Biomedical Engineering Reference
In-Depth Information
11.6.2
Power Dependence
Power dependence curves show that, at a given laser energy, as the photon flux
(laser power) is increased the electric field shift also increases and ultimately
saturates. The saturation demonstrates a limit to the optically generated electric
field most likely due to the saturation of the trapping of the ionized carriers. At
a fixed excitation wavelength, just above the WL (
860.1 nm), this saturation
was measured to be around 1.4 kV/cm and occurred for a power of 0.5 W/cm
2
.
Reproducing the same measurement on eight different QDMs within the same
sample we found an average saturation of 1.12 kV/cm (
λ
e
=
λ
e
=
860.1 nm).
An interesting observation is that at a laser energy of
λ
e
=
860.1 nm (above the
WL), the field shift appears to saturate at a higher limit than at
892.0 nm (below
the WL). This could be interpreted as a large decrease in absorption and possibly
a lower trapping rate.
F
o
does still appear to saturate and it is possible that at even
higher power the maximum
F
o
would approach that seen above the WL. It is also
interesting to note that the saturation of
F
o
at 1.12 kV/cm for excitation at 860.1 nm
is below the maximum
F
o
measured for excitation above the GaAs band edge. It
is possible that either again at higher powers
F
o
would approach the maximum
measured above the GaAs band edge. It could also be that the rate at which carriers
leave the device region, and therefore no longer contribute to
F
o
, is such that the
number of traps filled in steady state is below the maximum. Unfortunately at higher
powers the spectra become dominated by background PL making unambiguous
measurements of the optically induced shifts impossible with the current samples. It
is possible that with larger apertures we may be able to observe uniform saturation
for all wavelengths and future experiments are in the works.
λ
e
=
11.6.3
Applied Field Dependence
Not surprisingly, the optically generated field also displays a dependence on the
externally applied field itself. We can measure this effect by using interdot lines
associated with different charge states. As mentioned earlier in the chapter, due to
the Coulomb interaction, different charge states are shifted in energy. This results in
the interdot versions of these lines appearing more prominently at different applied
fields.
In Fig.
11.8
we plot the power dependence of the shift of two such indirect
lines (
1
0
1
X
0
1
0
1
1
X
+
) which are observed at different applied electric field
values (48.0 and 31.7 kV/cm, respectively). With a given laser power and energy,
for example 0.5 W/cm
2
and
and 860.1 nm, we find that for an applied electric
field of
F
A
=
31.7 kV/cm we have an optically created electric field value of
F
o
=
1.3 kV/cm; while for a higher applied field of
F
A
=
48.0 kV/cm we find a
lower value of
F
o
=
0.74 kV/cm. This is a relatively small effect, only 1% of the
applied field, and therefore we find no measureable deviation from linearity for the
