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Interacting Polaritons in two-dimensional Systems

The interaction of matter excitations with light confined in an optical cavity forms the basis of cavity quantum electrodynamics. When this interaction exceeds the individual decay rates, exciton-polaritons, composite quasiparticles formed by excitons strongly coupled to the cavity light field, emerge as the new eigenstates of the system. These particles inherit fascinating physical properties from their constituents, and allow for observation of superfluidity and Bose-Einstein condensation at elevated temperatures. However, the interaction between excitons that allowed for those spectacular observations are weak, such that very high densities of polaritons are required for the interactions to become relevant. It is an outstanding goal to realize strong interactions at the single polariton level, which would open up a completely new regime of strongly correlated phases of photons.

Fig. 1: Heterostructure hosting indirect excitons (IX) and its optical spectrum. As a function of gate voltage, the energies of direct exciton (DX, green) and IX (red) are shifted, changing the nature of the resulting polaritons (black spectra).
Figure 1: Heterostructure hosting indirect excitons (IX) and its optical spectrum. As a function of gate voltage, the energies of direct exciton (DX, green) and IX (red) are shifted, changing the nature of the resulting polaritons (black spectra).

Electrically tunable interactions of dipolar
polaritons

Research in our group explores two approaches to increase and observe such strong interactions among polaritons. They are based on the idea to equip the excitons with new properties by hybridizing with other quasiparticles. The first approach uses indirect excitons where electron and hole are spatially separated. They couple to excitons via electron tunneling and lead to a large dipole moment given by the separation distance between electron and hole times the electric charge. This results in dipole-dipole interactions, which lead to stronger repulsion between excitons [1]. Furthermore, this admixture of indirect excitons, and thereby the interaction strength, can be tuned in-situ by applying an out-of-plane electric field.

Interacting Polaritons in the quantum hall
regime

Another approach towards stronger interactions is based on excitons immersed in a two dimensional electron gas (2DEG). Through the dressing with electrons, the excitons become a probe for the electronic ground state of the 2DEG. In a perpendicular magnetic field, the 2DEG exhibits the celebrated Quantum Hall effect. The filling factor, which is the ratio between number of electrons and magnetic flux quanta, defines the state of the system. By a mechanism yet to be fully understood, the polaritons dressed by electrons in particular fractional filling factors (2/5 & 2/3) showed an order of magnitude stronger interactions compared to neighboring fillings [2]. This spectacular effect is especially interesting because it might reveal new properties of the Quantum Hall system so far inaccessible in linear optical spectroscopy.

Interacting Polaritons in the quantum hall regime
Figure 2: Quantum Hall Polaritons. Varying the magnetic field varies the 2DEG filling factor. Spectroscopy of the polaritons coupled to the 2DEG via excitonic dressing reveals properties of the electronic ground state.  

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