Intersubband absorption at normal incidence by m-plane ZnO/MgZnO Quantum Wells

Intersubband absorption at normal incidence is forbidden by the selection rules and requires oblique incidence operation or texturing of the surface of intersubband-based devices such as focal plane arrays, adding additional processing steps to their fabrication and therefore increasing complexity and costs. Here we demonstrate normal-incidence, polarization sensitive intersubband absorption by wurtzite ZnO/MgZnO quantum wells grown on an m-plane orientation. When grown in this non-polar plane, the ZnO/MgZnO quantum wells spontaneously assemble forming a V-groove profile in the direction perpendicular to the c-axis, i.e. along the a-direction. A stack of quantum wells featuring this morphology acts as a metamaterial that allows for intersubband absorption at normal incidence whenever the electric field of the light is polarized in the direction perpendicular to the c axis. This phenomenon occurs because when the electric field is perpendicular to the c-axis it is no longer contained in the plane of the quantum wells therefore allowing for a small intersubband absorption. On the contrary, if the electric field is parallel to the c-axis, the usual normal-incidence conditions are recovered and no absorption is observed.