Investigation of coherent acoustic phonons in terahertz quantum cascade laser structures using femtosecond pump-probe spectroscopy ‌ Bruchhausen, A, Lloyd-Hughes, J, Hettich, M, Gebs, R, Grossmann, M, Ristow, O, Bartels, A, Fischer, M, Beck, M, Scalari, G, Faist, J, Rudra, A, Gallo, P, Kapon, E, Dekorsy, T