Biomedical Engineering Reference
In-Depth Information
The examination of the optical intensity plots in Fig. 5.6 reveals that the TM [ 001 ]
mode indeed increases as the number of the QD layers is increased: SQD
QDM-
4. This is a direct consequence of the change in the biaxial strain component that
increases HH and LH intermixing as discussed earlier in Sect. 5.3.2 .
5.4.3
Increase in the TM [ 001 ] not Enough for Isotropic
Polarization
Figure 5.6 shows that the increase in the TM [ 001 ] mode only partially helps towards
an isotropic polarization response. This is in contrast to a general notion where
it is described that the increase in the TM [ 001 ] mode is mainly responsible for
the isotropic polarization. The reason for such understanding is that the previous
theoretical [ 8 ] or experimental studies [ 6 , 9 , 23 ] of the DOP have assumed only
one direction for the TE mode. However, the PL measurements shown in Fig. 5.1
of the [ 4 ] indicate that the TE modes along the [110] and [ 110] have significant
anisotropy in the plane of the QDM. Our theoretical model shows that in fact a major
contribution to achieve isotropic polarization response in these systems stems from
a suppressed TE [ 110 ] mode rather than an increased TM [ 001 ] mode. Figure 5.6 shows
that irrespective of QD geometry, the increase in the TM [ 001 ] mode is insufficient to
reverse the sign of the DOP
[
.
The TE mode is highly anisotropic in the plane of the QD with the magnitudes of
the TE [ 110 ]
110
]
modes becoming very different as the QDM size increases.
For the SQD system, TE [ 110 ]
and TE
110
[
]
and TM [ 001 ] is very weak. Hence the
measured and calculated DOP is highly anisotropic (close to 1.0), irrespective of the
direction for the TE mode. As the QDM size increases, the TM [ 001 ] mode increases
partially contributing to the reduction in the DOP. However, at the same time, the
TE [ 110 ] mode decreases drastically such that for the QDM-3 and QDM-4, it becomes
smaller than the TM [ 001 ] mode. This reverses the sign of the DOP [ 110 ] . The reason
for such a drastic decrease in the TE [ 110 ] mode is the orientation of the hole wave
functions along the [ 110] direction for the QDM-3 and QDM-4 systems.
TE
110
[
]
5.4.4
Hole Wave Function Confinements Reduces
the TE [ 110 ] Mode
Figure 5.7 plots the top views of the highest five valence band states H 1 , H 2 , H 3 ,
H 4 ,and H 5 for the QD systems SQD, QDM-3, and QDM-4. The five hole wave
functions for the SQD system have an almost uniform distribution inside the QD
region with nearly symmetric shape. Such symmetry will result in approximately
equal magnitude of the TE mode along the [110] and
110
directions, as evident in
Fig. 5.6 a. For the QDMs with six and nine QD layers, the distribution of the hole
[
]
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