Study of molecular vibrations and spectroscopy of nano-cavities

Abstract

The concept of Molecular Optomechanics was recently proposed, motivated by the fact that molecular vibrations behave as mechanical oscillators in their quantum ground state at room temperature, and that they can be coupled to plasmonic nanocavities to realise optomechanical systems. This thesis explores with theoretical and experimental works new topics in molecular optomechanics.

The first part of this Thesis develops a quantum model for anharmonic coupling between two vibrational modes of a molecule and computes the expected time evolution of vibrational populations after preparation of the system in a phonon-number state. This work is inspired by recent experiments in the host group demonstrating a technique to excite the first Fock state of a phonon in a crystal, and lays the basis for future experimental work on molecular systems.

Apart from studying the vibrational dynamics itself, molecular optomechanics also aims at engineering the coupling between localised plasmons and Raman-active molecular vibrations. To this aim, it is essential to perform high precision spectroscopy of both Stokes and anti- Stokes sideband with a broadly tunable excitation source. The second and third part of this Thesis focus on the design and implementation of such a setup. In the second part, I present the design and optimisation, with a ray tracing software, and then the fabrication and assembly of a tunable notch spectral filter, which allows blocking the reflected laser light from the sample under study while collecting both Stokes and anti-Stokes Raman sidebands. Characterisation of the device confirms that the extinction ratio is suitable for Raman spectroscopy without the need of additional interference filters, greatly simplifying tunable excitation and detection.

Finally, the third part of the Thesis describes the design, construction and first operational outcome of a cryogenic microscope for multimodal spectroscopy, allowing simultaneous excitation of plasmonic nanocavities with different wavelengths. This setup was instrumental in revealing hitherto elusive fluctuations of metal-induced luminescence, while monitoring the stability of the vibrational Raman signal.

Overall, this Thesis provides important contribution to our understanding of light-matter interactions and vibrational dynamics in the context of molecular optomechanics with plasmonic nanocavities.