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Electrodynamically confined microscale lasers [Elektronische Ressource] / vorgelegt von Rachit Sharma

176 pages
Electrodynamically ConfinedMicroscale LasersDen Naturwissenschaftlichen Fakult¨aten derFriedrich-Alexander-Universit¨at Erlangen-Nu¨rnbergzurErlangung des Doktorgradesvorgelegt vonRachit Sharmaaus Bhilai, IndienMax-Planck-Institut fu¨r die Physik des LichtsErlangen, 2009Als Dissertation genehmigt von den NaturwissenschaftlichenFakult¨aten der Universit¨at Erlangen-Nu¨rnbergTag der mu¨ndlichen Pru¨fung: 28.07.2009Vorsitzenderder Promotionskommission: Prof. Dr. Eberhard B¨anschErstberichterstatter: Prof. Dr. Lijun WangZweitberichterstatter: Prof. Dr. Min XiaoiAcknowledgmentsThis thesis would have been impossible without the significant helps and con-tributions from my various co-workers in the research group of Professor Wang atthe Max Planck Institute for the Science of Light, Erlangen. Apart from being agreat help in the research work, they have also influenced my life and education ina very positive way during my stay in Germany. Therefore, I would like to take thisopportunity to express my deep sense of gratitude towards them.First and foremost, I would like to thank Prof. Lijun Wang for giving me theopportunity to pursue full-time graduate studies in his group, thus providing mewith a chance to realize my long cherished dream of making an original contributionto science. Prof. Wang has been an immense source of motivation, encouragementand knowledge to me.
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Electrodynamically Confined
Microscale Lasers
Den Naturwissenschaftlichen Fakult¨aten der
Friedrich-Alexander-Universit¨at Erlangen-Nu¨rnberg
zur
Erlangung des Doktorgrades
vorgelegt von
Rachit Sharma
aus Bhilai, Indien
Max-Planck-Institut fu¨r die Physik des Lichts
Erlangen, 2009Als Dissertation genehmigt von den Naturwissenschaftlichen
Fakult¨aten der Universit¨at Erlangen-Nu¨rnberg
Tag der mu¨ndlichen Pru¨fung: 28.07.2009
Vorsitzender
der Promotionskommission: Prof. Dr. Eberhard B¨ansch
Erstberichterstatter: Prof. Dr. Lijun Wang
Zweitberichterstatter: Prof. Dr. Min Xiaoi
Acknowledgments
This thesis would have been impossible without the significant helps and con-
tributions from my various co-workers in the research group of Professor Wang at
the Max Planck Institute for the Science of Light, Erlangen. Apart from being a
great help in the research work, they have also influenced my life and education in
a very positive way during my stay in Germany. Therefore, I would like to take this
opportunity to express my deep sense of gratitude towards them.
First and foremost, I would like to thank Prof. Lijun Wang for giving me the
opportunity to pursue full-time graduate studies in his group, thus providing me
with a chance to realize my long cherished dream of making an original contribution
to science. Prof. Wang has been an immense source of motivation, encouragement
and knowledge to me. He granted me with enormous freedom to pursue research in
my field of interest and, at the same time, provided the right kind of guidance to
make sure that my efforts were always in accordance with scientific rigor. Despite
hisbusyschedule,hewasalwaysaccessibleforanysortofacademicornon-academic
help that I needed during the course of this project.
I also extend sincere thanks to my labmates Jan Schaefer and Dr. Jessica Mon-
dia. The three of us spent countless but enjoyable hours in the lab together while
setting up experiments and acquiring data. I specially thank Dr. Mondia for teach-
ing me various important laboratory skills during the initial phases of my doctoral
work and also for proof reading this thesis. I also thank Dr. Harald Schwefel for
proof reading the theoretical background chapter.
Furthermore, I thank Dr. Zehuang Lu who, on numerous occasions, helped in
solving the technical problems in the lab. In addition, thanks to Dr. Stefan Malzer
for helping me with the SEM images. Thanks also to Dr. Quanzhong Zhao for
helping me at various instances with the femtosecond micromachining setup. I also
thankProf. GottfriedDoehlerforhisusefulandfar-reachingsuggestions. Moreover,
I thank Ben Sprenger for helping us obtain the ZnO tetrapod samples. I also thank
all the members of the Wang group for their efficient help, creative suggestions, en-
lighteningdiscussions,andpleasantsocialinteractions. Finally,Ispeciallythankmy
parents and sisters whose support and encouragement have served as an invaluable
motivation during my doctoral research.
Rachit Sharmaiiiii
Zusammenfassung
Die konstante Weiterentwicklung von modernen optoelektronischen und ver-
wandtenTechnologieninRichtungkleinererBauteilebringteinensteigendenBedarf
an miniaturisierten Lichtquellen mit sich. Mikrolaser sind in dieser Hinsicht beson-
ders viel versprechend wegen ihrer mikroskopischen Gr¨oße, geringen Laserschwelle
und schmalen Ausgangsbandbreite. Die vorliegende Arbeit konzentriert sich auf die
Entwicklung und Untersuchung von drei solcher Laserquellen, und zwar einem ZnO
Tetrapoden Laser, einem Glyzerin Mikrotr¨opfchen Laser und CdSe/ZnS Quanten-
punkt Laser. Eine elektrodynamische Falle vom Endkappen-Typ wird verwendet,
um die Laser-aktiven Mikroteilchen r¨aumlich zu beschr¨anken. Ein gu¨tegeschalteter
Nd:YAG Laser (10 Hz, 10 ns) wird zur optischen Anregung verwendet. Wir zeigen
die experimentelle Realisierbarkeit der elektrodynamischen Isolation und Mikropo-
sitionierung von ZnO-basierten Nanostrukturen, um ihre intrinsischen optischen
Eigenschaften unter atmosph¨arischen Bedingungen zu untersuchen. Mit Hilfe einer
Elektrospray-Technikwirdeineverdu¨nnteL¨osungvonZnOTetrapoden(inMethanol)
in die elektrodynamische Falle gespru¨ht. Anschließend werden die Fallenparameter
der verdampfenden Methanol-L¨osung angepasst, bis eine einzelne ZnO Tetrapode
r¨aumlich isoliert in der Falle zuru¨ckbleibt. Laseraktivit¨at im UV (ca. 390 nm)
2bei einem Schwellstrahlungsfluss von 10 mJ/cm wird von einzelnen und mehreren
gleichzeitig gefangenen ZnO Tetrapoden mit typischen Beinl¨angen von 15-25 m
beobachtet. Daru¨ber hinaus wird die pr¨azise Mikromanipulation von gefangenen
Tetrapoden u¨ber eine L¨ange von 100m gezeigt. Wir demonstrieren außerdem Ra-
manLaseraktivit¨atinMikrotr¨opfchenausreinemGlyzerinundpr¨asentierenLangzeit
-messungen des typischen Laser-Blinkverhaltens (an/aus). Single- und Multimoden
Raman-Laseraktivit¨at (bei ca. 630 nm) werden bei Tr¨opfchendurchmessern von
10.3 m und 44.7 m erreicht und gezeigt. Typische Laserschwellen zwischen 200-
2390 mJ/cm werden gemessen. Das Lasersignal tritt in zeitlich getrennten und
fast symmetrischen H¨aufungen auf, die mit zunehmender Verdampfungsrate des
Tr¨opfchens an Frequenz zunehmen und an Dauer abnehmen. Durch eine Varia-
tion von Glyzerin-Konzentration und Pumpleistung gelingt es uns zu demonstri-
eren, dass das Blinkverhalten durch Doppelresonanz im verdampfenden Tr¨opfchen
verursachtwirdunddassesdurchKontrollederVerdampfungsratemanipuliertwer-
den kann. Schließlich demonstrieren wir Single- und Multimoden Laseraktivit¨at
(bei ca. 640 nm) von CdSe/ZnS-Quantenpunkt-dotierten Mikrotr¨opfchen bei 9 m
2und 34 m Tr¨opfchendurchmessern und bei Laserschwellen von ca. 50 mJ/cm .
Spektrale Blauverschiebungen der Lasermoden von bis zu 2 nm und des spek-
tralenVerst¨arkungsbereichsderQuantenpunktevon3.2nmwerdenbeizunehmender
Pumpleistung beobachtet. Außerdem deuten unsere Ergebnisse darauf hin, dass
die zur Laseraktivit¨at minimal ben¨otigte Quantenpunktkonzentration mehr als zwei
Gr¨oßenordnungenunterderbisherangenommenentheoretischenGrenzeliegenkann.ivv
Abstract
As modern-day optoelectronics and related technologies are constantly moving
towards smaller dimensions, there is an increasing need to develop efficient minia-
ture light sources. Microcavity lasers are very promising in this respect due to their
microscale sizes, low lasing thresholds, and narrow output linewidths. This work
focuses on the development and study of three such lasers, namely, the ZnO tetra-
pod laser, the glycerol microdrop Raman laser, and the CdSe/ZnS quantum dot
microdrop laser. An “end-cap” type electrodynamic trap is usedto spatially confine
the lasing microparticles. A Q-switched Nd:YAG laser (10 Hz, τ ∼10 ns) is used
for optical excitation. We experimentally show the viability of electrodynamically
isolating and micropositioning ZnO-based nanostructures to investigate their intrin-
sic optical nature under atmospheric conditions. An electrospray technique is used
to spray a dilute solution of ZnO tetrapods (in methanol) into the electrodynamic
trap. Subsequent tuning of trapping parameters, as the methanol evaporates, leads
to the stable confinement of a single ZnO tetrapod in free space. UV lasing (around
2390 nm), with threshold fluence around 10 mJ/cm , is observed from single and
multiple trapped ZnO tetrapods with typical leg lengths of 15-25 m. Moreover,
precise translational micromanipulation of a trapped tetrapod is shown up to a
range of 100 m. We further demonstrate Raman lasing from a trapped pure glyc-
erol microdrop and present long-term measurements of the lasing blinking (on/off)
behavior. Single and multimode Raman lasing (around 630 nm) are achieved and
shown for glycerol drops of 10.3m and 44.7m in diameter, respectively. Typical
2threshold fluences are measured to be between 200-390 mJ/cm . Lasing is found to
occur in temporally separated and nearly symmetric bursts which increase in fre-
quencyanddecreaseindurationastheevaporationrateofthedropisincreased. By
using drops of different glycerol concentrations and by varying the pump fluence,
we conclusively demonstrate that the Raman lasing blinking is caused by double
resonances in the evaporating drop and that it can be manipulated by controlling
the drop’s evaporation rate. Finally, we demonstrate single and multimode lasing
(around 640 nm) from CdSe/ZnS doped microdrops, of diameters 9m and 34m,
2respectively, at threshold pump fluences of around 50 mJ/cm . Blue-shifts of up to
2 nm for the lasing modes and 3.2 nm for the quantum dot gain profile are observed
with increasing pump fluences. Moreover, our results indicate that the minimum
quantum dot concentration required for lasing can be more than two orders of mag-
nitude lower than the previously reported theoretical limit.viContents
Acknowledgments i
Zusammensfassung iii
Abstract v
List of Figures xi
List of Tables xix
1 Introduction 1
1.1 Towards Microscale Lasers . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Semiconductor Nanowires and Tetrapods . . . . . . . . . . . . 3
1.1.2 Microcavity Raman Lasers . . . . . . . . . . . . . . . . . . . . 5
1.1.3 Quantum Dot Microcavity Lasers . . . . . . . . . . . . . . . . 7
1.2 Motivation and Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 Outline of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2 Theoretical Background 13
2.1 Basics of Quadrupole Ion Trapping . . . . . . . . . . . . . . . . . . . 13
2.2 Lasing Mechanism in ZnO Tetrapods . . . . . . . . . . . . . . . . . . 19
2.3 Theory of Whispering Gallery Modes in Spherical Microcavities . . . 20
2.3.1 The Mie Theory. . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3.2 The Ray Model . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.4 Fundamentals of Raman Scattering . . . . . . . . . . . . . . . . . . . 28viii CONTENTS
2.5 Brief Theory of Colloidal Quantum Dots . . . . . . . . . . . . . . . . 30
3 Experimental Details 33
3.1 The Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2 Pump Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.3 The Electrospray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3.1 Brief Theory of Electrospray Ionization . . . . . . . . . . . . . 35
3.3.2 Electrospray for the Experiment . . . . . . . . . . . . . . . . . 39
3.4 The Endcap Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.5 The Imaging System . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.6 Spectral Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.6.1 Signal Collection Optics . . . . . . . . . . . . . . . . . . . . . 44
3.6.2 The Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . 45
4 The Electrodynamically Confined Single ZnO Tetrapod Laser 49
4.1 ZnO Tetrapods: Preparation and Structural Properties . . . . . . . . 49
4.2 Electrodynamic Trapping of a Single ZnO Tetrapod . . . . . . . . . . 51
4.3 Optical Investigations of Trapped ZnO Tetrapods . . . . . . . . . . . 54
4.3.1 Photoluminescence and Raman Spectra . . . . . . . . . . . . . 54
4.3.2 UV Lasing in a Single ZnO Tetrapod . . . . . . . . . . . . . . 56
4.3.3 UV Lasing in Multiple ZnO Tetrapods . . . . . . . . . . . . . 60
4.4 Micromanipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4.1 Translational Control . . . . . . . . . . . . . . . . . . . . . . . 63
4.4.2 Charge Determination . . . . . . . . . . . . . . . . . . . . . . 65
4.4.3 Towards Rotational Control . . . . . . . . . . . . . . . . . . . 68
4.5 Study of ZnO Tetrapods on a Glass Substrate . . . . . . . . . . . . . 70
4.5.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . 70
4.5.2 Lasing on Substrate vs Lasing in Trap . . . . . . . . . . . . . 72
4.5.3 Q Factor Estimation of Lasing Modes . . . . . . . . . . . . . . 75
4.5.4 Transverse Whispering Gallery Modes on the Tapered Legs . . 76
4.6 Summary of the Chapter . . . . . . . . . . . . . . . . . . . . . . . . . 81