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 ultracold Fermi gases 1-5 . Optical creation of an exciton or a polariton in a twodimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity coupling favouring ultralow-mass polariton formation 6 and exciton-electron interactions leading to polaron or trion formation 7,8 . Here, we present cavity spectroscopy of gatetunable monolayer MoSe 2 (ref. 9) exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode 10,11 . 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 exciton-polaron state. Transition metal dichalcogenide (TMD) monolayers represent a new class of two-dimensional (2D) semiconductors exhibiting features such as strong Coulomb interactions 14 , locking of spin and valley degrees of freedom due to large spin-orbit coupling 9 and finite electron/exciton Berry curvature with novel transport and optical signatures 15,16 . Unlike quantum wells or 2D electron systems (2DES) in III-V semiconductors, TMD monolayers exhibit an ultralarge exciton binding energy E exc of order 0.5 eV (ref. 14) and strong trion peaks in photoluminescence (PL) that are redshifted from the exciton line by E T ∼ 30 meV (refs 9,17). These features provide a unique opportunity to investigate many-body physics associated with trion 18 formation as well as coupling of excitons to a 2DES 19 and to cavity photons 20,21 , provided that the experimental set-up allows for varying the electron density n e and light-matter coupling strength g c .Here, we carry out an investigation of Fermi polarons 1 in a charge-tunable MoSe 2 monolayer embedded in an open microcavity structure (Fig. 1a,b). Since E exc is much larger than all other relevant energy scales, such as the normal mode splitting (2g c ), E T and the Fermi energy (E F ), an optically generated exciton in a TMD monolayer can be considered as a robust mobile bosonic impurity embedded in a fermionic reservoir (Fig. 1c). The Hamiltonian describing the system iswhere the first line describes the coupling of 2D excitons, described by the exciton annihilation operator x k to a 0D cavity mode c 0 whose resonance frequency ω c can be tuned by applying a voltage (u p ) to a piezoelectric actuator that changes the cavity length. This part of the Hamiltonian corresponds to the elementary building block of the recent ground-breaking experiments based on coupled 0D-polariton systems 22 . The second line of the Hamiltonian describes the Feshbach-like physics associated with the bound-molecular (trion) channel and the corresponding effective interactions between the excitons and the electrons 1 . Thi...

Advent of new materials such as van der Waals heterostructures, propels new research directions in condensed matter physics and enables development of novel devices with unique functionalities. Here, we show experimentally that a monolayer of MoSe 2 embedded in a charge controlled heterostructure can be used to realize an electrically tunable atomically-thin mirror, that effects 90% extinction of an incident field that is resonant with its exciton transition. The corresponding maximum reflection coefficient of 45% is only limited by the ratio of the radiative decay rate to the linewidth of exciton transition and is independent of incident light intensity up to 400 Watts/cm 2 . We demonstrate that the reflectivity of the mirror can be drastically modified by applying a gate voltage that modifies the monolayer charge density. Our findings could find applications ranging from fast programmable spatial light modulators to suspended ultra-light mirrors for optomechanical devices.A plethora of ground-breaking experiments have established monolayers of transition metal dichalcogenides (TMD) such as MoSe 2 or WSe 2 as a new class of two dimensional (2D) direct 1

We have fabricated an encapsulated monolayer MoS 2 device with metallic ohmic contacts through a prepatterned hBN layer. In the bulk, we observe an electron mobility as high as 3000 cm 2 /Vs at a density of 7 × 10 12 cm −2 at a temperature of 1.7 K. Shubnikov-de Haas oscillations start at magnetic fields as low as 3.3 T. By realizing a single quantum dot gate structure on top of the hBN we are able to confine electrons in MoS 2 and observe the Coulomb blockade effect. By tuning the middle gate voltage we reach a double dot regime where we observe the standard honeycomb pattern in the charge stability diagram.Contrary to graphene, in monolayer molybdenum disulfide (MoS 2 ) inversion symmetry is broken. This, together with the presence of time-reversal symmetry, endows single layer MoS 2 with individually addressable valleys in momentum space at the K and K points in the first Brillouin zone. 1-4 This valley addressability facilitates the momentum state of electrons to be used for novel qubit architectures. Recent theoretical works have been exploring the possibility of using spin and valley states of gate-defined quantum dots in 2D MoS 2 as quantum bits. [5][6][7] In this manuscript, we describe the observation of Coulomb blockade in single and coupled dot in a high quality single layer MoS 2 . The high electronic quality of our monolayer MoS 2 results in the observation of Shubnikov-de Haas oscillations (SdHO) occurring at magnetic fields as low as 3.3 T. The 2DEG in the MoS 2 can be electrostatically depleted below the gate pattern with resistance values exceeding the resistance quantum h/e 2 by orders of magnitude. We observe Coulomb blockade resonances close to pinch-off indicating single electron tunneling in and out of the dot. By adjusting the gate voltages, we are able to tune the electrostatic landscape inside the dot and to form a double dot system within a single dot gate structure. [8][9][10][11] In Fig. 1(a), we show the schematic of a monolayer MoS 2 (∼ 0.7 nm thick) encapsulated between two hexagonal boron nitride (hBN) layers. The measured MoS 2 flake was exfoliated from natural bulk crystal (SPI supplies). The bottom hBN layer is ∼ 30 nm thick and works both as an atomically flat substrate and as a dielectric that isolates the MoS 2 from a graphite backgate. The graphite gate enables us to control electrostatically the electron density in MoS 2 with the voltage V bg . The layers thicknesses were determined by atomic force microscopy (AFM). The top hBN layer (∼50nm thick) has a) Electronic mail: pisonir@phys.ethz.ch been pre-patterned using E-beam lithography and reactive ion etching. 12 This enables us to evaporate metallic contacts (Ti/Au) on top of the MoS 2 layer where the hBN has been etched away, without exposing the channel region to organic residues remaining from the fabrication process. 13 Prior to metal evaporation, the heterostructure has been annealed in forming gas (Ar/H 2 ) at 300 • C for 30 minutes in order to remove most of the organic residues on top of the MoS 2 contact regions a...

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 realization 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 quasiparticles 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 timeresolved pump-probe measurements and observe polaron-polariton interaction enhancement by a factor of 50 (0.5 μeV μm 2) as compared to exciton-polaritons. Concurrently, we measure a spectrally integrated transmission gain of the probe field of ≳2 stemming from stimulated scattering of polaron-polaritons. We show theoretically that the nonequilibrium nature of optically excited quasiparticles favors 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 realization of nonequilibrium polariton condensates.

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