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Register a new userMacroscopic quantum self-trapping and Josephson oscillations of exciton-polaritons
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A textbook example of quantum mechanical effects is the coupling of two states through a tunnel barrier. In the case of macroscopic quantum states subject to interactions, the tunnel coupling gives rise to Josephson phenomena including Rabi oscillations, the a.c. and d.c. effects, or macroscopic self-trapping depending on whether tunnelling or interactions dominate. Non-linear Josephson physics, observed in superfluid helium and atomic condensates, has remained inaccessible in photonic systems due to the required effective photon-photon interactions. We report on the observation of non-linear Josephson oscillations of two coupled polariton condensates confined in a photonic molecule etched in a semiconductor microcavity. By varying both the distance between the micropillars forming the molecule and the condensate density in each micropillar, we control the ratio of coupling to interaction energy. At low densities we observe coherent oscillations of particles tunnelling between the two micropillars. At high densities, interactions quench the transfer of particles inducing the macroscopic self-trapping of the condensate in one of the micropillars. The finite lifetime of polaritons results in a dynamical transition from self-trapping to oscillations with pi phase. Our results open the way to the experimental study of highly non-linear regimes in photonic systems, such as chaos or symmetry-breaking bifurcations.
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Atomically-thin transition metal dichalcogenide crystals (TMDCs) hold great promise for future semiconductor optoelectronics due to their unique electronic and optical properties. In particular, electron-hole pairs (excitons) in TMDCs are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, the excitons can hybridise with cavity photons to form exciton polaritons (polaritons herein), which display both ultrafast velocities and strong interactions. The ability to manipulate and trap polaritons on a microchip is critical for future applications. Here, we create a potential landscape for room-temperature polaritons in monolayer WS$_2$, and demonstrate their free propagation and trapping. We show that the effect of dielectric disorder, which restricts the diffusion of WS$_2$ excitons and broadens their spectral resonance, is dramatically reduced in the strong exciton-photon coupling regime leading to motional narrowing. This enables the ballistic transport of WS$_2$ polaritons across tens of micrometers with an extended range of partial first-order coherence. Moreover, the dephasing of trapped polaritons is dramatically suppressed compared to both WS$_2$ excitons and free polaritons. Our results demonstrate the possibility of long-range transport and efficient trapping of TMDC polaritons in ambient conditions.
Atomically thin crystals of transition metal dichalcogenides are ideally suited to study the interplay of light-matter coupling, polarization and magnetic field effects. In this work, we investiagte the formation of exciton-polaritons in a MoSe2 monolayer, which is integrated in a fully-grown, monolithic microcavity. Due to the narrow linewidth of the polaritonic resonances, we are able to directly investigate the emerging valley Zeeman splitting of the hybrid light-matter resonances in the presence of a magnetic field. At a detuning of -54.5 meV (13.5 % matter constituent of the lower polariton branch), we find a Zeeman splitting of the lower polariton branch of 0.36 meV, which can be directly associated with an excitonic g factor of 3.94pm0.13. Remarkably, we find that a magnetic field of 6T is sufficient to induce a notable valley polarization of 15 % in our polariton system, which approaches 30% at 9T. Strikingly, this circular polarization degree of the polariton (ground) state exceeds the polarization of the exciton reservoir for equal magnetic field magnitudes by approximately 50%, as a consequence of enhanced relaxation of bosons in our monolayer-based system.
Observations of polariton condensation in semiconductor microcavities suggest that polaritons can be exploited as a novel type of laser with low input-power requirements. The low-excitation regime is approximately equivalent to thermal equilibrium, and a higher excitation results in more dominant nonequilibrium features. Although standard photon lasing has been experimentally observed in the high excitation regime, e-h pair binding can still remain even in the high-excitation regime theoretically. Therefore, the photoluminescence with a different photon lasing mechanism is predicted to be different from that with a standard photon lasing. In this paper, we report the temperature dependence of the change in photoluminescence with the excitation density. The second threshold behavior transited to the standard photon lasing is not measured at a low-temperature, high-excitation power regime. Our results suggest that there may still be an electron--hole pair at this regime to give a different photon lasing mechanism.
The electronic bandstructure of a solid is a collection of allowed bands separated by forbidden bands, revealing the geometric symmetry of the crystal structures. Comprehensive knowledge of the bandstructure with band parameters explains intrinsic physical, chemical and mechanical properties of the solid. Here we report the artificial polaritonic bandstructures of two-dimensional honeycomb lattices for microcavity exciton-polaritons using GaAs semiconductors in the wide-range detuning values, from cavity-photon-like (red-detuned) to exciton-like (blue-detuned) regimes. In order to understand the experimental bandstructures and their band parameters, such as gap energies, bandwidths, hopping integrals and density of states, we originally establish a polariton band theory within an augmented plane wave method with two-kind-bosons, cavity photons trapped at the lattice sites and freely moving excitons. In particular, this two-kind-band theory is absolutely essential to elucidate the exciton effect in the bandstructures of blue-detuned exciton-polaritons, where the flattened exciton-like dispersion appears at larger in-plane momentum values captured in our experimental access window. We reach an excellent agreement between theory and experiments in all detuning values.
We study driven-dissipative Bose-Einstein condensates in a two-mode Josephson system, such as a double-well potential, with asymmetrical pumping. We investigate nonlinear effects on the condensate populations and mode transitions. The generalized Gross-Pitaevskii equations are modified in order to treat pumping of only a single mode. We characterize the steady-state solutions in such a system as well as criteria for potential trapping of a condensate mode. There are many possible steady-states, with different density and/or phase profiles. Transitions between different condensate modes can be induced by varying the parameters of the junction or the initial conditions, or by applying external fields.
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