Coherent phonons in plasmonic nanostructures and surface phonon polariton resonators for enhanced light-matter interaction in the mid-IR

Abstract

Light exhibits a rich set of phenomena when interacting with matter, making it an invaluable tool for fundamental research and also for technological applications. However, in many instances, the size mismatch between matter components and the localization of light with conventional optical elements hampers a more fruitful exploitation of this interaction. The excitation of plasmon polaritons in metals enables us to circumvent the limits in the light localization imposed by far-field diffraction. Nevertheless, the light confinement at optical frequencies in plasmonic systems requires the storage of energy in the motion of free-electrons, which are subject to unavoidable ohmic losses. At lower frequencies, the large negative real permittivity of metals leads to a poor confinement of the near-field radiation. The high losses impede the application of plasmonic systems in the transmission and the guiding of sub-diffraction optical modes. Additionally, the poor confinement at longer wavelengths limits the study and the application of enhanced molecular sensing which requires spatial and spectral overlap between confined modes and vibrational transitions in chemical species. Thus, the search for alternative materials that can provide sub-diffraction light confinement with low losses across the electromagnetic spectrum becomes a major goal in nanophotonics research. In this thesis we explore how the fast electronic relaxation processes related to losses in plasmonic systems can be used to generate coherent acoustic phonons of tailored frequency and amplitude in metallic nanoantennas subject to dielectric mechanical constraints. The damping of coherent phonons through emission of surface acoustic waves (SAWs) is also explored, where narrow-frequency mechanical modulation of plasmonic resonances via the emitted SAWs is shown possible. Finally, we investigate the potential of sub-diffraction surface phonon polariton resonances in polar dielectric materials as an alternative to plasmonic systems for sensing in the mid-infrared range, where the detection of sub-nanometric film thicknesses is achieved.