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In this work fluorescence-based single molecule detection at low concetration is investigated, with an emphasis on the usage of active transport and waveguides.
Active transport allows to overcome the limits of diffusion-based systems in terms of the lowest detectable threshold of concentration.
The effect of flow in single molecule experiments is investigated and a theoretical model is derived for laminar flow.
Waveguides on the other hand promise compact detection schemes and show great potential for their possible integration into lab-on-a-chip applications. Their properties in single molecule experiments are analyzed with help of a method based on the reciprocity theorem of electromagnetic theory.
Wachstum und Charakterisierung von Quantenpunkt-Mikrotürmchen mit adiabatischer Modenanpassung
(2013)
Verschiedene Konzepte zur Realisierung einer geeigneten Umgebung für Licht-
Materie-Wechselwirkung konkurrieren um Anerkennung und eine ständige Optimierung
der Systemparameter findet statt. Das Konzept von Mikrotürmchen scheint
prädestiniert, da es viele anwendungsfreundliche Eigenschaften in sich vereint. Allerdings
stellt die drastische Abnahme des Q Faktors für kleiner werdende Durchmesser
d einen wesentlichen Limitierungsfaktor dieser Strukturen dar. Für viele Anwendungen
resultiert daraus ein Kompromiss aus hohem Q Faktor und kleinem
Modenvolumen der Strukturen, wodurch das volle Potential des Resonatorsystems
nicht ausgeschöpft werden kann. Ziel dieser Arbeit war es, die drastische Abnahme
des Q Faktors von Mikrotürmchen mit Durchmessern um 1μm aufzuheben und
dadurch Resonatoren mit d < 1μm für ausgeprägte Licht-Materie-Wechselwirkung
herzustellen.
Dazu wurde erstmalig beabsichtigt eine Modenanpassung in Mikrotürmchen vorgenommen.
Mittels Molekularstrahlepitaxie konnte eine Übergangsregion, bestehend
aus drei Segmenten, in diese Strukturen implementiert und so ein adiabatischer
Modenübergang zwischen der aktiven Mittelschicht und den Spiegelbereichen
vorgenommen werden. Der positive Einfluss dadurch ergab sich in einer signifikanten
Verbesserung des gemessenen Q Faktors für Durchmesser unter 1μm.
Für d = 0.85μm konnte ein Q Faktor von 14 400 bestimmt werden. Dies stellt damit
den höchsten je gemessenen Wert für Mikrotürmchen im Submikrometerbereich
dar. Dadurch wird ein Bereich mit Modenvolumina < 3 kubischen Wellenlängen erschlossen und ausgeprägte Wechselwirkungseffekte im Mikrotürmchensystem sind zu erwarten. Starke
Quantenpunkt-Licht-Kopplung konnte in diesen Strukturen nachgewiesen werden.
Die höchste Vakuum-Rabiaufspaltung betrug 85μeV und die Visibilität wurde zu
0.41 bestimmt. Im Zuge der weiteren Optimierung der Systemparameter für die starke
Kopplung wurde ein ex-situ Ausheilschritt auf die verwendete Quantenpunktsorte
angewendet. In magnetooptischen Untersuchungen konnte damit eine Verdopplung
der mittleren Oszillatorstärke auf einen Wert von 12 abgeschätzt werden.
Weiter konnte in adiabatischen Mikrotürmchen über einen großen Durchmesserbereich
von 2.25 bis 0.95μm eindeutiger Laserbetrieb des Quantenpunktensembles
nachgewiesen werden. Dabei konnte eine kontinuierliche Reduzierung der Laserschwelle
von über zwei Größenordnungen für kleiner werdende Durchmesser beobachtet
werden. Für Durchmesser < 1.6μm betrug der Beta-Faktor der Mikrolaser in
etwa 0.5. Sie zeigten damit beinahe schwellenloses Verhalten.
Zuletzt wurde der elektrische Betrieb von adiabatischen Mikrotürmchen gezeigt. Dafür
wurde eine dotierte Struktur mit adiabatischem Design hergestellt. Im Vergleich
zur undotierten Struktur fielen die gemessenen Q Faktoren in etwa um 5 000 geringer
aus. Die spektralen Eigenschaften sowohl des Resonators als auch einzelner
Quantenpunktlinien zeigten vernachlässigbare Abhängigkeit der Anregungsart (optisch
oder elektrisch) und zeugen von einem erfolgreichen Konzept zum elektrischen
Betrieb der Bauteile. Zeitaufgelöste Messungen erlaubten die Beobachtung von interessanten
Dynamiken der Rekombination von Ladungsträgern in den Proben. Als
Ursache dafür wurde ein hohes intrinsisches Feld, welches auf Grund des Designs
der Schichtstruktur entsteht, identifiziert. Weiter zeigte sich, dass sich das interne
Feld durch Anregungsart und extern angelegte Spannungen manipulieren lässt.
Die vorliegende Arbeit beschäftigt sich mit der Licht-Materie-Wechselwirkung in Quantenpunkt-Mikroresonatoren und deren vertikalen und lateralen Emissionseigenschaften. Quantenpunkte sind nanoskopische Strukturen, in denen die Beweglichkeit der Ladungsträger unterhalb der de-Broglie-Wellenlänge eingeschränkt ist, wodurch die elektronische Zustandsdichte diskrete Werte annimmt. Sie werden daher auch als künstliche Atome bezeichnet. Um die Emissionseigenschaften der Quantenpunkte zu modifizieren, werden sie im Rahmen dieser Arbeit als aktive Schicht in Mikrosäulenresonatoren eingebracht. Diese bestehen aus einer GaAs lambda-Kavität, die zwischen zwei Braggspiegeln aus alternierenden GaAs und AlAs Schichten eingefasst ist. Diese Resonatoren bieten sowohl eine vertikale Emission über Fabry-Perot Moden, als auch eine laterale Emission über Flustergaleriemoden. Die Licht-Materie-Wechselwirkung zwischen den Resonatormoden und lokalisierten Ladungsträgern in den Quantenpunkten, genannt Exzitonen, kann in zwei Regime unterteilt werden. Im Regime der starken Kopplung wird der spontane Emissionsprozess in einem Quantenpunkt reversibel und das emittierte Photon kann wieder durch den Quantenpunkt absorbiert werden. Die theoretische Beschreibung der Kopplung eines Exzitons an die Resonatormode erfolgt über das Jaynes-Cummings Modell und kann im Tavis-Cummings Modell auf mehrere Emitter erweitert werden. Ist die Dämpfung des Systems zu gross, so befindet man sich im Regime der schwachen Kopplung, in dem die Emissionsrate des Quantenpunkts durch den Purcell-Effekt erhöht werden kann. In diesem Regime können Mikrolaser mit hohen Einkopplungsraten der spontanen Emission in die Resonatormode und niedrigen Schwellpumpströmen realisiert werden. Zur Charakterisierung der Proben werden vor allem die Methoden der Mikro-Elektrolumineszenz und der Photonenkorrelationsmessungen eingesetzt.
This publication is dedicated to investigate strong light-matter coupling with excitons in 2D materials. This work starts with an introduction to the fundamentals of excitons in 2D materials, microcavities and strong coupling in chapter 2. The experimental methods used in this work are explained in detail in chapter 3. Chapter 4 covers basic investigations that help to select appropriate materials and cavities for the following experiments. In chapter 5, results on the formation of exciton-polaritons in various materials and cavity designs are presented. Chapter 6 covers studies on the spin-valley properties of exciton-polaritons including effects such as valley polarization, valley coherence and valley-dependent polariton propagation. Finally, the formation of hybrid-polaritons and their condensation are presented in chapter 7.
The focus of the work concerned the development of a series of MRI techniques that were specifically designed and optimized to obtain quantitative and spatially resolved information about characteristic parameters of the lung. Three image acquisition techniques were developed. Each of them allows to quantify a different parameter of relevant diagnostic interest for the lung, as further described below:
1) The blood volume fraction, which represents the amount of lung water in the intravascular compartment expressed as a fraction of the total lung water. This parameter is related to lung perfusion.
2) The magnetization relaxation time T\(_2\) und T*\(_2\)
, which represents the component of T\(_2\) associated with the diffusion of water molecules through the internal magnetic field gradients of the lung. Because the amplitude of these internal gradients is related to the alveolar size, T\(_2\) und T*\(_2\) can be used to obtain information about the microstructure of the lung.
3) The broadening of the NMR spectral line of the lung. This parameter depends on lung inflation and on the concentration of oxygen in the alveoli. For this reason, the spectral line broadening can be regarded as a fingerprint for lung inflation; furthermore, in combination with oxygen enhancement, it provides a measure for lung ventilation.
Optical antennas work similar to antennas for the radio-frequency regime and convert electromagnetic radiation into oscillating electrical currents. Charge density accumulations form at the antenna surface leading to strong and localized near-fields. Since most optical antennas have dimensions of a few hundred nanometers, their near-fields allow the focusing of electromagnetic fields to volumes much smaller than the diffraction limit, with intensities several orders of magnitude larger than achievable with classical diffractive and refractive optical elements. The task to maximize the emission of a quantum emitter, a point-like entity capable of reception and emission of single photons, is identical to the task to maximize the field intensity at the position of the quantum emitter. Therefore it is desirable to optimize the capabilities of focusing optical antennas.
Radio-frequency-antenna designs scaled to optical dimensions of several hundred nanometers show already a decent performance. However, optical frequencies lie near the plasma frequency of the metals used for optical antennas and the mass of electrons cannot be neglected anymore. This leads to new physical phenomena. Light can couple to charge density oscillations, yielding a so-called Plasmon. Effects emerge which have no equivalent in the very advanced field of radio-frequency-technology, e.g.~volume currents and shortened effective wavelengths. Additionally the conductivity is not infinite anymore, leading to thermal losses. Therefore, the question for the optimal geometry of a focusing optical antenna is not easy to answer. However, up to now there was no evidence that there exist better alternatives for optical antennas than down-scaled radio-frequency designs.
In this work the optimization of focusing optical antennas is based on an approach, which often proved successful for radio-frequency-antennas in complex applications (e.g.~broadband and isotropic reception): evolutionary algorithms. The first implementation introduced here allows a large freedom regarding particle shape and count, as it arranges cubic voxels on a planar, square grid. The geometries are encoded in a binary matrix, which works as a genome and enables the methods of mutation and crossing as mechanism of improvement. Antenna geometries optimized in this way surpass a comparable dipolar geometry by a factor of 2. Moreover, a new working principle can be deduced from the optimized antennas: a magnetic split-ring resonance can be coupled conductively to dipolar antennas, to form novel and more effective split-ring-antennas, as their currents add up constructively near the focal point.
In a next step, the evolutionary algorithm is adapted so that the binary matrices describe geometries with realistic fabrication constraints. In addition a 'printer driver' is developed which converts the binary matrices into commands for focused ion-beam milling in mono-crystalline gold flakes. It is shown by means of confocal two-photon photo-luminescence microscopy that antennas with differing efficiency can be fabricated reliably directly from the evolutionary algorithm. Besides, the concept of the split-ring antenna is further improved by adding this time two split-rings to the dipole-like resonance.
The best geometry from the second evolutionary algorithm inspires a fundamentally new formalism to determine the power transfer between an antenna and a point dipole, best termed 'three-dimensional mode-matching'. Therewith, for the first time intuitive design rules for the geometry of an focusing optical antenna can be deduced. The validity of the theory is proven analytically at the case of a point dipole in from of a metallic nano sphere.
The full problem of focusing light by means of an optical antenna can, thus, be reduced to two simultaneous mode-matching conditions -- on the one hand with the fields of a point dipole, on the other hand with a plane wave. Therefore, two types of ideal focusing optical antenna mode patterns are identified, being fundamentally different from the established dipolar antenna mode. This allows not only to explain the functionality of the evolutionary antennas and the split-ring antenna, but also helps to design novel plamonic cavity antennas, which lead to an enhanced focusing of light. This is proven numerically in direct comparison to a classical dipole antenna design.
Nano-antennas are an emerging concept for the manipulation and control of optical fields at the sub-wavelength scale. In analogy to their radio- and micro-wave counterparts they provide an efficient link between propagating and localized fields. Antennas operating at optical frequencies are typically on the order of a few hundred nanometer in size and are fabricated from noble metals. Upon excitation with an external field the electron gas inside the antenna can respond resonantly, if the dimensions of the antenna are chosen appropriate. Consequently, the resonance wavelength depends on the antenna dimensions. The electron-density oscillation is a hybrid state of electron and photon and is called a localized plasmon resonance. The oscillating currents within the antenna constitute a source for enhanced optical near-fields, which are strongly localized at the metal surface.
A particular interesting type of antennas are pairs of metal particles separated by a small insulating gap. For anti-symmetric gap modes charges of opposite sign reside across the gap. The dominating field-components are normal to the metal surface and due to the boundary conditions they are sizable only inside the gap. The attractive Coulomb interaction increases the surface-charge accumulation at the gap and enhanced optical fields occur within the insulating gap. The Coulomb interaction increases with decreasing gap size and extreme localization and strongest intensity enhancement is expected for small gap sizes.
In this thesis optical antennas with extremely small gaps, just slightly larger than inter-atomic distances, are investigated by means of optical and electrical excitation. In the case of electrical excitation electron tunneling across the antenna gap is exploited.
At the beginning of this thesis little was known about the optical properties of antennas with atomic scale gaps. Standard measurement techniques of field confinement and enhancement involving well-separated source, sample and detector are not applicable at atomic length-scales due to the interaction of the respective elements. Here, an elegant approach has been found. It is based on the fact that for closely-spaced metallic particles the energy splitting of a hybridized mode pair, consisting of symmetric and anti-symmetric mode, provides a direct measure for the Coulomb interaction over the gap. Gap antennas therefore possess an internal ruler which sensitively reports the size of the gap.
Upon self-assembly side-by-side aligned nanorods with gap sizes ranging from 2 to 0.5nm could be obtained. These antennas exhibit various symmetric and anti-symmetric modes in the visible range. In order to reveal optical modes of all symmetries a novel scattering setup has been developed and is successfully applied. Careful analysis of the optical spectra and comparison to numerical simulations suggests that extreme field confinement and localization can occur in gaps down to 0.5 nm. This is possibly the limit of plasmonic enhancement since for smaller gaps electron tunneling as well as non-locality of the dielectric function affect plasmonic resonances.
The strongly confined and intense optical fields provided by atomic-scale gaps are ideally suited for enhanced light-matter interaction. The interplay of intense optical-frequency fields and static electric fields or currents is of great interest for opto-electronic applications. In this thesis a concept has been developed, which allows for the electrical connection of optical antennas. By means of numerical simulations the concept was first verified for antennas with gap sizes on the order of 25 nm. It could be shown, that by attaching the leads at positions of a field minimum the resonant properties are nearly undisturbed. The resonance wavelengths shift only by a small amount with respect to isolated antennas and the numerically calculated near-field intensity enhancement is about 1000, which is just slightly lower than for an unconnected antenna.
The antennas are fabricated from single-crystalline gold and exhibit superior optical and electrical properties. In particular, the conductivity is a factor of 4 larger with respect to multi-crystalline material, the resistance of the gap is as large as 1 TOhm and electric fields of at least 10^8 V/m can be continuously applied without damage. Optical scattering spectra reveal well-pronounced and tunable antenna resonances, which demonstrates the concept of electrically-connected antennas also experimentally.
By combining atomic-scale gaps and electrically-connected optical antennas a novel sub-wavelength photon source has been realized. To this end an antenna featuring an atomic scale gap is electrically driven by quantum tunneling across the antenna gap. The optical frequency components of this fluctuating current are efficiently converted to photons by the antenna. Consequently, light generation and control are integrated into a planar single-material nano-structure. Tunneling junctions are realized by positioning gold nanoparticles into the antenna gap, using an atomic force microscope. The presence of a stable tunneling junction between antenna and particle is demonstrated by measuring its distinct current-voltage characteristic. A DC voltage is applied to the junction and photons are generated by inelastically tunneling electrons via the enhanced local density of photonic states provided by the antenna resonance. The polarization of the emitted light is found to be along the antenna axis and the directivity is given by the dipolar antenna mode. By comparing electroluminescence and scattering spectra of different antennas, it has been shown that the spectrum of the generated light is determined by the geometry of the antenna. Moreover, the light generation process is enhanced by two orders of magnitude with respect to a non-resonant structure.
The controlled fabrication of the presented single-crystalline structures has not only pushed the frontiers of nano-technology, but the extreme confinement and enhancement of optical fields as well as the light generation by tunneling electrons lays a groundwork for a variety of fundamental studies and applications.
Field localization down to the (sub-)nanometer scale is a prerequisite for optical spectroscopy with near-atomic resolution. Indeed, recently first pioneering experiments have achieved molecular resolution exploiting plasmon-enhanced Raman scattering. The small modal volume of antennas with atomic-scale gaps can lead to light-matter interaction in the strong coupling regime. Quantum electro-dynamical effects such as Rabi splitting or oscillations are likely when a single emitter is placed into resonant structures with atomic-scale gaps.
The concept of electrically-connected optical antennas is expected to be widely applied within the emerging field of electro-plasmonics. The sub-wavelength photon source developed during this thesis
will likely gain attention for future plasmonic nanocircuits. It is envisioned that in such a circuit the optical signal provided by the source is processed at ultrafast speed and nanometer-scales on the chip and is finally converted back into an electronic signal. An integrated optical transistor could be realized by means of photon-assisted tunneling. Moreover, it would be interesting to investigate, if it is possible to imprint the fermionic nature of electrons onto photons in order to realize an electrically-driven source of single photons. Non-classical light sources with the potential for on-chip integration could be built from electrically-connected antennas and are of great interest for quantum communication. To this end single emitters could be placed in the antenna gap or single electron tunneling could be achieved by means of a single-channel quantum point contact or the Coulomb-blockade effect.
An experimental setup for probing ultrafast dynamics at the diffraction limit was developed, characterized and demonstrated in the scope of the thesis, aiming for optical investigations while simultaneously approaching the physical limits on the length and timescale.
An overview of this experimental setup was given in Chapter 2, as well as the considerations that led to the selection of the individual components. Broadband laser pulses with a length of 9.3 fs, close to the transform limit of 7.6 fs, were focused in a NA = 1.4 immersion oil objective, to the diffraction limit of below 300 nm (FWHM).
The spatial focus shape was characterized with off-resonance gold nanorod scatterers scanned through the focal volume. For further insights into the functionality and limitations of the pulse shaper, its calibration procedure was reviewed. The deviations between designed and experimental pulse shapes were attributed to pulse-shaper artifacts, including voltage-dependent inter-layer as well as intra-layer LCD-pixel crosstalk, Fabry-Pérot-type reflections in the LCD layers, and space-time coupling. A pixel-dependent correction was experimentally carried out, which can be seen as an extension of the initial calibration to all possible voltage combinations of the two LCD layers.
The capabilities of the experimental setup were demonstrated in two types of experiments, targeting the nonlinearity of gold (Chapter 3) as well as two-dimensional spectroscopy at micro-structured surfaces (Chapter 4).
Investigating thin films, an upper bound for the absolute value for the imaginary part of the nonlinear refractive index of gold could be set to |n′′ 2 (Au)| < 0.6·10−16 m2/W, together with |n′ 2 (Au)| < 1.2·10−16 m2/W as an upper bound for the absolute value of the real part. Finite-difference time-domain simulations on y-shaped gold nanostructures indicated that a phase change of ∆Φ ≥ 0.07 rad between two plasmonic modes would induce a sufficient change in the spatial contrast of emission to the far-field to be visible in the experiment. As the latter could not be observed, this value of ∆Φ was determined as the upper bound for the experimentally induced phase change. An upper bound of 52 GW/cm2 was found for the damage threshold.
In Chapter 4, a novel method for nonlinear spectroscopy on surfaces was presented. Termed coherent two-dimensional fluorescence micro-spectroscopy, it is capable of exploring ultrafast dynamics in nanostructures and molecular systems at the diffraction limit. Two-dimensional spectra of spatially isolated hotspots in structured thin films of fluorinated zinc phthalocyanine (F16ZnPc) dye were taken with a 27-step phase-cycling scheme. Observed artifacts in the 2D maps were identified as a consequence from deviations between the desired and the experimental pulse shapes. The optimization procedures described in Chapter 2 successfully suppressed the deviations to a level where the separation from the nonlinear sample response was feasible.
The experimental setup and methods developed and presented in the scope of this thesis demonstrate its flexibility and capability to study microscopic systems on surfaces. The systems exemplarily shown are consisting of metal-organic dyes and metallic nanostructures, represent samples currently under research in the growing fields of organic semiconductors and plasmonics.
Laser spectroscopic gas sensing has been applied for decades for several applications
as atmospheric monitoring, industrial combustion gas analysis or fundamental research.
The availability of new laser sources in the mid-infrared opens the spectral fingerprint
range to the technology where multiple molecules possess their fundamental ro-vibrational
absorption features that allow very sensitive detection and accurate discrimination of
the species. The increasing maturity of quantum cascade lasers that cover this highly
interesting spectral range motivated this research to gain fundamental knowledge about
the spectra of hydrocarbon gases in pure composition and in complex mixtures as they
occur in the petro-chemical industry. The long-term target of developing accurate and fast
hydrocarbon gas analyzers, capable of real-time operation while enabling feedback-loops,
would lead to a paradigm change in this industry.
This thesis aims to contribute to a higher accuracy and more comprehensive understanding
of the sensing of hydrocarbon gas mixtures. This includes the acquisition of yet
unavailable high resolution and high accuracy reference spectra of the respective gases,
the investigation of their spectral behavior in mixtures due to collisional broadening of
their transitions and the verification of the feasibility to quantitatively discriminate the
spectra when several overlapping species are simultaneously measured in gas mixtures.
To achieve this knowledge a new laboratory environment was planned and built up to
allow for the supply of the individual gases and their arbitrary mixing. The main element
was the development of a broadly tunable external-cavity quantum cascade laser based
spectrometer to record the required spectra. This also included the development of a new
measurement method to obtain highly resolved and nearly gap-less spectral coverage as
well as a sophisticated signal post-processing that was crucial to achieve the high accuracy
of the measurements. The spectroscopic setup was used for a thorough investigation of
the spectra of the first seven alkanes as of their mixtures. Measurements were realized
that achieved a spectral resolution of 0.001 cm-1 in the range of 6-11 µm while ensuring an
accuracy of 0.001 cm-1 of the spectra and attaining a transmission sensitivity of 2.5 x 10-4
for long-time averaging of the acquired spectra.
These spectral measurements accomplish a quality that compares to state-of-the art
spectral databases and revealed so far undocumented details of several of the investigated
gases that have not been measured with this high resolution before at the chosen measurement
conditions. The results demonstrate the first laser spectroscopic discrimination of a
seven component gas mixture with absolute accuracies below 0.5 vol.% in the mid-infrared
provided that a sufficiently broad spectral range is covered in the measurements. Remaining
challenges for obtaining improved spectral models of the gases and limitations of the
measurement accuracy and technology are discussed.
Molekularstrahlepitaxie von niederdimensionalen GaInAs(N) Systemen für AlGaAs Mikroresonatoren
(2018)
Die Erforschung von Quantenpunkten mit ihren quantisierten, atomähnlichen Zuständen, bietet eine Vielzahl von Möglichkeiten auf dem Weg zum Quantencomputer und für Anwendungen wie Einzelphotonenquellen und Quantenpunktlasern. Vorangegangene Studien haben grundlegend gezeigt, wie Quantenpunkte in Halbleiterresonatoren integriert und mit diesen gekoppelt werden können. Dazu war es zum einen notwendig, die Quantenpunkte und ihr epitaktisches Wachstum besser zu verstehen und zu optimieren. Zum anderen mussten die Bragg-Resonatoren optimiert werden, sodass Güten von bis zu 165.000 realisiert werden konnten. Eingehende Studien dieser Proben zeigten im Anschluss einen komplexeren Zusammenhang von Q-Faktor und Türmchendurchmesser. Man beobachtet eine quasi periodische Oszillation des Q-Faktors mit dem Pillar Durchmesser. Ein Faktor für diese Oszillation ist die Beschaffenheit der Seitenflanken des Resonatortürmchens, bedingt durch die unterschiedlichen Eigenschaften von AlAs und GaAs bei der Prozessierung der Türmchen. Darüber hinaus wurden in der Folge auf den Grundlagen dieser Strukturen sowohl optisch als auch elektrisch gepumpte Einzelphotonenquellen realisiert.
Da in diesen Bauteilen auch die Lage des Quantenpunkts innerhalb des Resonatortürmchens einen erheblichen Einfluss auf die Effizienz der Kopplung zwischen Resonator und Quantenpunkt hat, war das weitere Ziel, die Quantenpunkte kontrolliert zu positionieren. Mit einer gezielten Positionierung sollte es möglich sein, ein Resonatortürmchen direkt über dem Quantenpunkt zu plazieren und den Quantenpunkt somit in das Maximum der optischen Mode zu legen.
Besondere Herausforderung für die Aufgabenstellung war, Quantenpunkte in einem Abstand von mind. der Hälfte des angestrebten Türmchendurchmessers, d.h 0,5 μm bis 2 μm, zu positionieren. Die Positionierung musste so erfolgen, dass nach dem Wachstum eines AlAs/GaAs DBR Spiegel über den Quantenpunkten, Resonatortürmchen zielgenau auf die Quantenpunkte prozessiert werden können. Es wurden geeignete Prozesse zur Strukturierung eines Lochgitters in die epitaktisch gewaschene Probe mittels Elektronenstrahllithographie entwickelt. Für ein weiteres Wachstum mittels Molekularstrahlepitaxie, mussten die nasschemischen Reinigungsschritte sowie eine Reinigung mit aktivem Wasserstoff im Ultrahochvakuum optimiert werden, sodass die Probe möglichst defektfrei überwachsen werden konnte, die Struktur des Lochgitters aber nicht zerstört wurde. Es wurden erfolgreich InAs-Quantenpunkte auf die vorgegebene Struktur positioniert, erstmals in einem Abstand von mehreren Mikrometern zum nächsten Nachbarn. Eine besondere Herausforderung war die Vorbereitung für eine weitere Prozessierung der Proben nach Quantenpunktwachstum. Eine Analyse mittels prozessierten Goldkreuzen, dass 30 % der Quantenpunkte innerhalb von 50 nm und 60 % innerhalb von 100 nm prozessiert wurden. In der Folge wurde mit der hier erarbeiteten Methode Quantenpunkte erfolgreich in DBR-Resonatoren sowie photonische Kristalle eingebaut
Die gute Abstimmbarkeit von Quantenpunkten und die bereits gezeigte Möglichkeit, diese in Halbleiterresonatoren einbinden zu können, machen sie auch interessant für die Anwendung im Telekommunikationsbereich. Um für Glasfasernetze Anwendung zu finden, muss jedoch die Wellenlänge auf den Bereich von 1300 nm oder 1550 nm übertragen werden. Vorangegangene Ergebnisse kamen allerdings nur knapp an die Wellenlänge von 1300nm. Eine fu ̈r andere Bauteile sowie für Laserdioden bereits häufig eingesetzte Methode, InAs-Quantenpunkte in den Bereich von Telekommunikationswellenla ̈ngen zu verschieben, ist die Verwendung von Stickstoff als weiteres Gruppe-V-Element. Bisherige Untersuchungen fokussierten sich auf Anwendungen in Laserdioden, mit hoher Quantenpunktdichte und Stickstoff sowohl in den Quantenpunkten als in den umgebenen Strukturen. Da InAsN-Quantenpunkte in ihren optischen Eigenschaften durch verschiedene Verlustmechanismen leiden, wurde das Modell eines Quantenpunktes in einem Wall (Dot-in-Well) unter der Verwendung von Stickstoff weiterentwickelt. Durch gezielte Separierung der Quantenpunkte von den stickstoffhaltigen Schichten, konnte e eine Emission von einzelnen, MBE-gewachsenen InAs Quantenpunkten von über 1300 nm gezeigt werden. Anstatt den Stickstoff direkt in die Quantenpunkte oder unmittelbar danach in die Deckschicht ein zu binden, wurde eine Pufferschicht ohne Stickstoff so angepasst, dass die Quantenpunkte gezielt mit Wellenlängen größer 1300 nm emittieren. So ist es nun möglich, die Emission von einzelnen InAs Quantenpunkten jenseits dieser Wellenlänge zu realisieren.
Es ist nun daran, diese Quantenpunkte mit den beschriebenen Mikroresonatoren zu koppeln, um gezielt optisch und elektrisch gepumpte Einzelphotonenquellen für 1300nm zu realisieren.