Biomedical Engineering Reference
In-Depth Information
Fig. 11.8
Power dependence of
F
o
measured using the
1
1
1
X
+
state as the probe. When exciting
above the WL peak PL energy there is a rapid saturation of
F
o
at around 1.4 kV/cm. This saturation
is found to occur much slower with power and at an apparently lower field for excitation below the
WL energy
field-dependent PL of the interdot exciton. If the effect were larger, we would expect
a noticeable quadratic dependence on the interdot lines.
We attribute this effect to the applied field dependence of the tunneling and
trapping rates of the electron and hole. At higher applied electric field, charges
are less easily trapped and tunnel more easily out of the device, therefore reducing
the optically created field. Though the saturation point of
F
o
appears to be nearly the
same, it is possible that there is a dependence of the maximum
F
o
whichcouldbe
attributed to the field dependence of the potential well created at the GaAs/AlGaAs
interface. As the field is increased the number of states available would decrease
leading to a reduction of the maximum
F
o
(Fig.
11.9
).
11.6.4
Time Dependence
In this section we investigate the dynamics of
F
o
with time-resolved measurements.
As previously discussed, with laser excitation above the WL energy, we optically
generate an electric field within a device containing the QDMs and observe the field
strength through a shift in the interdot PL spectra. The excitation above the WL
energy was seen to generate a shift of up to 3.5 kV/cm in the electric field dependent
PL spectra of QDMs, whereas for excitation below the WL energy only a negligible
shift was observed [
16
]. By modulating the above WL excitation, oscillations of
the optically generated electric field can be created. Monitoring the PL energy of
the interdot excitonic emission at different time delays from the creation to the
