![](bg2.png)
In this work, we focus on quantum dashes
(QDashes) of the InAs/InGaAlAs material system
grown on InP substrate by molecular beam epitaxy.
The system consists of 200 nm barrier
In
0.53
Ga
0.23
Al
0.24
As layer (lattice matched to InP),
InAs 2-5 monolayers leading to nucleation of
QDashes, 100 nm the same barrier and 10 nm InP.
The QDashes are rather non-uniform nanostructures
of high surface density of ~10
10
cm
-1
and elongated
along [1-10] crystallographic direction reaching
length of more than 50 nm (Sauerwald et al., 2005;
Reithmaier, Eisenstein and Forchel, 2007) (Fig. 1a).
A spectral range of emission can be tuned from 1200
to 2000 nm by controlling the amount of nominally
deposited InAs material, thus enabling their use in the
active region of nanophotonic or optoelectronic
devices devoted to operate at telecommunication
windows of O-band (1310 50 nm) and C-band
(1550 15 nm).
By using chemically etched submicrometer
structures like asymmetric mesas shown in Fig 1b),
the number of investigated nanostructures is limited
to a few tens of optically active QDashes on 200 x
400 nm
2
area and 300 nm height. In Fig. 1 c) we
demonstrated results of photoluminescence
experiment performed on both ensemble and etched
mesa structure showing well isolated spectral features
for the mesa, which can be assigned to optical
transitions from excitonic complexes confined in the
QDash.
2 POLARIZATION ANISOTROPY
OF QUANTUM DASH IN
ASYMMETRIC MESA
In this section, we study the influence of mesa
structure geometry on the polarization anisotropy of
Figure 2: a) Polarization resolved emission of quantum dash
exciton for rectangular mesa of 400 x 200 nm
2
oriented
paralallel and b) perpendicular to the QDash axis showing
DOLP of 0.5 and -0.2, respectively. c) A size dependence
of DOLP measured for several excitons using fixed lateral
aspect ratio.
excitonic emission around 1550 nm spectral range.
All results can be quantified by a degree of linear
polarization (DOLP) defined as
,
where
is emission intensity from QDash
polarized along [1-10] ([110]) in-plane directions.
First, we examined the reference sample, which is
200 x 200 nm
2
and we obtained a non-zero
polarization anisotropy exciton line, which is
typically in a range from 0.2 to 0.3 (Musiał,
Podemski, et al., 2012; Mrowiński et al., 2016). Such
noticeable anisotropy seen for QDashes results from
the anisotropic quantum confinement due to the in-
plane elongated geometry, which leads to a mixing in
the valence-band and thus induces a difference in the
oscillator strengths for the linearly polarized emission
from mixed exciton bright states (Musiał,
Kaczmarkiewicz, et al., 2012; Tonin et al., 2012;
Singh, Kumar and Singh, 2017). Polarization
anisotropy originated from the electronic structure is
then an intrinsic contribution. Next, we performed
similar measurements for asymmetric mesa
structures, namely rectangular one of 400 x 200 nm
2
size, oriented both along the QDash elongation axis
[1-10] and perpendicular to it. Such configurations
results in DOLP of about 0.55 and -0.20, respectively
(Fig. 2a,b). This effect has also been examined in
function of the mesa size. In Fig. 2c) we present the
measured DOLP of exciton emission for several
mesas of fixed lateral aspect ratio of 2:1 oriented
along QDash main axis and for increasing its base
dimension up to 900 x 450 nm
2
. It shows that the mesa
influence on the DOLP decreases with the increasing
size approaching 0.26 for the largest one, which is
close to the value obtained for reference and in-plane
symmetric mesa.
3 FDTD SIMULATIONS
We perform simple modelling using commercial-
grade simulator (‘Lumerical Solutions, Inc.) based on
FDTD to explain the experimental results concerning
the polarization anisotropy. A spontaneous emission
(SE) rate of exciton can be described in a dipole-
approximation as
,
where
is local density of optical states,
is
transition dipole moment and
is electric field
amplitude for the far-field mode at the position of the
emitter. In the weak-coupling regime, the main
contribution to the anisotropy is expected for the
electric field, which is localized in the asymmetric
mesa structures depending on its polarization. The
Highly Linearly Polarized Emission from Quantum Dash Excitons - Modelling and Experiment at the 3rd Telecom Window
239