Intersubband polarons in oxides

TL;DR

This paper reports the observation of intersubband polarons in ZnO/MgZnO quantum wells, demonstrating ultrastrong coupling between ISB transitions and LO phonons at high electron densities, opening new avenues in oxide-based ultrastrong coupling phenomena.

Contribution

It provides the first experimental evidence of intersubband polarons in oxides with ultrastrong coupling regimes, using dense 2DEGs in ZnO/MgZnO quantum wells.

Findings

Coupling strength can reach 1.5 times the LO-phonon frequency.

ISB polaron frequency exceeds three times the bare ISB transition.

Observation of intersubband polarons at high 2DEG densities.

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.