Light-matter interaction in plasmonic and dielectric nanoparticle arrays

Abstract

When light interacts with a metal, it can induce collective oscillations of the free electrons within the metal. These oscillations are known as plasmons, hence plasmonics is the research field studying the interactions between light and metals on a nanoscale. In case of metallic nanoparticles, the light can directly couple to the plasmons, giving rise to localized surface plasmon resonances (LSPRs) in the nanoparticles. These resonances are remarkable as their electric field is highly confined in hotspots around the particle, enabling efficient coupling to quantum emitters. In this thesis, nanoparticles arranged in arrays are studied. These nanoparticle arrays interacting with light form surface lattice resonances which are hybrid modes of the LSPRs and the diffracted orders stemming from the array geometry. We combine the arrays with organic dye molecules and study them with regard to strong coupling and lasing phenomena. Although plasmonic nanoparticles couple efficiently to light, they suffer from ohmic losses. One way to circumvent these losses is by using high-index dielectric materials that provide low losses and high scattering abilities. Reduced losses can lead to narrower resonances and longer lifetimes of the modes. In Publication I we combine amorphous silicon nanoparticle arrays with organic dye molecules. By varying the dye concentration, we show that the dielectric nanoparticle arrays are in the strong coupling regime with the organic dye molecules. Finite-difference time-domain simulations are performed supporting the experimental results. In Publication II and Publication III we study how complex unit cell geometries influence the lasing behaviour in gold nanoparticle arrays. We arrange particle clusters into arrays and combine these with dye molecules. We excite the system optically with a laser and study the output from the array in real space, momentum space as well as angular resolved spectra. In Publication II we study quadrumer arrays and show by combining polarization resolved measurements with theory that the lasing mode is a bound state in continuum (BIC) and that it has a topological charge of q = 1. In Publication III we study hexamer arrays. By varying the hexamer size we tune the lasing mode and observe lasing in a bright mode as well as in BICs with topological charges of q = +1, -1, and -2. By calculating the modes with the T-matrix method, we show that the losses of the modes change with changing hexamer sizes. The modes with the lowest losses are favoured for lasing. In Publication IV we study lasing in gold nanoparticle arrays consisting of supercells. We create these supercells by leaving specific sites of a regular square array empty. This creates a supercell of the size of 13 unit cells of the underlying square lattice. As a result of this second lattice period, additional bands are formed. In the lasing experiments we observe simultaneous lasing from different modes. By a theoretical mode analysis we show that the lasing modes stem from high-order high-symmetry points namely the 74th Γ- and the 106th X-point of the supercell.