The locking of the electron spin to the valley degree of freedom in transition metal dichalcogenide (TMD) monolayers has seen them emerge as a promising platform in valleytronics [1,2]. When embedded in optical microcavities the large oscillator strengths of excitonic transitions in TMDs allow the formation of polaritons which are part-light part-matter quasiparticles [3][4][5][6][7]. Here, we report that polaritons in MoSe2 show an efficient retention of the valley pseudospin contrasting them with excitons and trions in this material. We find that the degree of the valley pseudospin retention is dependent on the photon, exciton and trion fractions in the polariton states. This allows us to conclude that in the polaritonic regime, cavity-modified exciton relaxation inhibits loss of the valley pseudospin. The valley addressable exciton-trion-polaritons presented here offer robust valley polarised states with the potential for valleytronic devices based upon TMDs embedded in photonic structures and valley-dependent nonlinear polariton-polariton interactions.Single layers of transition metal dichalcogenides (TMDs) are two-dimensional (2D) direct band-gap semiconductors which exhibit pronounced exciton resonances with binding energies of around 0.5 eV [8][9][10]. The monolayer nature of TMDs gives rise to strongly confined excitons with Bohr radii of around 1 nm and large oscillator strengths evident from optical absorption as strong as 15% [8]. By embedding TMD monolayers in optical microcavities this huge oscillator strength has allowed the realisation of the strong light-matter coupling regime and the formation of part-light part-matter polariton eigenstates [3][4][5][6][7]. Polaritonic states inherit properties such as a strong nonlinear interaction and low effective mass from the constituent exciton and photon components. In other material systems, this has led to the experimental realisation of a wealth of rich nonlinear phenomena such as Bose-Einstein condensation [11], superfluid-like behavior [12] and optical spin switching [13]. The observation of exciton-polaritons in TMDs creates new opportunities in engineering the polariton-polariton interaction [14,15] in 2D materials. Moreover, TMD based polaritons are expected to inherit additional degrees of freedom of valley pseudospin and finite Berry curvature from their constituent excitons and trions which can be utilised in new valley-polaritonic implementations [16].In this article we report on the valley addressability of polaritons in MoSe 2 monolayers embedded in tunable microcavities. We report clear valley polarisation of both exciton-and trion-polaritons which show a strong dependence of the polarisation degree on the cavity detuning. In the bare flake fast exciton depolarisation occurs due to the Maialle-Silva-Sham (MSS) mechanism [17][18][19]. We demonstrate that in the strong coupling regime this depolarisation mechanism can be overcome and robust valleypolarised polariton states with much higher polarisation degrees can be achieved. We support this...

Exciton-polaritons are quasiparticles with mixed photon and exciton character that demonstrate rich quantum phenomena, novel optoelectronic devices and the potential to modify chemical properties of materials. Organic semiconductors are of current interest for their room-temperature polariton formation. However, within organic optoelectronic devices, it is often the 'dark' spin-1 triplet excitons that dominate operation. These triplets have been largely ignored in treatments of polariton physics. Here we demonstrate polariton population from the triplet manifold via triplettriplet annihilation, leading to polariton emission that is longer-lived (>μs) even than exciton emission in bare films. This enhancement arises from spin-2 triplet-pair states, formed by singlet fission or triplet-triplet annihilation, feeding the polariton. This is possible due to state mixing, which -in the strong coupling regime-leads to sharing of photonic character with states that are formally non-emissive. Such 'photonic sharing' offers the enticing possibility of harvesting or manipulating even states that are formally dark.

Two-dimensional transition metal dichalcogenides (TMDs) provide a unique possibility to generate and read-out excitonic valley coherence using linearly polarized light, opening the way to valley information transfer between distant systems. However, these excitons have short lifetimes (ps) and efficiently lose their valley coherence via the electron-hole exchange interaction. Here, we show that control of these processes can be gained by embedding a monolayer of WSe2 in an optical microcavity, forming part-light-part-matter exciton-polaritons. We demonstrate optical initialization of valley coherent polariton populations, exhibiting luminescence with a linear polarization degree up to 3 times higher than displayed by bare excitons. We utilize an external magnetic field alongside selective exciton-cavity-mode detuning to control the polariton valley pseudospin vector rotation, which reaches 45° at B = 8 T. This work provides unique insight into the decoherence mechanisms in TMDs and demonstrates the potential for engineering the valley pseudospin dynamics in monolayer semiconductors embedded in photonic structures.

Monolayer transition metal dichalcogenides (TMDs) hold great promise for future information processing applications utilizing a combination of electron spin and valley pseudospin. This unique spin system has led to observation of the valley Zeeman effect in neutral and charged excitonic resonances under applied magnetic fields. However, reported values of the trion valley Zeeman splitting remain highly inconsistent across studies. Here, we utilize high quality hBN encapsulated monolayer WSe 2 to enable simultaneous measurement of both intervalley and intravalley trion photoluminescence. We find the valley Zeeman splitting of each trion state to be describable only by a combination of three distinct g-factors, one arising from the exciton-like valley Zeeman effect, the other two, trion specific, g-factors associated with recoil of the excess electron. This complex picture goes significantly beyond the valley Zeeman effect reported for neutral excitons, and eliminates the ambiguity surrounding the magneto-optical response of trions in tungsten based TMD monolayers.