Multisubband Plasmons in Doped ZnO Quantum Wells

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/MgxZn1-xO multiple quantum wells, couple into a single collective oscillation: the multisubband plasmon (MSP). With 2D electron densities up to 4 x 101(3) 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/MgxZn1-xO for infrared optoelectronic applications, which can be extended to the THz range with appropriate design of the quantum wells.