Coherently-driven exciton polaritons and directional cQED effects in the quantum dot-micropillar system [original]

Coherently-driven exciton polaritons and

directional cQED effects in the quantum

dot-micropillar system

vorgelegt von

Diplom-Ingenieur

Caspar Hopfmann

geb. in Leipzig

von der Fakult¨at II - Mathematik und Naturwissenschaften

der Technischen Universit¨at Berlin

zur Erlangung des akademischen Grades

Doktor der Naturwissenschaften

Dr.rer.nat.

genehmigte Dissertation

Promotionsausschuss:

Vorsitzender: Prof. Dr. Andreas Knorr

Gutachter: Prof. Dr. Stephan Reitzenstein

Gutachter: PD Dr. Jacek Kasprzak

Tag der wissenschaftlichen Aussprache: 13. Juni 2017

Berlin 2018

Abstract

Cavity quantum electrodynamic (cQED) systems have attracted extensive research

interest in the past decades as the main driving force of quantum optics. First

experimental demonstrations focused on the enhancement of spontaneous emission

and optical nonlinearities related to a single quantum emitter confined in a micro-

cavity. Major technological effort was needed to enter the strong coupling regime of

single emitter cQED to study for instance the non-harmonicity of resulting Jaynes-

Cummings ladder and to perform quantum non-demolition measurements at the

heart of quantum optics. Corresponding milestone experiments paved the way to-

wards the realization of a wide variety of non-classical light sources bringing the

concept of photonic quantum networks to a practical level.

cQED systems can be realized using atomic, superconducting and semiconductor

platforms. In view of future applications in the quantum information technology, the

solid state platform of microcavities with embedded quantum emitters is the most

attractive one as it promises low-cost mass production as well as interoperability

with the integrated electronic circuit technology of today. Therefore, quantum dot

(QD)-microcavity systems are very attractive for further investigations in this field.

Indeed, even though cQED effects have been studied for many years now, there are

still fundamental aspects remaining to be explored. One example is the exploration

of a strongly coupled cQED system under coherent optical excitation which is one

of the primary goals of this work.

The work presented in this thesis aims at a deeper understanding of cQED effects in

semiconductor based implementations of this central topic of quantum optics. It in-

cludes comprehensive spatially-resolved studies of the QD-micropillar system in the

both weak and strong coupling regime. For this purpose an advanced 90◦excitation

and detection scheme, suitable for efficient wavelength-independent driving of the

coupled exciton (X) and cavity mode (C) system, was implemented. This flexible and

powerful technique enables access to the three-dimensional emission characteristics

of QD-micropillars providing important new insight into cQED effects which is the

central aspect of the first part of this thesis. The performed studies show for instance

a pronounced anticorrelation (correlation) of the directional emission characteristics

down to the single emitter level in the weak (strong) coupling regime demonstrating

directly their fundamentally different nature. Related investigations of the lasing

regime present a straightforward characterization method to demonstrate lasing ac-

tion in high-βmicrolasers without the need for time consuming studies employing

photon statistics.

The second part of the thesis addresses another very important yet unexplored fun-

damental aspect of cQED. The main focus is on coherently-driven strongly-coupled

QD-microcavity systems. Of particular interest is the regime where the coherent

excitation dresses the X-C polariton at high excitation powers. This regime is found

at the crossover from a quantum (Jaynes Cummings) to a semi-classical (Mollow

Triplet-like) system and can be observed between the limiting cases of an anhar-

monic and a harmonic ladder solely by varying the excitation strength. An in-

dispensable condition for the first observation of this transitory regime is driving

the system through the matter (X) component, which distinguishes this work from

previous studies. Counterintuitively, significant cavity losses with respect to the cou-

pling strength are required to create the highly coherent state of the laser-dressed

polariton. Moreover, this work pioneers in resonance fluorescence (RF) studies of

strongly-coupled QD-microcavity systems and reveals that strong X-C coupling sup-

presses the RF substantially. Additionally, injection pulling of a single polariton is

observed for the first time in the QD-microcavity system, which links this classical

effect of injection locking, which is also found in macroscopic lasers, to the quantum

regime. Complementary studies on long range off-resonant X-C coupling, mutual

coupling of two QD Xs as well as temperature stability of the coherent coupling

regime deepen the fundamental understanding of the strong light-matter interaction

in semiconductor systems.

Kurzdarstellung

Kavit¨

atsquantenelektrodynamische (cQED) Systeme sind die treibende Kraft der

Quantenoptik und haben in den vergangenen Dekaden weitreichendes wissenschaft-

liches Interesse auf sich gezogen. Erste Experimente demonstrierten die verst¨

arkte

spontane Emission und optische Nichtlinearit¨

aten von einem einzelnen Quantene-

mitter, eingebettet in einer Mikrokavit¨

at. Großer technologischer Aufwand musste

betrieben werden, um das Regime der starken Kopplung eines einzelnen Emitters

zu erreichen und somit, unter anderem, die anharmone Jaynes-Cummings-Leiter zu

untersuchen und zerst¨

orungsfreie Quantenmessungen durchzuf¨

uhren, die den Kern

der Quantenoptik ausmachen. Dazugeh¨

orige experimentelle Meilensteine haben den

Weg f¨

ur die Realisierung einer großen Bandbreite von nichtklassischen Lichtquel-

len bereitet, die die Umsetzung von photonischen Quantennetzwerken erm¨

oglichen.

cQED Systeme k¨

onnen realisiert werden mithilfe von atomaren, supraleitenden oder

Halbleiter basierten Quantenemittern. Mit Blick auf zuk¨

unftige Quanteninformati-

onstechnologien sind Halbleitermikrokavit¨

aten mit eingeschlossenen Quantenemit-

tern eines der vielversprechensten Festk¨

orpersysteme, da diese kosteng¨

unstige Mas-

senfertigung als auch Kompatibilit¨

at zur derzeitigen Mikroelektronik versprechen.

Daher sind Quantenpunkt (QD)-Mikrokavit¨

aten sehr attraktive Kandidaten f¨

ur wei-

terf¨

uhrende Untersuchungen in diesem Teil der Quantenphysik. Auch wenn cQED

Effekte nun seit vielen Jahren untersucht werden, so gilt es doch noch einige funda-

mentale Aspekte zu beleuchten. Ein Beispiel ist die Untersuchung eines stark gekop-

pelten cQED Systems mithilfe koh¨

arenter Anregung, was auch eines der prim¨

aren

Ziele dieser Arbeit ist.

Der Inhalt dieser Arbeit zielt auf die Vertiefung des Verst¨

andnisses von cQED Effek-

ten in Halbleiter basierten Implementationen dieses zentralen Teils der Quantenoptik

ab. Sie beinhaltet umfassende ortsaufgel¨

oste Studien von QD-Mikros¨

aulenresonator-

en sowohl in dem Regime der schwachen als auch der starken Kopplung. Zu diesem

Zweck wurde ein einmaliges 90◦Anregungs- und Detektionsschema, welches sowohl

f¨

ur koh¨

arente als auch inkoh¨

arente Anregung von gekoppelten Exzitonen (X) und

iii

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