Biomedical Engineering Reference
In-Depth Information
We want to conclude this section with this remark that the recent advancements in
the growth techniques have allowed to precisely control the shape of QDs, leading
to the fabrication of strongly elongated QD-like nano-structures [
50
]. These offer
an enhanced exciton oscillator strength and allow the realization of single exciton-
single photon coupling to build the fundamental blocks for the solid state quantum
information [
19
]. Since our calculations indicate a strong dependence of DOP
on the elongation of QDs, so this parameter can be exploited to achieve tailored
polarization response for a desired operation.
5.6
Conclusions
In summary, this topic chapter presents a detailed analysis of the electronic
properties and polarization response of the multi-layer QD molecules (QDMs) by
performing a set of systematic multi-million atom tight binding calculations. Our
theoretical results follow the trends of the experimental measurements on the QDMs
containing three, six, and nine QD layers. We provide in-depth physical insight
of the underlying fundamental physics by analyzing the strain profiles, the band
edge diagrams, and the wave function plots as a function of the QDM size. The
polarization properties of the QDMs indicate that the value of the DOP decreases
with the size of the QDM, and therefore an isotropic polarization response can be
achieved by engineering the number of QD layers inside a QDM. Furthermore,
the large in-plane anisotropy (TE
110
TE
110
), consistent with the experimental
evidence, is explained in terms of hole wave fu
nc
tion confinements that occur inside
HH pockets at the QD interfaces along the [110] direction. We suggest that the
isotropic polarization response (DOP
110
∼
0) from the multi-layer QDMs is due
to two factors: (i) the
r
eduction of the TE
110
-mode due to the hole wave function
confining along the [110] direction and (ii) the increase in the TM
001
-mode due
to enhanced LH-HH intermixing. Our results presented in this chapter for various
geometry configurations serve as a guidance for the experimentalists to design future
QD-based optical devices. A flip in the sign of the DOP as the size of QDM increases
indicates significant potential to achieve polarization insensitive response from the
multi-layer QDM systems.
Acknowledgments
I am indebted to many colleagues with whom I have had the pleasure to work
with, and in particular I wish to gratefully acknowledge Gerhard Klimeck (Purdue University
USA), Takashi Kita (Kobe University Japan), Timothy B. Boykin (University of Alabama in
Huntsville USA), Eoin P. O'Reilly (Tyndall National Institute Ireland), Stefan Schulz (Tyndall
National Institute Ireland), and Shaikh S. Ahmed (Southern Illinois University USA). The use
of computational resources from the National Science Foundation (NSF) funded Network for
Computational Nanotechnology (NCN) through
https://nanohub.org
is acknowledged. The NEMO
3-D software package is developed by several researchers at Jet Propulsion Labs (JPL) and
Purdue University under supervision of Prof. Gerhard Klimeck whom work has been cited in the
corresponding references. The open source tools based on NEMO 3-D simulator are available at
