PhD Defense by Andreas Barg

Optomechanics with soft-clamped silicon nitride membranes and carrier-mediated forces in coupled quantum wells

The field of quantum cavity optomechanics is fueled by the development of high-performance micro- and nanofabricated devices. These devices aim to enhance the interaction between optical and mechanical resonators by improving their respective quality factors and exploring new coupling mechanisms. Thanks to recent progress in the field, the dream to efficiently prepare and control quantum states of mechanical motion for quantum information processing is soon to become a reality.  

In this work, we present ultrahigh-quality-factor silicon nitride membranes with phononic crystal structures for quantum optomechanics. The membranes are periodically patterned, with a defect in the center that hosts localized mechanical modes and enables “soft clamping”: without a rigid silicon frame as a boundary for these modes, the curvature in the membrane remains small during the motion, significantly reducing intrinsic dissipation. Indeed, we measure mechanical quality factors of more than 200 million and long coherence times enabling several quantum-coherent oscillations, even at room temperature. This extraordinary performance, alongside their low effective masses of a few nanograms, render our membranes excellent candidates for quantum cavity optomechanics, as well as mass and force sensing applications.  

We place a patterned membrane inside a high-finesse optical cavity mounted in a 4-K-liquid-helium cryostat and optically cool a localized defect mode via radiation pressure induced dynamical backaction to the vicinity of the quantum ground state of motion. Raman sideband thermometry reveals the mean phonon occupancy of the mode to be 0.55(1), which is close to the backaction limit. Our setup thereby manifests itself as a quantum-enabled system, allowing for complex protocols such as the heralded generation of one-phonon Fock states by filtering and detecting single scattered photons.  

In a parallel line of research, we study carrier-mediated forces in semiconductor nanomembranes. By embedding coupled quantum wells in the membranes, lifetimes of optically generated electron-hole pairs reach up to 750 ns, which is comparable to the period of mechanical oscillation. As a result, the strong forces due to the piezoelectric effect and the deformation potential resonantly drive a bending mode of the membrane to an amplitude about three orders of magnitude larger than expected from radiation pressure. The forces are controlled using a bias voltage across the quantum wells that tunes the carrier lifetime. In addition to exploring potentially much more efficient optomechanical coupling mechanisms, this work may provide a new path towards optoelectromechanical hybrid devices.