Bloch oscillations of exciton-polaritons in topological lattices

• Applicants: Professor Dr. Sebastian Klembt; Professor Dr. Ulf Peschel
• Subject Area: Theoretical Condensed Matter Physics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 546922926

The aim of the project is to investigate the influence of lateral structures on the dynamics of exciton-polaritons, especially regarding Bloch-oscillations and topological effects. In semiconductor cavities, whose spectral transmission maximum is close to the exciton resonance of the material enclosed in the cavity, strong coupling between photons and excitons occurs. The excitation, dynamics, and relaxation of the resulting quasiparticles, the exciton-polaritons, are significantly influenced by lateral patterning of the optical cavity. A periodic modulation of the resonator mirror causes exciton-polaritons to behave similarly to electrons in a two-dimensional solid. Since exciton-polaritons, unlike electrons, are bosonic in nature, this makes it possible to investigate topological effects in a bosonic system, which is also relatively easy to access experimentally due to the strong coupling to the light field. In close cooperation between experiment and theory the project focuses on the investigation of topological effects and Bloch-oscillations in two-dimensional structured exciton-polaritonic systems. The generation of exciton-polaritons will be stimulated both by means of an incoherent, i.e. strongly blue-shifted, and by a coherent excitation with a laser wavelength close to the resonance of the optical system. While in the first case, which is easier to realize experimentally, a condensation in an energetically lower-lying state must take place first, a coherent excitation allows the direct injection of the desired quasi-particles. With coherent excitation, significantly higher densities of exciton-polaritons are achieved, so that the repelling particle-particle interaction becomes relevant and must be included in the investigations. A special focus will be on the influence of refractive index gradients on field propagation in an otherwise periodic structure. In the previous project, we were already able to prove the occurrence of Bloch oscillations for one-dimensional lattices, i.e. formed exciton-polaritons do not exclusively follow the gradient but return to the starting point. Overall, they perform oscillations in one-dimensional and complex periodic movements in two-dimensional lattices. In addition, transitions into higher bands occur, the so-called Zener tunneling. Numerical simulations suggest that the topology of the underlying lattice plays a crucial role, which will be investigated in the new project. The condensate that spreads under the influence of the structured cavity can now be observed using a streak camera, and the coherent form of excitation that has since been realized now also allows the systematic investigation of non-linear effects.

• DFG Programme: Research Grants