Interacting Fermi and Bose Exciton-Polaron-Polaritons

Abstract

When a quantum impurity is immersed in a bath with which it interacts, it becomes dressed by the excitations of the bath. The dressed quantum impurity can then be described as a quasiparticle with renormalised properties, commonly called the polaron-- an important concept first suggested by Landau in 1933. In this thesis, we seek to understand the modifications of the interactions experienced by an exciton-polariton impurity when it is immersed in a bath of electrons or a bath of opposite-spin exciton-polaritons. Exciton-polaritons are formed from the hybridisation of photons and excitons. Therefore, they inherit the light mass from the former and the interaction properties from the latter. The motivation of our research is twofold. Firstly, polarons have been proposed to play a crucial role in physical phenomena with potential applications such as high-Tc superconductivity and itinerant ferromagnetism. Thus, the understanding of polarons has become a highly sought after goal in physics today. Secondly, polariton interactions are a crucial ingredient for many proposed applications. These include both classical and quantum optical communication technology which the world will become highly-dependent on in the forseeable future. Our experiments are performed on a type of 2D semiconductor known as monolayer molybdenum diselenide (MoSe2) which is strongly coupled to a zero-dimensional (0D) open fiber cavity at cryogenic temperatures (T = 4 K). In order to ensure strong-coupling between the excitons and the photons, the van der Waals (vdW) semiconductor heterostructures and the distributed Bragg reflector (DBR) mirror coatings are designed after optimisation with transfer matrix simulations. The concave dimpled surfaces on fibers are fabricated with low surface roughness using carbon dioxide (CO2) laser ablation. We investigate the system in two regimes with time-resolved pump-probe spectroscopy: a quantum impurity in a fermionic bath of electrons and in a bosonic bath of opposite-spin exciton-polaritons. In the fermionic case, we show that the impurity-impurity interactions are sizably enhanced-- in one device by a factor of 50 when itinerant electrons are injected into the system such that the exciton-polariton impurities become dressed by electron-hole pair excitations out of the Fermi sea. The enhancement comes from a phase-space filling effect due to the saturation of the bath. Furthermore, we observe optical amplification of the lower-polariton mode in the presence of the electrons. There is a clear dependence of the amplification magnitude on the polarisation of the pumped and probed polariton modes. In the bosonic case, we demonstrate the formation of Bose polarons by immersing a minority spin-down exciton-polariton population into a bath of spin-up exciton-polaritons. The optical spectrum shows clear signatures of the splitting of a single polariton branch into an attractive Bose polaron and a repulsive Bose polaron, corresponding to a redshift and blueshift respectively. These are formed due to the dressing of the spin-down exciton-polariton with the Bogoliubov excitations of the resonantly pumped non-equilibrium condensate of spin-up exciton-polaritons. We furthermore investigate the impurity-impurity interactions by increasing the impurity density beyond the single impurity limit. We demonstrate the tunability of the magnitude as well as sign of both the impurity-bath and impurity-impurity interactions.