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Interacting polaritons in two-dimensional electron systems

Periodic Reporting for period 4 - POLTDES (Interacting polaritons in two-dimensional electron systems)

Reporting period: 2020-05-01 to 2020-10-31

The three principal research objectives of the ERC project were (a) demonstration that interactions between polaritons and electrons could lead to novel physics, (b) enhancement of polariton-polariton interactions in two-dimensional electron systems (2DES) to realize strongly correlated photons, and (c) investigation of novel physics in van der Waals heterostructures of transition metal dichalcogenides (TMD). The underlying theme for all research vectors was the new physics emerging from interactions in two dimensional (2D) systems hosting itinerant electrons and exciton-polariton excitations. Strong correlations in nonequilibrium systems represents a frontier for condensed-matter physics. As is the case of most basic condensed-matter projects, the primary goal is to explore new physics in systems where there is potential for applications, such as low-threshold coherent light sources or electrically controlled superconductivity in 2D semiconductors.

While we detail the principal achievements of the project below, we highlight three of the most important results here. These include (1) Identification of exciton-polarons (polaron-polaritons) as elementary optical excitations in an interacting exciton-electron (polariton-electron) system, (2) Demonstration of strong nonlinear optical response of fractional quantum Hall states, and (3) Observation of incompressible Mott state of electrons in a twisted molybdenum diselenide homobilayer embedded in a van der Waals heterostructure.
A) Realization of interacting polariton-electron system. The advances we have made during the ERC project have revealed new phenomena that originate from interactions between exciton-polaritons and itinerant electrons. Our first achievement was the identification of elementary optical excitations in this system: we demonstrated that introduction of an exciton in a degenerate 2DES, results in dressing of the mobile exciton with electronic density fluctuations. The resulting quasi-particles, termed exciton-polarons, couple non-perturbatively to a cavity mode, leading to the formation of polaron-polaritons.
Armed with this new understanding, we theoretically addressed polariton manipulation using external electric or magnetic fields. Our calculations indicated that neutral photonic excitations respond to gauge fields. More specifically, we found that polaritons could be manipulated using dc or ac electric fields. To verify our predictions, we carried out an experimental study where we subjected polaritons in a gallium arsenide (GaAs) 2DES, to external fields. Our experiments unequivocally demonstrated polariton acceleration using external electric or magnetic fields.
A hallmark of 2DES response to an external magnetic field is Shubnikov de Haas (SdH) oscillations. In our effort to further identify the nature of the magnetic field response of optical excitations dressed by a 2DES in a TMD monolayer, we carried out optical spectroscopy at high magnetic fields and observed SdH oscillations of exciton-polarons.
Our findings pave the way for exploration of a degenerate Bose-Fermi mixture consisting of a 2DES and exciton-polariton Bose-Einstein condensate. We made a preliminary effort to understand its potential for exploration of strongly correlated phases. Our findings suggest a promising avenue for polariton mediated superconductivity: polariton-electron interactions lead to a polariton mode softening, which in turn enhances the attractive electron-electron interactions.

B) Towards strongly interacting polaritons. We addressed two principal requirements for observing strong correlations between polaritons – namely, the implementation of artificial gauge fields and strong polariton-polariton interactions. First, we implemented a new method to effect an electrically tuneable artificial gauge potential for photons using the magneto-electric Stark effect. What makes this achievement particularly important is that such an effective gauge potential can be implemented in any polarizable medium; while an electric field gradient effects an artificial magnetic field on polaritons, a time-dependent electric field induces an artificial electric field for photonic excitations.
To enhance polariton-polariton interactions, we have pursued four parallel approaches. The first approach was based on using a fiber cavity structure: this allowed us to tightly confine polaritons and thereby demonstrate photon antibunching in resonant transmission of a polariton mode, paving the way for investigating the quantum regime of polaritons. To further enhance the requisite nonlinearity, we focused on dipolar polaritons with large permanent dipole ensuring strong repulsive interactions. We measured a factor of 7 enhancement of the interactions as we increased the size of the polariton dipole moment.
Furthermore, we investigated polariton formation in a high mobility GaAs 2DES exhibiting fractional quantum Hall states (FQHE), where we observed a strong dependence of polariton spectrum on the filling factor. Using collinear four-wave-mixing spectroscopy, we demonstrated that the Kerr nonlinearity in this system is dramatically enhanced whenever the 2DES is in =2/5 and =2/3 FQHE states. In addition to enhancing polariton interactions, these ground-breaking experiments demonstrated the potential of nonlinear optical spectroscopy for investigating exotic states of matter. Finally, we investigated polariton interactions in a fiber-cavity incorporating a TMD heterostructure. We observed that exciton-polaron-polaritons interact 50 times stronger than bare exciton-polaritons, demonstrating how to overcome the small nonlinearity of TMD monolayers.

C) Novel physics of van der Waals heterostructures. To establish TMD heterostructures as a platform for quantum photonics, we followed up on our theoretical analysis and carried out an experiment demonstrating a TMD monolayer as an atomically thin mirror. Subsequently, we investigated the magnetic response of electrons and holes in monolayer TMDs. These experiments revealed giant paramagnetic response of both electrons and holes, indicating the key role played by strong interactions.
The consequences of strong electron interactions in TMD heterostructures show up most spectacularly in twisted bilayer structures. We used such a structure to demonstrate that a small twist angle between two molybdenum diselenide layers lead to the formation of a moire potential: strong electron interactions in turn ensure that for unit filling of each layer, the electrons form an incompressible Mott state. During the last 6 months of the project, we developed and implemented a new optical spectroscopic technique that provides direct evidence for the emergence of charge order in strongly correlated electronic systems.
a) 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.

b) Observation of incompressible Mott insulator state arising from strong electronic correlations in a 2D material (see the attached image).

c) Very large nonlinear optical response of fractional quantum Hall states.

d) The new optical spectroscopic technique we developed that allows for detecting electronic charge order in a 2D material.
In a twisted molybdenum selenide homobilayer, electrons in each layer form Mott states