Biomedical Engineering Reference
In-Depth Information
10.1
Introduction
A prototypical quantum dot molecule (QDM) [
1
] consists of a stacked pair of single
quantum dots (QDs) coupled by charge tunneling (i.e., electron or hole tunnel-
ing) [
2
]. Excitons in QDMs can be coherently excited and controlled by pulsed
electromagnetic fields. At the same time, for self-assembled systems embedded in
Schottky structures, an external electric field applied along the growth direction
can be used to localize charges in either dot. The molecular excitonic states of a
QDM can be labeled with either exciton spin and charge isospin degrees of freedom.
The electrically controlled interdot coupling effectively rotates the system effective
isospin, giving rise to a quantum superposition (molecular states) of the individual
excitons [
3
,
4
]. This exciting behavior allows for the implementation and coherent
manipulation of excitons in QDMs. If such delicate control of the interdot coupling
is achieved, QDs and coupled QD arrays offer a robust and scalable architecture for
the next generation of optical sensing and quantum information devices.
A fundamental process that occurs in optically excited molecular systems
is F orster (fluorescence) resonant energy transfer (FRET). This process occurs
between a donor (D) excited state and an acceptor (A) state of a molecule
in the ground state. In the case of a QDM the fluorescence occurs upon the
recombination of an exciton on each single QD. A basic requirement for this process
is the spectral overlap between the donor emission and the acceptor absorption
spectra, respectively. Within a QDM, FRET effectively couples two single QDs
via long-range dipole-dipole interactions between the donor and acceptor excitonic
levels. Since FRET plays a fundamental role in the charge dynamics in molecular
interactions, key questions are whether FRET could be a relevant and controllable
coupling mechanism for QDs, and which types of optical signatures can be expected
in realistic self-assembled QDMs.
10.2
Forster Resonant Energy Transfer
Excitation energy transfer within a molecular complex is a directed transfer of
electrostatic energy in the absence of charge transfer. This process arises from
non-radiative multi-pole Coulomb interactions among the conformational states of
the molecule involving optically excited states. Furthermore, it produces excitation
delocalization within the whole complex, which might be interpreted as exciton
hopping between distant sites the molecule [
5
]. Excitation transfer kinetics can be
modeled as a coherent process, or as an open quantum systems problem within the
context of the equation of motion for the excitonic density matrix. Let us introduce
the notion of excitation energy transfer among two molecules
Q
1
and
Q
2
. Consider
each molecule as a two-level quantum system, with the levels being the ground
state and the first excited state of each molecule,
g
1
,
e
1
and
g
2
,
e
2
, respectively, see
Fig.
10.1
.
