Biomedical Engineering Reference
In-Depth Information
11.6
Optically Generated Electric Field
The ability to modulate an applied electric field is necessary for many experiments
but can often be difficult to implement [
35
-
37
]. One possibility would be to
modulate the applied electric field by means of an optically created electric field.
Using photoionization of optically created excitons it is possible to rapidly generate
an electric field in the devices studied here. Moreover, using the interdot exciton
recombination it is possible to precisely characterize this field. In the following
sections we will use this tool to investigate the detailed dependence on energy,
power, applied field, and time of the photogenerated field in these QDM devices.
As discussed earlier, by embedding the QDMs in a Schottky diode structure, the
bound energy levels of an individual QD are easily tuned with respect to each other
by means of an applied electric field [
2
]. By monitoring the interdot exciton states,
we can directly observe the effect of the optically created electric field as shown
in Fig.
11.5
b. In contrast, for an intradot exciton, we observe only a small shift,
if any, due to the QCSE. For the samples used in these measurements the GaAs
barrier between the QDs within the molecule was 4 nm and the average interdot
Stark shift was 0.74 meV/kV/cm, up to 100 times larger than that of the intradot
exciton. As discussed in the previous section, this large enhancement comes from
the displacement of the electron and hole and is proportional to the distance between
the two dots.
Fig. 11.5
(
a
) Schematic representation of the device structure and the interdot (
blue dashed arrow
)
and intradot (
blue solid arrow
) exciton recombination. Once the laser energy is higher than that
of the WL (
red arrows
) the absorbed photons may create large numbers of e-h pairs which may
ionize. These ionized charges may then be trapped within the device resulting in an electric field
which opposes the applied field. (
b
) Example data showing the observed shift in the interdot PL
energy. If we measure the field at which the interdot PL is a fixed energy (
Δ
E
) away from the
intradot PL we find that this occurs at a different field (
F
) depending on the laser power. We
could conversely measure the change in PL energy at a fixed applied field
Δ
