Studies

Author YURY MALEVICH

Dissertation title:"Transient anisotropic photoconductivity and terahertz pulse generation from semiconductors"
Fields of science: Physical Sciences, Physics (02P)
Scientific supervisor:  dr. Ramūnas Adomavičius

Defence of the dissertation: 2017-01-10

SUMMARY:
The study of transient photoconductivity in semiconductors excited by femtosecond laser pulses is of fundamental importance for understanding of photocarrier dynamics on a subpicosecond time scale. These investigations are of great practical interest because the ultrafast electronic processes have a profound impact on operation of semiconductor  devices operating on a subpicosecond time scale. The purpose of this work has been to study the influence of the optical alignment of photocarriers over momenta on the transient photoconductivity in cubic semiconductors excited by femtosecond laser pulses and to investigate how this effect is manifested in emission of THz radiation from the semiconductor surface.
The transient photoconductivity in InAs excited by ultrashort laser pulses with different photon energy has been calculated with use of the transport equation for photocarriers in collisionless approximation. It has been found that the nonparabolicity of the electron dispersion law as well as the optical alignment of the photoexcited carrier momenta result in anisotropic photocurrent with a component perpendicular to the dc electric field even in semiconductors with a cubic symmetry. This component of the photocurrent is profoundly revealed on a ballistic stage of photocarrier movement that is during the first few hundreds of femtoseconds after the photoexcitation and causes the emission of terahertz radiation pulses with an amplitude dependent on the angle between the optical field and the crystallographic axes. In the case of InAs the contribution of this component in terahertz emission can be comparable to the contribution of the photocurrent component directed perpendicular to the crystal surface and explains experimental results of both the azimuthal anisotropy of the emitted terahertz pulse amplitude and its dependence on the exciting photon energy. Monte Carlo simulations of the anisotropic transient photoconductivity in cubic semiconductors In0,53Ga0,47As and InAs excited by a femtosecond laser pulse have been performed. Calculations show that the degree of anisotropy of the transient photocurrent reaches its peak when the excess energy of the optically excited electrons approaches the threshold for the intervalley transfer. It has been also found that when the electrons are excited near the threshold energy for the intervalley transfer, the component of the transient photocurrent directed along dc field can become negative for a short time after 33 photoexcitation. The anisotropy of the transient photoconductivity has been observed experimentally on (001) In0,53Ga0,47As sample by optical pump – terahertz probe technique. It has been found that in the first few picoseconds after excitation the optically induced change of THz transmission depends on the direction of THz field relative to polarization of the optical pump pulse and crystallographic axes of the semiconductor. On rotation of the sample around normal to the excited semiconductor surface the fourfold symmetry of THz transmission is observed. It has been established experimentally that the  degree of photoconductivity anisotropy depends nonmonotonically on the exciting photon energy that is consistent with the results of the Monte Carlo simulation.

Dissertation title: “Fiber laser based technologies for high energy femtosecond wavelength-tunable optical parametric chirped pulse amplification systems”
Fields of science: Physical Sciences, Physics (02P)
Scientific supervisor: dr. Andrejus Michailovas.
Scientific consultant: dr. Nerijus Rusteika
Defence of the dissertation:  2017-02-28

 

Development of optical parametric chirped pulse amplification (OPCPA) systems is a rapidly emerging field in ultrafast laser technology. This doctoral dissertation comprises theoretical modelling and experimental investigations which led to the realization of an OPCPA system with a novel frontend. A unique concept opened a path for the development of compact femtosecond high-energy tunable hybrid laser systems, incorporating advantages of fiber and solid-state laser technologies, which may be adopted in a variety of ultrafast laser applications. During the doctoral studies a novel optically synchronised dual-wavelength all-in-fiber laser source for seeding both femtosecond Yb and picosecond Nd-based solid-state lasers was proposed and experimentally demonstrated. Incorporation of pulse temporal and spectral stretching techniques in a picosecond all-in-fiber laser together with the methods of narrowband Nd ions based solid-state and broadband optical parametric amplification enabled the realisation of a new technique of femtosecond pulse generation. Method to improve the output pulse contrast in a chirped pulse amplification system by applying the temperature gradient on a chirped fiber Bragg grating stretcher and thus controlling the dispersion parameters was investigated numerically and experimentally. The doctoral work was concluded by implementing the developed novel methods into an experimental realisation of a compact femtosecond high-energy wavelength tunable Vis NIR OPCPA system.

Author: TOMAS TOLENIS

Dissertation title: “Modelling, formation and characterization of nano-sculptured thin films”
Fields of science: Technological Sciences, Material Engineering (08T)
Scientific supervisor: dr. Ramutis Drazdys

Defence of the dissertation:  2017-06-09

SUMMARY:
Investigation and characterization of nano-sculptured thin films are presented in thesis. Investigation includes the formation of columnar structures by glancing angle deposition, evaporation of porous silica layers and conformal evaporation of metal on biological surfaces. Results indicate, that columnar structures exhibit birefringence, which can be controlled by changing deposition angle. Such coatings can be used during the formation of waveplates. Also, chiral structures can be deposited by slowly rotating the substrate, placed at glancing angle. Such coatings exhibited the selectivity for circular polarization. Vertical columns can be formed by using fast substrate rotation. The refractive index of such coatings can be tailored by changing the angle of deposition. Antireflection and high reflection coatings have been produced by controlling the refractive index of substituting layers. Investigation shows, that silica coatings exhibit high resistivity to laser radiation and can be used in high power optical systems. Also, conformal evaporation of metal on butterfly wing provides a way to replicate, study and exploit hidden morphologies of biological surfaces.

Author: VYTENIS PRANCULIS

Dissertation title:  “Charge transfer dynamics in bulk-heterojunction organic solar cells”
Fields of science: Technological Sciences, Material Engineering (08T)
Scientific supervisor: prof. habil. dr. Vidmantas Gulbinas

 

Defence of the dissertation: 2017-06-12 

 

SUMMARY:
The work presented in this thesis focuses on the extraction of charges photogenerated in organic bulk-heterojunction solar cells. Charge migration dynamics were observed experimentally on a vast time range – from sub-ps to ms – as well as in a variety of devices in order to form a comprehensive view on the entire process. Role of coherence and delocalization on charge transfer at donor-acceptor interface and the initial electron migration was analyzed by measuring the electric field drop in the solar cell after the ultrafast photoexcitation. Time-resolved electric field induced second harmonic generation (TREFISH) method was used to obtain sub-ps time resolution. Experimental data was then used as a basis for the Stochastic Schrödinger Equation (SSE) simulations. Obtained results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces. Subsequent electron and hole migration was investigated using TREFISH method combined with integrated photocurrent (IPC) measurements on a vast pool of devices. Experimental results over different donor-acceptor ratio devices in combination with time dependent mobility modelling and Monte-Carlo calculations enabled the separation of extraction of electrons and holes. It was found that charge extraction strongly depends on the fraction of the corresponding material in the blend. Balanced carrier mobility did not ensure the most efficient extraction. Rather, fast motion of electrons was found to be essential for efficient charge carrier separation helping to avoid geminate recombination. Mobility of the photogenerated charges was observed to decrease over orders of magnitude in the time span from their generation to extraction. This drop in mobility was found to originate from carrier relaxation within their respective density of states (DOS). Furthermore, a remarkable distribution of the photocurrent over energy was found, in which the most relaxed charge carriers counteract the net photocurrent. Morphology optimization using the solvent additive 1,8-diiodooctane (DIO) was found to double the charge pair separation efficiency and the short-circuit current. Carrier extraction at low internal electric field was slightly faster from the cells prepared with DIO, which can reduce recombination losses and enhance fill factor.

Author: VALDEMAR STANKEVIČ

Dissertation title: “Formation and characterization of micro -opto -mechanical 3d devices for sensor application in transparent materials”
Fields of science: Technological Sciences, Material Engineering (08T)
Scientific supervisor:  dr. Gediminas Račiukaitis 

 

This dissertation focuses on the investigation of the etching selectivity in bulk fused silica modified with ultrashort laser pulses and fabrication of the complex 3D microfluidic devices. Here, the combination of a few topics is presented. The femtosecond laser-induced chemical etching technique (FLICE) was used for microchannels inscription in bulk fused silica. The broad range of the highly focused femtosecond pulses parameters was investigated for optimizing the microchannels etching rate. It was demonstrated that the optimal etching conditions were achieved for the channels fabricated with the multipulse irradiation and pulse duration exceeding ~ 700 fs. Microchannels inscribed with the tightly focused pulses of the 1030 nm wavelength showed ~ 20% better etching performance comparing to the 515 nm wavelength pulses. The etching selectivity was induced due to the volume nanostructures that were oriented perpendicularly to the laser beam polarisation. The nature of the nanogratings is not completely understood thus this phenomenon is under debate. By investigating the morphology of such nanogratings with the SEM microscope, it has been observed that the orientation of the nanogratings was slightly tilted with respect to the scan direction. This remark followed the discovery of the new phenomenon that demonstrated the nanogratings tilt dependence on the different scanning directions when laser polarisation was kept constant. The phenomenon was investigated under the different laser radiation parameters, and a theoretical model was proposed. It was demonstrated enhanced etching of the structures fabricated with the contour scanning method in combination with the linear or circular polarisation which also followed the minimized surface roughness (Ra~ 75 nm) of the microchannels fabricated with the optimal 200 nJ pulse energy and ~ 1 mm/s scanning speed. The practical applications of the FLICE technique were demonstrated by fabricating three different 3D microfluidic devices for fluid pumping, microchannels for ellipsometric application and challenging double-sided aspheric microfluidic lens for light focusing from the Y-shaped waveguide.

Author: ALDONA BALČIŪNAITĖ

Dissertation title: “New materials for alkaline fuel cells: synthesis, characterization and properties”

Defence of the dissertation:  2017-07-07

 

SUMMARY:
The thesis is related to the intensively developing research in the field of fuel cells and nanomaterials and is devoted to the search of new effective materials, that can be used for development and design of direct borohydride fuel cells (DBFCs) in order to improve the performance of existing fuel cells or create novel fuel cells with high efficiency. The aim of the work was the fabrication of efficient catalysts, their characterization and application for direct borohydride fuel cells. The AuM (M = Ni, Co, Cu) catalysts deposited on the titanium (Ti) or titania nanotube arrayed (TiO2-NTs) surfaces were fabricated by means of simple and low-cost electrochemical, electroless metal deposition and galvanic displacement techniques. With the aim of formation of effective catalysts, the immobilization of Au nanoparticles should be made on the underlayers deposited on the Ti and TiO2-NTs surfaces with the thickness of ca. 0.2-0.4 μm for Ni, ca. 0.5-1.1 μm for Co and ca. 1.5 μm for Cu. The optimal time period of immersion for Au deposition varies from 0.5 to 5 minutes. The Au(M)/Ti Au(M)/TiO2-NTs catalysts were fabricated with the Au nanoparticles sized ca. 10-100 nm and having the Au loadings from ca. 0.9 to 59.4 gAu cm–2. The prepared Au(M)/Ti and Au(M)/TiO2-NTs catalysts have a significantly higher electrocatalytic activity towards the oxidation of sodium borohydride as compared to that of the bare Au, M/Ti and M/TiO2-NTs electrodes. The Au(M)/Ti and the Au(M)/TiO2-NTs catalysts with the Au loading ca. 0.9-57.6 and 8.2-59.4, respectively, gAu cm–2 exhibit ca. 7-16 times higher electrocatalytic activity towards the electrooxidation reaction of sodium borohydride in an alkaline medium as compared with that of bare Au electrode. The highest mass activity (140 mA gAu–1) towards the oxidation reaction of sodium borohydride was obtained on the Au(Ni)/Ti catalyst with the Au loading of 0.9 gAu cm–2. Direct alkaline NaBH4-H2O2 single fuel cell tests were carried out by employing the prepared M/Ti, M/TiO2-NTs, Au(M)/Ti and Au(M)/TiO2-NTs catalysts as the anodes. It has been found that the highest peak power density up to 224 mW cm-2 was аttаined аt a temperature of 25 ºC using the Au(Co)/Ti catalyst with the Au loading of 57.6 µg cm–2 as the anode. The highest specific peak power density of 102.6 mW µgAu–1 at a temperature of 25 °C was attained using Au(Ni)/Ti with the Au loading of 0.9 µg cm–2 as an anode. The Au(M)/Ti and Au(M)/TiO2-NTs catalysts are promising materials and can be used as anodes in direct sodium borohydride fuel cells.

Author: JELENA KOVGER

Dissertation title: “Study on titanium anodic films decoration with visible light absorbing semiconductor nanostructures”

Defence of the dissertation: 2017-09-19

 

New nanostructure materials, their order colonies, layered films, synthesis and construction of heterostructures allow scientists to develop at those days new or improved properties of materials that will determine the future of scientific and technological progress. The thesis is related to prospective studies of nanostructured TiO2 layers and film composites with other semiconductor nanoparticles based on their environmental friendliness, low cost, chemical and mechanical resistance. The aim of the work was the investigate the possibilities of anodic TiO2 film nanotubes for electrochemical decoration of lower band gap semiconductor nanoparticles. The methods for uniform decoration of anatase TiO2 nanotube (Ntb) films with Cu2O nanoparticles were proposed: with weakly acidic copper acetate electrolyte and alternating current. Growth of pure Cu2O crystals in the TiO2 Ntbs was based on thermodynamic analysis of the solution. It has been discovered that TiO2 Ntb films decorated with Cu2O nanoparticles are able to absorb not only the UV light, but visible light as well. By increasing amount of Cu2O deposited in the TiO2 nanotubes, the TiO2 Ntb – Cu2O absorption edge can be moved up to 2.1 eV. The possibility of electrochemical decoration of TiO2 Ntb film with semiconducting copper selenide nanoparticles in aqueous solutions and the peculiarities of the process were investigated. The effect of hydrogen doping of the Ti/TiO2 Ntb electrode for uniform decoration of TiO2 nanotubes with semiconducting nanoparticles was used for the first time. It was determined that TiO2 Ntb – Cu2O heterostructures are characterised by significantly lower dependence on the angle of incidence of excitation beam – this can be especially useful for solar cell circuits. Investigated optical properties and light absorption features of TiO2 Ntb decorated with copper selenide (Cu3Se2 and Cu2-xSe) nanoparticles. Optimized composition of hydrothermal processing solution and conditions for decoration of TiO2 Ntb film surfaces and nanotubes with nanoleafed MoS2. The uniform formation of crystalline MoS2 on the TiO2 surface is achieved with low concentration ammonium heptamolybdate and urea solutions within the temperature of 220 to 225 °C. It was determined that Ti/TiO2 Ntb – MoS2 electrodes catalyse hydrogen release from acidic solutions and the reaction is characterized by stability.

Author: ROMUALDAS STRIELA

Dissertation title: "Daugiafunkcinių piridino darinių sintezė ir savybių tyrimas"

Defence of the dissertation:  2017-09-22

 

Pyridine ring bearing compounds are found in many natural products, active pharmaceuticals and functional materials. According MDL Drug Data Report pyridine fragment is the most abundant heterocyclic fragment found in the drug molecules and is present in more than 7000 existing drugs. Bipyridine and terpyridine carcasses are well known for their chelating ability, which is exploited in many fields including enantioselective catalysis and chemotherapy. 1,4-Dihydropyridine can be used as core fragment for fluorescent sensors. Pyridine ring is also used as electron acceptor for electron transfer in organic light emitting diodes. The aim of this work is the a development of synthetic methods for 5-hydroxyomeprazole, bromocyclopropylpyridines, orelanine and terpyridine building blocks. The existing methods for each of the target system have many flaws and the 5-hydroxyomeprazole has no synthetic method at all. There was a chemical synthesis pathway for 5-hydroxyomeprazole developed. It was found that methyl 2,4-dichloro-5,6-dimethylpyridine-3-carboxylate reaction with two equivalents of sodium methoxide product distribution depends on reaction temperature. The procedure for regeneration this reaction side products to the starting compound was developed. It was found that during hydrodehalogenation reaction of 2-chloro-5,6-dimethyl-4-methoxypyridine-3-carboxylate under 80 atmospheres pressure, the hydrolysis of 4-methoxy group occurs. Non-aqueous Sandmeyer reaction conditions for bromo(chloro)cyclopropylpyridines were developed. It was found that Sandmeyer reaction of aminocyclopyridines with copper(II) bromide and alkylnitrite undergoes both cationic and radiacal pathway. 2-pyridinediazonium ion formed upon reaction of 2-amino-5-cyclopropylpyridine is apparently unstable in rather polar solvents and undergoes heterolytic cleavage. The produced highly reactive cation reacts with any available nucleophile. Unpublished formation of ω-halogenoalcoxypyridines was observed. Novel improved total synthesis of orelanine was developed. For this synthesis novel an efficient aqueous synthetic pathway for 3,4-dimethoxypyridine was applied and transition metal catalysis free 3,4-dimethoxypyridine C-C homocoupling method was developed. Synthesis of ,6''-bis(bromomethyl)-4'-(4-nitrophenyl)-2,2':6',2''-terpyridine utilizing Kröhnke pyridine synthesis method was developed.

Author: JULIJANAS ŽELUDEVIČIUS

Dissertation title:  “Optimization of pulsed fiber lasers, nonlinear pulse combining and optical frequency conversion”
Fields of science: Technological Sciences, Material Engineering (08T)
Scientific supervisor:  dr. Kęstutis Regelskis

Defence of the dissertation: 2017-09-29

 

The research described in this doctoral thesis was aimed at optimization of pulse characteristics achievable from fiber lasers and amplifiers. Pulse generation and amplification in ytterbium doped fused silica fibers is investigated. Also, optical frequency conversion and combining of pulsed beams in quadratic susceptibility crystal is investigated. The thesis consists of literature review chapter and four research chapters. The first research chapter is dedicated to investigation of the novel ultrashort pulse generation in fiber method, which allows for operation without using conventional saturable absorbers. In the second chapter, pulse amplification in fibers is studied. Nonlinear fiber chirped pulse amplification system, in which self-phase modulation is utilized both in pulse stretching and amplification stages, is investigated and optimized. In the third chapter, second harmonic generation of broadband pulsed radiation from fiber lasers is considered. Promising method for efficient second harmonic generation by inducing temperature gradient along a nonlinear crystal is investigated. In the fourth chapter, two methods for combining of pulsed radiation in quadratic susceptibility crystal are presented. In one case, pulsed beams from four fiber amplifiers are combined into single sum-frequency beam, in which pulses are multiplexed in time. In the other case, two separated in time pulses are combined into single sum-frequency pulse.

Author: NIJOLĖ REMEIKAITĖ-NIKIENĖ

Dissertation title: “Distribution of organic matter and metals in the south-eastern Baltic Sea (Lithuanian zone)”

Defence of the dissertation: 2017-09-29

 

Processes supplying organic matter and hazardous substances to seawater are especially intensive in coastal areas and semi-enclosed seas. An understanding of the characteristics, nature of the organic matter, its cycling, distribution of pollutants and the major controlling factors in the coastal ecosystems can provide interesting insights into the occurrence of natural and anthropogenic processes in the lagoon and its drainage basin. The main task of this work was to assess the distribution pattern of organic matter and metals and to evaluate an extent of contamination of the south-eastern Baltic Sea. In this work, the stable carbon and nitrogen isotopes ratios were used in order to evaluate sources and composition of organic matter as well as the seasonal and temporal variability of organic matter. The distribution pattern of metals (Pb, Cu, Cd, Ni, Cr, Zn) in sediments of the SE Baltic Sea and the most polluted sites were evaluated by applying different geochemical indices (EF, Igeo, CF, Cd, Eir, RI). During this study, the metal (Cd, Cu, Pb and Zn) concentrations in biota tissue were assessed and the accumulation capacity of each element by mussels Macoma balthica and Dreissena polymorpha was evaluated.

Author: IGNAS NEVINSKAS

Dissertation title:  “Emission of terahertz pulses from the narrow-gap semiconductor structures”
Fields of science: Physical Sciences, Physics (02P)
Scientific supervisor: prof. habil. dr. Arūnas Krotkus

Defence of the dissertation: 2017-12-08

 

Pulsed Terahertz radiation emission from the surfaces of semiconductors is typically achieved when Ti:Sapphire (λ = 800 nm) femtosecond laser pulses are impinging them at an incline angle, and the generated Terahertz pulse is registered in reflection geometry with a photoconductive antenna comprising of a short carrier lifetime material. Further improvements in terms of compactness and cost-effectiveness of these Terahertz Time-Domain Spectroscopy (THz-TDS) systems would require telecommunication wavelength, λ = 1.55 µm (0.8 eV), fiber-based femtosecond lasers activating the generating and registering semiconducting Terahertz components. The narrow-gap A3B5 semiconductors (Arsenides and Antimonides) are great candidates as surface THz emitters, since the photoexcited electrons with 0.8 eV quanta in these materials can have high excess kinetic energies and, thus, high initial velocities. In addition, their narrow forbidden energy gap induces the anisotropy of a conduction band, due to its strong interaction with a valence band. Photoconduction, consequentially, becomes anisotropic in the A3B5 narrow-gap semiconductors that are sliced along low crystallographic symmetry planes. The lateral photocurrent components provide another advantage, namely, the capability of implementing the line-of-sight optical geometry, greatly convenient for a fiber-laser-based THz-TDS system. Alternative solution enhancing the lateral photocurrent components is the external magnetic field that has a strong influence on the low effective mass electrons. Finally, a GaInAsBi-based photoconductive antenna operating as a THz detector at λ = 1.55 µm is demonstrated.

Author: DAINIUS PAVILONIS

Dissertation title: “Magnetoresistance and electrical resistance relaxation in La-Sr(Ca)-Mn-O nanostructured films”
Fields of science: Physical Sciences, Physics (02P)
Scientific supervisor: prof. dr. Nerija Žurauskienė

Defence of the dissertation:  2017-12-19

 

SUMMARY:
The goal of this thesis was to investigate, how chemical composition and morphology of La-Sr(Ca)-Mn-O nanostructured films influences the resistivity and magnetoresistance values in wide range of temperatures and magnetic fields, and to clear up origin of memory effects occurring after the magnetic field pulse affected the films is switched off. It was demonstrated that the magnetoresistance of nanostructured manganite films can be analyzed using modified Mott’s hopping model taking into account contributions of both crystallites and grain boundaries. It was found that resistance relaxation process in these films after magnetic field is switched off has two different components: ‘fast’ (> 100 µs), and ‘slow’ (≥ 1 ms). The characteristic time constants and relative parts of the remnant resistivity of these processes were systematically analyzed in a wide range of temperatures and magnetic fields using Kolmogorov-Avrami-Fatuzo and Kohlrausch–Williams–Watts models. The obtained results enabled to suggest the development of fast high pulsed magnetic field sensors operating at two ranges of temperatures: close to room and cryogenic. It was also shown that the treatment of nanostructured manganite films by annealing them in an argon atmosphere at 100-200 ⁰C temperatures accelerates their ageing processes and stabilizes their resistance and magnetoresistance.