Microcavity polaritons are composite half-light half-matter quasi-particles, which have recently been demonstrated to exhibit rich physical properties, such as non-equilibrium Bose-Einstein condensation, parametric scattering and superfluidity. At the same time, polaritons have some important advantages over photons for information processing applications, since their excitonic component leads to weaker diffraction and stronger inter-particle interactions, implying, respectively, tighter localization and lower powers for nonlinear functionality. Here we present the first experimental observations of bright polariton solitons in a strongly coupled semiconductor microcavity. The polariton solitons are shown to be non-diffracting high density wavepackets, that are strongly localised in real space with a corresponding broad spectrum in momentum space. Unlike solitons known in other matter-wave systems such as Bose condensed ultracold atomic gases, they are non-equilibrium and rely on a balance between losses and external pumping. Microcavity polariton solitons are excited on picosecond timescales, and thus have significant benefits for ultrafast switching and transfer of information over their light only counterparts, semiconductor cavity lasers (VCSELs), which have only nanosecond response time

We demonstrate that the tunable potential introduced by a surface acoustic wave on a homogeneous polariton condensate leads to fragmentation of the condensate into an array of wires which move with the acoustic velocity. Reduction of the spatial coherence of the condensate emission along the surface acoustic wave direction is attributed to the suppression of coupling between the spatially modulated condensates. Interparticle interactions observed at high polariton densities screen the acoustic potential, partially reversing its effect on spatial coherence.

Today, surface acoustic waves (SAWs) and bulk acoustic waves are already two of the very few phononic technologies of industrial relevance and can been found in a myriad of devices employing these nanoscale earthquakes on a chip. Acoustic radio frequency filters, for instance, are integral parts of wireless devices. SAWs in particular find applications in life sciences and microfluidics for sensing and mixing of tiny amounts of liquids. In addition to this continuously growing number of applications, SAWs are ideally suited to probe and control elementary excitations in condensed matter at the limit of single quantum excitations. Even collective excitations, classical or quantum are nowadays coherently interfaced by SAWs. This wide, highly diverse, interdisciplinary and continuously expanding spectrum literally unites advanced sensing and manipulation applications. Remarkably, SAW technology is inherently multiscale and spans from single atomic or nanoscopic units up even to the millimeter scale. The aim of this Roadmap is to present a snapshot of the present state of surface acoustic wave science and technology in 2019 and provide an opinion on the challenges and opportunities that the future holds from a group of renown experts, covering the interdisciplinary key areas, ranging from fundamental quantum effects to practical applications of acoustic devices in life science.

We report on the two-dimensional gap-soliton nature of exciton-polariton macroscopic coherent phases (PMCP) in a square lattice with a tunable amplitude. The resonantly excited PMCP forms close to the negative mass M point of the lattice band structure with energy within the lattice band gap and its wave function localized within a few lattice periods. The PMCPs are well described as gap solitons resulting from the interplay between repulsive polariton-polariton interactions and effective attractive forces due to the negative mass. The solitonic nature accounts for the reduction of the PMCP coherence length and optical excitation threshold with increasing lattice amplitude. DOI: 10.1103/PhysRevLett.111.146401 PACS numbers: 71.36.+c, 42.65.Yj, 63.20.kk, 73.21.Cd The periodic spatial modulation of a medium creates an artificial band structure with energy gaps and anomalous (i.e., negative) dispersion. In the presence of nonlinearity, spatially self-localized states may appear within the energy gaps as the result of the interplay between the anomalous dispersion and interparticle interactions. This takes place when the kinetic energy contribution [E K ¼ À 2 @ 2 =ð2m b 2 Þ] due to localization of particles with a negative mass Àm b within a radius compensates the repulsive interparticle interaction energy E I . These states, known as gap solitons (GSs), are metastable solutions of the Gross-Pitaevskii equation [1]. GSs have been explored in optical fibers [2], nonlinear photonic crystals [3-6], atomic Bose Einstein condensates (BECs) in optical lattices [7,8], and, very recently, also in the hybrid lightmatter polariton system [9]. Polaritons result from the strong coupling of photons and quantum well (QW) excitons in a semiconductor microcavity (MC). Being bosonic light-matter quasiparticles, they advantageously combine features from both species. Namely, the small mass arising from the photonic component allows them to form polariton macroscopic coherent phases (PMCPs) at low densities and high temperatures, while the interexcitonic interactions provide a nonlinearity several orders of magnitude stronger than in purely photonic systems [10]. While GSs in one-dimensional (1D) potentials have been extensively studied [2,3,5,[7][8][9], GSs in 2D lattices have so far only been reported for purely photonic systems [4,6]. GSs in 2D potentials are qualitatively different from their 1D counterparts, for example, opening the way to the realization of novel topological phases [6,11].In this Letter, we demonstrate the formation and manipulation of GSs of PMCPs in a 2D tunable lattice. The studies were carried out in PMCPs resonantly excited in a tunable square lattice created by surface acoustic waves (SAWs). While PMCPs in a homogeneous MC normally appear at the lowest energy state with zero in-plane momentum, PMCPs in a shallow (i.e., low amplitude) lattice have a GS character and are excited via the accumulation of particles at critical points of negative mass and energy above the ground state [12]. The PMCP forms clo...

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.