in comparison to the ML-glass samples.