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Defended Dissertations in 2018


ELENA ADOMAITIENĖ

Author: ELENA ADOMAITIENĖ
Dissertation title: Development of methods for controlling equilibrium and synchrony of nonlinear dynamical system
Fields of science: Physics (02P)
Scientific supervisor: habil. dr.  Arūnas Vytautas Tamaševičius
Defence of the dissertation: January 8, 2018

SUMMARY:
Stabilization of unstable equilibrium (UEQ) of dynamical systems is an important problem in basic science and engineering applications, if periodic or chaotic oscillations are unacceptable behaviours. There is a need for adaptive, i.e. model-independent and reference-free methods, automatically tracing and stabilizing unknown UEQ. Therefore fast feedback methods using unstable filter control, stable filter control and proportional feedback control techniques, as well as synergetic methods, based on combined unstable feedback control and stable filter control techniques also with an auxiliary differentiator in the feedback loop for stabilizing equilibrium with a priori unknown coordinates of both weakly and strongly damped dynamical systems, were developed. Synchronization is a universal and very common phenomenon. Though in the most cases synchronization plays a positive role, sometimes it has a negative impact. Strong synchronization in the human brain is an example. It is widely believed that synchrony of spiking neurons in a neuronal population causes the symptoms of the Parkinson’s disease. Therefore, the development of the methods and practical techniques for controlling, more specifically, for suppressing synchrony of coupled oscillators, in general, and particularly with possible application to neuronal arrays, is of great importance. Methods for destroying synchrony or suppressing oscillations in mean-field coupled oscillators were developed.

SAULIUS FRANKINAS
Author: SAULIUS FRANKINAS
Dissertation title: Controlling of temporal and spectral characteristics of ultrashort fiber lasers by nonlinear effects
Fields of science: Physics (02P)
Scientific supervisor:  dr. Andrejus Michailovas
Defence of the dissertation: April 27, 2018

SUMMARY:
The opportunity to tune spectral and temporal characteristics of the fiber laser extends the application field of fiber lasers. One of the techniques, which allows tuning the central wavelength of the system is a continuum generation by pulses of a few hundred femtoseconds and the optical parametric amplification. By developing fiber systems for applications that do not require a high pulse energy, much more attention is paid to the compactness and reliability of the system. Therefore, techniques of controlling spectral and temporal characteristics of pulses that can be integrated into the fiber system design are developing intensively. Therefore, the research described in this doctoral thesis was aimed at the controlling of temporal and spectral pulse characteristics in soliton mode-locked fiber oscillators and in fiber chirped pulse amplification systems. Also experiments of chirped pulse amplification in large diameter photonic crystal fibers were described. These researches were important for development of fiber laser systems, which temporal, energetic and spectral characteristics of pulses are suitable for pumping of optical parametric amplifier operating at 1 MHz repetition rate. Finally, implementation of the developed methods and techniques into a wavelength-tunable laser system that generates pulses with the duration less than 150 fs in the spectral range from 700 nm up to 2040 nm is presented.
IEVA KULAKAUSKAITĖ

Author: IEVA KULAKAUSKAITĖ
Dissertation title: Synthesis of magnetic nano-sorbents, their characterization and investigation of sorptive properties
Fields of science: Chemistry (03P)
Scientific supervisor: dr. Galina Lujanienė
Defence of the dissertation: June 1, 2018

SUMMARY:
The main objective of environmental protection is to reduce heavy metal pollution and to find efficient sorbents for. Regenerated, mechanically stable and environmentally friendly magnetic nanosorbents used to remoce heavy metals are more promising than macro- or micro- materials. After analyzing their sorptive and regenerative properties, establishing favorable sorption conditions, sorbents can be applied not only in water treatment technologies, protecting the environment from toxic elements, but also concentrating metals from various aqueous media. This dissertation estimates the application of magnetic nanosorbents (magnetite, magnetic graphene oxide, magnetic Prussian blue and magnetic Prussian blue with graphene oxide) to remove heavy metals from liquid media. The dissertation consists of the literature review and three research chapters. The first section of the research results is intended for the characterization of synthetic nanosorbents by Mössbauer, XRD, FTIR, RAMAN, SEM, and TEM methods. Also, their magnetic properties are determined. In the second chapter, studies of the dependence on the sorption efficiency of heavy metals (Cu (II), Co (II), Ni (II) and Pb (II) are shown. Results show effect of initial metal ion concentration, time and sorbent dosage on sorption effieciency, as well as competitive sorption and regeneration studies. Isotherm (Langmuir, Freudlich, Dubinin-Radushkevish) and kinetics (pseudo-first, pseudo-second-particle diffusion) models are used for data analysis. The third chapter examines correlation of data with the statistical model. The dispersion analysis (ANOVA) was used to determine influence of three variables on sorption efficiency. The correspondence between experimental and theoretical data is presented.

SIMONAS INDRIŠIŪNAS
Author: SIMONAS INDRIŠIŪNAS
Dissertation title: Formation of light harvesting structures for photovoltaics using laser interference ablation
Fields of science: Technological Sciences, Material Engineering (08T)
Scientific supervisor: dr. Gediminas Račiukaitis
Defence of the dissertation: June 15, 2018

SUMMARY:
In this PhD thesis, application of patterning with interfering laser beams for the fabrication of light harvesting structures for the photovoltaic elements was investigated. Interference patterning technique was used to pattern antireflective silicon nitride layer on silicon solar cells, crystalline or multicrystalline silicon substrates and thin metal films. Using the irradiation fluence below the ablation threshold, silicon nitride was locally converted to silicon oxide/oxy-nitride. This resulted in increased absorption of the element in the visible spectral range and improved electric characteristics of the solar cell. Interference patterning technique, combined with anisotropic or isotropic wet chemical etching, was used for the fabrication of the inverted pyramid pattern in crystalline silicon and periodic dimple pattern in monocrystalline silicon. The optimal irradiation conditions were determined. Fabrication of sub-micrometre patterns was investigated. In this case, heat diffusion in the volume between the maximal intensity spots plays a significant role. A three-dimensional model for heat diffusion simulation was introduced. In the last chapter of this thesis, improvements of the interference patterning setup are discussed. Methods to ensure uniform large area fabrication and to increase interference patterning flexibility are discussed.
MILDA TAMOŠIŪNAITĖ
Author: MILDA TAMOŠIŪNAITĖ
Dissertation title: Impact of external effects on terahertz applications for telecommunications
Fields of science: Physics (02P)
Scientific supervisor: prof. dr. Gintaras Valušis
Defence of the dissertation: September 28, 2018

SUMMARY:
World mobile data traffic is growing nearly exponentially. Available bandwidth is one of the factors limiting data transmission rate. Current wireless technologies, coupled with the newest 5G technology, is expected to meet only 81 percent of the planned need. One of the promising candidates is the Terahertz (THz) Band (100 GHz – 10 THz), but the THz waves are very vulnerable in the atmosphere. In this work, the possibilities and limitations of the THz wireless networks were examined in terms of the atmospheric attenuation, mostly rain attenuation and variations of atmospheric refractive index. Also, one of the most probable technological scenarios (short distances and quasi-optical transmission) was investigated. The results were compared with data from a laboratory-controlled rain experiment. In order to overcome small gain of individual THz Band antennas, very large antenna arrays might be required. The zone plate-based antennas are considered as a reduced size and cost, easy-integrated planar solution for telecommunication systems in THz bands. Therefore, the zone plate design, in which reflection-reducing and focusing features are compatible on one 500 µm thick silicon substrate, was consistently optimized and produced using laser-ablation. Focusing performance of the binary zone plate for 0.6 THz with dual function antireflective layer was close to diffraction limited operation.
VYTAUTAS CĖPLA
Author: VYTAUTAS CĖPLA
Dissertation title: Hydrogels for biofabrication: synthesis, chemical and physical characterization
Fields of science: Physics (02P)
Scientific supervisor: dr. Ramūnas Valiokas, prof. dr. May Griffith
Defence of the dissertation: October 29, 2018

SUMMARY:
Regenerative medicine needs the next generation of biocompatible materials that not only function as physical scaffolds supporting cells and tissues formation, but they should also interactively promote or correct the vital processes. In this work, polymer coatings and biosynthetic materials (hydrogels) were developed and characterised that display controlled chemical and physical properties as well as biocompatibility. For the first time, PEG hydrogel coatings on glass were synthesized without chemical and photochemical initiators. They were characterised by physical methods. It was shown that such PEG hydrogels can be modified by extracellular matrix protein micropatterns suitable for controlling the mechanobiological responses of the cells. Also, it was found that despite of the high water content (~85% weight) and low crosslinking degree, collagen hydrogels used for implant production possess a physically defined surface. They are suitable as substrates for fabrication of biochip-like architectures by contact lithography techniques. Spectroscopy and scanning probe techniques were used for hydrogel characterization in their hydrated state. Atomic force and fluorescent microscopy were employed for analysis of the fabricated microstructures. Finally, microscopic protein hydrogel scaffolds were obtained by combining chemical synthesis and microfabrication techniques for applications as three-dimensional cell and tissue architectures. It was found that such hydrogels adopt and retain the defined shape with nanometer precision. The developed coatings, biochip-like architectures and three-dimensional scaffolds are promising in the fields of biology, drug research and regenerative medicine.
VYTAUTAS JAKŠTAS
Author: VYTAUTAS JAKŠTAS
Dissertation title: Compact terahertz emitters and detectors based on AlGaN/GaN heterostructures
Fields of science: Physics (02P)
Scientific supervisor: dr. Irmantas Kašalynas
Defence of the dissertation: November 8, 2018

SUMMARY:
The rapidly expanding applications of THz-frequency radiation encourages the research of new materials and technological solutions that allow the creation of compact, frequency-tunable electric THz radiation sources and high-speed sensors operating at room temperature. The main goal of the dissertation is to develop high-electron mobility transistors (HEMTs) and “bow-tie” diodes from AlGaN/GaN heterostructures and to investigate their availability for the emission and detection of THz radiation. A simpler technology for the manufacturing of AlGaN/GaN samples is introduced in dissertation and the electrical parameters of such-made components (high electron mobility transistors (HEMT) and Schottky diodes) are comparable to those of commercially available AlGaN/GaN components. THz radiation of commercial THz source was successfully detected by the manufactured AlGaN/GaN transistor and “bow-tie” diode sensors. After examining the radiation spectra of a large-area HEMT with a lattice-type gate electrode, one of the observed peaks was attributed to the first mode of the ungraded plasmon. A possibility to investigate residual impurities in AlGaN/GaN heterostructures by measuring electroluminescence spectra was demonstrated.
SERGEJ ŠEMČUK

Author: SERGEJ ŠEMČUK
Dissertation title: Application of graphene oxide based nanocomposites and Šaltiškiai clay for radionuclides removal from contaminated solutions
Fields of science: Physics (02P)
Scientific supervisor: dr. Galina Lujanienė
Defence of the dissertation: November 23, 2018

SUMMARY:
The environment constantly suffers from human anthropogenic activity. Release of radionuclides into the environment harmfully affects humanity and nature. Therefore, their removal from contaminated liquid media is one of the most important purposes of the environmental protection.Recently, various nanosorbents have been widely used for this purpose due to their efficiency, eco-friendly and specific properties. At the same time, analytical methods are being improved and new promising materials for safe waste disposal are being proposed. The storage of radioactive waste for a long-term requires ensuring the minimum environmental impact, predicting and monitoring of the released radionuclides into the environment and their variations. The dissertation describes the results of a study of the removal of radionuclides using the graphene oxide and the nanocomposites based on it (Prussian blue graphene oxide, magnetic graphene oxide, magnetic Prussian blue graphene oxide and chitosan graphene oxide). The dissertation consists of three sections: literature review, materials and methods, results and discussion. The second section describes the methods of synthesis of nanocomposites and the conditions for radionuclide sorption experiments. Synthesized nanosorbents were characterized by FTIR, XRD, SEM, TEM and RAMAN methods. Using the produced nanocomposites, the sorption experiments were conducted with 134Cs (I), 60Co (II), 241Am (III), 239Pu (IV) and 239Pu (V) as well as Cu (II), Co (II), Ni (II) and Pb (II) as stable radionuclide analogues. The batch technique was used to study the dependence of concentration, initial pH and contact time on the sorption efficiency. Results of these studies are discussed in the third section. Also, the mechanisms of Pu isotope sorption on Šaltiškiai clay minerals were investigated and the changes during the association depending on Pu (IV) spreading in the environment were studied.

ARMANDAS BALČYTIS
Author: ARMANDAS BALČYTIS
Dissertation title: Fabrication of subwavelength functional components by means of lithographic methods
Fields of science: Technological Sciences, Material Engineering (08T)
Scientific supervisor: dr. Raimondas Petruškevičius
Defence of the dissertation: December 5, 2018

SUMMARY:
The purpose that directs the broad field of lithographic fabrication is to provide reliable methods for producing structures with maximally strictly defined geometries. In general terms, the dimensions of functional component features or their arrangements dictate which physical phenomena they will be capable of affecting or interfacing with, as well as the duration of various transport processes. For a considerable range of photonic phenomena structures have to be made on the scale of a single optical wavelength or below, hence, among all of the available approaches only a select few fabrication methods possess the requisite resolution to exert fine control over relevant optics-related interactions. This work is centered on leveraging nanoscale high-precision fabrication methods, mainly electron and ion beam lithography as well as physical vapor deposition or reactive ion etching auxiliary pattern transfer techniques, to produce various structures and components exhibiting a range of functionalities relating to radiation interaction with mater - including integrated photonic waveguide resonators for optical bio-sensing, micro-optical elements for shaping custom polarization or topological charge wavefronts, and plasmonic metasurfaces for spectrally selective absorbance enhancement. In addition, miniaturization here is leveraged to provide novel capabilities for thermometric detection, molecular sensing and even in the quantum science of ultracold atom trapping.