Biomedical Engineering Reference
In-Depth Information
10
10
−
where the Rabi frequency in the regime where,
δ
V
F
,isgivenby
10
10
2
2
Ω
=
(
δ
+
V
F
)
+
8
Ω
,
(10.18)
R
with the width of the gap at the anticrossing in Fig.
10.7
bgivenby2
√
2
.
Integration of the time-dependent probability density gives the average exciton
population,
p
1
10
=
Ω
2
R
, which has a maximum at
10
10
V
F
. We readily
notice that
p
1
10
increases monotonically with laser power for different values of
the detuning. The dependence of
p
1
10
on the laser detuning and excitation power
suggests a high degree of tunability of the FRET optical signatures via optical
parameters only. In this sense, our results suggest that the identification of FRET
signatures in QDMs, using monoexciton PL experiments, is attainable for realistic
parameters and experimental conditions.
2
Ω
/
Ω
δ
=
−
10.9
FRET in Biexciton Optical Signatures
Increasing the QDM laser excitation power results in the possibility of pumping
additional exciton states outside the direct exciton manifold [
33
]. On the one hand,
excited states of single electron-hole pairs exist a few meV above the lowest
transition energy and correspond to excited states of holes or electrons [
25
,
34
].
On the other hand, longitudinal optical (LO) phonons might be resonant at
35 meV
above the lowest transition; for the parameters considered in the model, and for a
sufficiently narrow laser line-width and narrow exciton transitions due to cryogenic
temperatures,
T
∼
4 K, render those processes negligible under suitable experimental
conditions [
35
,
36
]. More probable situations arise when considering relaxation
and exciton dephasing due to longitudinal acoustic (LA) phonons and pumping
of charged excitons and biexcitons [
32
,
37
,
38
]. As we would prove in Chap. 4
of this dissertation, indirect exciton states would result to be more prone to LA
phonon exciton dephasing in QDMs, rather than their direct counterparts, which
are mostly involved in FRET processes. Charged excitons, on the other hand, can
be sufficiently suppressed by controlling carrier capture from the back contacts of
the Schottky structure. However, biexciton transitions in QDMs,
2
20
X
,
0
02
X
and
1
11
X
,
are more favorable due to the existence of more intermediate transition cascades
leading to their formation. Direct transitions such as
≤
0
00
X
2
20
X
0
02
X
are usually
forbidden by selection rules. The most probable pathways for pumping biexciton in
single QDs involves the transitions
→
(
)
00
00
X
10
10
X
20
20
X
→
→
00
00
X
01
01
X
02
02
X
→
→
00
00
X
01
01
X
01
01
X
11
11
X
→
(
)
→
,
(10.19)
