A Feshbach resonance occurs when the energy of two interacting free particles comes into resonance with a molecular bound state. When approaching this resonance, marked changes in the interaction strength between the particles can arise. Feshbach resonances provide a powerful tool for controlling the interactions in ultracold atomic gases, which can be switched from repulsive to attractive [1][2][3][4] , and have allowed a range of many-body quantum physics e ects to be explored 5,6 . Here we demonstrate a Feshbach resonance based on the polariton spinor interactions in a semiconductor microcavity. By tuning the energy of two polaritons with anti-parallel spins across the biexciton bound state energy, we show an enhancement of attractive interactions and a prompt change to repulsive interactions. A mean-field two-channel model quantitatively reproduces the experimental results. This observation paves the way for a new tool for tuning polariton interactions and to move forward into quantum correlated polariton physics.A semiconductor microcavity is a unique system where exciton-polaritons emerge from the strong coupling between an exciton and a photon. The demonstration of Bose-Einstein condensation of exciton-polaritons in a semiconductor microcavity 7 has attracted much attention and opened a wide field of research on polariton quantum fluids, such as superfluidity 8 , quantum vortices 9 and Bogoliubov dispersion [10][11][12] . Many more examples could be proposed to highlight the fact that polaritons provide a concrete realization of a bosonic interacting many-body quantum system, complementing the work performed on ultracold atom systems.Furthermore, polaritons exhibit a polarization degree of freedom, with a one-to-one connection to two counter circular polarizations for their photonic part. The different excitonic content of both polarization states results in asymmetric spinor interactions. Such spinor interactions offer a wide range of effects and a very rich physics to explore in semiconductor microcavities [13][14][15][16][17][18] .In this work, we demonstrate a Feshbach resonance in a polariton semiconductor microcavity. Feshbach biexcitonic resonant scattering is investigated through spectrally resolved circularly polarized pump-probe spectroscopy on a III-V based microcavity (Methods). To bring the energy of a two-lower polariton state into resonance with the biexciton state we change the cavity exciton detuning (Fig. 1a,c). We evidence the resonant polariton scattering by probing the anti-parallel spin polariton interactions when scanning the two-polariton energy across the bound biexciton state. We clearly show the enhancement of polariton interactions and the change of their character from attractive to repulsive. Moreover, we observe a decrease of the polariton resonance amplitude when the lower polariton energy is in the vicinity of the biexciton energy. The results are modelled by numerical simulations based on a meanfield two-channel model that includes coupling between polaritons and biexcito...

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 nearcomplete valley polarization of electrons in MoSe 2 monolayer with a density 1.6 × 10 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 attractiveFermi-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 MoSe 2 , which paves the way for valleytronics applications. Hall effect [10,11] as well as a modification of the exciton spectrum [12,13]. Investigation of one of the most interesting features of this material system, namely the valley pseudospin degree of freedom [14,15], has been hampered by the difficulty in obtaining a high-degree of valley polarization of free electrons or holes [16]. While circularly polarized excitation ensures that the excitons are generated in a single valley [17][18][19], significant transfer of valley polarization from excitons to itinerant electrons or holes has not been observed.Here, we report a strong paramagnetic response of a two dimensional electron system (2DES) in a charge-tunable monolayer MoSe 2 sandwiched between two hexagonal boron-nitride (hBN) layers (Fig. 1A). Figure 1B shows the corresponding single-particle energy-band diagram when an external arXiv:1701.01964v1 [cond-mat.mes-hall]

Nanocavity lasers are commonly characterized by the spontaneous coupling coefficient β that represents the fraction of photons emitted into the lasing mode. While β is conventionally discussed in semiconductor lasers where the photon lifetime is much shorter than the carrier lifetime (class-B lasers), little is known about β in atomic lasers where the photon lifetime is much longer than the other lifetimes and only the photon degree of freedom exists (class-A lasers). We investigate the impact of the spontaneous coupling coefficient β on lasing properties in the class-A limit by extending the well-known Scully–Lamb master equation. We demonstrate that in the class-A limit all the photon statistics are uniquely characterized by β and that the laser phase transition-like analogy becomes transparent. In fact, β perfectly represents the “system size” in phase transition. Finally, we investigate the laser-phase transition analogy from the standpoint of a quantum dissipative system. Calculating a Liouvillian gap, we clarify the relation between β and the continuous phase symmetry breaking.

We report on spinor polariton interactions in GaAs based microcavities. This investigation is carried out by means of heterodyne polarized pump-probe spectroscopy. We show the dependence of the energy renormalization of the lower and upper polariton resonances with cavity detuning for different polariton densities. We use the exciton-photon based Gross-Pitaevskii equation to model the experiment for both lower and upper polariton modes. The theoretical results reproduce qualitatively the experimental observations revealing the magnitude and sign of the parallel and antiparallel spin interaction strength. We evidence the strong influence of the biexciton resonance on the antiparallel spin polariton energy shift and provide the exciton-biexciton coupling constant. We derive our results in the lower polariton basis using the Gross-Pitaevskii equation, from which we express analytically the spinor polariton interactions and identify the clear role of the biexciton resonance.

We determine experimentally the spinor interaction constants of lower polaritons α1 and α2 using a resonant pump-probe spectroscopy with a spectrally narrow pump pulse. Our experimental findings are analyzed with the Bogoliubov-type theory and a mean-field two channel model based on the lower polariton and biexciton basis. We find an enhancement of the attractive interaction and a dissipative non-linearity of lower polaritons with anti-parallel spins in the vicinity of the biexciton resonance when the energy of two lower polaritons approaches energetically that of the biexciton. These observations are consistent with the existence of a scattering resonance between lower polaritons and biexcitons (polaritonic Feshbach resonance).