Biomedical Engineering Reference
In-Depth Information
−
ξ
=
with the constraint
0. The transformed Hamiltonian has diagonal matrix
elements containing the detunings of the exciton transition energies with respect
to the laser excitation energy,
H
D
ξ
i
j
=
∑
i
δ
|
|
=
−
i
i
, with
δ
E
i
h
ω
, whereas the
i
i
couplings to the radiation field are striped of the
e
±
i
ω
t
terms.
10.6
Electrically Tunable Molecular Coupling
Level anticrossing spectroscopy (LACS) is an invaluable tool for probing the optical
signatures of quantum coupling in QD aggregates. It consists of performing cryo-
genic resonant or non-resonant photoluminescence spectroscopy while applying
external perturbations which shift the exciton levels of the system [
3
,
4
,
25
]. In this
manner, all the different possible resonances of connecting different transitions give
a direct measure of the electronic and molecular structure of the system. In single
QDs and QDMs, this is typically achieved by embedding them in an
n
i
Schottky
junction. In this way the QD system is subject to an external axial electric field con-
trolled by a bias voltage between the
n
−
contact and the Schottky gate, see Fig.
10.3
.
The strongest signature of molecular coupling arises from interdot tunneling
coupling [
3
,
4
,
26
]. This manifests as an anticrossing in the eigenvalue spectrum
and indicates the onset of carrier delocalization over the two QDs. Figure
10.4
shows the hallmark of this delocalization, a QDM under optical excitation generates
an exciton, or electron (blue) hole (red) pair; initially, emission would arise from
recombination of a single neutral direct exciton as in Fig.
10.4
a; as we vary the
electric field, the conduction band levels of both QDs become resonant via the Stark
shift, allowing electron tunneling, as shown in Fig.
10.4
b. In this regime, the QDM
−
Fig. 10.3
Level anticrossing spectroscopy schematics. (
Left panel
) Vertically stacked QD layers
are processed into Schottky photo diodes. Opaque shadow masks are patterned with apertures
that isolate single QDMs for optical probing. (
Right panel
) Full device schematics with realistic
dimensions used in experiments. An applied bias voltage provides an axial electric field
F
that
lifts the conduction and valence band edges. Upper inset by Krenner, H.J., et al.: Phys. Rev. Lett.
94
, 057402 (2005). (2011) American Physical Society. Main figure, courtesy of Prof. E.A. Stinaff,
Ohio University
