Plasmons are oscillations of charge carriers in metallic nanostructures that confine light in the nanometer length-scale. Colloidal dispersions and self-assembled plasmonic nanoparticles can become an integral part of optoelectronics and light harvesting technologies like plasmonic solar cells, various types of sensors, and photocatalysts. The connection between the properties of plasmons and the static morphology of the metallic nanostructures (size, shape, spatial arrangement, chemical environment) has been studied over several decades. More recently the focus is on the effect of dynamical morphological changes on plasmons, as the ones induced with confined acoustic vibrations in the gigahertz (GHz) frequency range. Coupling between plasmonic and mechanical degrees of freedom paves the way for the so-called acousto-plasmonic devices, which can be utilized for various types of sensing and signal-processing applications.
For realistic applications, plasmonic nanostructures are synergistically combined with molecular ligands, polymers, or inorganic substrates, crystalline surfaces and nanomembranes. During this project, we have studied all these three types of hybrid plasmonic materials. More specifically, we studied:
1) Nanocomposites of plasmonic nanostructures embedded in polymers, which have a broad range of applications due to their temperature- and pressure-tunable mechanical and thermal properties, photothermal conversion and photo-actuation, the ability to employ a multifarious toolbox for nanofabrication, and their potential use in flexible devices.
2) Ligand-protected plasmonic nanoparticles. These are nanoparticles whose surface is decorated with molecules that drive self-assembly into ‘super-crystalline’ structures, i.e. colloidal crystals. An interesting aspect, examined during this project, is the ability of light and plasmons to energize the ligands and promote self-assembly.
3) Plasmonic nanostructures on surfaces and nanomembranes, which can be used as efficient sources of acoustic phonons (surface acoustic waves, SAW, and Lamb waves, respectively) for acoustoplasmonic devices. In future signal-processing devices, the high operational frequency needs to be combined with ultrasmall device size. Thus, plasmonics may offer new types of trasnducers between electromagnetic signals and acoustic phonons for timing, delaying or spectral filtering.
For all these systems, studies of acoustic phonons in plasmonic nanostructures are essential for heat management, mechanical properties, and optomechanics. The aim of this project is to elucidate the interaction between plasmons and mechanical motions in nanostructures using Brillouin Light Scattering (BLS) spectroscopy. More precisely, the objectives are: (1) to prepare and characterize plasmonic nanostructures, such as plasmonic nanorods dispersed in polymers and ligand-protected self-assembled plasmonic nanospheres, (2) to study their optical properties and identify the energy and symmetry of their plasmonic resonances, (3) to study their confined acoustic vibrations with BLS at various photon wavelengths, and (4) to study the interaction between plasmonic and mechanical resonances and understand how the plasmonic near-fields modify the selection rules for optomechanical coupling - the interaction giving rise to the BLS signal.
