Mirror-Coupled Nanoantennas with Hexagonal-Boron Nitride Spacers

Abstract

Transparent dielectric materials are employed in a wide range of optical resonators. Combining plasmonic nanoantennas with such resonators enables the formation of nanoscale optical resonances. The novel nano-optics observed by combining these heterostructures with optically active media is laying the foundations for devices such as ultra-high efficiency optical switches. In this thesis, mirror-coupled nanoantennas consisting of the layered dielectric hexagonal-Boron Nitride encapsulated between gold nanoparticles and a gold substrate, are probed using high-angle optical scattering measurements. During illumination of individual nanoparticles, light is trapped at the nanoscale inside the hBN, between particle and gold beneath. As the thickness of hBN is decreased, the coupling between nanoparticle and substrate changes dramatically. Here, the inert properties of hBN are used to remove extraneous influences on scattering spectra, thereby revealing new nanoscale light confinement mechanisms and sub-nanometre structural changes. In the first experiment presented, submicron-thick hBN crystals embedded in gold form planar Fabry-Perot half-microcavities. Gold nanoparticles on top of these microcavities form previously unidentified angle- and polarization-sensitive nanoresonator modes that are tightly laterally confined by the nanoparticle. Comparing dark-field scattering with reflection spectroscopies shows plasmonic and Fabry-Perot-like enhancements magnify subtle interference contributions, which lead to unexpected redshifts in the dark-field spectra, explained by the presence of these new modes. In the second experiment presented, the thickness of hBN is reduced down to a singleatom- thick layer leading to greatly enhanced field intensities and confinement when compared to thicker layers, via plasmonic coupling. By comparing results to an analytic model, resultant ultra-sensitive scattering signals from nanoparticles atop these thinner layers reveal field interactions sub-nanometre structural changes. Finally, photoluminescence measurements on defects in as-grown monolayer WS2 on gold lead to clear emission despite significant substrate quenching. Measurements using nanoplatelets of the metal halide perovskite methyl-ammonium lead-iodide, lead to suppression of resonant modes and a blue-shifting behaviour correlated with light emission.