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Optoelectronics in the deep-ultraviolet (DUV) regime is still a growing research field that requires significant effort to understand the material properties and optimize the device structures to realize efficient DUV devices. Aluminum gallium nitride (AlGaN) is perhaps the most studied semiconductor to replace the environmentally hazardous mercury lamps; however, the external quantum efficiency of AlGaN based DUV devices is insufficient to replace the existing old-fashioned mercury UV lamps. Despite the tunability in the bandgap of AlGaN, the excessive strain accumulation associated with increased alloying of Al in AlGaN and the poor dopant activation due to the relatively large ionization energy of the donors and acceptors are not favorable for realizing efficient DUV emitters. In addition, the crossover among the light hole, heavy hole and split-off bands in the valance band for Al-rich AlGaN suppresses the transverse-electric polarization, which further worsens the external quantum efficiency. Furthermore, for DUV photodetection, commercially available Si-photodetectors suffer from poor responsivity for wavelengths shorter than 400 nm in contrast to the visible spectrum. Hence, the-state-of-art photodetectors in the DUV regime also need a significant upgrade, particularly for high-speed applications. Firstly, we utilized the high quantum confinement in plasma-assisted MBE grown ultrathin GaN QDisks to realize DUV (λ ≈ 260 nm) emission using a binary compound (GaN) in contrast to conventionally used ternary compound (AlGaN). More importantly, we experimentally demonstrated TE-dominant DUV emission, unlike Al-rich AlGaN, illustrating a unique pathway for realizing efficient DUV vertical emitters. Secondly, inspired by the light manipulation technique practiced in nature, we presented iridocytes on giant clams (Tridacna maxima), known for their symbiotic relationship with algae as a color downconverting material for DUV photodetection. Investigating the structural and optical properties of iridocytes found in Tridacna maxima, we established a robust UV communication allowing the data transfer rate of 100 Mbit/s within the forward error correction limit for modulated 375 nm-laser diode. Using a similar matrix implemented to 375 nm-laser, with high-power UV-C LED (λ ≈ 278 nm), we could establish an optical wireless communication that could allow a data-transmission rate of tens of Mbit/s within the forward error correction limit.

Název práce: Nelineární optické vlastnosti křemíkových nanostruktur Autor: Karel Žídek Katedra (ústav): Katedra chemické fyziky a optiky Vedoucí disertační práce: Doc. RNDr. František Trojánek, Ph.D. E-mail vedoucího: trojanek@karlov.mff.cuni.cz Abstrakt: Disertační práce se zabývá nelineárními optickými jevy a ultrarychlým vývojem luminis- cence křemíkových nanokrystalů. Pomocí metody optického hradlování signálu (časové rozlišení až 250 fs) porovnáváme ultrarychlý vývoj luminiscence křemíkových nanokrystalů s různými ve- likostmi (v řádu jednotek nanometrů) a také s rozdílnými formami pasivace. Pro nanokrystaly, kde po excitaci dominuje vliv zachytávání nosičů do povrchových stavů nanokrystalu, navrhujeme teoretický popis závislosti rychlosti těchto procesů na vlastnostech nanokrystalů. Dále v práci podrobně zkoumáme působení Augerovy rekombinace, která se projevuje jak v časově rozlišené, tak i v časově integrované emisi vzorků. Experimentální data velmi dobře popisuje námi navržený model na bázi kinetických rovnic. Závěr práce se zaměřuje na zkoumání ultrarychle dohasínající stimulované emise. U stávajících metod měření optického zisku (VSL a SES) navrhujeme jejich rozšíření pro...

In this work, optical pump – near-infrared probe and near-infrared pump – mid-infrared probe spectroscopy are used for the investigation of pressure-induced insulator-to-metal transitions in transition metal oxide compounds. The materials under study are α-Fe₂O₃, also known as hematite, and VO₂. Both materials undergo pressure-induced metallization. However, the physical mechanisms of this phase transition are very different for these systems and have not been fully understood up to now. Using ultrafast pump-probe spectroscopy we obtain an insight into the evolution of the band structure and electron dynamics across the insulator-to-metal transition. In the case of VO₂, our near-infrared pump – mid-infrared probe experiments reveal a non-vanishing pumping threshold for photo-induced metallization even at our highest pressures around 20 GPa. This demonstrates the existence of localized charge carriers and the corresponding persistence of a band gap. Besides the threshold behaviour for photo-induced metallization, the carrier relaxation time scale, and the linear reflectivity and transmissivity have been studied under pressure increase. An anomaly in the threshold behaviour as well as the linear reflectivity and transmissivity at a critical pressure around 7 GPa indicates band gap filling under pressure. This is further supported by results obtained under decompression, where the changes of the linear reflectivity turned out to be almost fully reversible. The observations on VO₂ are highly reproducible and can be explained in terms of a pressure-induced bandwidth-driven insulator-to-metal transition. Fe₂O₃ has been studied via optical pump – near-infrared probe spectroscopy up to pressures of 60 GPa. In the pressure range up to 40 GPa, the changes of the response can be explained by photo-induced absorption and bleaching. The pressure dependent study of the relaxation dynamics allows to identify cooling of the electron system as origin of the picosecond relaxation process. A sharp anomaly found in the response of Fe₂O₃ at 40 GPa indicates a strong rearrangement of the electronic band structure which could be explained by an insulator-to-metal phase transition induced by pumping. The successful demonstration of pump-probe experiments in diamond anvil cells using pulses from optical to mid-infrared wavelengths and reaching pressures of several tens of GPa is a good basis for further experimental high-pressure studies. Our results obtained on VO₂ and Fe₂O₃ can serve as a benchmark for the development of advanced material models. / In der vorliegenden Arbeit wird der druckinduzierte Isolator–Metall-Phasenübergang in den Übergangsmetalloxiden α-Fe₂O₃ (Hämatit) und VO₂ mittels ultraschneller Anrege-Abfrage-Spektroskopie (engl. pump-probe spectroscopy) untersucht. Hämatit wird dazu im sichtbaren Spektralbereich angeregt und im nahen Infrarot (NIR) abgefragt, bei VO₂ wurde zur Anregung NIR und zur Abfrage mittleres Infrarot (MIR) verwendet. Beide Materialien werden bei hinreichend hohem Druck metallisch, wobei die jeweils dem Isolator–Metall-Phasenübergang zugrundeliegenden Mechanismen verschieden und noch nicht vollständig verstanden sind. Dies motiviert den Einsatz von ultraschneller Anrege-Abfrage-Spektroskopie, die einen Einblick in die Änderung der Bandstruktur und der Ladungsträgerdynamik während des Isolator–Metall-Übergangs gewährt. Beim Überschreiten eines Schwellenwertes der Anregung wird VO₂ photoinduziert metallisch. In unseren NIR-MIR Anrege-Abfrage-Experimenten zeigt sich, dass der Schwellenwert auch bei den höchsten Drücken dieser Messreihe (ca. 20 GPa) nicht verschwindet. Dies weist auf die Existenz lokalisierter Ladungsträger hin und damit verbunden auf das Fortbestehen der Bandlücke. Neben dem Schwellenwert für photoinduzierte Metallisierung wurden auch die Druckabhängigkeiten der Relaxationsdynamik der Ladungsträger sowie des linearen Reflexions- und Transmissionsvermögens untersucht. Eine Anomalie im druckabhängigen Verlauf des Anrege Schwellenwertes sowie des linearen Reflexions- und Transmissionsvermögens bei einem kritischen Druck von ca. 7 GPa deutet darauf hin, dass durch das Anlegen von Druck Zustände innerhalb der Bandlücke induziert werden. Diese Interpretation wird auch durch während der Dekompression gewonnene Messdaten unterstützt. Die druckinduzierte Änderung des linearen Reflexionsvermögens erwies sich als nahezu vollständig reversibel. Unsere Beobachtungen an VO₂ sind reproduzierbar und lassen sich als druckinduzierter, Bandbreiten-getriebener Isolator–Metall-Übergang nachvollziehen. Fe₂O₃ wurde mittels Anrege-Abfrage-Spektroskopie bei Drücken bis zu 60 GPa untersucht. Änderungen im Druckbereich bis 40 GPa können als Wechselspiel eines photo-induzierten Absorptionsbandes und der photoinduzierten Unterdrückung eines anderen Absorptionskanals erklärt werden. Die druckabhängige Untersuchung der Relaxationsdynamik ermöglicht es, der Relaxation auf der Zeitskala weniger Pikosekunden Kühlungsdynamik als Ursache zuzuordnen. Eine scharfe Anomalie im qualitativen Verlauf des Anrege-Abfrage-Signals von Fe₂O₃ bei einem Druck von 40 GPa weist auf deutliche Änderungen in der elektronischen Bandstruktur hin, welche als Signatur eines photoinduzierten Isolator–Metall Phasenübergangs interpretiert werden können. Die erfolgreiche Demonstration von Anrege-Abfrage-Experimenten in Diamantstempeldruckzellen mit Laserimpulsen vom sichtbaren Spektralbereich bis hin zum mittleren Infrarot und bei Drücken von 20 GPa bis zu 60 GPa liefert die solide Basis für weitergehende Hochdruck-Experimente. Die an VO₂ und Fe₂O₃ erzielten Ergebnisse sind eine gute Grundlage für die Weiterentwicklung der theoretischen Beschreibung solcher Materialsysteme.

This thesis deals with infrared studies of impurity states, ultrafast carrier dynamics as well as coherent intersubband polarizations in semiconductor quantum structures such as quantum wells and superlattices, based on the GaAs/AlGaAs material system. In the first part it is shown that the 2pz confined impurity state of a semiconductor quantum well develops into an excited impurity band in the case of a superlattice. This is studied by following theoretically the transition from a single to a multiple quantum well or superlattice by exactly diagonalizing the three-dimensional Hamiltonian for a quantum well system with random impurities. Intersubband absorption experiments, which can be nearly perfectly reproduced by the theory, corroborate this interpretation, showing that at low temperatures in the low doping density regime all optical transitions originate from impurity transitions. These results also require reinterpretation of previous experimental data. The relaxation dynamics of interminiband transitions in doped GaAs/AlGaAs superlattices in the mid-IR are studied. This involves single-color pump-probe measurements to explore the dynamics at different wavelengths, which is performed with the Rossendorf freeelectron laser (FEL), providing picosecond pulses in a range from 3-200 µm and are used for the first time within this thesis. In these experiments, a fast bleaching of the interminiband transition is observed followed by thermalization and subsequent relaxation, whose time constants are determined to be 1-2 picoseconds. This is followed by an additional component due to carrier cooling in the lower miniband. In the second part, two-color pump-probe measurements are performed, involving the FEL as the pump source and a table-top broad-band tunable THz source for probing the transmission changes. These measurements allow a separate specification of the cooling times after a strong excitation, exhibiting time constants from 230 ps to 3 ps for different excitation densities and miniband widths. In addition, the dynamics of excited electrons within the minibands is explored and their contribution quantitatively extracted from the measurements. Intersubband absorption experiments of photoexcited carriers in single quantum well structures, measured directly in the time-domain, i.e. probing coherently the polarization between the first and the second subband, are presented. From the data we can directly extract the density and temperature dependence of the intersubband dephasing time between the two lowest subbands, ranging from 50 up to 400 fs. This all optical approach gives us the ability to tune the carrier concentration over an extremely wide range which is not accessible in a doped quantum well sample. By varying the carrier density, many-body effects such as the depolarization and their influence on the spectral position as well as on the lineshape on the intersubband dephasing are studied. Also the difference of excitonic and free-carrier type excitation is discussed, and indication of an excitonic intersubband transition is found.

Chromophore is a molecule or a part of a molecule which is responsible for its appearance color. This definition has been evolving over time with the progress of science. Contemporary scientific advances have expanded its meaning: to an inclusive level, chromophore is an irreducible collective of fundamental particles, which can represent the photophysical (optical physical) properties of the macroscopic matter. Previous studies have already found that the same molecule can have different photophysical properties under different condensed states. Therefore, it is straight forward to conclude that the definition of chromophore should take such extrinsic influencing interactions of this given molecule into consideration, thus simply taking the smallest unit such as a molecule is not accurate. A good example is quantum dots. Same species of quantum dots possess the identical smallest chemical unit but can emit very differently due to quantum confinement effect, thus defining the smallest unit as the chromophore is apparently fallacious. In solid polymeric compositions, the chemical unit or building blocks may differ from the spectroscopic unit depending on how these chemical units interacts within their ensemble to evolve new properties such as a new transition dipole. As thus, understanding the evolution of photophysical behaviors between the targeted unit and neighbors is of much importance to determine whether they should be considered as one chromophore or many. This requires a thorough understanding towards the evolution of photophysical properties of a collective, and the construction of such collective will need to pay extra attention to, as any structural factor could have changed some photophysical interactions of the collective. The introductory chapter discusses the material platform and fundamental photophysics investigated in this dissertation. Chromophore assembly (CA) as a sylloge of several classes of self-assembled materials, including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), porous organic polymers (POPs). Among them, MOF-based CAs (MOF-CAs) featuring with the ease of synthesis, demonstrate incomparable promises to construct such collective with several appealing characteristics, including component diversity, chemical stability, structural porosity, and post-synthetic versatility (Chapter 1.1). As for here, the main target to achieve using these assemblies is to understand the interaction between adjacent chemical monomeric units, therefore their spatial arrangements are of the paramount importance. As modern theory discovered, both ordered and random systems can be very important for novel quantum material developments. Both crystalline and amorphous arrangements of monomeric units can be achieved by adopting different classes of materials. MOF-CAs could achieve the precise control of spatial arrangement including distance, direction, and dihedral angle by its crystalline structures, whereas porous organic polymer-based CAs (POP-CAs) could feature a total randomness. Photophysics, as the research topic targeting the firsthand knowledge gained by interrogating the information provided by the propagating light after its interaction with matters, could provide crucial knowledge of the targeted matter. Hence, photophysical properties could provide fundamental understanding of the targeted matter (Chapter 1.2). State-of-the-art spectroscopic methods and instrumentation have made it possible to critically examine new structures to correlate photophysics with the chemical structure of their assemblies. By combining multiple spectroscopic techniques along with theoretical study, several correlations between the electronic properties of the matter, such as structural features, have been investigated. To illustrate, some unique topology-dependent photophysical behaviors found in chromophore assemblies are introduced (Chapter 1.3). In this dissertation, the feasibility of using specific types of MOF-CAs to conduct unique photophysical studies has been carefully chosen and verified (Chapter 2). Next, with the help of first principles computations, the nature of several electronic excited states as a function of different extent of Van der Waals or electronic interaction in MOF-CAs is unveiled, and experimentally studied with several environmental variates (Chapter 3). The knowledge was then articulated to devise a strategy to improve resonance energy transfer process in MOF-CAs. Here, low electronic symmetry of linker and directionally aligned transition dipoles of their collective ensembled are found beneficial to improve such photophysical process in a bottom-up manner (Chapter 4). Then, a series of MOFs were rationally designed to examine the feasibility and extent of a nonlinear excitonic process, singlet fission, to promote the generation of carriers usable for many applications including light-harvesting applications. The outcome demonstrated MOF-CA is a powerful tool to design such materials and is more capable in terms of its tunability (Chapter 5). At last, a set of randomly oriented CAs in POP were examined for underlying excited state dynamic process that highlights a thermal activated delayed fluorescence (TADF) involving S1 and low-lying T2 excited states (Chapter 6). This dissertation has highlighted unique yet tunable excited-state features and photophysical processes within the well-defined molecular ensemble realized via porous frameworks. These photophysical properties differ from those of their respective molecular system in their solubilized forms. Studies in this dissertation demonstrates a reliable platform to investigate multibody chromophore systems and suggested several valuable discoveries and lights the way for the study of novel chromophore assembly systems.

Successful enzyme catalysis depends on many noncovalent interactions between the enzyme, cofactors, and substrate that poise the system to access a productive transition state. Motions on a variety of timescales contribute to this, but some controversy exists surrounding the role of ultrafast dynamics on catalysis. Site-specific 2D IR spectroscopy using probes of vibrational dynamics provides the opportunity to explore ultrafast motions in an enzyme active site owing to the technique’s spatial and temporal resolution. In this work, a series of aryl-azide vibrational labels were assessed using a variety of 2D IR techniques for their sensitivity to solvent and energy transfer processes, and their ability to be adapted to experiments in biomacromolecules. One of these labels, 4-azido-N-phenylmaleimide, is a substrate analog for the promiscuous ene-reductase from Pyrococcus horikoshii (PhENR). The label was covalently attached in two orientations in the enzyme active site, occupying the same position as native substrates based on X-ray crystallography and molecular dynamics simulations. FTIR and 2D IR spectroscopy were used to identify close-lying conformational states based on the strong anharmonic coupling of the label, revealing that the active site itself modulates the probe’s internal vibrational coupling. More commonly used analogous aryl-nitrile labels, however, were not sensitive to such small structural and lineshape changes. This demonstrates the importance of thoughtful label design to maximize the amount of information that can be gleaned from 2D IR studies. Using the methods herein—both spectroscopic and biochemical—provides a strategy for probing ultrafast motions that could possibly be catalytically relevant.

In dieser Arbeit werden elektronische und strukturelle Dynamiken durch Femtosekundenpuls injizierte kleine Polaronen in Lithiumniobat betrachtet, sowie die Auswirkungen auf die nichtlineare Optik mit Schwerpunkt auf die Holographie und den mittleren infraroten Spektralbereich untersucht.

New methods to measure and quantify tissue molecular composition and metabolism are a major driver of discovery in basic and clinical research. Optical methods are well suited for this task based on the non-invasive nature of many imaging and spectroscopy techniques, the variety of exogenous fluorescent probes available, and the ability to utilize label-free endogenous absorption signatures of tissue chromophores including oxy- and deoxy-hemoglobin, water, lipid, collagen, and glucose. Despite significant advances in biomedical imaging, there remain challenges in probing tissue information in a fast, wide-field, and non-invasive manner. Moreover, quantitative in vivo mapping of endogenous biomarkers such as water and lipids remain relatively less explored by the biomedical optics community due to their characteristic extinction spectra, which have distinct spectral features in the shortwave infrared, a wavelength band that has been traditionally more challenging to measure. The work presented in this dissertation was focused on developing instrumentation and algorithms for non-invasive quantification of tissue optical properties, fluorophore concentrations, and chromophore concentrations in a wide-field imaging format. All of the imaging methods and algorithms developed in this thesis extend the capability of the emerging technique called Spatial Frequency Domain Imaging (SFDI). First, a new imaging technique based on SFDI is presented that can quantify the quantum yield of exogenous fluorophores in tissue. This technique can potentially provide a new non-invasive means for in vivo mapping of local tissue environment such as temperature and pH. Next, an angle correction algorithm was developed for SFDI for more accurate estimation of tissue optical properties as well as chromophore concentrations in highly curved tissue, including small animal tumor models. Next, a wide-field label-free optical imaging system was developed to simultaneously measure water and lipids using the shortwave infrared (SWIR) wavelength region. Last, to break the bottleneck of processing speed in optical property inversion, new deep learning based models were developed to provide over 300× processing speed improvement. Together, these projects substantially extend the available contrasts and throughput of SFDI, providing opportunities for new preclinical and clinical applications. / 2020-10-22T00:00:00Z

Les impulsions laser ultrabrèves nous permettent de suivre en temps réel les phénomènes ultrarapides au sein de la matière à l’échelle microscopique. C’est précisément pour l’invention de la chimie à l’échelle femtoseconde, ou femtochimie, qu’Ahmed Zewail se vit décerner le prix Nobel de chimie en 1999. Depuis les utilisateurs du laser cherchent à augmenter la résolution temporelle, c’est-à-dire réduire la durée des impulsions laser. Aujourd’hui, nous savons générer des flashs lumineux à l’échelle attoseconde dans le domaine spectral de l’extrême ultraviolet (XUV) mais l’efficacité de génération reste faible et le développement de sources laser attosecondes intenses constitue un sujet de recherche très actif sur le plan international.Notre groupe au LOA se concentre sur la génération d’impulsions attoseconde sur miroir plasma en régime relativiste. Pour cela, il cherche à développer une source d’impulsions femtosecondes à forte cadence et fort contraste et suffisamment énergétiques pour atteindre des intensités relativistes (>> 10^18W/cm2) lorsqu’elles sont fortement focalisées sur un plasma surdense. Un plasma surdense réfléchit la lumière incidente et par conséquent agit comme un miroir qui se déplaçant à vitesse relativiste et qui comprime l’impulsion incidente, produisant ainsi un flash attoseconde par cycle optique. En utilisant des impulsions proches d’un cycle optique, il est donc envisageable de générer une seule impulsion attoseconde intense pendant l’interaction.Dans la première partie de mon travail de thèse, j’ai réalisé un compresseur nonlinéaire pour réduire la durée des impulsions issues d’une chaîne à double dérive de fréquence (10mJ, 25fs, 1kHz) à phase enveloppe-porteuse (CEP) stabilisée. En propageant les impulsions du laser à haute intensité dans une fibre creuse remplie de gaz rare, j’ai réussi à générer des impulsions de 1.3 cycle optique avec une puissance crête autour de 1TW avec une CEP stabilisée. Dans un deuxième temps, j’ai mis en forme spatialement et temporellement les impulsions issues du compresseur à fibre pour générer à la fois des impulsions attosecondes intenses et des faisceaux d’électrons énergétiques sur un miroir plasma à gradient de densité contrôlé. Ces expériences nous permis, pour la première fois, de mettre en évidence la production d’impulsions attosecondes isolées dans l’XUV, l’émission corrélée de faisceaux d’électrons énergétiques en régime relativiste ainsi qu’un nouveau régime d’accélération d’électrons à très long gradient plasma. / Very short light pulses allow us to resolve ultrafast processes in molecules, atoms and condensed matter. This started with the advent of Femtochemistry, for which Ahmed Zewail received the Novel Prize in Chemistry in 1999. Ever since, researcher have been trying to push the temporal resolution further and we have now reached attosecond pulse durations. Their generation, however, remains very challenging and various different generation mechanisms are the topic of heated research around the world.Our group focuses on attosecond pulse generation and ultrashort electron bunch acceleration on solid targets. In particular, this thesis deals with the upgrade of a high intensity, high contrast, kHz, femtosecond laser chain to reach the relativistic interaction regime on solid targets. Few cycle driving laser pulses should allow the generation of intense isolated attosecond pulses. A requirement to perform true attosecond pump-probe exeriments.To achive this, a HCF postcompression scheme has been conceived and implemented to shorten the duration of a traditional laser amplifier. With this a peak intensity of 1TW was achieved with near-single-cycle pulse duration. For controlled experiments, a vacuum beamline was developed and implemented to accurately control the laser and plasma conditions on target.During the second part of this thesis, this laser chain was put in action to drive relativistic harmonic generation on solid targets. It was the first time ever that this has been achieved at 1 kHz. By CEP gating the few-cycle-pulses, single attosecond pulses were generated. This conclusion has been supported by numerical simulations. Additionally a new regime to accelerate electron bunches on soft gradients has been detected.