In this thesis we investigate experimentally certain aspects of the interaction of terahertz (THz) radiation with intersubband transitions and excitonic transitions in semiconductor quantum wells. The first part deals with a more fundamental view on an intersubband transition in a symmetric, undoped GaAs/AlGaAs multiple quantum well. After optical excitation of carriers, the considered electronic conduction intersubband transition is probed in the low-intensity linear regime using broadband THz pulses. These pulses are detected via field-resolved electro-optic sampling. While the sample’s terahertz absorption shows the expected single peak of the resonant intersubband transition, the differential transmission spectra, i.e. the photoexcitation-induced changes in transmission, display strong Fano signatures. On the basis of a microscopic theory, we show that they originate from a phase sensitive superposition of THz current and ponderomotive current. The latter one results from the wiggling motion of carriers induced by the accelerating THz field. Our findings demonstrate for the first time that the ponderomotive contribution has to be taken into account also at the lowest THz intensities. The following issues consider the interaction with THz pulses of higher intensity from the free-electron laser (FEL) of the Forschungszentrum Dresden-Rossendorf. In one experiment we investigate efficient second order sideband generation in the GaAs/AlGaAs multiple quantum well mentioned above. To this end a near-infrared laser tuned to excitonic interband transitions is mixed inside the sample with the inplane polarized FEL beam to create the sum- and difference-frequencies between them. We compare the sideband efficiencies for the THz beam tuned to the interexcitonic heavy-hole light-hole transition and to the intraexcitonic heavy-hole 1s-2p transition. In the latter case we achieve a ten times higher n=+2 low-temperature efficiency around 0.1%. This value is comparable to previous studies in the literature, but our approach involves different transitions in a much simpler geometry. At room temperature the efficiency drops only by a factor of 7 for low THz powers. The last part of this thesis addresses another fundamental quantum-mechanical phenomenon: the splitting of an absorption line in a strong THz field. In the same abovementioned quantum well sample the FEL wavelength is tuned near the intraexcitonic 1s-2p heavy-hole transition. The THz radiation induces a power-dependent splitting of the heavy-hole 1s exciton absorption line which manifests itself in the transmitted spectrum of a broadband near-infrared probe beam. The FEL-wavelength-dependent strength of this so-called Autler-Townes splitting is discussed on the basis of a simple two-level model.

This dissertation consists of three major studies. The first study, described in Chapter 3, focuses on the experimental work we carried out; experimental study of the electronic transition dipole moment matrix elements (TDMM) for the and electronic transitions of the sodium dimer molecule. Here we obtained the electronic transition dipole moments through Autler-Townes and resolved fluorescence spectroscopy and compared them to the theory. The second study, described in Chapter 4, is on sodium dimer ion-pair states. In this work, we calculated the radiative lifetimes and electronic transition dipole moments between Na2 ion-pair states ( , , , ) and state. This study was published in 2015. The last study, described in Chapter 5, is the total lifetime (bound-bound plus bound-free) and transition dipole moment calculations of the ion-pair electronic states, , of the lithium dimer molecule. / Physics

<p>In this dissertation is exemplified how different laser based methods are applied in high-resolution spectroscopic studies of internal properties of diatomic molecules.</p><p>A molecular beam apparatus assembly is described, where a laser ablation source is combined with a time-of-flight mass spectrometer. Compounds investigated with this equipment are hafnium sulfide and hafnium oxide. The molecules are excited and ionized applying the resonant two-photon ionization (R2PI) scheme, which is a sensitive absorption and detection technique for probing the population of an excited state.</p><p>By means of the DC Stark effect, permanent electric dipole moments of HfS in the <i>D</i> ¹Π state and HfO in the <i>b</i> ³Π₁ state are determined while the molecules are exposed to a static electric field. Under field-free conditions low temperature rotationally resolved spectra are recorded, generating line positions from which molecular parameters are derived.</p><p>The R2PI method, modified with an adjustable delay time, is also used in lifetime measurements of individual rotational levels of the HfS <i>D</i>¹Π and HfO<i> b</i> ³Π₁ states. Oscillator strengths for transitions from the ground state are calculated, and in this connection basic concepts like Einstein coefficients, line strengths and Hönl-London factors, are surveyed. Theoretical calculation of lifetimes is discussed in view of the fact that a commonly available computer program (LEVEL 7.5 by Le Roy) gives erroneous output.</p><p>Some coherence and quantum interference related phenomena, such as electromagnetically induced transparency (EIT) and Autler-Townes (AT) splitting, are presented in the latter part of this thesis. Fundamental concepts and relations are introduced and explained. The driven three-level cascade system is elucidated, including some of its experimental applications to alkali metal dimers, Na₂ and Li₂.</p><p>A triple resonance spectroscopy experiment is described in terms of a three-laser, four-level inverted-Y excitation scheme, implemented in Na₂. The accompanying density matrix formalism, providing the basis for theoretical simulations, is accounted for. From analysis of the results an absolute value of the electric dipole moment matrix element (transition moment) is extracted, using the AC Stark effect.</p><p>Some recently reported unexpected experimental results and unforeseen features, occurring in Doppler broadened samples and related to the open character of molecular systems, are briefly commented.</p>

In this dissertation is exemplified how different laser based methods are applied in high-resolution spectroscopic studies of internal properties of diatomic molecules. A molecular beam apparatus assembly is described, where a laser ablation source is combined with a time-of-flight mass spectrometer. Compounds investigated with this equipment are hafnium sulfide and hafnium oxide. The molecules are excited and ionized applying the resonant two-photon ionization (R2PI) scheme, which is a sensitive absorption and detection technique for probing the population of an excited state. By means of the DC Stark effect, permanent electric dipole moments of HfS in the D 1Π state and HfO in the b 3Π1 state are determined while the molecules are exposed to a static electric field. Under field-free conditions low temperature rotationally resolved spectra are recorded, generating line positions from which molecular parameters are derived. The R2PI method, modified with an adjustable delay time, is also used in lifetime measurements of individual rotational levels of the HfS D 1Π and HfO b 3Π1 states. Oscillator strengths for transitions from the ground state are calculated, and in this connection basic concepts like Einstein coefficients, line strengths and Hönl-London factors, are surveyed. Theoretical calculation of lifetimes is discussed in view of the fact that a commonly available computer program (LEVEL 7.5 by Le Roy) gives erroneous output. Some coherence and quantum interference related phenomena, such as electromagnetically induced transparency (EIT) and Autler-Townes (AT) splitting, are presented in the latter part of this thesis. Fundamental concepts and relations are introduced and explained. The driven three-level cascade system is elucidated, including some of its experimental applications to alkali metal dimers, Na2 and Li2. A triple resonance spectroscopy experiment is described in terms of a three-laser, four-level inverted-Y excitation scheme, implemented in Na2. The accompanying density matrix formalism, providing the basis for theoretical simulations, is accounted for. From analysis of the results an absolute value of the electric dipole moment matrix element (transition moment) is extracted, using the AC Stark effect. Some recently reported unexpected experimental results and unforeseen features, occurring in Doppler broadened samples and related to the open character of molecular systems, are briefly commented.

In this thesis we present a study about strong light-matter interaction in a broad single GaAs/AlGaAs quantum well representing a 3-level system. In particular we investigate the AC-Stark effect, where we observe in THz absorption spectra an Autler-Townes splitting as well as a Mollow-triplet. Compared to previous work, we showed for the first time an all-THz pump-probe experiment in the THz regime below the Reststrahlenband. Furthermore, we observe a strong frequency shift in the absorption energy of the first intersubband transition depending on the charge carrier density in the quantum well. The Autler-Townes splitting as well as the absorption frequency shift can be potentially exploited for THz-modulation applications. Beyond nonlinear optics many interesting effects occur in the strong light-matter interaction regime such as Rabi oscillations, coherent population trapping, lasing without inversion, electromagnetically induced transparency (EIT) and the AC-Stark effect. Our quantum well represents a 3-level system in which we investigate a splitting behaviour in the absorption spectrum of the first and second intersubband transition. Especially a splitting for the first intersubband transition is predicted also for electromagnetically induced transparency, while the second intersubband transition is pumped with a strong varying electric field. Naturally, a fundamental question is, how to distinguish EIT and an Autler-Townes duplet since both result in a spectrally transparent window. The method of choice for investigations combines narrowband pulses in the THz range provided by a free-electron laser and broadband THz pulses generated in a GaP crystl within a THz time-domain spectroscopy setup. In this unique configuration we perform time-resolved pump and probe spectroscopy experiments by pumping resonantly the second intersubband transition at 3.4 THz to induce a splitting of the second and third subband. Broadband THz pulses then probe an absorption splitting of about 0.2 THz related to the first intersubband transition at ≈ 2.3 THz as well as a splitting of the second intersubband transition (Mollow triplet). Analyzing experiments and using a theoretical criteria to distinguish EIT and Autler-Townes splitting, we conclude to observe an Autler-Townes doublet instead of an EIT effect. / In dieser Arbeit berichten wir über die starke Licht-Materie Wechselwirkung in 3-Niveau system anhand eines einzelnen, breiten GaAs/AlGaAs Quantentopfes. Insbesondere untersuchen wir den AC-Stark Effekt und beobachten eine Aufspaltung des Absorptionsspektrums durch das Autler-Townes Dublett und das Mollow Triplett. Im direkten Vergleich mit vorangegangenen Arbeiten zeigen wir zum ersten Mal ein reines THz Anrege-Abfrage Experiment mit Frequenzen unterhalb des Reststrahlenbandes. Weiterhin beobachten wir eine starke Frequenzverschiebung der Absorptionsenergie des ersten Intersubbandübergangs in Abhängigkeit von der Ladungsträgerdichte im Quantentopf. Sowohl das Autler-Townes Dublett als auch die Verschiebung der Absorptionsfrequenz ermöglichen potentielle Anwendung im Bereich der THz-Modulation. Im Bereich der starken Licht-Materie Wechselwirkung sind viele interessante Effekte beobachtbar wie Rabi Oszillationen, coherent population trapping, Lasern ohne Inversion, elektromagnetisch induzierte Transparenz (EIT) und der AC-Stark Effekt. Unser Quantentopf stellt ein 3-Niveau System dar, in welchem wir eine Aufspaltung der Absorption bezüglich des ersten und zweiten Intersubbandübergangs beobachten. Insbesondere für den ersten Intersubbandübergang ist auch eine Absorptionsaufspaltung durch den EIT Effekt vorhergesagt, während der zweite Intersubbandübergang durch ein starkes, elektrisches Wechselfeld angeregt wird. Es stellt sich dann die Frage, wodurch sich die Effekte EIT und Autler-Townes splitting unterscheiden, weil beide durch ein spektrales transparentes Fenster gekennzeichnet sind. Die von uns gewählte Methode verknüpft schmalbandige, starke elecktrische Wechselfelder im THz-Bereich eines freien Elektronen Lasers und breitbandigen THz-Pulsen, welche durch nichtlineare optische Effekte in einem THz Zeit-Bereichs Spektroskopie Aufbaus erzeugt werden. In dieser einzigartigen Konfiguration führen wir zeitaufgelöste Anrege-Abfrage Spektroskopie Experimente durch, in dem wir den zweiten Intersubbandübergang bei 3, 4 THz nahezu resonant anregen und das zweite und dritte Subband aufspalten. Mit breitbandigen THz Pulsen fragen wir dann die Absorptionsaufspaltung von ca. 0, 2 THz des ersten Intersubbandübergangs bei ≈ 2, 3 THz und des zweiten Intersubbandübergangs (Mollow-Triplett) ab. Nach Auswerten der Experimente und theoretischer Kriterien für die Unterscheidung zwischen EIT und Autler-Townes splitting schlussfolgern wir, ein Autler-Townes Dublett zu beobachten.

In this thesis we investigate experimentally certain aspects of the interaction of terahertz (THz) radiation with intersubband transitions and excitonic transitions in semiconductor quantum wells. The first part deals with a more fundamental view on an intersubband transition in a symmetric, undoped GaAs/AlGaAs multiple quantum well. After optical excitation of carriers, the considered electronic conduction intersubband transition is probed in the low-intensity linear regime using broadband THz pulses. These pulses are detected via field-resolved electro-optic sampling. While the sample’s terahertz absorption shows the expected single peak of the resonant intersubband transition, the differential transmission spectra, i.e. the photoexcitation-induced changes in transmission, display strong Fano signatures. On the basis of a microscopic theory, we show that they originate from a phase sensitive superposition of THz current and ponderomotive current. The latter one results from the wiggling motion of carriers induced by the accelerating THz field. Our findings demonstrate for the first time that the ponderomotive contribution has to be taken into account also at the lowest THz intensities. The following issues consider the interaction with THz pulses of higher intensity from the free-electron laser (FEL) of the Forschungszentrum Dresden-Rossendorf. In one experiment we investigate efficient second order sideband generation in the GaAs/AlGaAs multiple quantum well mentioned above. To this end a near-infrared laser tuned to excitonic interband transitions is mixed inside the sample with the inplane polarized FEL beam to create the sum- and difference-frequencies between them. We compare the sideband efficiencies for the THz beam tuned to the interexcitonic heavy-hole light-hole transition and to the intraexcitonic heavy-hole 1s-2p transition. In the latter case we achieve a ten times higher n=+2 low-temperature efficiency around 0.1%. This value is comparable to previous studies in the literature, but our approach involves different transitions in a much simpler geometry. At room temperature the efficiency drops only by a factor of 7 for low THz powers. The last part of this thesis addresses another fundamental quantum-mechanical phenomenon: the splitting of an absorption line in a strong THz field. In the same abovementioned quantum well sample the FEL wavelength is tuned near the intraexcitonic 1s-2p heavy-hole transition. The THz radiation induces a power-dependent splitting of the heavy-hole 1s exciton absorption line which manifests itself in the transmitted spectrum of a broadband near-infrared probe beam. The FEL-wavelength-dependent strength of this so-called Autler-Townes splitting is discussed on the basis of a simple two-level model.

The quantum interference effects, such as the Autler-Townes (AT) effect and electromagnetically induced transparency (EIT) applied to molecular systems are the focus of this Dissertation in the context of high resolution molecular spectroscopy. We demonstrate that the AT effect can be used to manipulate the spin character of a spin-orbit coupled pair of molecular energy levels serving as a \textit{gateway} between the singlet and triplet electronic states. We demonstrate that the singlet-triplet mixing characters of the \textit{gateway} levels can be controlled by manipulating the coupling laser \textit{E} field amplitude. We observe experimentally the collisional population transfer between electronic states $G^1\Pi_g (v=12, J=21, f)$ and $1^3\Sigma _g^-(v=1, N=21, f)$ of $^7$Li$_2$. We obtain the Stern-Vollmer plot according to the vapor pressure dependence of collisional transfer rate. The triplet fluorescence from the mixed \textit{gateway} levels to the triplet $b^3\Pi_u(v'=1,J'= / Physics

FAPEMIG - Fundação de Amparo a Pesquisa do Estado de Minas Gerais / O conhecimento teórico e experimental acumulado nas últimas décadas sobre pontos quânticos semicondutores (PQs) impulsiona o surgimento de um grande número de propostas atuais de utilizá-los em sistemas híbridos. A habilidade de controlar suas propriedades optoeletrônicas, bem como o domínio de sofisticadas técnicas de fabricação, tornaram-os candidatos ideais para formar junções com supercondutores (SCs), cujas características individuais são também notáveis. Essas junções podem ser simples, com um único PQ acoplado a um SC, ou múltiplas: a conexão de dois terminais SCs através de um PQ (junção tipo Josephson) e a junção de dois PQs através de um SC. Esta última forma um separador de pares de Cooper, dispositivo sugerido como fonte de partículas emaranhadas, que depende fundamentalmente da ocorrência de reflexão de Andreev cruzada (CAR) nas interfaces da junção. Junções de PQs com SCs e com supercondutores topológicos também tem sido propostas em sistemas de dois níveis formando qubits para a computação quântica trivial e topológica. Embora o estudo das junções PQ-SC esteja em evidência atualmente, a revisão da literatura mostra que a análise do regime transiente foi pouco explorada. Por isso, abordamos neste trabalho o transporte de cargas em uma junção PQ-SC-PQ com resolução temporal. Utilizando técnicas de função de Green de não-equilíbrio, particularmente, o formalismo de Kadanoff-Baym, escrevemos um conjunto de equações diferenciais acopladas, solucionado numericamente. Analisando as oscilações de Rabi que surgem na evolução temporal da corrente elétrica e das ocupações dos PQs, fomos capazes de identificar assinaturas de mecanismos de espalhamento através da junção, isto é, o tunelamento direto e a CAR. Adicionalmente, propusemos a expansão deste sistema pela sua inserção na estrutura de um fotodiodo, aplicando luz laser sobre um dos PQs. Os resultados mostram que ocorre a separação dos níveis opticamente excitados em dubletos de Autler-Townes, para acoplamentos fracos entre os PQs. Consequentemente, é observada a ocorrência de CAR, mediada pelo laser aplicado, através dos níveis que compõe o dubleto. Os resultados tem dependência também com a tensão fonte-dreno aplicada ao dispositivo, que pode estar no limite de bias alta (HB) ou zero (ZB). Este é o primeiro trabalho a analisar a separação de pares de Cooper assistida por fótons em uma junção PQ-SC-PQ, em regime de não-equilíbrio. Embora os resultados experimentais ainda sejam escassos, a inclusão de um SC em um fotodiodo de PQs híbrido permite novos mecanismos de formação de fotocorrente, abrindo novas possibilidades de aplicações desse sistema. / The theoretical and experimental knowledge accumulated in the last decades on semiconductors quantum dots (QDs) impulses the emergence of many current proposals for using them in hybrid systems. The ability to control their optoelectronic properties, as well as the control of fabrication techniques, made them the perfect candidates to compose junctions with superconductors (SCs), whose individual characteristics are also remarkable. These junctions can be simple, with a single QD coupled to a SC, or multiple: a connection of two superconducting terminals through a QD (a Josephson-like junction) and the junction of two QDs through a SC. The latter is known as a Cooper-pairs splitter, a device suggested as a source of entangled particles, for which is required the occurrence of crossed Andreev reflection (CAR) on the interfaces of the junction. Junctions of QDs with SCs and with topological SCs have also been proposed in two-level systems as qubits for both trivial and topological quantum computation. Despite the study of QD-SC junctions being currently in evidence, the literature review shows that the analysis of transient regime was little explored. Therefore, we address in this work the topic of time-dependent charge transport in a QD-SC-QD junction. By using non-equilibrium Green functions techniques, particularly, the Kadanoff-Baym formalism, we write down a set of coupled differential equations, which is numerically solved. Examining the Rabi oscillations that appears on the time evolution of electric current and QDs occupations, we were able to identify signatures of the scattering mechanisms through out the junction, i. e., direct tunnelling and CAR. Additionally, we propose to use this system as a photodiode, with the aid of a laser beam over one of the QDs. The results show the splitting of the optically excited states in Autler- Townes doublets, for a weak coupling between the QDs. Hence, CAR mediated by the applied laser was observed through the energy levels that compose the doublet. The results depend also with the source-drain potential applied to the device, which can be high bias (HB) or zero bias (ZB). The present work is the first to analyse the splitting of Cooper pairs assisted by photons in a QD-SC-QD junction, in nonequilibrium regime. Although the experimental results are still sparse, the inclusion of a SC in a QD hybrid photodiode allows new mechanisms of photocurrent formation, creating possibilities in future applications. / Tese (Doutorado)