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We develop a comprehensive theoretical model for the interaction strength between a pair of exciton-polaritons in microcavity devices. Ab initio numerical calculations for dipolar polaritons in one dimension are used as a starting point to build a Born-Oppenheimer theory that generally applies to generic -- dipolar or non-dipolar polaritons -- in both one and two dimensions. This theory anticipates that the strong coupling to the cavity mode leads to a drastic enhancement of the polariton interactions as compared to bare excitons, and predicts unexpected scaling laws in the interaction strength as a function of system parameters. Comparisons with available experimental data are drawn, and specific suggestions to validate it with new experiments are made. Promising strategies towards the observation of a strong polariton blockade regime are finally sketched., Comment: Main manuscript: 18 pages, 6 figures. Appendix: 2 pages, 2 figures

The elementary optical excitations of a two-dimensional electron or hole system have been identified as exciton-Fermi-polarons. Nevertheless, the connection between the bound state of an exciton and an electron, termed trion, and exciton-polarons is subject of ongoing debate. Here, we use an analogy to the Tavis-Cummings model of quantum optics to show that an exciton-polaron can be understood as a hybrid quasiparticle -- a coherent superposition of a bare exciton in an unperturbed Fermi sea and a bright collective excitation of many trions. The analogy is valid to the extent that the Chevy Ansatz provides a good description of dynamical screening of excitons and provided the Fermi energy is much smaller than the trion binding energy. We anticipate our results to bring new insight that could help to explain the striking differences between absorption and emission spectra of two-dimensional semiconductors., Comment: 5 pages

Optically generated electron-hole pairs can probe strongly correlated electronic matter, or, by forming exciton-polaritons within an optical cavity, give rise to photonic nonlinearities. The present paper theoretically studies the properties of electron-hole pairs in a two-dimensional electron liquid in the fractional quantum Hall regime. In particular, we quantify the effective interactions between optical excitations by numerically evaluating the system's energy spectrum under the assumption of full spin and Landau level polarization. Optically most active are those pair excitations which do not modify the correlations of the electron liquid, also known as multiplicative states. In the case of spatial separation of electrons and holes, these excitations interact repulsively with each other. However, when the electron liquid is compressible, other non-multiplicative configurations occur at lower energies. The interactions of such dark excitations strongly depend on the liquid, and can also become attractive. For the case of a single excitation, we also study the effect of Landau level mixing in the valence band which can dramatically change the effective mass of an exciton., Comment: 8 + 4 pages, 3 figures

Two dimensional semiconductors provide an ideal platform for exploration of linear exciton and polariton physics, primarily due to large exciton binding energy and strong light-matter coupling. These features, however, generically imply reduced exciton-exciton interactions, hindering the realisation of active optical devices such as lasers or parametric oscillators. Here, we show that electrical injection of itinerant electrons into monolayer molybdenum diselenide allows us to overcome this limitation: dynamical screening of exciton-polaritons by electrons leads to the formation of new quasi-particles termed polaron-polaritons that exhibit unexpectedly strong interactions as well as optical amplification by Bose-enhanced polaron-electron scattering. To measure the nonlinear optical response, we carry out time-resolved pump-probe measurements and observe polaron-polariton interaction enhancement by a factor of 50 ($0.5 \mu$eV $\mu$m$^2$) as compared to exciton-polaritons. Concurrently, we measure a spectrally integrated transmission gain of the probe field of $\gtrsim 2$ stemming from stimulated scattering of polaron-polaritons. We show theoretically that the non-equilibrium nature of optically excited quasiparticles favours a previously unexplored interaction mechanism stemming from a phase-space filling in the screening cloud, which provides an accurate explanation of the strong repulsive interactions observed experimentally. Our findings show that itinerant electron-exciton interactions provide an invaluable tool for electronic manipulation of optical properties, demonstrate a new mechanism for dramatically enhancing polariton-polariton interactions, and pave the way for realisation of nonequilibrium polariton condensates.

Optically generated excitons dictate the absorption and emission spectrum of doped semiconductor transition metal dichalcogenide monolayers. We show that upon increasing the electron density, the elementary optical excitations develop a roton-like dispersion, evidenced by a shift of the lowest energy state to a finite momentum on the order of the Fermi momentum. This effect emerges due to Pauli exclusion between excitons and the electron Fermi sea, but the robustness of the roton minimum in these systems is a direct consequence of the long-range nature of the Coulomb interaction and the nonlocal dielectric screening characteristic of monolayers. Finally, we show that the emergence of rotons could be related to hitherto unexplained aspects of photoluminescence spectra in doped transition metal dichalcogenide monolayers., Comment: Supplemental material available as ancillary file

Mobile quantum impurities interacting with a fermionic bath form quasiparticles known as Fermi polarons. We demonstrate that a force applied to the bath particles can generate a drag force of similar magnitude acting on the impurities, realizing a novel, nonperturbative Coulomb drag effect. To prove this, we calculate the fully self-consistent, frequency-dependent transconductivity at zero temperature in the Baym-Kadanoff conserving approximation. We apply our theory to excitons and exciton polaritons interacting with a bath of charge carriers in a doped semiconductor embedded in a microcavity. In external electric and magnetic fields, the drag effect enables electrical control of excitons and may pave the way for the implementation of gauge fields for excitons and polaritons. Moreover, a reciprocal effect may facilitate optical manipulation of electron transport. Our findings establish transport measurements as a novel, powerful tool for probing the many-body physics of mobile quantum impurities., Comment: 18 + 11 pages, 4 figures

For applications exploiting the valley pseudospin degree of freedom in transition metal dichalcogenide monolayers, efficient preparation of electrons or holes in a single valley is essential. Here, we show that a magnetic field of 7 Tesla leads to a near-complete valley polarization of electrons in MoSe2 monolayer with a density 1.6x10^{12} cm^{-2}; in the absence of exchange interactions favoring single-valley occupancy, a similar degree of valley polarization would have required a pseudospin g-factor exceeding 40. To investigate the magnetic response, we use polarization resolved photoluminescence as well as resonant reflection measurements. In the latter, we observe gate voltage dependent transfer of oscillator strength from the exciton to the attractive-Fermi-polaron: stark differences in the spectrum of the two light helicities provide a confirmation of valley polarization. Our findings suggest an interaction induced giant paramagnetic response of MoSe2, which paves the way for valleytronics applications.

The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking experiments with ultra-cold Fermi gases. Optical creation of an exciton or a polariton in a two-dimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity-coupling favoring ultra-low mass polariton formation and exciton-electron interactions leading to polaron or trion formation. Here, we present cavity spectroscopy of gate-tunable monolayer MoSe$_2$ exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode. As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive-exciton-polaron resonance to the lower-energy attractive-polaron manifold. Simultaneous observation of polariton formation in both attractive and repulsive branches indicate a new regime of polaron physics where the polariton impurity mass is much smaller than that of the electrons. Our findings shed new light on optical response of semiconductors in the presence of free carriers by identifying the Fermi polaron nature of excitonic resonances and constitute a first step in investigation of a new class of degenerate Bose-Fermi mixtures.

Interacting Bose-Fermi systems play a central role in condensed matter physics. Here, we analyze a novel Bose-Fermi mixture formed by a cavity exciton-polariton condensate interacting with a two-dimensional electron system. We show that that previous predictions of superconductivity [F.P. Laussy, Phys. Rev. Lett. 10, 104 (2010)] and excitonic supersolid formation [I.A. Shelykh, Phys. Rev. Lett. 14, 105 (2010)] in this system are closely intertwined- resembling the predictions for strongly correlated electron systems such as high temperature superconductors. In stark contrast to a large majority of Bose-Fermi systems analyzed in solids and ultracold atomic gases, the renormalized interaction between the polaritons and electrons in our system is long-ranged and strongly peaked at a tunable wavevector, which can be rendered incommensurate with the Fermi momentum. We analyze the prospects for experimental observation of superconductivity and find that critical temperatures on the order of a few Kelvins can be achieved in heterostructures consisting of transition metal dichalcogenide monolayers that are embedded in an open cavity structure. All optical control of superconductivity in semiconductor heterostructures could enable the realization of new device concepts compatible with semiconductor nanotechnology. In addition the possibility to interface quantum Hall physics, superconductivity and nonequilibrium polariton condensates is likely to provide fertile ground for investigation of completely new physical phenomena., Comment: 20 pages, 6 figures

Self-assembled quantum dots are ideal structures in which to test theories of open quantum systems: Confined exciton states can be coherently manipulated and their decoherence properties are dominated by interactions with acoustic phonons. We here describe the interaction of a pair of un-coupled, driven, quantum dot excitons with a common phonon environment, and find that this coupling effectively generates two kinds of interaction between the two quantum dots: An elastic coupling mediated by virtual phonons and an inelastic coupling mediated by real phonons. We show that both of these interactions produce steady state entanglement between the two quantum dot excitons. We also show that photon correlations in the emission of the quantum dots can provide a signature of the common environment. Experiments to demonstrate our predictions are feasible with the state-of-the-art technology and would provide valuable insight into quantum dot carrier-phonon dynamics.

We present a theoretical and experimental study of photonic and electronic transport properties of a voltage biased InAs semiconductor double quantum dot (DQD) that is dipole-coupled to a superconducting transmission line resonator. We obtain the Master equation for the reduced density matrix of the coupled system of cavity photons and DQD electrons accounting systematically for both the presence of phonons and the effect of leads at finite voltage bias. We subsequently derive analytical expressions for transmission, phase response, photon number and the non-equilibrium steady state electron current. We show that the coupled system under finite bias realizes an unconventional version of a single-atom laser and analyze the spectrum and the statistics of the photon flux leaving the cavity. In the transmission mode, the system behaves as a saturable single-atom amplifier for the incoming photon flux. Finally, we show that the back action of the photon emission on the steady-state current can be substantial. Our analytical results are compared to exact Master equation results establishing regimes of validity of various analytical models. We compare our findings to available experimental measurements., Comment: 13 pages, 9 Figures, 1 Table