Biomedical Engineering Reference
In-Depth Information
5.1
Introduction
Deployment of semiconductor quantum dots (QDs) in the active region of optical
devices offers unique electronic and optical properties which can be exploited to
design several optoelectronic technologies ranging from lasers [
1
] to semicon-
ductor optical amplifiers (SOAs) [
2
] or single photon sources [
3
], where they
have successfully overcome critical challenges such as extremely low threshold,
high speed response, or entangled photon emission, respectively. However, in
these applications, a critical design parameter is the polarization response of
QDs, typically characterized in terms of either degree of polarization [DOP
=
(TE
TM)] [
4
,
5
] or TM/TE ratio [
6
,
7
], where TE mode is measured
along a direction in the plane of the QD, and TM mode is measured along the
growth [001] direction for the GaAs(001) QDs. Engineering of QD nanostructures
to achieve isotropic polarization (DOP
−
TM)/(TE
+
0) is critical for the implementation of
several optoelectronic devices, for example semiconductor optical amplifier (SOA).
InAs QDs grown by the Stranski-Krastonov (SK) self-assembly growth process
typically exhibit very poor polarization response (DOP
∼
0.8) due to the large
compressive biaxial strain surrounding the flat shapes of the QDs. The strain-
induced splitting between the heavy hole (HH) and the light hole (LH) valence
bands leads to a dominant HH character in the few top most valence band states,
thus significantly suppressing the TM mode. Therefore, the previous studies of the
single InAs QDs have reported very high values of the DOP, typically larger than
0.8 [
4
,
6
,
8
,
9
].
The polarization response of InAs QDs is influenced by several parameters such
as crystal/atomic symmetry, QD shape and aspect ratio (AR
≥
height/base), and
composition profile. The at
o
mistic asymmetry of the underlying zincblende crystals
implies that the [110] and [110] directions are inequivalent. This lowers the overall
symmetry of a perfectly circular dome-shaped QD from C
∞
v
to C
2
v
. As a result,
TE mode in the plane of the QD does not remain symmetric and significant in-
plane anisotropy may be observed even for an ideal circular-base InAs QD [
10
].
The anisotropy of the in-plane TE mode can be described as:
=
(
TE
[
]
−
TE
[
110
]
)
110
Pol
||
=
(5.1)
(
TE
[
110
]
+
TE
[
]
)
110
TE
[
110
]
, so a single value of the DOP is not sufficient to charac-
terize the polarization response of the QD systems. This, in the past studies [
4
,
10
],
has led us to define a direction-dependent value of the DOP,
Since TE
]
=
[
110
DOP
[
n
]
=
(
TE
[
n
]
−
TM
[
001
]
)
(5.2)
(
TE
[
n
]
+
TM
[
001
]
)
where the direction, [
n
] = [110] or [110], associated with the DOP
[
n
]
is same as
the direction of the TE
[
n
]
-mode in the plane of the QD.
