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...
Using an angle-resolved heterodyne four-wave-mixing technique, we probe the low momentum excitation spectrum of a coherent polariton gas. The experimental results are well captured by the Bogoliubov transformation which describes the transition from single particle excitations of a normal fluid to soundlike excitations of a superfluid. In a dense coherent polariton gas, we find all the characteristics of a Bogoliubov transformation, i.e., the positive and negative energy branch with respect to the polariton gas energy at rest, soundlike shapes for the excitations dispersion, intensity, and linewidth ratio between the two branches in agreement with the theory. The influence of the nonequilibrium character of the polariton gas is shown by a careful analysis of its dispersion.
We present the results of a detailed time resolved luminescence study carried out on a very high quality InGaAs quantum well sample where the contributions at the energy of the exciton and at the band edge can be clearly separated. We perform this experiment with a spectral resolution and a sensitivity of the set-up allowing to keep the observation of these two separate contributions over a broad range of times and densities. This allows us to directly evidence the exciton formation time, which depends on the density as expected from theory. We also evidence the dominant contribution of a minority of excitons to the luminescence signal, and the absence of thermodynamical equilibrium at low densities.PACS numbers: 71.35.Cc,71.35.Ee,73.21.Fg,78.47.+p,78.67.De Excitons in quantum wells form quite an appealing quasiparticle showing a large range of optical properties that have proven at the same time technologically useful, and physically interesting [1]. A large part of this interest is linked with the appearance of excitonic resonances in absorption up to room temperature. It is also well known, since the seminal work of Weisbuch et al [2], that free excitons appear to dominate the luminescence response of semiconductor quantum wells at low temperatures. Part of the origin of this effect lies in the breakdown of the translational symmetry which brings a very efficient recombination channel to free excitons in quantum wells [3,4].Interestingly, in the low density regime, time resolved luminescence (TR-PL) in quantum wells is observed to be always dominated by light coming at the exciton energy, even under non resonant excitation. The observations of this dominant contribution are so numerous that only a very partial list of references may be given here [5,6,7,8,9] (in order to be specific, we only consider here the case of quantum wells grown on GaAs substrates). A double question has then been debated for more than 10 years in the literature: first how do free electron hole pairs bind into excitons and second does indeed the luminescence at very short time proceed from bound excitons. A brief survey of the literature allows to find that experimentalists have reported formation times ranging from less than 10 ps up to about 1 ns [7,8,9, 10] and theoretical values range from 100 ps [11,12,13] to over 20 ns [14]. Clearly, the origin of this spreading in the reported values lies in the poor sensitivity of the experiments used in general to probe the exciton formation process, except for the case of the recent terahertz absorption experiments [10]. On the theoretical side, binding of an electron hole pair into an exciton requires, at low temperatures, the emission of an acoustic phonon, which brings long formation time due to the small coupling of acoustic phonons to excitons.The long formation time of excitons, together with the observation of luminescence at the exciton energy at the shortest times [7,15] led Kira et al [16] to introduce the idea that a free electron hole plasma, properly including Coulomb correla...
Non-linear interactions in coherent gases are not only at the origin of bright and dark solitons and superfluids; they also give rise to phenomena such as multistability, which hold great promise for the development of advanced photonic and spintronic devices. In particular, spinor multistability in strongly coupled semiconductor microcavities shows that the spin of hundreds of exciton-polaritons can be coherently controlled, opening the route to spin-optronic devices such as ultrafast spin memories, gates or even neuronal communication schemes. Here we demonstrate that switching between the stable spin states of a driven polariton gas can be controlled by ultrafast optical pulses. Although such a long-lived spin memory necessarily relies on strong and anisotropic spinor interactions within the coherent polariton gas, we also highlight the crucial role of non-linear losses and formation of a non-radiative particle reservoir for ultrafast spin switching.
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.
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