Highly Linearly Polarized Emission from Quantum Dash Excitons

Modelling and Experiment at the 3

Telecom Window

Paweł Mrowiński

, Sven Höfling

2,3

, Johannes P. Reithmaier

and Grzegorz Sęk

LOSN, Faculty of Fundamental Problems of Technology, Departement of Experimental Physics,

Wrocław University of Science and Technology, Wybrzeze Wyspianskiego 27, Wrocław, Poland

Technische Physik & W. C. Röntgen-Center for Complex Material Systems, Universität Würzburg, Germany

SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, KY16 9SS, St. Andrews, U.K.

Institute of Nanostructure Technologies and Analytics (INA), CINSaT, University of Kassel,

Heinrich-Plett-Str. 40, 34132 Kassel, Germany

Keywords: Photoluminescence, Spectroscopy, Exciton, InAs, Quantum Dash, Polarization Anisotropy, FDTD,

Telecommunication.

Abstract: This work is focused on controlling the polarization anisotropy of emission from single self-assembled

InAs/InGaAlAs quantum dashes grown by molecular beam epitaxy on InP substrate. We studied the degree

of linear polarization of excitonic emission for submicrometer mesa photonic structures of asymmetric in-

plane geometry. We present both experimental and numerical analysis performed at 1550 nm wavelength (3

telecommunication window for optical fibers), and we discussed the impact of anisotropy of the dielectric

confinement, which paves the way towards a single photon source characterized by a degree of linear

polarization exceeding 0.9.

1 INTRODUCTION

Single nanostructures like quantum dots or dashes

embedded in semiconductor matrix have been found

as promising candidates for quantum emitters such as

single photon sources (Michler et al., 2000; Fiore et

al., 2007; Dusanowski et al., 2014) or entangled

photon pairs (Akopian et al., 2006; Stevenson et al.,

2006). By modifying their external environment, one

can obtain enhancement or inhibition of spontaneous

emission rate due to Purcell effect (Purcell, 1946).

Advanced microstructures allow also for controlling

the polarization state of the emitted light when

transition dipole moment of excitons couples with the

optical far field modes (Munsch et al., 2012; Foster et

al., 2015). By measuring the polarization anisotropy

of emission, it is possible to analyse the coupling

selectivity in terms of polarization, and by using a

simulation tool based on Finite-Difference Time-

Domain (FDTD) one can design a unique

microstructure that enables to outcouple photons with

a well-defined polarization state (Konishi et al., 2011;

Mrowiński et al., 2016).

Figure 1: a) SEM image of uncapped InAs quantum dash

sample b) chemically etched mesa of submicrometer in-

plane size and 300 nm height c) photoluminescence spectra

of both planar and processed samples showing isolated

spectral lines for mesas due to limited number of optically

active nanostructures.

238

Mrowi

nski, P., Höﬂing, S., Reithmaier, J. and S˛ek, G.

Highly Linearly Polarized Emission from Quantum Dash Excitons - Modelling and Experiment at the 3rd Telecom Window.

DOI: 10.5220/0006635602380241

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 238-241

ISBN: 978-989-758-286-8

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

0.53

0.23

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

-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

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

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

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

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

. 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, 



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

Figure 3: a) Simple model used for FDTD simulations of

the optical field confinement inside the mesa structure for

the far field modes. b) Simulation results of the polarized

electric field intensity distributions using two representative

projections for asymmetric mesa structure of 400 x 200 nm

and 300 nm height.

polarization degree of freedom is taken into account

in the scalar product where the linearly polarized

H(V) dipoles couples to the parallel component of the

localized electric field.

We calculated the electric field intensity

distribution inside a mesa structure using linearly

polarized pulsed excitation normal to the sample

surface, which is schematically shown in Fig. 3a). A

spectral range is centred at 1550 nm. By using

simplified geometry of InAs rectangular mesa on the

InP substrate, we performed calculations for 400 x

200 nm

in-plane size and 300 nm height. In Fig. 3b)

we demonstrate Fourier transformed results of

polarized electric field distributions in a linear scale

for both the in-plane projections and cross-sections

along the longer mesa edge. We find that parallel

configuration exhibits a noticeable localization of the

field while cross-polarized case shows excluded field

from the inside. By comparing the squared absolute

amplitude values in the middle point of the mesa. We

obtain about three times higher value for co-polarized

component, which gives an optical DOLP defined as

















































 of 0.32. Such mesa

induced polarization anisotropy influences the

Figure 4: Optical DOLP calculated for mesa structures of

high in-plane asymmetry and height of a) 300 nm and b)

500 nm showing qualitatively different distributions and

maximum DOLP as high as 0.85.

emitter polarization and in the case of single QDash

with intrinsic DOLP of 0.25 it results in anisotropy of

SE rate on the level of 0.53 when the mesa is oriented

parallel to QDash axis, or -0.08 when it is

perpendicular to it. These results are in good

agreement with the experimentally obtained DOLP

for exciton emission for asymmetric mesa structures.

We can now use this modelling to find the limits

of tailoring the polarization anisotropy of emission at

1550 nm wavelength by using asymmetric mesa

structures of this kind. In Fig. 4 a) we demonstrate the

calculated cross-sectional distribution of optical

DOLP (mesa induced, i. e. for a symmetric quantum

dot with no intrinsic DOLP) for mesa structure of 700

x 100 nm

in-plane size and 300 nm height. The

distribution is rather uniform and optical DOLP is

more than 0.85 inside the mesa. By exploiting a

QDash as a quantum emitter embedded parallel in

such a mesa structure, it would be possible to obtain

anisotropy of SE rate above 0.9 due to additional

intrinsic anisotropy of the emitter. In Fig. 4 b) we

examined also mesa of 700 x 200 nm

and 500 nm

height and we find a non-trivial distribution of optical

DOLP showing the highest value of 0.9 at 50 nm

below the top edge and lower than 0.6 in a lower

region. Although high DOLP can be obtained in this

configuration, it is crucial to control the position of

QDash precisely along z-direction making fabrication

procedure more problematic.

4 CONCLUSIONS

We demonstrated both experimental and numerical

studies on developing highly linearly polarized

emission from a single InAs quantum dashes emitting

in the spectral range of the 3

telecommunication

window, i. e. at 1550 nm. We investigated anisotropic

submicrometer mesa structures and we found their

influence on a degree of linear polarization of exciton

emission of about 0.3 by using parallel or

perpendicular alignment with respect to the

elongation axis of a quantum dash. Next, we

presented simulations based on FDTD numerical

modelling of polarized electric field intensity

distributions inside a mesa structure, and thus we

found a quantitative agreement with the experiment

by calculating the polarization anisotropy of the

spontaneous emission rates. Such methodology

allowed proposing mesa structures of higher in-plane

asymmetries to find conditions for the degree of

linear polarization of more than 0.85 for a symmetric

quantum dot or above 0.9 for quantum dashes.

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

240

ACKNOWLEDGEMENTS

P. Mrowiński acknowledges financial support from

National Science Centre in Poland within Grant No.

2015/17/N/ST7/03858.

REFERENCES

Akopian, N. et al. (2006) ‘Entangled Photon Pairs from

Semiconductor Quantum Dots’, Physical Review

Letters, 96(13), p. 130501.

Dusanowski, Ł. et al. (2014) ‘Single photon emission at

1.55 μ m from charged and neutral exciton confined in

a single quantum dash’, Applied Physics Letters.

American Institute of Physics Inc., 105(2), p. 21909.

Fiore, A. et al. (2007) ‘Telecom-wavelength single-photon

sources for quantum communications’, Journal of

Physics: Condensed Matter, 19(22), pp. 225005–

225014.

Foster, A. P. et al. (2015) ‘Linearly Polarized Emission

from an Embedded Quantum Dot Using Nanowire

Morphology Control’, Nano Letters, p.

150216083859008.

Konishi, K. et al. (2011) ‘Circularly polarized light

emission from semiconductor planar chiral

nanostructures’, Physical Review Letters, 106(5), pp.

1–4.

‘Lumerical Solutions, Inc.’ (no date). Available at:

http://www.lumerical.com/tcad-

products/fdtd/%0A%0A.

Michler, P. et al. (2000) ‘A Quantum Dot Single-Photon

Turnstile Device’, Science, 290(5500), pp. 2282–2285.

Mrowiński, P. et al. (2016) ‘Tailoring the

photoluminescence polarization anisotropy of a single

InAs quantum dash by a post-growth modification of its

dielectric environment’, Journal of Applied Physics,

120(7), p. 74303.

Munsch, M. et al. (2012) ‘Linearly polarized, single-mode

spontaneous emission in a photonic nanowire’,

Physical Review Letters, 108(7), pp. 1–5.

Musiał, A., Kaczmarkiewicz, P., et al. (2012) ‘Carrier

trapping and luminescence polarization in quantum

dashes’, Physical Review B - Condensed Matter and

Materials Physics, 85(3), p. 35314.

Musiał, A., Podemski, P., et al. (2012) ‘Height-driven

linear polarization of the surface emission from

quantum dashes’, Semiconductor Science and

Technology, 27, p. 105022.

Purcell, E. M. (1946) ‘Spontaneous Emission Probabilities

at Radio Frequencies’, Phys. Rev., 69(11–12), p. 681.

Reithmaier, J. P., Eisenstein, G. and Forchel, A. (2007)

‘InAs/InP quantum-dash lasers and amplifiers’,

Proceedings of the IEEE, 95(9), pp. 1779–1790.

Sauerwald, A. et al. (2005) ‘Size control of InAs quantum

dashes’, Applied Physics Letters, 86(25), p. 253112.

Singh, R., Kumar, R. and Singh, V. (2017) ‘Optical

anisotropy and the direction of polarization of exciton

emissions in a semiconductor quantum dot : Effect of

heavy- and light-hole mixing’, Chinese Physics B,

26(8), p. 87303.

Stevenson, R. M. et al. (2006) ‘A semiconductor source of

triggered entangled photon pairs.’, Nature, 439(7073),

pp. 179–82.

Tonin, C. et al. (2012) ‘Polarization properties of excitonic

qubits in single self-assembled quantum dots’, Physical

Review B - Condensed Matter and Materials Physics,

85(15), pp. 1–10.

Highly Linearly Polarized Emission from Quantum Dash Excitons - Modelling and Experiment at the 3rd Telecom Window

241