Spin-orbit coupling is a fundamental mechanism that connects the spin of a charge carrier with its momentum 1 . Likewise, in the optical domain, a synthetic spin-orbit coupling is accessible, for instance, by engineering optical anisotropies in photonic materials 2 . Both, akin, yield the possibility to create devices directly harnessing spin-and polarization as information carriers 3 . Atomically thin layers of transition metal dichalcogenides provide a new material platform which promises intrinsic spin-valley Hall features both for free carriers, two-particle excitations (excitons), as well as for photons 4 . In such materials, the spin of an exciton is closely linked to the high-symmetry point in reciprocal space it emerges from (K and K' valleys) 5,6 . Here, we demonstrate, that spin, and hence valley selective propagation is accessible in an atomically thin layer of MoSe2, which is strongly coupled to a microcavity photon mode. We engineer a wire-like device, where we can clearly trace the flow, and the helicity of exciton-polaritons expanding along a channel. By exciting a coherent superposition of K and K' tagged polaritons, we observe valley selective expansion of the polariton cloud without neither any applied external magnetic fields nor coherent Rayleigh scattering. Unlike the valley Hall effect for TMDC excitons 7 , the observed optical valley Hall effect (OVHE) 8 strikingly occurs on a macroscopic scale, and clearly reveals the potential for applications in spin-valley locked photonic devices.Spin-valley locking is a striking feature of free charge carriers and excitons emerging in monolayers of transition metal dichalcogenides (TMDCs) 6,9 . It originates form the strong spin-orbit interaction, which arises from the heavy transition metals in TMDCs and the broken inversion symmetry of the crystal lattice. This leads to inverted spin orientations at opposite K points at the corners of the hexagonal Brillouin zone, for both conduction band electrons and valence band holes. As a result, the K and K' valleys can be selectively addressed by σ + and σcircular polarized light 10,11 , which is referred to as valley-polarization. Likewise, coherent superpositions of both valleys can be excited by linear polarized light, which is referred to as valley coherence. The outstanding control of the valley pseudospin has attracted great interest in exploiting this degree of freedom to encode and process information by manipulating free charge carriers 12 and excitons 7,13,14 , which has led to the emerging field of valleytronics 4 . However, exciton spin-valley applications are strongly limited by the depolarization mechanisms due to the strong Coulomb exchange interaction of electrons and holes, as well as by the limited exciton diffusion and propagation lengths.

We study the condensation of exciton-polaritons in a two-dimensional Lieb lattice of micropillars. We show selective polariton condensation into the flatbands formed by S and Px,y orbital modes of the micropillars under non-resonant laser excitation. The real space mode patterns of these condensates are accurately reproduced by the calculation of related Bloch modes of S-and Pflatbands. Our work emphasizes the potential of exciton-polariton lattices to emulate Hamiltonians of advanced potential landscapes. Furthermore, the obtained results provide a deeper inside into the physics of flatbands known mostly within the tight-binding limit.Dispersionless energy bands or flatbands (FBs) appear in a large variety of condensed matter systems and are linked to a wide range of topological many-body phenomena such as graphene edge modes [1], the fractional quantum Hall effect [2][3][4][5] and flat band ferromagnetism [6][7][8].There is a variety of two-dimensional lattices that support flat energy bands [9][10][11], with the so-called Lieb lattice being on of the most straightforward examples [12]. Lieb lattices have been studied extensively in recent years and flatband states have been observed in photonic [13][14][15] as well as cold atom systems [16]. Creating artificial lattices in order to emulate and simulate complex many-body systems with additional degrees of freedom has attracted considerable scientific interest [17][18][19]. Exciton-polariton gases in periodic lattice potential landscapes have emerged as a very promising solid state system to emulate many-body physics [20,21]. Polaritons are eigenstates resulting of strong coupling between a quantum well exciton and a photonic cavity mode. The excitonic fraction provides a strong nonlinearity while the photonic part results in a low effective mass, allowing the formation of driven-dissipative BoseEinstein condensation [22,23]. These so-called quantum fluids of light [24] can be placed in an artificial lattice potential landscape using a variety of well developed semiconductor etching techniques [9,25,26], thin metal films [27], surface acoustic waves [28], or optically imprinted lattices [29,30]. In this work we investigate the polariton photoluminescence (PL) emission in a two-dimensional Lieb lattice ( Fig. 1(a)). Due to destructive interference of next neighbor tunneling J, flatbands form. Fig. 1(b) shows a tightbinding calculation of the first Brillouin zone (BZ) band structure, with the flatband dispersion highlighted in red. High symmetry points of the BZ are found in the inset.The two-dimensional polaritonic Lieb lattice was fabricated using an electron beam lithography process * sebastian.klembt@physik.uni-wuerzburg.de and a consecutive reactive ion etching step on an AlAs λ/2-cavity with three stacks of four 13 nm wide GaAs quantum wells (QWs) placed in the antinode of the electric field, with a 32.5 (36) fold AlAs/Al 0.20 Ga 0.80 As top (bottom) distributed Bragg reflector (DBR) (Fig. 1(c,d)). The Rabi splitting of the sample is 9.5 meV. Only the top D...

Topologically locked for emission The output power from a laser system can be increased by forming an array of lasers; however, because the individual lasers are independent, the resultant output may not be coherent. Dikopoltsev et al . report on the realization of a topological vertical-cavity surface-emitting laser (VCSEL) array. The topological nature of the array-based laser emission was achieved through a combination of topological in-plane propagation of evanescent light linking the vertical cavity surface-emitting lasers of the array. The topological features of the array force injection locking, making all emitters (30 in this case) act as a single coherent laser. This development will be important for realizing large-scale coherent laser arrays. —ISO

Interacting bosonic particles in artificial lattices have proven to be a powerful tool for the investigation of exotic phases of matter as well as phenomena resulting from non-trivial topology. Exciton-polaritons, bosonic quasi-particles of light and matter, have shown to combine the on-chip benefits of optical systems with strong interactions, inherited form their matter character. Technologically significant semiconductor platforms, however, strictly require cryogenic temperatures for operability. In this paper, we demonstrate exciton-polariton lasing for topological defects emerging form the imprinted lattice structure at room temperature. We utilize a monomeric red fluorescent protein derived from DsRed of Discosoma sea anemones, hosting highly stable Frenkel excitons. Using a patterned mirror cavity, we tune the lattice potential landscape of a linear Su-Schrieffer-Heeger chain to design topological defects at domain boundaries and at the edge. In spectroscopic experiments, we unequivocally demonstrate polariton lasing from these topological defects. This progress promises to be a paradigm shift, paving the road to interacting Boson many-body physics at ambient conditions.

Topological concepts have been applied to a wide range of fields in order to successfully describe the emergence of robust edge modes that are unaffected by scattering or disorder. In photonics, indications of lasing from topologically protected modes with improved overall laser characteristics were observed. Here, we study exciton-polariton microcavity traps that are arranged in a one-dimensional Su-Schrieffer-Heeger lattice and form a topological defect mode from which we unequivocally observe highly coherent polariton lasing. Additionally, we confirm the excitonic contribution to the polariton lasing by applying an external magnetic field. These systematic experimental findings of robust lasing and high temporal coherence are meticulously reproduced by a combination of a generalized Gross-Pitaevskii model and a Lindblad master equation model. Thus, by using the comparatively simple SSH geometry, we are able to describe and control the exciton-polariton topological lasing, allowing for a deeper understanding of topological effects on microlasers.