The purpose of this thesis is to investigate the nonlinear phenomena known as doubly-stimulated, non-degenerate two-photon emission (ND-2PE) in Gallium Arsenide (GaAs). 2PE refers to the simultaneous emission of two-photons as electrons move from the conduction band in a direct gap semiconductor to the valence band. Following the same path for describing one-photon emission (1PE) we describe 2PE as a product of the irradiance, and the negative of the loss which in this case is two-photon absorption, , the negative coming from the population inversion. We attempt to observe 2PE by using a frequency non-degenerate pump-probe experiment in which a third beam optically excites a 4 µm thick GaAs sample. We use nondegenerate beams in hopes of utilizing the 3-orders of magnitude enhancement seen in twophoton absorption (2PA) by going to extreme nondegeneracy (END) to enhance 2PE. GaAs is chosen due to the availability of the appropriate wavelengths, the maturity of the GaAs technology, its use in optoelectronic devices and its ability to be electrically pumped. During the experimental development we learn how to effectively etch and manipulate thin GaAs samples and model the transmission spectrum of these samples using thin film transmission matrices. We are able to match the measured transmission spectrum with the theoretical transmission spectrum. Here we etch the bulk GaAs left on the sample leaving only the 4 µm thickness of molecular beam epitaxial grown GaAs plus additional layers of aluminum gallium arsenide (AlGaAs). These samples were grown for us by Professor Gregory Salamo of the University of Arkansas. iv Using the pump-probe experiment on the 4 µm GaAs sample, we measure the change of the 2PA due to the presence of optically excited carriers. The goal is to reduce the 2PA signal to zero and then invert the 2PA signal indicating an increase in transmission indicative of 2PE when the population is inverted. Our results show that we achieve a 45% reduction in the 2PA signal in a 4 μm thick GaAs sample due to the excited carriers. Unfortunately, we currently cannot experimentally determine whether the reduction is strictly due to free-carrier absorption (FCA) of our pump or possibly due to a change in the two-photon absorption coefficient. We measure the transmission of various wavelengths around the bang gap of GaAs as a function of excitation wavelength and achieve a transmittance of ~80% which we attribute to possibly be one photon gain (1PG) at 880 nm. We also go to cryogenic temperatures to concentrate the carriers near the bottom of the conduction band and improve the theoretical gain coefficient for 2PE. Unfortunately, we do not observe a measurable change in 2PA with the addition of optically excited carriers. Along with FCA of our infrared pump we suspect that the difficulties in this first set of experiments are also a result or radiative recombination due to amplified spontaneous emission reducing our free carrier density along with the fact that 4 m is too thick for uniform excitation. We now have 1 m samples from Professor Gregory Salamo which we hope will give better and more definitive results

No description available.

Die Erzeugung spinpolarisierter Ladungsträger in einem Halbleiter gilt als Grundvoraussetzung zur Realisierung spintronischer Bauelemente. Einen möglichen Ansatz zu deren Realisierung stellen Ferromagnet/Halbleiter(FM/HL)-Hybridstrukturen dar, deren Herstellung jedoch mit einigen Schwierigkeiten verbunden ist. Durch die Vermischung des ferromagnetischen Materials mit dem Halbleiter werden die elektronischen Eigenschaften der Hybridstruktur verändert und die Spininjektionseffizienz stark verringert. Durch das gezielte Einfügen einer dünnen Oxidschicht in den FM/HL-Grenzübergang kann die Diffusion unterdrückt, die Kristallqualität verbessert und die Effizienz der Struktur erhöht werden. Diese Arbeit beschäftigt sich mit dem Wachstum und der Charakterisierung dünner Oxidschichten, hergestellt mittels Molekularstrahlepitaxie. Zwei Seltenerdoxide, La2O3 und Lu2O3, werden auf GaAs-Substraten gewachsen und die Kristallqualität der Schichten miteinander verglichen. Mit der Heusler-Legierung Co2FeSi als Injektorschicht wird eine FM/Oxid/HL-Hybridstruktur auf Basis einer La2O3/GaAs(111)B-Struktur realisiert und magnetisch und elektrisch charakterisiert. Ein häufig verwendetes Barrierenmaterial in FM/HL-Hybridstrukturen ist Magnesiumoxid (MgO). In dieser Arbeit werden dünne MgO-Schichten auf GaAs(001) an der PHARAO-Wachstumsanlage am BESSY II erzeugt. Dies geschieht durch getrenntes Verdampfen von metallischem Mg bzw. Einleiten von molekularem Sauerstoff in die Wachstumskammer. Um die Oxidation des Halbleitersubstrats zu verhindern, wird vor dem MgO-Wachstum eine dünne Mg-Schicht abgeschieden. Abhängig von der Dicke dieser Schicht sind zwei in-plane-Orientierungen des MgO relativ zum GaAs kontrolliert einstellbar. Darüber hinaus werden Hybridstrukturen mit Eisen Fe als Injektorschicht und schrittweise erhöhter MgO-Schichtdicke gewachsen. Die Eindiffusion von Fe in das GaAs-Substrat nimmt mit zunehmender MgO-Schichtdicke um mehrere Größenordnungen ab. / The generation of spin-polarized charge carriers in a semiconductor is a basic building block for the implemention of spintronic devices. A feasible approach to their implementation are ferromagnet/semiconductur(FM/SC) hybrid structures, whose fabrication is associated with some issues. The intermixing of the ferromagnetic material with the semiconductor leads to distortion of the electrical properties of the hybrid structure and the spin injection efficiency is reduced. By intentionally inserting a thin oxide layer into the FM/SC interface diffusion can be suppressed while the crystal quality and the spin injection efficiency of the structure are both increased. In this thesis the growth and characterization of thin oxide films fabricated by molecular beam epitaxy are discussed. Two rare earth oxides, La2O3 and Lu2O3, are grown on GaAs substrates and their crystal qualities are compared. Based on La2O3/GaAs(111)B full FM/SC hybrid structures are grown with the Heusler alloy Co2FeSi as injection layer and characterized by magnetic and electrical means. Another material used as a barrier in FM/SC hybrid structures is magnesium oxide (MgO). Here, thin MgO layers are grown on GaAs(001) at the PHARAO system at BESSY II. The growth is conducted by the separated evaporation of metallic Mg and introducing molecular oxygen into the growth chamber. To avoid oxidation of the semiconducting substrate a thin Mg layer is deposited prior to the MgO growth. Depending on the Mg layer thickness two different MgO in-plane orientations can be achieved with respect to the GaAs substrate. Furthermore, FM/SC hybrid structures with iron Fe as injection layer are grown while the MgO layer thickness is increased gradually. The indiffusion of Fe into the GaAs substrate is suppressed by several orders of magnitude with increasing MgO layer thickness.

Thema dieser Arbeit ist die Synthese von GaAs Nanodrähten mittels Molekularstrahlepitaxie. Dabei wird das Wachstum mittels Au- und jenes mittels selbst-induziertem VLS-Mechanismus verglichen. Die Au-induzierte Methode ist als vielseitiger Ansatz für die Herstellung von Nanodrähten bekannt. Darüberhinaus wird seit Neuerem der selbst-induzierte Mechanismus untersucht, bei dem Galliumtropfen die Rolle des Goldes übernehmen, um eine etwaige Verunreinigung mit Au von vornherein auszuschliessen. Mit beiden Wachstumsmethoden erzielen wir GaAs Nanodrähte mit großem Aspektverhältnis und epitaktischer Beziehung zum Si(111) Substrat. Während des Au-induzierten Wachstums entsteht eine parasitäre Schicht zwischen den Drähten, die mittels des selbst-induzierten Mechanismus vermieden werden. Alle GaAs Drähte sind vollständig relaxiert. Die durch die Gitterfehlanpassung (4,1\% zwischen GaAs und Si) verursachte Verspannung wird durch Versetzungen an der Grenzfläche abgebaut. Selbst-induzierte Drähten zeigen ausschließlich unpolare Seitenfacetten, während verschiedene polare Facetten für Au-induzierte Nanodrähte beschrieben werden. Mittels VLS-Nukleationstheorie könnne wir den Einfluss des Tropfenmaterials auf die Stabilität der verschiedenen Seitenfacetten erklären. Optoelektronische Anwendungen benötigen lange Minoritätsladungsträgerlebensdauern bei Raumtemperatur. Daher wurden mit (Al,Ga)As Hüllen ummantelte GaAs Nanodrähte mittels zeitaufgelöster PL vermessen. Das Ergebnis sind 2,5 ns für die selbst-induzierten aber nur 9 ps für die Au-induzierten Nanodrähte. Durch temperaturabhängige PL Messungen kann eine charakteristische Aktivierungsenergie von 77 meV nachgewiesen werden, die nur in den Au-induzierten Nanodrähten vorliegt. Dies suggeriert, dass sich Au aus den Tröpfchen in die GaAs Nanodrähte einbaut und dort als tiefes, nichtstrahlendes Rekombinationszentrum fungiert. / In this work the synthesis of GaAs nanowires by molecular beam epitaxy (MBE) using the vapour-liquid-solid (VLS) mechanism is investigated. A comparison between Au- and self-assisted VLS growth is at the centre of this thesis. While the Au-assisted method is established as a versatile tool for nanowire growth, the recently developed self-assisted variation results from the exchange of Au by Ga droplets and thus eliminates any possibility of Au incorporation. By both methods, we achieve nanowires with epitaxial alignment to the Si(111) substrates. Caused by differences during nanowire nucleation, a parasitic planar layer grows between the nanowires by the Au-assisted method, but can be avoided by the self-assisted method. Au-assisted nanowires grow predominantly in the metastable wurtzite crystal structure, while their self-assisted counterparts have the zincblende structure. All GaAs nanowires are fully relaxed and the strain arising from the lattice mismatch between GaAs and Si of 4.1\% is accommodated by misfit dislocations at the interface. Self-assisted GaAs nanowires are generally found to have vertical and non-polar side facets, while tilted and polar nanofacets were described for Au-assisted GaAs nanowires. We employ VLS nucleation theory to understand the effect of the droplet material on the lateral facets. Optoelectronic applications require long minority carrier lifetimes at room temperature. We fabricate GaAs/(Al,Ga)As core-shell nanowires and analyse them by transient photoluminescence (PL) spectroscopy. The results are 2.5 ns for the self-assisted nanowires as well as 9 ps for the Au-assisted nanowires. By temperature-dependent PL measurements we find a characteristic activation energy of 77 meV that is present only in the Au-assisted nanowires. We conclude that most likely Au is incorporated from the droplets into the GaAs nanowires and acts as a deep, non-radiative recombination centre.

L'optomécanique est la science des interactions entre la lumière et les mouvements mécaniques. Ce rapport de thèse décrit des expériences réalisées avec des microdisques fabriqué dans différents résonateurs semi-conducteurs III-V: l'Arséniure de Gallium (GaAs), l'Arséniure d'Aluminium Gallium (AlGaAs) et l'Arséniure d'Indium Phosphide (InGaP). Ces matériaux sont compatibles avec les fonctionnalités de l’optoélectronique et procurent un couplage optomécanique géant. Pour améliorer les performances des résonateurs en GaAs, nous avons développé des méthodes de traitement de surface permettant de réduire la dissipation optique par un facteur dix et ainsi d'atteindre un facteur de qualité de six millions. En plus de ces études sur le GaAs, nous avons réalisés une étude comparative des interactions optomecaniques dans des microdisques d'InGaP et d'AlGaAs, et nous avons mis en évidences leurs résonances optomécaniques. Finalement, nous avons réalisé des mesures de force avec des résonateurs en GaAs, démontrant un nouveau principe de détection basé sur notre étude de leur la trajectoire dans l'espace de phase et leur bruit de phase / Optomechanics studies the interaction between light and mechanical motion. This PhD thesis reports on optomechanical experiments carried with miniature disk resonators fabricated out of distinct III-V semiconductors: Gallium Arsenide (GaAs), Aluminium Gallium Arsenide (AlGaAs) and Indium Gallium Phosphide (InGaP). These materials are compliant with optoelectronics functionalities and provide giant optomechanical coupling. In order to boost performances of GaAs resonators, we implemented surface control techniques and obtained a ten-fold reduction of optical dissipation, attaining a Q of six million. On top of GaAs, we performed a comparative investigation of optomechanical interactions in InGaP and AlGaAs disk resonators, and demonstrated their operation as optomechanical oscillators. Finally, we carried out optomechanical force sensing experiments with GaAs resonators, analyzing a new sensing principle in light of the phase space trajectory and phase noise of the corresponding oscillators

In dieser Arbeit wird die ultraschnelle Dynamik von Leitungsbandelektronen in Halbleitermaterialien mit Hilfe nichtlinearer Terahertz-Spektroskopie erforscht. Insbesondere wird n-dotiertes Galliumarsenid bei mittleren Dotierdichten zwischen 10^(16) cm^(-3) und 10^(17) cm^(-3) untersucht. Für die Erzeugung intensiever THz Strahlung wurde eine neuartige Quelle entwickelt, die THz Transienten mit nur einer Oszillationsperiode und maximalen Feldamplituden von mehr als 400 kV/cm liefert. Diese THz-Quelle benutzt ultrakurze optische Laserpulse aus einem Ti:Saphir Oszillator. Zusätzlich wurde ein neuartiger zwei-Farben Anrege-Abtast Experimentierplatz aufgebaut, der zweidimensionale, zeitaufgelöste Messungen im mittleren und fernen Infrarotbereich ermöglicht. Feldionisation flacher, neutraler Störstellen im Galliumarsenid-Gitter mittels intensiver, ultrakurzer THz Impulse und die anschliessende kohärente, strahlende Rekombination von Elektronen in die Störstellen-Grundzustände bei Raumtemperatur wird gezeigt. Der superradiante Zerfall der nichtlinearen Polarisation führt zur Abstrahlung eines kohärenten Signals mit Lebensdauern von über einer Pikosekunde. Solche nichtlinearen Signale, die 10-fache Lebensdauern im Vergleich zum linearen Fall aufweisen, wurden in dieser Arbeit zum ersten Mal gemessen. Bei niedrigen Temperaturen und THz Feldstärken unter 5 kV/cm werden Rabi-Oszillationen an Übergängen in flachen Störstellen demonstriert. Zum ersten Mal konnte die polare Elektron-LO-Phonon Wechselwirkung im quantenkinetischen Regime direkt gemessen werden. Die quasi-instantane Beschleunigung von Leitungsbandelektronen im polaren Galliumarsenid-Gitter und die anschließende Messung der Transmission im mittleren Infrarot-Bereich, zeigen eine Modulation der Transmission entlang der Anrege-Abtast Verzögerung mit der Frequenz des LO Phonons. Diese Oszillation ist ein direktes Maß der relativen Phase zwischen der Elektronenbewegung und der umgebenden Phonon Wolke. Quantenkinetische Modellrechnungen reproduzieren vollständig die beobachteten Effekte. / In this thesis, the ultrafast dynamics of conduction band electrons in semiconductors are investigated by nonlinear terahertz (THz) spectroscopy. In particular, n-doped gallium arsenide samples with doping concentrations in the range of 10^16cm^(-3) to 10^17 cm^(-3) are studied. A novel source for the generation of intense THz radiation is developed which yields single-cycle THz transients with field amplitudes of more then 400 kV/cm. The THz source uses ultrashort optical laser pulses provided by a Ti:sapphire oscillator. In addition, a two-color THz-pump mid-infrared-probe setup is implemented, which allows for two-dimensional time-resolved experiments in the far-infrared wavelength range. Field ionization of neutral shallow donors in gallium arsenide with intense, ultrashort THz pulses and subsequent coherent radiative recombination of electrons to impurity ground states is observed at room temperature. The superradiant decay of the nonlinear polarization results in the emission of a coherent signal with picosecond lifetimes. Such nonlinear signals, which exhibit a lifetime ten times longer than in the linear regime are observed for the first time. At low temperatures and THz field strengths below 5 kV/cm, Rabi flopping on shallow donor transitions is demonstrated. For the first time, the polar electron-LO phonon interaction is directly measured in the quantum kinetic transport regime. Quasi-instantaneous acceleration of conduction band electrons in the polar gallium arsenide lattice by the electric field of intense THz pulses and subsequent probing of the mid-infrared transmission reveals a modulation of the transmission along the THz-mid-infrared delay coordinate with the frequency of the LO phonon. These modulations directly display the relative phase between the electron motion and its surrounding virtual phonon cloud. Quantum kinetic model calculations fully account for the observed phenomena.

Carrier, spin, charge current, and spin current injection by one- and two-color optical schemes are investigated within 30-band k·p theory. Parameters of the band model are optimized to give full-Brillouin zone band structures for GaAs and Ge that give accurate Γ-point effective masses and gyromagnetic factors and give access to the L valley, and to the E₁ and E₁+Δ₁ critical points in the linear optical absorption. Calculations of one- and two-photon carrier and spin injection and two-color current injection are performed for excitation energies in the range of 0—4 eV in GaAs and 0—3.5 eV in Ge. Significant spin and spin current injection occurs with 30% spin polarization in GaAs and Ge at photon energy matching the E₁ critical point. Further, the anisotropy and disparity of the current injection between parallel and perpendicular linearly-polarized beam configurations are calculated. For light propagating along a <111> crystal axis, anisotropic contributions in coherent current control and two-photon spin injection give rise to normal current components and in-plane spin components. In Ge, contributions from the holes to spin, electrical current, and spin current injection are investigated. Optical orientation results in 83% spin-polarized holes at the band edge. The effects of carrier dynamics in Ge are treated within a rate-equation model. The detection of spin dynamics in a pump-probe setup is considered, and the Fermi-factor approach is justified for electrons but not for holes. Carrier and current injection are further investigated in single-layer and bilayer graphene within the tight-binding model. In single-layer graphene, the linear-circular dichroism in two-photon absorption yields an absorption coefficient that is twice as large for circularly polarized light compared to linearly polarized light. Coherent current injection is largest for co-circularly polarized beams and zero for cross-circularly polarized beams. For linearly polarized beams, the magnitude of the injected current is independent of beam polarizations. In contrast, the injected current in bilayer graphene shows disparity between parallel and perpendicular configurations of the beams. The resulting angular dependence of the current is a macroscopic, measurable consequence of interlayer coupling in the bilayer.

Carrier, spin, charge current, and spin current injection by one- and two-color optical schemes are investigated within 30-band k·p theory. Parameters of the band model are optimized to give full-Brillouin zone band structures for GaAs and Ge that give accurate Γ-point effective masses and gyromagnetic factors and give access to the L valley, and to the E₁ and E₁+Δ₁ critical points in the linear optical absorption. Calculations of one- and two-photon carrier and spin injection and two-color current injection are performed for excitation energies in the range of 0—4 eV in GaAs and 0—3.5 eV in Ge. Significant spin and spin current injection occurs with 30% spin polarization in GaAs and Ge at photon energy matching the E₁ critical point. Further, the anisotropy and disparity of the current injection between parallel and perpendicular linearly-polarized beam configurations are calculated. For light propagating along a <111> crystal axis, anisotropic contributions in coherent current control and two-photon spin injection give rise to normal current components and in-plane spin components. In Ge, contributions from the holes to spin, electrical current, and spin current injection are investigated. Optical orientation results in 83% spin-polarized holes at the band edge. The effects of carrier dynamics in Ge are treated within a rate-equation model. The detection of spin dynamics in a pump-probe setup is considered, and the Fermi-factor approach is justified for electrons but not for holes. Carrier and current injection are further investigated in single-layer and bilayer graphene within the tight-binding model. In single-layer graphene, the linear-circular dichroism in two-photon absorption yields an absorption coefficient that is twice as large for circularly polarized light compared to linearly polarized light. Coherent current injection is largest for co-circularly polarized beams and zero for cross-circularly polarized beams. For linearly polarized beams, the magnitude of the injected current is independent of beam polarizations. In contrast, the injected current in bilayer graphene shows disparity between parallel and perpendicular configurations of the beams. The resulting angular dependence of the current is a macroscopic, measurable consequence of interlayer coupling in the bilayer.

Polarization entangled photon pairs on demand are considered to be an important building block of quantum communication technology. It has been demonstrated that semiconductor quantum dots (QDs), which exhibit a certain spatial symmetry, can be used as a triggered, on-chip source of polarization entangled photon pairs. Due to limitations of the growth, the as-grown QDs usually do not exhibit the required symmetry, making the availability of post-growth tuning techniques essential. In this work first the QD-morphology of hundreds of QDs is correlated with the optical emission of neutral excitons confined in GaAs/AlGaAs QDs. It is presented how elastic anisotropic stress can be used to partially restore the symmetry of self-assembled GaAs/AlGaAs and InGaAs/GaAs QDs, making them as candidate sources of entangled photon pairs. As a consequence of the tuning of the QD-anisotropy we observe a rotation of the polarization of the emitted light. The joint modification of polarization orientation and QD anisotropy can be described by an anticrossing of the so-called bright excitonic states. Furthermore, it is demonstrated that anisotropic stress can be used to tune the purity of the hole states of the QDs by modifying the degree of heavy and light hole mixing. This ability might be interesting for applications using the hole spin as a so-called quantum bit.

Time-resolved Faraday rotation (TRFR) is a pulsed laser pump/probe optical measurement used to characterize electron spin dynamics in semiconductor materials. A Mach-Zehnder type interferometer with orthogonally polarized arms is presented as a device for TRFR measurement that is superior to optical bridge detection, the traditional measuring technique, since Faraday rotation can be passively optically amplified via interference. Operation of the interferometer is analyzed under ideal conditions. Corrections to the ideal case stemming from imperfectly aligned optics, finite polarization extinction ratios, and an imperfect recombination optic are analyzed using a matrix transformation approach. The design of the interferometer is presented and chronicled. A description of the single-beam active control system utilized to stabilize the interferometer by continuous corrections to the optical path length of one arm with a piezoelectric actuator is given. Optical amplification by increasing the power in either arm of the interferometer is demonstrated and TRFR measurements taken with the interferometer at ambient temperatures are compared with measurements taken with the optical bridge. We find the interferometer to offer a detection limit on the order of 50 mrad at room temperature, which is five times more sensitive than the optical bridge. Isolation and stabilization of the interferometer were also successful in reducing signal noise to a level comparable with the optical bridge. Our results demonstrate that the interferometer is a better detection device for Faraday rotation under ambient conditions. In the immediate future, improvements to the control system should be made and experiments should be performed with high-quality samples at cryogenic temperatures to confirm that the interferometer performs as favorably under those conditions.