Exciton-polaritons in van der Waals heterostructures embedded in tunable microcavities. by Dufferwiel, S, Schwarz, S, Withers, F, Trichet, A, Li, F, Sich, M, Del Pozo-Zamudio, O, Clark, C, Nalitov, A, Solnyshkov, D, Malpuech, G, Novoselov, K, Smith, J, Skolnick, M, Krizhanovskii, D, Tartakovskii, A