We have developed a method to grow and characterize the state of the art non-polar ZnO/(Zn,Mg)O multi-quantum wells on m-plane ZnO substrates as a prerequisite for applications based on intersubband transitions. The epilayer interfaces exhibit a low roughness, and the layer thickness remains constant within one monolayer in these heterostructures. The optical properties have been studied in the UV and IR domains by means of photoluminescence and absorption experiments, respectively. In the UV, the photoluminescence is very well described by an excitonic transition, with the clear effect of quantum confinement as a function of the well thickness in the absence of the internal field. In the IR, the intersubband transitions can be precisely modeled if a large depolarization shift is taken into account. Overall, we demonstrate a very good control in the design and fabrication of ZnO quantum wells (QWs) for intersubband transitions. Our result gives a clear understanding of the ISBTs in ZnO QWs.

Intersubband (ISB) transitions are of high significance for light-emitting and light-detecting devices in the infrared and, when involving large electron densities, for plasmonics and strong light-matter coupling physics. Here it is observed that the simultaneously occurring fundamental and excited-state ISB transitions in highly-doped, m-plane ZnO/Mg x Zn 1−x O multiple quantum wells, couple into a single collective oscillation: the multisubband plasmon (MSP). With 2D electron densities up to 4 × 10 13 cm −2 , an outstanding regime is reached in which the observed MSP frequency is three times larger than that of the fundamental ISB transition as a result of depolarization. This effect is analyzed using a dielectric tensor for ZnO including the interaction of the light with the lattice, the in-plane free electrons, and the off-plane MSP. The impact of the broadening of the MSP and its interaction with phonons is discussed. The results presented here show the potential of ZnO/Mg x Zn 1−x O for infrared optoelectronic applications, which can be extended to the THz range with appropriate design of the quantum wells.

Unexpected light propagation effects, such as negative refraction, have been reported in artificial media. Leveraging on the intersubband resonances in heterostructured semiconductors, we show that all possible optical regimes, ranging from classical dieletric and metal to hyperbolic metamaterial types 1 and 2, can be achieved. As a demonstration, we prove that the negative refraction effect can occur at a designed frequency by controlling the electronic quantum confinement.