Experimentelle Physik II

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http://hdl.handle.net/2003/41

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Untersuchung der elektronischen Eigenschaften (Ladungs- und Spinzustände) in Halbleiterquantenstrukturen mittels linearer und nichtlinearer Spektroskopie (zeitintegriert und zeitaufgelöst), besonders auch unter Zuhilfenahme externer elektrischer und magnetischer Felder. Untersuchung der optischen Moden in Resonatorstrukturen sowie der Modifikation der Licht-Materie-Wechselwirkung, die durch den Einschluss von Licht in diesen Kavitäten erreicht werden kann. Untersuchung neuartiger semimagnetischer Halbleitermaterialien, die die Basis für die Umsetzung neuartiger Konzepte in der Spintronik darstellen.

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Mode locking of hole spin coherences in CsPb(Cl, Br)3 perovskite nanocrystals
(2023-02-08) Kirstein, E.; Kopteva, N. E.; Yakovlev, D. R.; Zhukov, E. A.; Kolobkova, E. V.; Kuznetsova, M. S.; Belykh, V. V.; Yugova, I. A.; Glazov, M. M.; Bayer, M.; Greilich, A.
The spin physics of perovskite nanocrystals with confined electrons or holes is attracting increasing attention, both for fundamental studies and spintronic applications. Here, stable lead halide perovskite nanocrystals embedded in a fluorophosphate glass matrix are studied by time-resolved optical spectroscopy to unravel the coherent spin dynamics of holes and their interaction with nuclear spins of the 207Pb isotope. We demonstrate the spin mode locking effect provided by the synchronization of the Larmor precession of single hole spins in each nanocrystal in the ensemble that are excited periodically by a laser in an external magnetic field. The mode locking is enhanced by nuclei-induced frequency focusing. An ensemble spin dephasing time of a nanosecond and a single hole spin coherence time of T2 = 13 ns are measured. The developed theoretical model accounting for the mode locking and nuclear focusing for randomly oriented nanocrystals with perovskite band structure describes the experimental data very well.
Exciton-phonon interactions in quantum dots and 2D materials
(2023) Schindler, Janina J.; Aßmann, Marc-Alexander; Cinchetti, Mirko
Excitons are fundamental electronic excitations in semiconductors. They may couple to their crystal environment, namely to phonons which are elementary vibrational excitations of the crystal lattice. The coupling between excitons and phonons is essential, since it ultimately determines the radiative features of semiconducting materials and their relevance for diverse applications in optoelectronics, magneto-optics, spin- and, e.g., valleytronics. Understanding and exploiting the exciton-phonon interactions are therefore crucial for gaining insight into the physics of low-dimensional semiconductors. The focus of this thesis lies on investigating exciton-phonon interactions in self-assembled quantum dots and uncapped and hBN-encapsulated transition metal dichalcogenide monolayers by photoluminescence and inelastic laser-light scattering spectroscopy. The first part of the thesis deals with a novel Fano-type quantum interference in InGaAs/GaAs quantum dots between the bright and dark exciton states and a continuum composed of two orthogonally linear-polarized acoustic phonons. It is sensitive to external factors such as magnetic field strength and direction and optical pumping intensity. The Fano interaction, observed between the excitonic spin transition and the acoustic phonon continuum, provides a valuable method for probing weak couplings in two-level quantum systems and offers insights into previously hidden optically inactive states in semiconductor nanostructures. In the second part, different kinds of charge carrier-phonon interactions in van der Waals heterostructures are studied. In a WSe2 monolayer, the interlayer electron-phonon interaction leads to a significant increase in the excitonic emission intensity which is attributed to a double resonance phenomenon. Additionally, phonon-polariton anticrossings at the neutral and negatively charged exciton resonances are revealed, as well as an upconversion of a dark intervalley exciton into a bright intravalley exciton. The energy gain associated to this upconversion is described by a cooling of the resident electrons or by an exciton scattering with L- or K-valley phonons. Moreover, through tuning the electron doping levels via the hBN thickness, the fine structure of excitonic complexes in MoS2 heterostructures is explored, and remarkably enhanced g-factors are obtained in the ternary alloy MoWSe2. The control and manipulation of excitonic properties and interactions, such as the excitonic g-factor and their intricate couplings to phonons, provide opportunities for further advancements in spintronics and quantum information processing. The results also highlight that the interactions between excitons and phonons must be studied thoroughly in order to exploit the full potential of these semiconductor materials and to allow for tailoring their functional and structural properties.
Spin noise spectroscopy from the perspective of scattered light and noise formation principles in variety of systems
(2024) Kamenskii, Aleksandr; Greilich, Alex; Narevicius, Edvardas
This work is dedicated to a tool to get information about the spin system ground state with the help of optics: spin noise spectroscopy. Its primary goal is to characterize the virtually undisturbed spin dynamics by obtaining parameters of spin fluctuations in thermal equilibrium. The homodyne detection scheme with phase stabilization is utilized to improve the responsiveness of the polarization analysis in spin noise spectroscopy. By providing power to the local oscillator, it is possible to overcome the electronic noise and effectively increase the acquired signal - without additional disturbance in the probed spin system while the beam bypasses the sample. This possibility allows the work with lower probing power densities, approaching the desired nonperturbing regime. An improvement larger than half an order of magnitude is present for a bulk n-doped GaAs for small probe intensities. Phase manipulation and stabilization make it achievable to choose the desired parameter - Faraday rotation, ellipticity or a mixture - from the experiment. It avoids otherwise necessary modifications in the arrangement of the optical components. This improved technique examine further the fundamental characteristics of the spin noise signal construction by obtaining various angular dependencies of the scattered light. The distributed Bragg reflector forming a microcavity amplifies the light-matter interplay, making it possible to detect spin noise from an ensemble of $n$-doped (In, Ga)As/GaAs quantum dots reliably and extract weak effects. As a proof of principle, we performed an observation of pure scattered field (outside of the transmitted light aperture), as well as extraction of the primary electron spin properties, g-factor and spin dephasing time. We also studied the impact of the microcavity on the spatial and spectral dispersion of the scattered light intensity. Additionally, the interplay of two beam resonance excitation was considered for the potential signal enlargement capabilities. From that point, the interest in the work is shifted in the direction of probing new systems and their characterization. The fundamentals of the spin noise spectroscopy is transcription of the spin-system magnetization on the angle of the Faraday rotation, which should be sufficient to be measurable. The presence of such a sufficiency can not be extracted from linear magneto-optical effects, which is especially crucial in inhomogeneously broadened systems, exhibiting the spin noise gain effect. In this part, the connection between the spin noise gain effect and the behavior of the nonlinear resonant Faraday effect is established, allowing us to predict the applicability of the spin noise spectroscopy to this type of paramagnet. The experimental evidence is based on intraconfigurational (4f-4f) transitions of the trivalent rare-earth ions of neodymium and ytterbium in fluorite-based crystals, approving the theoretical estimations. At last, spin noise spectroscopy is applied to materials conventionally studied by means of the electronic paramagnetic resonance spectroscopy -- dielectrics with paramagnetic impurities, which were thought of as inapplicable for the spin noise spectroscopy before. Such belief was founded on their low specific Faraday rotation for strong optical transitions. This work demonstrates that for forbidden intraconfigurational transitions, one can see the spin noise spectroscopy due to the spin noise gain effect, which is proportional to a relation of inhomogeneous linewidth to the homogeneous one and can be as high as 10^8 in the mentioned system. The requirements for the optical setup to unlock such measurements are discussed, along with the potential applications. Finally, a method is present to simplify the identification of obtained spectra components. Due to the discovered fact that the sum of squares of the magnetic resonance frequencies stays the same for any direction of the magnetic field in cubic crystals with anisotropic impurity centers, if the magnitude of the field is constant, one can match the peaks to corresponding types of centers. The relation between the invariant and the g-tensor components were derived for various kinds of centers and proofed experimentally with the spin noise spectroscopy on a cubic CaF2-Nd3+ crystal.
On-chip phonon-magnon reservoir for neuromorphic computing
(2023-12-14) Yaremkevich, Dmytro D.; Scherbakov, Alexey V.; De Clerk, Luke; Kukhtaruk, Serhii M.; Nadzeyka, Achim; Campion, Richard; Rushforth, Andrew W.; Savel'ev, Sergey; Balanov, Alexander G.; Bayer, Manfred
Reservoir computing is a concept involving mapping signals onto a high-dimensional phase space of a dynamical system called “reservoir” for subsequent recognition by an artificial neural network. We implement this concept in a nanodevice consisting of a sandwich of a semiconductor phonon waveguide and a patterned ferromagnetic layer. A pulsed write-laser encodes input signals into propagating phonon wavepackets, interacting with ferromagnetic magnons. The second laser reads the output signal reflecting a phase-sensitive mix of phonon and magnon modes, whose content is highly sensitive to the write- and read-laser positions. The reservoir efficiently separates the visual shapes drawn by the write-laser beam on the nanodevice surface in an area with a size comparable to a single pixel of a modern digital camera. Our finding suggests the phonon-magnon interaction as a promising hardware basis for realizing on-chip reservoir computing in future neuromorphic architectures.
Large scale purification in semiconductors using Rydberg excitons
(2023-12-12) Bergen, Martin; Walther, Valentin; Panda, Binodbihari; Harati, Mariam; Siegeroth, Simon; Heckötter, Julian; Aßmann, Marc
Improving the quantum coherence of solid-state systems is a decisive factor in realizing solid-state quantum technologies. The key to optimize quantum coherence lies in reducing the detrimental influence of noise sources such as spin noise and charge noise. Here we demonstrate that we can utilize highly-excited Rydberg excitons to neutralize charged impurities in the semiconductor Cuprous Oxide - an effect we call purification. Purification reduces detrimental electrical stray fields drastically. We observe that the absorption of the purified crystal increases by up to 25% and that the purification effect is long-lived and may persist for hundreds of microseconds or even longer. We investigate the interaction between Rydberg excitons and impurities and find that it is long-ranged and based on charge-induced dipole interactions. Using a time-resolved pump-probe technique, we can discriminate purification from Rydberg blockade, which has been a long-standing goal in excitonic Rydberg systems.
Spectroscopic analysis of structural and chemical properties of solid thin film lubricants
(2023) Thomann, Carl Arne; Debus, Jörg; Westphal, Carsten
Upon tribological loading, especially at elevated temperatures or in humidity, chemical transitions and structural processes are known to take place in solid thin film lubricants like amorphous carbons (a-C) and molybdenum disulphide (MoS2) and to critically limit their performance. To accommodate those restrictions, a selection of element-modifications is applied on a regular basis. General aim is to analyse structural and chemical properties in a-C and MoS2 thin films. The understanding of those is a prerequisite for the scientific conception around tribo-film formation, which is identified as critical part of the tribological system. For this, Raman microscopy is augmented by a setup for optical temperature tuning via a pulsed laser. This work summarises the Raman-accessible film properties and the changes after element-modification and thermal impact. In a-C studies of optical temperature tuning, two ordering mechanisms are found, which are affected by element-modification. As initial reaction upon heating, graphitic clusters within an amorphous matrix increase in number and, subsequently, in size upon further heating. A Five Stage-model of structural relaxation in a-C is proposed. For MoS2, it is found that the formation of tribo-films is strongly dependent on the working environment, the tribo-film gains temperature resistance over untouched thin film material, and tribo-film formation is affected by element-modification. In studies of optical temperature tuning, an initial ordering process of amorphous MoS2 and subsequent chemical reaction to distinct oxides were found. The results are useful for identification of defect mechanisms and possibly for gauging the status of wear in a-C and MoS2 thin films. Element-modification in a-C with previously unknown effects may now be interpreted within a found Five Stage-model; similarly, the behaviour of MoS2 spectra upon heating can now be evaluated.
The squeezed dark nuclear spin state in lead halide perovskites
(2023-10-21) Kirstein, E.; Smirnov, D. S.; Zhukov, E. A.; Yakovlev, D. R.; Kopteva, N. E.; Dirin, D. N.; Hordiichuk, O.; Kovalenko, M. V.; Bayer, M.
Coherent many-body states are highly promising for robust quantum information processing. While far-reaching theoretical predictions have been made for various implementations, direct experimental evidence of their appealing properties can be challenging. Here, we demonstrate optical manipulation of the nuclear spin ensemble in the lead halide perovskite semiconductor FAPbBr3 (FA = formamidinium), targeting a long-postulated collective dark state that is insensitive to optical pumping after its build-up. Via optical orientation of localized hole spins we drive the nuclear many-body system into this entangled state, requiring a weak magnetic field of only a few milli-Tesla strength at cryogenic temperatures. During its fast establishment, the nuclear polarization along the optical axis remains small, while the transverse nuclear spin fluctuations are strongly reduced, corresponding to spin squeezing as evidenced by a strong violation of the generalized nuclear squeezing-inequality with ξs < 0.5. The dark state corresponds to an ~35-body entanglement between the nuclei. Dark nuclear spin states can be exploited to store quantum information benefiting from their long-lived many-body coherence and to perform quantum measurements with a precision beyond the standard limit.
Hybrid coherent control of magnons in a ferromagnetic phononic resonator excited by laser pulses
(2024-01-23) Scherbakov, Alexey V.; Carr, Alex D.; Linnik, Tetiana L.; Kukhtaruk, Serhii M.; Armour, Andrew D.; Nadzeyka, Achim; Rushforth, Andrew W.; Akimov, Andrey V.; Bayer, Manfred
We propose and demonstrate the concept of hybrid coherent control (CC) whereby a quantum or classical harmonic oscillator is excited by two excitations: one is quasiharmonic (i.e., harmonic with a finite lifetime) and the other is a pulsed broadband excitation. Depending on the phase relation between the two excitations, controlled by the detuning of the oscillator eigenfrequencies and the wave forms of the quasiharmonic and broadband excitations, it is possible to observe Fano-like spectra of the harmonic oscillator due to the interference of the two responses to the simultaneously acting excitations. Experimentally, as an example, the hybrid CC is implemented for magnons in a ferromagnetic grating where GHz coherent phonons act as the quasiharmonic excitation and the broadband impact arises from pulsed optical excitation followed by spin dynamics in the ferromagnetic nanostructure.
From spectroscopy to coherent control: photon echoes of excitons in organic-inorganic perovskites and (In, Ga)As quantum dots
(2023) Grisard, Stefan; Akimov, Ilya A.; Reiter, Doris E.
Photon echoes emerge from the delayed optical response of inhomogeneous ensembles of emitters upon resonant laser excitation. In semiconductors, they allow to uncover internal scattering and interaction dynamics on the picosecond timescale, while also holding promise as the realization of quantum optical memories in future quantum networks. Based on these two fields of application, this work uses photon echoes to investigate two material systems: Organic-inorganic perovskites and (In,Ga)As quantum dots. Organic-inorganic perovskites have attracted significant attention for their exceptional performance in photovoltaics and light-emitting applications. However, a comprehensive understanding of coherent light-matter interactions in this material class and in particular the role of excitons close to the band gap remained elusive. This work reveals that excitons dominate the nonlinear optical response of MAPbI3 and FAPbI3 single crystals and are subject to strong inhomogeneous broadening even at cryogenic temperatures. Compositional substitution is found to induce spatial band gap fluctuations on the nanometer scale that localize excitons accompanied by an extension of their coherence time by two orders of magnitude. Furthermore, exciton interactions are studied through polarization-dependent photon echoes, uncovering the formation of a biexciton state and the contribution of spin-dependent many-body interactions to nonlinear optical spectra. Subsequently, the focus shifts to confined excitons in (In,Ga)As semiconductor quantum dot ensembles, that represent an ideal platform to explore new approaches on how to coherently transfer, manipulate, and retrieve optical information to a solid state on picosecond timescales. First, it is demonstrated that collective Rabi rotations of the photon echoes from a quantum dot ensemble can be observed when a spatially uniform excitation profile is used. In this way, internal mechanisms of decoherence under strong laser excitation are identified. Thereafter, the photon echo sequence is expanded by two control pulses, providing all-optical control over the emission time, spectral response, and polarization state of photon echoes from quantum dots. Here, the interplay of temporally sorted multi-wave-mixing signals is exploited.
Magneto-optical effects in hybrid plasmonic nanostructures
(2023) Klompmaker, Lars; Akimov, Ilya; Hövel, Heinz
This thesis focuses on magneto-optical effects and their enhancement at optical resonances in hybrid plasmonic nanostructures. One of the main goals is to gain a better understanding of the transverse magnetic routing of light emission (TMRLE) regarding both components of the hybrid plasmonic-semiconductor model system used to investigate this novel effect. Here, the TMRLE describes the routing of light emitted from excitons in a diluted magnetic semiconductor (DMS) quantum well (QW), where the selection rules of the exciton optical transitions are modified by an external magnetic field to have a non-zero transverse spin along the magnetic field direction. By placing the light source near a surface, it can couple to subwavelength evanescent optical fields, such as surface plasmon polaritons (SPPs), which possess a strong transverse spin and spin-momentum locking. This translates the spin of the emitter into a routed wave along the surface and directional emission into the far-field. Firstly, the temperature dependence of the routing from the DMS QW, used as strongly polarizable light source, is investigated. The findings reveal a significant decline in the achievable emission routing for increasing temperatures, but also the emergence of the light-hole emission, which is routed in the opposite direction to the main heavy-hole emission. Additionally, alternative non-DMS-based QW structures are explored as potential candidates for achieving temperature-independent emission routing. Secondly, the influence of the plasmonic nanograting, the other constituent of the hybrid structure, on the enhanced routing is demonstrated. The emission directionality is investigated for various grating periods and slit widths, which also reveals the usually hard-to-detect weak coupling between the QW excitons and the SPPs as a large contributor to the emission directionality spectrum. Lastly, the thesis explores the transverse magneto-optical Kerr effect (TMOKE) for light reflected from or transmitted through a magnetite-based plasmonic waveguide structure. Here, the hybridization of the plasmonic and magnetic waveguide modes leads to a wide-band enhancement of the TMOKE signal in transmission.
Photon echo from trions in semiconductor quantum wells and quantum dots
(2023) Kosarev, Alexander; Akimov, Ilya; Cinchetti, Mirko
The present work is dedicated to the study of the ultrafast coherent dynamics of the excitonic complexes localized in semiconductor nanostructures. We consider the optical control of the optical and spin coherence of the negatively charged excitons using the photon echo technique. In the first two chapters, we overview the basic knowledge of the research area and the experimental techniques. In the third chapter, we considered the freezing of the dephasing in the inhomogeneously broadened ensembles of InGaAs quantum dots driven by the application of a resonant control pulse with an area of multiple of 2p. The freezing of dephasing allows us to vary the timing of the photon echo by up to 5 ps, which even slightly exceeds the echo duration. In the fourth chapter, we consider the resident electron dynamics in the CdTe quantum well. Long-lived three-pulse photon echoes in combination with the pump-probe Kerr rotation allowed us to analyze comprehensively the resident electron dynamics since they are differently impacted by the hopping of the resident electrons between the different localization sites. From the data, we evaluate the hopping rate all-optically. In the fifth chapter, we demonstrate the long-lived three-pulse photon echoes from the ensembles of the InGaAs quantum dots. It allowed us to extend the coherent optical response by at least an order of magnitude. We show that the transverse g-factor of the heavy hole in the trion state strongly impacts the temporal evolution of the system, and therefore modifies the long-lived three-pulse photon echo signal.
Tuning the nuclei-induced spin relaxation of localized electrons by the quantum Zeno and anti-Zeno effects
(2023-09-07) Nedelea, Vitalie; Leppenen, Nikita V.; Evers, Eiko; Smirnov, Dmitry S.; Bayer, Manfred; Greilich, Alex
Quantum measurement back action is fundamentally unavoidable when manipulating electron spins. Here we demonstrate that this back action can be efficiently exploited to tune the spin relaxation of localized electrons induced by the hyperfine interaction. In optical pump-probe experiments, powerful probe pulses suppress the spin relaxation of electrons on Si donors in an InGaAs epilayer due to the quantum Zeno effect. By contrast, an increase of the probe power leads to a speed-up of the spin relaxation for electrons in InGaAs quantum dots due to the quantum anti-Zeno effect. The microscopic description shows that the transition between the two regimes occurs when the spin dephasing time is comparable to the probe pulse repetition period.
Polarization- and time-resolved nonlinear optical spectroscopy of excitons in Cu2O
(2023) Farenbruch, Andreas; Yakovlev, Dmitri; Reiter, Doris
Excitons are Coulomb-bound states of a negatively charged electron in the conduction band and a positively charged hole in the valence band of a semiconductor or an insulator. Their hydrogen-like absorption spectrum was first detected in 1951 in cuprous oxide Cu2O. In 2014, a spectral scan using an ultra-narrow bandwidth laser extended the P exciton series in a natural Cu2O crystal to a principal quantum number of n = 25. This breakthrough opened up the field of Rydberg physics in semiconductors leading to numerous experimental and theoretical studies investigating their behavior in external fields as well as exciton-plasma and exciton-exciton interactions. The present study reports on second harmonic generation (SHG) spectroscopy of dark- and bright excitons in Cu2O. SHG is forbidden for laser light propagating along the high-symmetry [110] and [001] crystal axis. By applying a magnetic field up to 10 T perpendicular to the light direction, SHG becomes allowed due to the Zeeman- (ZE) and magneto-Stark effect (MSE). The polarization selection rules for both mechanisms are derived from point group theory considerations. The linear polarization angles of the incoming and outgoing light can be controlled in order to differentiate between both SHG mechanisms. The spin-forbidden dark paraexcitons are activated by admixture of the M = 0 component of the bright orthoexcitons in a magnetic field. The Rydberg series of dark paraexcitons up to a quantum number of n = 6 is detected using this method. Due to the electron-hole exchange interaction, the paraexcitons are generally located energetically below the orthoexcitons. This order is found to be reversed for the n = 2 state due to mixing of the yellow 2S orthoexciton with the green 1S orthoexciton. The blue series of excitons involves the same valence band as the yellow series but the second-lowest conduction band. Accessing these states using linear optical spectroscopy is challenging due to the high absorption in this spectral range. MSE and ZE-induced SHG has been shown to be a suitable investigation method enabling the detection of blue 1S, 2S, and 2P excitons and magneto-excitons up to n=8. By analyzing their magnetic-field shift and polariton effect, key properties of blue excitons, such as the resonance energies, the exciton radius, the band gap, the reduced exciton mass, and the anisotropic conduction band mass, are obtained. Difference frequency generation with two-photon excitation (2P-DFG) allows experimental investigations of excitons in the time domain. The pulses of the first laser induce a coherent exciton population by a two-photon excitation process. The pulses of the second laser stimulate the emission of photons with the energy difference between the excitons and the stimulating photons. By delaying the pulses of the second laser, the 2P-DFG signal is measured as a function of time. This technique is used to measure the coherence times of 1S and higher n excitons and magnetic-field-induced quantum beats of the three 1S orthoexciton states.
Optical excitation and detection of high-frequency coherent phonons and magnons
(2022) Kobecki, Michal; Bayer, Manfred; Cinchetti, Mirko
The goal of this work is to obtain better control over all-optical excitation and detection of high-frequency collective excitations, in particular - phonons and magnons. Phonons- a collective movement of atoms in the lattice, and magnons- excitations of a spin system in magnetically ordered materials, recently became the prospective alternative to electrons in quantum computing or in general, information technology. In the scope of this thesis, we present a novel approach to the resonant excitation of fundamental magnon mode in a thin ferromagnetic film of Iron-Gallium alloy, as well as enhanced detection sensitivity of propagating coherent phonon wave packet exploiting giant photo-elasticity of exciton-polaritons in GaAs/AlAs superlattice. Additionally, a possible way to miniaturize the all-optical set-ups for manipulation of collective excitations is proposed by implementing a passively mode-locked semiconductor laser diode.
Measuring coherence properties of exciton-polaritons with homodyne detection
(2022) Lüders, Carolin; Aßmann, Marc; Sperling, Jan
Für die Quantentechnologie sind hybride Systeme gefragt, die verschiedene physikalische Systeme verbinden, z.B. ein Materiesystem zur Informationsverarbeitung und Licht zur Kommunikation. Für die Verbindung zwischen Halbleitern und Licht erforscht die Halbleiter-Quantenoptik, wie Licht den Quantenzustand des Halbleiters beeinflusst und wie der Zustand des Halbleiters über das emittierte Licht gemessen werden kann. Zur Messung des Quantenzustands von Licht wird in der Quantenoptik die vielseitige Methode der optischen Homodyn-Tomographie (OHT) verwendet. Ihre Anwendung auf die Emission von Halbleitern wird jedoch häufig durch das Fehlen einer festen Phasenreferenz für nicht-resonante Lumineszenz und durch die schnellen Zeitskalen des Systems verhindert. Diese Herausforderungen werden in dieser Arbeit angegangen. Wir stellen die Anwendung von OHT auf Halbleiterlumineszenz ohne feste Phasenreferenz vor, um die Kohärenz-Eigenschaften und den Quantenzustand zu untersuchen. Dabei ermöglichen ein gepulster Lokaloszillator und schnelle Detektoren eine hohe Zeitauflösung. Als Testumgebung für die Methode untersuchen wir die Emission eines Exziton-Polariton-Kondensats in einer GaAs-Mikrokavität. Konkret zeigt diese Arbeit, welche Informationen durch die Verwendung von einem, zwei und drei Homodyn-Detektionskanälen gewonnen werden können. Mit einem Kanal wird die Photonenkorrelationsfunktion zweiter Ordnung g(2)(0) gemessen, mit zwei Kanälen messen wir die phasengemittelte Husimi-Funktion und quantifizieren den Grad der Quantenkohärenz im Polaritonensystem, und mit drei Kanälen rekonstruieren wir die regularisierte P-Funktion abhängig von postselektierten Anfangsbedingungen und verfolgen den zeitlichen Zerfall der Quantenkohärenz.
The Landé factors of electrons and holes in lead halide perovskites: universal dependence on the band gap
(2022-06-02) Kirstein, E.; Yakovlev, D. R.; Glazov, M. M.; Zhukov, E. A.; Kudlacik, D.; Kalitukha, I. V.; Sapega, V. F.; Dimitriev, G. S.; Semina, M. A.; Nestoklon, M. O.; Ivchenko, E. L.; Kopteva, N. E.; Dirin, D. N.; Nazarenko, O.; Kovalenko, M. V.; Baumann, A.; Höcker, J.; Dyakonov, V.; Bayer, M.
The Landé or g-factors of charge carriers are decisive for the spin-dependent phenomena in solids and provide also information about the underlying electronic band structure. We present a comprehensive set of experimental data for values and anisotropies of the electron and hole Landé factors in hybrid organic-inorganic (MAPbI3, MAPb(Br0.5Cl0.5)3, MAPb(Br0.05Cl0.95)3, FAPbBr3, FA0.9Cs0.1PbI2.8Br0.2, MA=methylammonium and FA=formamidinium) and all-inorganic (CsPbBr3) lead halide perovskites, determined by pump-probe Kerr rotation and spin-flip Raman scattering in magnetic fields up to 10 T at cryogenic temperatures. Further, we use first-principles density functional theory (DFT) calculations in combination with tight-binding and k ⋅ p approaches to calculate microscopically the Landé factors. The results demonstrate their universal dependence on the band gap energy across the different perovskite material classes, which can be summarized in a universal semi-phenomenological expression, in good agreement with experiment.
Extending the time of coherent optical response in ensemble of singly-charged InGaAs quantum dots
(2022-06-10) Kosarev, Alexander; Trifonov, Artur V.; Yugova, Irina A.; Yanibekov, Iskander I.; Poltavtsev, Sergey; Kamenskii, Aleksandr; Scholz, Sven; Sgroi, Carlo Alberto; Ludwig, Arne; Wieck, Andreas; Yakovlev, Dmitri; Bayer, Manfred; Akimov, Ilya
The ability to extend the time scale of the coherent optical response from large ensembles of quantum emitters is highly appealing for applications in quantum information devices. In semiconductor nanostructures, spin degrees of freedom can be used as auxiliary, powerful tools to modify the coherent optical dynamics. Here, we apply this approach to negatively charged (In,Ga)As/GaAs self-assembled quantum dots which are considered as excellent quantum emitters with robust optical coherence and high bandwidth. We study three-pulse spin-dependent photon echoes subject to moderate transverse magnetic fields up to 1 T. We demonstrate that the timescale of coherent optical response can be extended by at least an order of magnitude by the field. Without magnetic field, the photon echo decays with T2 = 0.45 ns which is determined by the radiative lifetime of trions T1 = 0.26 ns. In the presence of the transverse magnetic field, the decay of the photon echo signal is given by spin dephasing time of the ensemble of resident electrons T2,e ∼ 4 ns. We demonstrate that the non-zero transverse g-factor of the heavy holes in the trion state plays a crucial role in the temporal evolution and magnetic field dependence of the long-lived photon echo signal.
SPP2402-Projektantrag "Greybox-Modellierungen zum Einlaufverhalten beschichteter Werkzeuge im Fräsprozess als dynamisches Lastkollektiv auf Basis von Operando-, IN- und Ex-situ-Analysen"
(2022-10-17) Debus, Jörg
Die Datenpunkte für die drei Kurven der Abb. 3a liegen in der txt-Datei "Abb3a" vor. Sie enthält vier Spalten mit der Struktur (x y y y), wobei die erste Spalte die gemeinsamen Daten der x-Achse enthält. Die anderen drei Spalten umfassen die jeweiligen y-Werte der drei Kurven (input, denoised, target). Für die Abb. 3b mit ihren 3 Panels liegen drei txt-Dateien vor, gemäß dem Verschleißzustand (initial, 9800 mm^3, 54800 mm^3). Jede Datei enthält sechs Spalten mit xy-Daten für jeden der drei verwendeten Cluster. Die ersten beiden Spalten enthalten die xy-Datenpunkte für den 1. Cluster, die dritte und vierte Spalte die xy-Daten für den 2. Cluster und die letzten beiden Spalten enthalten die xy-Daten des 3. Clusters. Die txt-Dateien für die Abb. 3c besitzen die gleiche Struktur wie die der Abb. 3b. Zusätzlich sind die xy-Datenpunkte für die Regressionskurve, h(Delta E), des Panels für 54800 mm^3 in der txt-Datei "Abb3c_54800_fitkurve" enthalten. Die txt-Datei zur Abbildung 3d enthält vier Spalten (x y x y). Die ersten beiden Spalten enthalten die Datenpunkte für den initialen Zustand und die beiden letzten Spalten beschreiben den Zustand mit dem Materialabtragsvolumen 54800 mm^3. Die txt-Datei zum Histogramm der Abb. 3e enthält drei Spalten, welche die Prädiktor-Wichtigkeit für die Amplitude (1. Spalte), Kristallitgröße h (2. Spalte) und Hintergrundintensität (3. Spalte) enthalten. Die Datei zur Abbildung 3f enthält vier Spalten (x y x y). Die ersten beiden Spalten gehören zu der grünen Kurve und die dritte und vierte Spalte zu der schwarzen Kurve der Abbildung 3f.
Coherent spin dynamics and carrier-nuclear interaction in lead halide perovskite crystals
(2022) Kirstein, Johan Erik; Greilich, Alex; Cinchetti, Mirko
In the present work, the charge carrier spin dynamics in lead halide perovskites APbX3, with X = {I,Br,Cl} are studied. The lead halide perovskites are distinguished with respect to the groups of purely inorganic perovskites with A = Cs and hybrid organic inorganic ones with A = {MA,FA} (methylammonium and formamidinium), as well as their size, i. e. macroscopic single crystals or nano crystals. The spin dynamics of the charge carriers, electrons and holes, are mainly investigated using picosecond resolution Kerr and Faraday spectroscopy. In addition, the observation of the spin dynamics is complemented by exciton spectroscopy (polarization-resolved photoluminescence, reflection, and transmission) and time-resolved differential reflectometry. Nanosecond spin dynamics are observed and the underlying interaction mechanisms are revealed by the experimental techniques, with a focus on interaction of charge carrier and nuclear spins.
Nonlinear optical functionalities of VO2- and GaN-based nanocomposites
(2022) Mundry, Jan; Betz, Markus; As, Donat Josef
This thesis presents fundamental research and concepts for active photonic elements operating in the telecom wavelength regime. The aim of the study is to determine the characteristics of the investigated nanostructures and to evaluate the implementation of the proposed materials in potential optical devices. In the first part of this thesis the optical properties as well as the photonic application of vanadium dioxide (VO2) nanocrystals (NCs) are studied. VO2 exhibits an easily accessible insulator-to-metal phase transition (IMT) near ambient temperatures. Upon excitation it undergoes an atomic rearrangement that is accompanied by a substantial modification of the complex dielectric function. When VO2 undergoes the IMT, the near-infrared transmission peaks of a moderate-finesse etalon containing a sub-wavelength layer of VO2 NCs are found to markedly shift in their spectral position and peak transmissivity. Both heat deposition and optical excitation permit to actively control the etalon’s functionality. Much less is known about the nonlinear optical properties of VO2 beyond the established IMT. To this end the nonlinear optical response of a thin film of VO2 NCs is investigated with open aperture z-scans involving femtosecond near-infrared pulses. A pronounced saturable absorption on the short-wave side of the resonance as well as a marked reverse saturable absorption in the telecom window are observed. The results hold promise for the use of VO2 nanocrystals as a saturable absorber, e.g., to mode-locked near-infrared lasers. In the second part a semiconductor heterostructure based on hexagonal ultranarrow GaN/AlN multi-quantum wells (MQWs) is investigated. The tailored inter-miniband (IMB) transition is characterized in terms of its linear as well as ultrafast nonlinear optical properties using the established pump-probe scheme. In line with theoretical predictions for LO-phonon scattering, a fast relaxation is found for resonant IMB excitation. In stark contrast, significantly larger relaxation times are observed for photon energies addressing the above barrier continuum. The last section reports on a new type of nonlinear metasurface taking advantage of these telecom-range IMB transitions. The heterostructure is functionalized with an array of plasmonic antennas featuring cross-polarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second harmonic generation at normal incidence which is completely absent for an antenna array without the heterostructure underneath.

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