The authors report the growth of ZnO/ZnMgO multiple quantum well (MQW) structures by plasmaassisted molecular beam epitaxy. A set of three MQW samples with different well thicknesses were grown on c-plane sapphire substrates. Structural and optical properties of the samples were characterized by reflection high-energy electron diffraction, high-resolution x-ray diffraction (XRD) and photoluminescence measurements. Clear superlattice satellite peaks and thickness fringes observed in XRD measurements indicate the formation of periodic structure with good interfacial quality and high crystalline quality. Mid infrared absorptions around 3 pm are observed from Fourier transform infrared spectroscopy measurement The polarization-dependent absorption proves that the absorptions are originated from intersubband transitions. (C) 2015 Elsevier B.V. All rights reserve