Studies

SUMMARY:  This thesis describes the generation of ultrashort high peak power laser pulses using optical parametric and stimulated Raman scattering chirped pulse amplifiers (OPCPA and TSRCPA) pumped by a picosecond Yb:YAG laser. The investigations were carried out by experimental optimization of designs and characterization of the output parameters of the pump laser, OPCPA and TSRCPA. The thesis brings together several advanced and rapidly developing solid-state laser technologies. The first part of the thesis presents a Yb:YAG laser system for pumping OPCPA and TSRCPA, generating pulses with a pulse width of 1.15 ps and an energy of 20 mJ. A method for controlling the envelope of single-picosecond pulses using cascade generation of the second harmonic is also described. The second part is devoted to generating a broadband seed for OPCPA and TSRCPA in the visible and infrared spectral range, respectively. The third and fourth parts of the thesis describe the amplification of chirped continuum pulses in optical parametric and stimulated Raman scattering amplifiers. In the first case, broadband pulses with a pulse width of 20 fs, an energy of 2 mJ were generated at a central wavelength of 790 nm. In the second case, pulses with a pulse width of 145 fs and a power of 0.6 mJ were generated at a central wavelength of 1135 nm. The demonstrated TSRCPA efficiency of 55% highlights the advantages of this amplification method over optical parametric amplification, especially when the wavelength is tuned close to the pump laser wavelength.

SUMMARY: 137Cs and plutonium isotopes are found in surface soils bound to soil components. The rate of radionuclide migration in the soil depends on many factors, such as the chemical forms of the radionuclides, the geochemical composition of the soil, physico-chemical conditions, atmospheric precipitation, vegetation and species, microorganism activity, animal and human activities. In order to ensure adequate radiation protection and to develop effective remediation measures for contaminated sites, it is necessary to collect and analyze data on long-term measurements of radionuclide concentrations, describing past and present pollution in the soil and its evolution. The aim of the research summarized in this doctoral dissertation is to establish the factors determining the sorption/desorption and vertical transport of 137Cs and Pu isotopes in soils of different types and different moisture regime (relatively dry or periodically flooded soils). The sorption capacity of plutonium in different soil types with different particle sizes, organic matter and stable elements was evaluated. Factors determining sorption/desorption of Pu and 137Cs in non-boggy and waterlogged soils were analyzed. The sequential extraction method has been applied in practice to evaluate the migration potential of 137Cs and Pu in non-boggy and waterlogged soils.

SUMMARY: Conventional combustion technologies, which emit high levels of harmful particles, not only pose a threat to air pollution, health and the climate, but alsotheir operation requires enormous natural resources that are unfortunately not eternal. Therefore, the demand for alternative energy sources has mobilized scientists around the world to search for such energy sources. Fuel cells can operate more efficiently than conventional combustion engines and are more environmentally friendly energy sources than combustion engines. However, although fuel cells prototypes are already being applied in various industries (aviation, energy, electronics etc.), research into their wider applicability and performance improvement is still relevant to this day. One of the main disadvantages of fuel cells which limiting their wider practical use is cost. One of the components influencing the cost of fuel cells is the price of precious metals contained in the electrocatalytic materials used for the cathode and anode of fuel cells. Therefore, in recent decades, a lot of attention has been paid to the development of simple technologies that would allow the formation of efficient catalysts with significantly lower amounts of precious metals or to avoid the use of precious metals altogether. To achieve this goal, various additives have been introduced, such as secondary metals or metal oxides in catalysts, which use makes it possible to reduce the amount of precious metal without losing the activity of the catalyst. This study is related to the preparation of new efficient catalysts that can be used as cathode and/or anode materials in direct alcohol fuel cells. In this study metal oxide-based substrates were formed for the preparation of catalysts with the aim to reduce the amount of precious metals and to improve the efficiency of the catalysts. The influence of catalyst formation conditions on the activity of catalysts is taken into account in the work. The electrocatalytic activity of the formed catalysts was evaluated for the oxidation and oxygen reduction reactions of alcohols (ethanol, methanol, glycerol, ethylene glycol).

SUMMARY: Annotation The protection of the environment and utilization of energy is currently the most important challenge to our planet and here the need for clean renewable energy arises. Among alternative energy sources, the development of hydrogen infrastructure would be a major step towards a clean energy future. Hydrogen gas does not exist naturally in nature - it needs to be extracted from natural gas or produced. Nowadays, more than 500 billion cubic meters (or about 45 million tonnes) of hydrogen are extracted worldwide each year. However, 96% of hydrogen is extracted from natural gas, oil, or coal. Thus, the main way to produce hydrogen is still heavily dependent on fossil fuels and does not solve the problem of climate change at all. Electrochemical water splitting is one of the most reliable and effective ways for the sustainable production of pure hydrogen. It is known that platinum and platinum group metals and their derivatives are the most efficient electrocatalysts for hydrogen release from water. However, the practical use of precious metal electrodes is limited by their high cost. In the last decade, electrocatalysts based on carbon, transition metal carbides, nitrides, phosphides, sulfides and selenides capable of splitting water molecules have been intensively researched and developed. One such electrocatalyst is MoS2 – a stable, non-toxic, affordable material, that exhibits great potential in catalysis, sensing, electrochemical operations, and environmentally related fields. MoS2 is a typical transition metal dichalcogenide (TMD) compound with a two-dimensional S−Mo−S triatomic layer structure. It has attracted attention not only because it is capable of catalyzing water and has the greatest potential to replace Pt, but also because it is chemically stable, tunable electronic structure, is easily synthesized, and is inexpensive compared to Pt-based catalysts..

SUMMARY: Synchronization in large populations of interacting dynamical units is the focus of intense research in physical, technological and biological systems. In neural networks, under normal conditions, it is responsible for cognition and learning, while excessive synchronization can cause abnormal brain rhythms associated with various neurological diseases. Numerous control algorithms have been developed to suppress unwanted synchronized network oscillations. It is known that an improperly designed stimulation system can damage neural tissue or the electrode itself. The mean absolute value of the stimulating current can be chosen as a performance measure for the optimization of stimulating waveform parameters in order to minimize this damage. This choice has another advantage. The optimal waveform which ensures entrainment of a spiking neuron to an external periodic stimulation is determined only by the distance between absolute extrema of the phase response curve and its amplitude. This allows to estimate the stimulation parameters empirically. The same method can be applied to a network of interacting neurons exhibiting collective periodic oscillations. The effect of high-frequency stimulation on a system of two interacting populations of QIF neurons is explained using mean-field equations averaged over the stimulation period. Such methodology can serve as an effective tool for developing stimulation algorithms to control synchronization processes in large-scale neural networks.

SUMMARY: This doctoral thesis is dedicated to address low bulk sensitivity and poor referencing capabilities of microring resonators. To overcome the latter various microring resonator geometries and concepts are investigated to increase the light and matter interactions. In this work numerical simulations of novel microring resonator geometries, based on modulation of the effective refractive index in the core material are presented. Here, subwavelength hollow core defects are introduced inside the core material to increase the surface area of the resonator and the light–matter interaction. In addition, to address devices where resonance shifts are too large to observe and absorbing materials are used, novel microring resonators with no free spectral range as well as resonators working with highly absorbing materials such as metals are shown. Finally, to simplify the detection mechanism, self-referencing sensors are processed and shown.

SUMMARY: The aim of this PhD thesis was to synthesize photoelectrochemically active WO3 layers of different morphology and thickness and to investigate their performance in PEC synthesis of strong oxidants in sulfate and chloride electrolytes. To achieve this, following tasks were formulated:  1. To investigate how different alcohols (methanol, ethanol, isopropanol and butanol) used as reductants in chemical bath deposition as well as annealing temperature influence the crystallization of WO3 phase, morphology of the coatings and their PEC activity;  2. To synthesize highly porous nanostructured WO3 layers of increasing thickness (from ~ 0.5 up to ~ 10 μm) and investigate their PEC activity;  3. To investigate the energy conversion efficiency of PEC generation of reactive chlorine species (ClO- + ClO2-) and persulfate (S2O82-) on WO3 coatings having different thickness and morphology;  4. To investigate the competition between photoanodic oxidation of water and anions as well as stability of different WO3 coatings under conditions of photoelectrolysis in sulfate and chloride electrolytes; 5. To test the antimicrobial effect of photoelectrochemical generation of strong oxidants using WO3 photoanode.

SUMMARY: The objectives of this PhD thesis were to study the properties of the optical response for various hybrid plasmonic modes based on SPP, TPP and surface lattice resonances. The main contribution of the studies was related to the hybrid TPP-SPP mode generation in TIRE setup and application of these modes for plasmonic sensing with bio-related materials. This PhD thesis consist of introduction, literature overview, methods, results and conclusions, summary in Lithuanian and the references. In this PhD thesis the unambiguous experimental evidence of strong coupling between the TPP and SPP resonances in the hybrid TPP-SPP mode was demonstrated for the first time, by tuning of the incident light spectra with the optical filters. The hybrid TPP-SPP mode and single TPP mode application for biosensing was demonstrated. Also direct laser writing technique was employed for the fabrication of a lattice array of gold microbumps which support narrow (~10 nm) hybrid lattice plasmon polaritons modes. The application of strong coupling in plasmonic modes reduces the energy losses in return producing narrow plasmonic resonances.

SUMMARY:  In this thesis, the investigation of laser glass processing methods, based on material cracking, is presented. Two methods were investigated – intra-volume modification and mechanical separation, and bottom-up processing. It was demonstrated that glass scribing can be enhanced by inducing directional transverse cracks using axicon-generated asymmetrical Bessel-like laser beams. Experimental and theoretical research showed that such beams can be generated via aberration-control or angular spectrum modulation of the axicon-generated beam. In this work, the influence of the axicon shape and its rotation around the axis, perpendicular to the beam propagation axis, was investigated. Variously shaped amplitude and phase masks were investigated and compared. Laser-fabricated axicons were investigated as an alternative to traditional elements. High material removal efficiency over 100 µm3/µJ was demonstrated using the second method and nanosecond laser pulses or GHz bursts of picosecond pulses with the overall burst duration in the nanosecond scale. Technologies for cutting long contours, fabrication of chamfers and gas nozzles for laser-plasma accelerators were developed. Developed laser processing methods were thoroughly compared to traditional by evaluating the quality and flexural strength of cut samples.

SUMMARY: The natural water bodies, as well as terrestrial ecosystems, are directly loaded with radionuclides of anthropogenic origin after incidents at various nuclear power facilities. Due to their harmful effects on the environment and living organisms, it is essential to quickly and reliably estimate the concentrations of radionuclides when monitoring the ecosystem effects of existing and future nuclear facilities. Therefore, one of the most essential needs at present is the assessment of water reservoir pollution and the development of effective and inexpensive water treatment methods. The purpose of the scientific research summarized in the dissertation is to investigate and compare the conditions for the concentration and separation of cesium and plutonium, important elements from a radioecological point of view, from an aqueous medium, using materials of biological origin (biosorbents) or chemically modified biowaste, to determine the sorption capacity of biosorbents and the prospects for their use in environmental studies. The sorption capacity and distribution coefficient of 10 sorbents for cesium and plutonium ions at different pH values were evaluated. The functional groups of sorbents involved in the sorption process of cesium and plutonium ions were determined, and the binding mechanisms were identified. Natural and modified mosses and sawdust were tested under real conditions, the conditions suitable for the determination of the investigated ions in freshwater were determined, and the best sorbents suitable for the sorption of cesium and plutonium ions were found.

SUMMARY: The natural water bodies, as well as terrestrial ecosystems, are directly loaded with radionuclides of anthropogenic origin after incidents at various nuclear power facilities. Due to their harmful effects on the environment and living organisms, it is essential to quickly and reliably estimate the concentrations of radionuclides when monitoring the ecosystem effects of existing and future nuclear facilities. Therefore, one of the most essential needs at present is the assessment of water reservoir pollution and the development of effective and inexpensive water treatment methods. The purpose of the scientific research summarized in the dissertation is to investigate and compare the conditions for the concentration and separation of cesium and plutonium, important elements from a radioecological point of view, from an aqueous medium, using materials of biological origin (biosorbents) or chemically modified biowaste, to determine the sorption capacity of biosorbents and the prospects for their use in environmental studies. The sorption capacity and distribution coefficient of 10 sorbents for cesium and plutonium ions at different pH values were evaluated. The functional groups of sorbents involved in the sorption process of cesium and plutonium ions were determined, and the binding mechanisms were identified. Natural and modified mosses and sawdust were tested under real conditions, the conditions suitable for the determination of the investigated ions in freshwater were determined, and the best sorbents suitable for the sorption of cesium and plutonium ions were found.

SUMMARY: This work presents THz excitation spectroscopy technique where various semiconductors and their structures are investigated. Until now this methodology was mostly used to investigate A3B5 direct bandgap semiconductors. It was shown that the conduction band subsidiary valley position can be determined from THz excitation spectra. The generation mechanism is related to semiconductor properties and the ballistic nature of excited carriers. Similar principles were used to investigate heterostructures, where their band offset parameter is important for engineering novel opto- and micro- electronic devices. The THz excitation spectroscopy was adapted to determine such a parameter and later the investigation of GaAsBi/GaAs and GaInAsBi/InP heterostructures was completed. Also, indirect band gap and layered crystals were investigated by THz excitation spectroscopy. Here germanium and transition metal dichalcogenides showed that it is possible to determine the direct band gap of such materials, while conduction band subsidiary valley position and lower lying valence band position were determined in GaSe crystals. Lastly, thin layers of semimetal bismuth were investigated, where THz generation mechanism depended on the crystallinity of the layer.

SUMMARY: Biomolecules are substances present in living organisms that play an essential role in biochemical processes. The ability to control the interactions between biomolecules, to observe the processes occurring at interfaces is highly relevant to many scientific fields. However, understanding biomolecular processes at the molecular level requires unique research methods. One such research method is Surface-Enhanced Raman Spectroscopy (SERS). Nonetheless, SERS has several fundamental drawbacks that limit the application of the method in the study of biomolecules at interfaces. The main disadvantage of SERS is the requirement to use roughened or nanostructured Au, Ag, or Cu surfaces to amplify the Raman signal. In this case, metals can directly interact with biomolecules, modifying the entire system. Therefore, in 2010 an alternative research technique was proposed – Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS), which allows obtaining detailed information about molecular vibrations, but at the same time avoids the main drawbacks of SERS. SHINERS has opened up new possibilities in studying adsorption, catalysis, charge transfer and other processes on smooth, single-crystal surfaces, also on two-dimensional materials. This dissertation presents the molecular-level characterization of interfacial biomolecules by using SHINERS method. The work discusses the possibilities and challenges of this method in studying heterogeneous molecular systems and reveals the structural peculiarities and the function of different biomolecules.