Phonon-induced Transparency in Quantum Dot Molecules

Quantum dot molecules (QDMs) formed by vertically stacked quantum dots provide a rich test ground for the investigation of elementary semiconductor excitations in a zero dimensional system. A high level of control over the mutual interactions between charges, spins and photons has been achieved with the enormous tunability of electronic states in QDMs [1]. In this work, we investigate the interaction of phonons with the QDM electronic states and demonstrate that, contrary to their usual dephasing role, phonons can actually increase control over a quantum system. This novel employment of phonons arises from the formation of a resonance-enhanced polaron. It is revealed via a Fano effect that arises from the interference of two competing optical absorption pathways, which results in a transparency of the system. One pathway involves a discrete electronic excitation with charges localized in separate quantum dots. The other pathway is for a polaron state, a hybrid of an electronic excitation with an optical phonon that results in a continuum of energy states. The pathways are coupled by the tunneling of a single hole, resulting in a Fano interference in the absorption lineshape characterized by stark dips (transparency) and peaks in the absorption. We show that the phonon-induced transparency is highly controllable by electric field, excitation energy and power. \newline [1] M. Scheibner, et. al., \textit{Essential concepts in the optical properties of quantum dot molecules}, Solid State Comm. \textbf{149}, 1427-1435 (2009).