Lithuanian Journal of Physics 

About: Lithuanian Journal of Physics is an academic journal published by Lithuanian Academy of Sciences. The journal publishes majorly in the area(s): Laser & Terahertz radiation. It has an ISSN identifier of 1648-8504. It is also open access. Over the lifetime, 760 publications have been published receiving 3155 citations.

Ultrafast supercontinuum generation in bulk condensed media

Abstract: Nonlinear propagation of intense femtosecond laser pulses in bulk transparent media leads to a specific propagation regime, termed femtosecond filamentation, which in turn produces dramatic spectral broadening, or super-broadening, termed supercontinuum generation. Femtosecond supercontinuum generation in transparent solids represents a compact, efficient and alignment-insensitive technique for generation of coherent broadband radiation at various parts of the optical spectrum, which finds numerous applications in diverse fields of modern ultrafast science. During recent years, this research field has reached a high level of maturity, both in understanding of the underlying physics and in achievement of exciting practical results. In this paper we overview the state-of-the-art femto-second supercontinuum generation in various transparent solid-state media, ranging from wide-bandgap dielectrics to semiconductor materials and in various parts of the optical spectrum, from the ultraviolet to the mid-infrared spectral range. A particular emphasis is given to the most recent experimental developments: multioctave supercon-tinuum generation with pumping in the mid-infrared spectral range, spectral control, power and energy scaling of broadband radiation and the development of simple, flexible and robust pulse compression techniques, which deliver few optical cycle pulses and which could be readily implemented in a variety of modern ultrafast laser systems.

3D artificial polymeric scaffolds for stem cell growth fabricated by femtosecond laser

Abstract: Femtosecond Laser Induced Polymerization is an attractive direct writing technique for rapid three-dimensional (3D) micro and nanofabrication in diverse applications. Recently, it has been successfully applied for 3D scaffold fabrication required in biomedicine applications. However, there are still a lot of investigations to be done before it can be used for practical applications in tissue engineering or regenerative medicine. In this work, experimental results on production of artificial polymeric scaffolds for stem cell growth are presented. Parameters (average laser power, sample scanning speed, and developing conditions) for microfabrication in biocompatible photopolymers AKRE (AKRE37) and ORMOSIL (SZ2080) are experimentally determined. 3D custom form and size artificial scaffolds were successfully microfabricated. Adult stem cell growth on them was investigated in order to test their biocompatibility. The results of myogenic stem cell culture expansion were compared to the control growth of the same cells on the scaffolds manufactured out of commonly used biocompatible photopolymers ORMOCER (Ormocore b59) and Poly-Ethylen Glycol Di-Acrylate (PEG-DA-258). Preliminary results show FLIP technique to have potential in fabrication of artificial 3D polymeric scaffolds for cell proliferation experiments. These are the first steps in transferring FLIP fabrication method from laboratory tests to flexible manufacturing of individual scaffolds out of biocompatible and biodegradable polymers. Keywords: femtosecond laser, two-photon polymerization, 3D micro-fabrication, biocompatible photopolymers, artificial scaffolds

Resistivity of non-galilean-invariant fermi- and non-fermi liquids

Abstract: While it is well-known that the electron-electron (ee) interaction cannot affect the resistivity of a Galilean-invariant Fermi liquid (FL), the reverse statement is not necessarily true: the resistivity of a non-Galilean-invariant FL does not necessarily follow a T 2 behavior. The T 2 behavior is guaranteed only if Umklapp processes are allowed; however, if the Fermi surface (FS) is small or the electron-electron interaction is of a very long range, Umklapps are suppressed. In this case, a T 2 term can result only from a combined – but distinct from quantum-interference corrections – effect of the electron-impurity and ee interactions. Whether the T 2 term is present depends on (i) dimensionality [two dimensions (2D) vs three dimensions (3D)], (ii) topology (simply- vs multiply-connected), and (iii) shape (convex vs concave) of the FS. In particular, the T 2 term is absent for any quadratic (but not necessarily isotropic) spectrum both in 2D and 3D. The T 2 term is also absent for a convex and simply-connected but otherwise arbitrarily anisotropic FS in 2D. The origin of this nullification is approximate integrability of the electron motion on a 2D FS, where the energy and momentum conservation laws do not allow for current relaxation to leading – second – order in T /EF (EF is the Fermi energy). If the T 2 term is nullified by the conservation law, the first non-zero term behaves as T 4 . The same applies to a quantum-critical metal in the vicinity of a Pomeranchuk instability, with a proviso that the leading (first non-zero) term in the resistivity scales as T^(D+2)/3 (T^(D+8)/3). We discuss a number of situations when integrability is weakly broken, e. g., by inter-plane hopping in a quasi-2D metal or by warping of the FS as in the surface states of topological insulators of the Bi2 Te3 family. The paper is intended to be self-contained and pedagogical; review of the existing results is included along with the original ones wherever deemed necessary for completeness.

Organic dye doped microstructures for optically active functional devices fabricated via two-photon polymerization technique

Abstract: Femtosecond Laser Two-Photon Polymerization (LTPP) is a fabrication technique based on ultra-localized polymerization reaction initiated by nonlinear absorption of tightly focused light beam. It offers possibility to form three-dimensional (3D) micro- and nanostructures out of photopolymers. The point-by-point photostructuring allows fabrication of objects directly from Computer Aided Design (CAD) models and thereby the geometry of required structure can be changed flexibly. The smallest structural elements, also called voxels (volumetric pixels), of 200 nm lateral dimensions can be achieved with high repeatability. In this article, we present 3D microstructures fabricated out of hybrid zirconium-silicon containing hybrid sol-gel photopolymer ORMOSIL (SZ2080) doped with conventionally used fluorescent dyes: rhodamine 6G (R6G), fluorescein, DCM LC6500, and coumarin 152. The structural quality of the microobjects was investigated by Scanning Electron Microscopy (SEM). Interior of doped 3D micro- and nanostructures has been diagnosed with a custom made scanning fluorescence microscope. Additionally, fluorescing artificial scaffolds, which could be used for cell growth and cell tracking, were manufactured. Finally, the model of Distributed Feedback Dye Laser (DFBL) was successfully fabricated and this demonstrated the possibility to manufacture optically active elements from doped photopolymers. Keywords: direct laser writing, doped polymers, 3D fabrication, distributed feedback dye laser

Femtosecond visible light induced two-photon photopolymerization for 3D micro/nanostructuring in photoresists and photopolymers

Abstract: Laser two-photon polymerization (LTPP) has been widely reported as a tool for three-dimensional micro/nanofabrication. Femtosecond lasers are employed to form nanostructures in photosensitive resins with subwavelength resolution. We demonstrate high throughput large scanning area LTPP system based on linear motor driven stages combined with Yb:KGW high repetition rate (312.5 kHz) amplified laser as irradiation source (515 nm second harmonic’s wavelength). Femtosecond green light can be focused to a smaller diffraction limited spot and provides higher structuring resolution comparing to commonly used Ti:sapphire lasers (operating at NIR wavelengths) used for LTPP. Additionally, shorter irradiation wavelength enables to process more of widely used photosensitive materials. The system capacitates production of nanostructures having 200 nm lateral resolution with high repeatability. By modifying focusing optics there is a possibility to scale up the fabrication: reduction of resolution results in shortening of fabrication time. The system enables formation of 3D structures with size varying from tens of microns to tens of millimetres. Most of the materials commonly used for photopolymerization technology (various blends of acrylates, hybrid organic–inorganic materials, and epoxy resins) are well suitable for processing with the constructed LTPP system. Keywords: two-photon absorption, laser processing, photopolymerization, micro/nanofabrication, three-dimensional structures, tissue engineering