Towards hybrid optomechanics in a fiber-based fabry-perot cavity

Hybrid optomechanical systems which associate a cavity optomechanical system and a two-level system have recently emerged as a route towards strong optomechanical coupling, which is challenging to achieve in a pure optomechanical system.
Motivated by this context, we decided to develop an experimental platform based on a fiber Fabry-Perot cavity (FFPC) and a mechanical resonator operated in a membrane-in-the-middle (MIM) configuration, aimed at studying the interaction between an optomechanical system and a two-level system embedded within the mechanical resonator. In this thesis I describe the conception, assembly and characterization of this experimental platform.
We used CO2 laser ablation to fabricate fiber mirrors with a geometry optimized for realizing hybrid optomechanical systems in the MIM configuration. We formed a FFPC between two fiber mirrors, held in a titanium cage that we specially designed for operating a MIM system within a 4He bath cryostat and in high vacuum, while maintaining both a high level of tunability and a high level of mechanical stability. The resulting cavity compares well with other state-of-the-art FFPCs, and most importantly it is mechanically stable enough for us to stabilize its length to within 5 pm using the Pound-Drever-Hall locking technique. The lock remains stable for many hours, both at room temperature and at 4 K, which to the best of our knowledge has not been previously achieved with a high finesse tunable FFPC.
Hexagonal boron nitride (hBN) is a 2D material which has been identified as a promising candidate for realizing hybrid mechanical resonators due to its superior mechanical properties and because it was found to host bright strain-coupled quantum emitters. We fabricated suspended hBN drum micromechanical resonators by transferring exfoliated flakes of hBN on top of a hole in a thin silicon nitride membrane, and inserted one of these hBN drums in the middle of the FFPC, forming a MIM system. We then measured the dispersive and dissipative effect of the position of the hBN drum on the resonance of the cavity, which is a signature of the optomechanical interaction in a MIM system. From this measurement we estimate a linear dispersive optomechanical coupling on the order of 6 GHz/nm.
We expect our forthcoming study of dynamical optomechanical effects in this MIM system to provide valuable data on the mechanical properties of hBN and to help answer some of the open questions about the dynamics of 2D material membranes. We then aim to observe the impact of a quantum emitter positioned within the hBN drum on its optomechanical interaction with the cavity field.