On the physics of polariton interactions

An exciton-polariton is a quasi-particle that emerges from the strong coupling between an exciton and a photon. Recently, the studies of the exciton-polariton have been receiving a great deal of attention in terms of both fundamental physics and potential applications. The very small polariton effective mass and the interactions brought respectively by the photon and excitonic content of polaritons enable a wide range of interesting physical phenomena including the realization of Bose-Einstein condensate, superfluidity, and quantum vortices. In addition to the interest in the basic physics, several device applications of semiconductor microcavities such as polariton switching, bistability, and stochastic resonance have also been proposed. In these researches, the interactions between exciton-polaritons, which is a source of nolinearity, are central. In this thesis, we explore the various aspects of the polariton interactions in semiconductor microcavities. We employ nonlinear spectroscopies as experimental techniques and compare our experimental results with different theoretical models. Firstly, we study lower-lower (upper-upper) polariton self-interactions and lower-upper polariton cross interactions. The self- and crossinteractions are identified in four-wave mixing two-dimensional Fourier spectra, which are followed by theoretical analyses based on a third-order perturbation theory and on a nonperturbative simulation of Gross-Pitaevskii equations. Secondly, using pump-probe spectroscopy, we measure the spin dependent nature of exciton-polariton interactions, which is called spinor interaction. The two spin projections of exciton-polaritons give rise to a spin anisotropy of the polariton interactions. In particular, we show that the polariton interactions with anti-parallel spins presents a scattering resonance behavior via an exciton molecule (biexciton), which we call polaritonic Feshbach resonance. The measurements of the spinor polariton interactions are compared with numerical simulations based on spinor Gross-Pitaevskii equations including the exciton-biexciton coupling. Finally, we explore the decoherence effect induced by the interaction of polaritons. Focusing on the delay dependence of the experimental pump-probe spectra, we find that the excitation induced dephasing (EID) plays an important role in the dynamics of exciton-polaritons. The delay dependence of the pump-probe spectra clearly probes that the coherent and incoherent parts of excitons temporally behave in a different way. These experimental features can be well reproduced only with the excitonic Bloch equations (EBE) approach, which is a theoretical framework that can include the incoherent population of excitons. In the last part of this thesis, a future perspective of the research is discussed while showing preliminary experimental results of pump-probe spectroscopy with a spectrally narrowband pump pulse

Giant paramagnetism induced valley polarization of electrons in charge-tunable monolayer MoSe2

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

Dephasing effects on coherent exciton-polaritons and the breakdown of the strong coupling regime

International audienceUsing femtosecond pump-probe spectroscopy, we identify excitation-induced dephasing as a major mechanism responsible for the breakdown of the strong coupling between excitons and photons in a semiconductor microcavity. The effects of dephasing are observed on the transmitted probe-pulse spectrum as a density-dependent broadening of the exciton-polariton resonances and the emergence of a third resonance at high excitation density. A striking asymmetry in the energy shift between the upper and the lower polaritons is also evidenced. Using the excitonic Bloch equations, we quantify the respective contributions to the energy shift of many-body effects associated with Coulomb fermion exchange and photon assisted exchange processes and the contribution to collisional broadening

Lower-Upper Polariton Interactions Proved with Two-Dimensional Fourier Transform (2DFT) Spectroscopy in Semiconductor Microcavity

International audienceTwo-dimensional Fourier transform (2DFT) spectroscopy is a powerful tool commonly used for investigating anharmonic coupling between different molecular vibrational states [1]. We applied this technique to a GaAs based semiconductor microcavity for investigating interactions among lower (LP) and upper (UP) polaritons. The observed 1-quantum and 2-quatum 2DFT experimental spectra are presented in Fig. 1 (a, c). Both spectra show four peak groups that correspond to LP-LP and UP-UP (diagonal peaks) and LP-UP and UP-LP interactions (off-diagonal peaks). Inside each peak, a complex structure can be found

Lower and Upper Polariton Spinor Interactions in GaAs Microcavities

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