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Topological phononics through nano-optomechanical interactions

Periodic Reporting for period 3 - TOPP (Topological phononics through nano-optomechanical interactions)

Reporting period: 2021-01-01 to 2022-06-30

The project aims at uncovering topological behavior in nanophotonic systems, in particular by breaking time-reversal symmetry through nano-optomechanical interactions. Such behavior is normally alien to both photons and phonons as they do not interact with magnetic fields, in contrast to electrons. It can lead to highly sought-after functionality such as one-way transport and states that are protected against scattering from disorder. As such, it would be appealing to introduce these principles in nanophotonic and nanomechanical systems, which are rich in applications related to sensing and information processing. Enabled by radiation pressure control techniques and nanoscale system design, we aim to induce topological behavior for light and sound at the nanoscale.
In the first phase of the project, we demonstrated fundamental phenomena that enable topological behavior and developed building blocks of nano-optomechanical topological devices.
In an enabling experiment, we demonstrated synthetic magnetic gauge fields for phonon transport in a nano-optomechanical system (Nature Nanotechnology, 2020). Such gauge fields underpin the celebrated Aharonov-Bohm and Quantum Hall effects for electrons, but were not established yet in the nanomechanical domain. We showed that on-chip nanomechanical resonators can be coupled via time-modulated radiation pressure of nanoconfined optical fields, and that this induces transfer of a nanomechanical vibration that carries the characteristic nonreciprocal phase of a magnetic gauge field. We developed a full theory of topological phononic insulators based on the effect. Through design and nanofabrication, we started scaling the number of coupled optical and mechanical modes to study emergent topological behaviour as system size is increased. We are developing the experimental control techniques required to optically address such large numbers of mechanical degrees of freedom, tailored to the properties of the developed systems. Moreover, we study the inclusion of non-Hermitian dynamics (controlled gain and loss) and nonlinearity in these optomechanical systems with broken time-reversal symmetry. In this context, we developed a theory of optomechanical nonlinearity that quantifies the enhancement of such nonlinearity in multimode systems (arXiv:2007.09106).
Based on the gained insights of synthetic magnetism in multimode optomechanical systems, we discovered two new, related effects: Optomechanically induced birefringence and optomechanically induced Faraday rotation. These provide new mechanisms to control polarization and create magnet-free isolation and circulation in a broad class of cavity systems. In collaboration with the group of Andrea Alù (CUNY), we published this theory in Physical Review Letters.
Finally, we explored a new type of photonic topological insulator in the developed nanophotonic silicon platform, based on symmetry breaking and spin-orbit coupling in photonic crystals. We directly observed the associated topologically protected edge states and characterized their dispersion, spin, and backscattering-free routing for the first time (Science Advances, 2020). In collaboration with the group of Kobus Kuipers (TUD), we used phase-resolved near-field microscopy to visualize backscattering in topological and regular waveguides, and quantify the degree of topological protection when edge states propagate around sharp corners (arXiv:2008.06497).
The aforementioned results go beyond the state of the art, in the sense that they show in theory and experiment that nonreciprocal and topological behavior can be induced in nanoscale optomechanical systems. They use new forms of spatial and time-reversal symmetry breaking to achieve such effects. As we scale system size and merge the concepts of time-reversal symmetry breaking, non-Hermiticity, and nonlinearity in the continuation of the project, we expect to uncover rich emerging behavior in these systems.
Artist impression of optically-induced coupling between nanomechanical resonators
Light can induce transfer of vibrations from one nanobeam to another, mimicking a magnetic field