Skip to main content

Interacting polaritons in two-dimensional electron systems

Periodic Reporting for period 3 - POLTDES (Interacting polaritons in two-dimensional electron systems)

Reporting period: 2018-11-01 to 2020-04-30

We had identified the principal research objectives of the ERC project as (a) demonstration that interactions between polaritons and electrons could lead to novel transport signatures including light-induced superconductivity, (b) enhancement of polariton-polariton interactions in two-dimensional electron systems (2DES) in gallium arsenide (GaAs) to realize strongly correlated photonic systems, and (c) investigation of novel polariton physics in monolayers of transition metal dichalcogenides (TMD). During the first 30 months of the project, we have made ground-breaking advances on all three fronts. An unexpected but much welcomed development was the remarkable speed with which the quality of TMD materials improved, which placed them as the principal material system of the Project.
Realization of interacting polariton-electron system.
Arguably, the most important achievement of the ERC project is the theoretical and experimental identification of exciton-polarons and polaron-polaritons that emerge as the elementary quasi-particles in strongly interacting polariton-electron system [1]. Motivated by the recent advances in ultra-cold atom physics, we demonstrated that introduction of an exciton in a degenerate 2DES, results in dressing of the mobile exciton with density fluctuations of the Fermi sea of electrons. The resulting quasi-particles, termed exciton-polarons, differ from a bound-state of an exciton and an electron, termed a trion. Unlike trions, exciton-polarons are many-body excitations and yet they have a finite quasi-particle weight. As a consequence, exciton-polarons can couple non-perturbatively to a cavity mode, leading to the formation of polaron-polaritons. Remarkably, the theoretical model we developed to analyze the many-body system provides quantitative agreement with experimental absorption spectra [1].
Even though polarons are neutral excitations, they can be manipulated using external electric or magnetic fields. We arrived at this striking, counter-intuitive prediction by carrying out a full diagrammatic calculation based on Baym-Kadanoff conserving approximation [2]. The consequences of our calculations are nothing short of spectacular since they indicate that neutral photonic excitations respond to gauge fields and the strength of this response is determined by an effective fractional charge. More specifically, we find that polaritons can be manipulated using dc or ac electric fields and they are subject to a Lorentz force when the magnetic field is non-zero. Conversely, we showed that forces applied on polaritons lead to an electrical transport response.
In order to identify the best material system to pursue polariton mediated superconductivity, we carried out a theoretical analysis fully taking into account screening of electron-electron and electron-polariton interactions. Our findings suggest that TMD monolayers provide a much more promising avenue for pursuing superconductivity [3]. We also demonstrated that increasing the polariton density leads to a polariton mode softening, with potentially very interesting consequences such as the formation of an excitonic supersolid.

Realization of strongly correlated polaritons.
Broadly speaking, the two principal requirements for observing strong correlation between photons/polaritons include the implementation of artificial gauge fields and polariton-polariton interactions strong enough to lead to polariton blockade effect. We addressed both of these challenges during the first 30 months of the project. Our initial efforts were aimed at implementation of a tuneable artificial gauge potential using the magneto-electric Stark effect: we demonstrated experimentally [4] that the effect of crossed external electric (E) and magnetic (B) fields on excitons or polaritons is to shift the minimum of the dispersion away from k=0. This shift can be described as a gauge potential A = a E x B, where a is the linear polarizability. To demonstrate the dependence of A on the orientation of electric and magnetic fields, we implemented a novel interference set-up that allowed us to determine the phase shift acquired by polaritons propagating in the medium. The next step in our experiments is to introduce an electric field gradient which will effect an artificial magnetic field on polaritons. A time-dependent external electric field on the other hand induces an artificial electric field for polaritons.
In order to reach the polariton blockade regime we focused on dipolar polaritons where large permanent dipole associated with each polariton ensures strong repulsive interactions between them. The system that we implemented consists of two undoped coupled quantum wells (QW) embedded in a microcavity: the wide QW hosts direct excitons that couple nonper
A truly unexpected outcome of the research has been the possibility ot manipulate neutral optical excitations using external electric and magnetic fields. This opens up completely new possibilities for realizing effective gauge fields for polaritons.

Armed with a new understanding of the interacting polariton-electron system, we will tackle the principal goal of the project, namely, the demonstration of polariton mediated superconductivity.