Title: Intersubband polarons in oxides,
Abstract: Intersubband (ISB) polarons result from the interaction of an ISB transition
and the longitudinal optical (LO) phonons in a semiconductor quantum well (QW).
Their observation requires a very dense two dimensional electron gas (2DEG) in
the QW and a polar or highly ionic semiconductor. Here we show that in
ZnO/MgZnO QWs the strength of such a coupling can be as high as 1.5 times the
LO-phonon frequency due to the very dense 2DEG achieved and the large
difference between the static and high-frequency dielectric constants in ZnO.
The ISB polaron is observed optically in multiple QW structures with 2DEG
densities ranging from $5\times 10^{12}$ to $5\times 10^{13}$ cm$^{-2}$, where
an unprecedented regime is reached in which the frequency of the upper ISB
polaron branch is three times larger than that of the bare ISB transition. This
study opens new prospects to the exploitation of oxides in phenomena happening
in the ultrastrong coupling regime.