We report on the spin properties of bright polariton solitons supported by an external pump to compensate losses. We observe robust circularly polarized solitons when a circularly polarized pump is applied, a result attributed to phase synchronization between nondegenerate TE and TM polarized polariton modes at high momenta. For the case of a linearly polarized pump, either σ þ or σ − circularly polarized bright solitons can be switched on in a controlled way by a σ þ or σ − writing beam, respectively. This feature arises directly from the widely differing interaction strengths between co-and cross-circularly polarized polaritons. In the case of orthogonally linearly polarized pump and writing beams, the soliton emission on average is found to be unpolarized, suggesting strong spatial evolution of the soliton polarization. The observed results are in agreement with theory, which predicts stable circularly polarized solitons and unstable linearly polarized solitons.

Exciton-polaritons in semiconductor microcavities form a highly nonlinear platform to study a variety of effects interfacing optical, condensed matter, quantum and statistical physics. We show that the complex polariton patterns generated by picosecond pulses in microcavity wire waveguides can be understood as the Cherenkov radiation emitted by bright polariton solitons, which is enabled by the unique microcavity polariton dispersion, which has momentum intervals with positive and negative group velocities. Unlike in optical fibres and semiconductor waveguides, we observe that the microcavity wire Cherenkov radiation is predominantly emitted with negative group velocity and therefore propagates backwards relative to the propagation direction of the emitting soliton. We have developed a theory of the microcavity wire polariton solitons and of their Cherenkov radiation and conducted a series of experiments, where we have measured polariton-soliton pulse compression, pulse breaking and emission of the backward Cherenkov radiation.

We report propagating bound microcavity polariton soliton arrays consisting of multipeak structures either along (x) or perpendicular (y) to the direction of propagation. Soliton arrays of up to five solitons are observed, with the number of solitons controlled by the size and power of the triggering laser pulse. The breakup along the x direction occurs when the effective area of the trigger pulse exceeds the characteristic soliton size determined by polariton-polariton interactions. Narrowing of soliton emission in energymomentum space indicates phase locking between adjacent solitons, consistent with numerical modeling which predicts stable multihump soliton solutions. In the y direction, the breakup originates from inhomogeneity across the wave front in the transverse direction which develops into a stable array only in the solitonic regime via phase-dependent interactions of propagating fronts. [6,7]. In many aspects, solitons behave like artificial particles. They may repel or attract depending on their relative phase as was shown in optical fibers [8] and cold atom systems [9]. Multisoliton complexes can form when localized initial perturbations split into multiple peaks [10,11]. Furthermore, solitons can selforganize into stable patterns with an equilibrium spacing [12] or can scatter in a collision [13].Recently, polaritons, hybrid light-matter particles forming in the strong coupling regime in semiconductor microcavities [14,15], have been shown to exhibit many interesting nonlinear hydrodynamic phenomena such as superfluidity [16] and integer [17] and half vortices [18]. Microcavity polaritons are an open system far from equilibrium. Bright polariton solitons have been observed [19], which exist when an external pump fully compensates photonic losses and the decay of the excitonic coherence and are therefore termed dissipative. Polariton solitons can be manipulated on a picosecond time scale, which is promising for the development of miniature polaritonic circuits and logic gates [20]. Dark soliton trains in a 1D conservative microcavity system (no pump) were recently predicted theoretically [21], whereas dissipative polariton soliton patterns in microcavities remain unexplored.Semiconductor optical resonators are prone to growth defects and imperfections [22], and, in contrast to atomic condensates [11], the observation of multiple solitons in such systems is a challenging task, even in well-studied systems such as vertical-cavity surface-emitting lasers (VCSELs) [23,24]. In contrast to solitons in VCSELs, microcavity polariton solitons are excited at high momenta. Combined with large energy blueshifts due to the giant optical nonlinearity, this makes polariton solitons less sensitive to photonic disorder, enabling our observation of stable multisoliton patterns.Here we demonstrate the formation of dissipative polariton soliton patterns. The interplay between bistability of the external pump field, polariton-polariton scattering, and polariton negative effective mass along the propagation direction e...

We explore nonlinear transitions of polariton wavepackets, first, to a soliton and then to a standing wave polariton condensate in a multi-mode microwire system. At low polariton density we observe ballistic propagation of the multi-mode polariton wavepackets arising from the interference between different transverse modes. With increasing polariton density, the wavepackets transform into single mode bright solitons due to effects of both inter-modal and intra-modal polariton-polariton scattering. Further increase of the excitation density increases thermalisation speed leading to relaxation of the polariton density distribution in momentum space with the resultant formation of a non-equilibrium condensate manifested by a standing wave pattern across the whole sample.