Domas Paipulas

A femtosecond laser-induced two-photon photopolymerization technique for structuring microlenses

Light-initiated quasi-instant solidification of a liquid polymer is attractive for its ultra-precise spatial and temporal control of the photochemical reaction. In this paper we present microlenses structured by femtosecond laser-induced photopolymerization. Due to nonlinear phenomena the fabrication resolution is not restricted to the diffraction limit for the applied laser excitation wavelength but is determined by the intensity of a focused beam. Furthermore, pin-point structuring enables one to produce three-dimensional structures of any form from the photopolymer. The smallest structural elements of 200 nm lateral dimensions can be achieved reproducibly by using high numerical aperture oil immersion focusing optics (NA = 1.4). Axial resolution (which is fundamentally a few times worse than lateral resolution due to the distribution of light intensity in the focal region) can be controlled to a precision of a few hundred nanometers by decreasing the scanning step. In our work we applied the commercially available and widely used zirconium–silicon based hybrid sol–gel photopolymer (Ormosil, SZ2080). Arrays of custom-parameter spherical microlenses for microscopy applications have been fabricated. Their surface roughness, focal distance and imaging quality were tested. The obtained results show potential for fast and flexible fabrication of custom-parameter microlenses by the proposed technique.

High 90% efficiency Bragg gratings formed in fused silica by femtosecond Gauss-Bessel laser beams

Direct laser write of volume Bragg gratings with diffraction efficiency (absolute) ∼90% is demonstrated using Gauss-Bessel laser beams in fused silica glass. Axial multiplexing of ∼ 90 μm long segments of modified optical material was demonstrated and thick Bragg gratings of aspect ratio depth/period ≈234 were achieved with period d = 1.5 μm. Typical fabrication scanning speeds were up to 50 mm/s for gratings with cross sections up to five millimeters made within 1 h time. Potential applications of high efficiency Bragg gratings in a low nonlinearity medium such as silica are discussed.

Microactuation and sensing using reversible deformations of laser-written polymeric structures

We investigate reversible deformations of polymeric microstructures fabricated using direct laser writing three-dimensional lithography upon immersion in various solvents. Swelling and shrinkage of sub-micrometre size features are induced by interaction with surrounding solvent and such deformations can be exploited to create larger structures whose size, shape, and other structural parameters depend on the surroundings. We describe diffractive optical elements, micro-mechanical sensors and also hybrid deformable structures, that can be used to implement micro-actuation, micro-sensing, and other functionalities highly sought for micro-optical, micro-mechanical, and micro-fluidic systems.

Femtosecond laser fabrication of hybrid micro-optical elements and their integration on the fiber tip

Femtosecond laser photo-polymerization of zirconium-silicon based sol-gel photopolymer SZ2080 is used to fabricate micro-optical elements with a single and hybrid optical functions. We demonstrate photo-polymerization of the solid immersion and Fresnel lenses. Gratings can be added onto the surface of lenses. The effective refractive index of polymerized structures can be controlled via the volume fraction of polymer. We used woodpile structure with volume fraction of 0.65-0.8. Tailoring of dispersion properties of micro-optical elements by changing filling ratio of polymer are discussed. Direct write approach is used to form such structures on a cover glass and on the tip of an optical fiber. Close matching of refractive indices between the polymer and substrate in visible and near infra red spectral regions (nSZ2080 = 1.504, nglass = 1.52) is favorable for such integration. The surface roughness of laser-polymerized resits was ~30 nm (min-max value), which is acceptable for optical applications in the visible range. For the bulk micro-optical elements the efficiency of 3D laser polymerization is increased by a factor ~ (2 - 4) × 102 times (depends on the design) by the shell-formation polymerization: (i) contour scanning for definition of shell-surface, (ii) development for removal of nonfunctional resist, and (iii) UV exposure for the final volumetric polymerization of an enclosed volume.

Large Scale Laser Two-Photon Polymerization Structuring for Fabrication of Artificial Polymeric Scaffolds for Regenerative Medicine

We present a femtosecond Laser Two‐Photon Polymerization (LTPP) system of large scale three‐dimensional structuring for applications in tissue engineering. The direct laser writing system enables fabrication of artificial polymeric scaffolds over a large area (up to cm in lateral size) with sub‐micrometer resolution which could find practical applications in biomedicine and surgery. Yb:KGW femtosecond laser oscillator (Pharos, Light Conversion. Co. Ltd.) is used as an irradiation source (75 fs, 515 nm (frequency doubled), 80 MHz). The sample is mounted on wide range linear motor driven stages having 10 nm sample positioning resolution (XY—ALS130‐100, Z—ALS130‐50, Aerotech, Inc.). These stages guarantee an overall travelling range of 100 mm into X and Y directions and 50 mm in Z direction and support the linear scanning speed up to 300 mm/s. By moving the sample three‐dimensionally the position of laser focus in the photopolymer is changed and one is able to write complex 3D (three‐dimensional) structures. An illumination system and CMOS camera enables online process monitoring. Control of all equipment is automated via custom made computer software “3D‐Poli” specially designed for LTPP applications. Structures can be imported from computer aided design STereoLihography (stl) files or programmed directly. It can be used for rapid LTPP structuring in various photopolymers (SZ2080, AKRE19, PEG‐DA‐258) which are known to be suitable for bio‐applications. Microstructured scaffolds can be produced on different substrates like glass, plastic and metal. In this paper, we present microfabricated polymeric scaffolds over a large area and growing of adult rabbit myogenic stem cells on them. Obtained results show the polymeric scaffolds to be applicable for cell growth practice. It exhibit potential to use it for artificial pericardium in the experimental model in the future.We present a femtosecond Laser Two‐Photon Polymerization (LTPP) system of large scale three‐dimensional structuring for applications in tissue engineering. The direct laser writing system enables fabrication of artificial polymeric scaffolds over a large area (up to cm in lateral size) with sub‐micrometer resolution which could find practical applications in biomedicine and surgery. Yb:KGW femtosecond laser oscillator (Pharos, Light Conversion. Co. Ltd.) is used as an irradiation source (75 fs, 515 nm (frequency doubled), 80 MHz). The sample is mounted on wide range linear motor driven stages having 10 nm sample positioning resolution (XY—ALS130‐100, Z—ALS130‐50, Aerotech, Inc.). These stages guarantee an overall travelling range of 100 mm into X and Y directions and 50 mm in Z direction and support the linear scanning speed up to 300 mm/s. By moving the sample three‐dimensionally the position of laser focus in the photopolymer is changed and one is able to write complex 3D (three‐dimensional) structures....

Combination of thermal extrusion printing and ultrafast laser fabrication for the manufacturing of 3D composite scaffolds

We present a novel approach to manufacturing 3D microstructured composite scaffolds for tissue engineering applications. A thermal extrusion 3D printer – a simple, low-cost tabletop device enabling rapid materialization of CAD models in plastics – was used to produce cm-scale microporous scaffolds out of polylactic acid (PLA). The fabricated objects were subsequently immersed in a photosensitive monomer solution and direct laser writing technique (DLW) was used to refine its inner structure by fabricating a fine mesh inside the previously produced scaffold. In addition, a composite material structure out of four different materials fabricated via DLW is presented. This technique, empowered by ultrafast lasers allows 3D structuring with high spatial resolution in a great variety of photosensitive materials. A composite scaffold made of distinct materials and periodicities is acquired after the development process used to wash out non-linked monomers. Another way to modify the 3D printed PLA surfaces was also demonstrated - ablation with femtosecond laser beam. Structure geometry on macro- to micro- scales could be finely tuned by combining these fabrication techniques. Such artificial 3D substrates could be used for cell growth or as biocompatible-biodegradable implants. To our best knowledge, this is the first experimental demonstration showing the creation of composite 3D scaffolds using convenient 3D printing combined with DLW. This combination of distinct material processing techniques enables rapid fabrication of diverse functional micro-featured and integrated devices. Hopefully, the proposed approach will find numerous applications in the field of tissue engineering, as well as in microelectromechanical systems, microfluidics, microoptics and others.

Multiscale 3D manufacturing: combining thermal extrusion printing with additive and subtractive direct laser writing

A novel approach for efficient manufacturing of three-dimensional (3D) microstructured scaffolds designed for cell studies and tissue engineering applications is presented. A thermal extrusion (fused filament fabrication) 3D printer is employed as a simple and low-cost tabletop device enabling rapid materialization of CAD models out of biocompatible and biodegradable polylactic acid (PLA). Here it was used to produce cm- scale microporous (pore size varying from 100 to 400 µm) scaffolds. The fabricated objects were further laser processed in a direct laser writing (DLW) subtractive (ablation) and additive (lithography) manners. The first approach enables precise surface modification by creating micro-craters, holes and grooves thus increasing the surface roughness. An alternative way is to immerse the 3D PLA scaffold in a monomer solution and use the same DLW setup to refine its inner structure by fabricating dots, lines or a fine mesh on top as well as inside the pores of previously produced scaffolds. The DLW technique is empowered by ultrafast lasers - it allows 3D structuring with high spatial resolution in a great variety of photosensitive materials. Structure geometry on macro- to micro- scales could be finely tuned by combining these two fabrication techniques. Such artificial 3D substrates could be used for cell growth or as biocompatible-biodegradable implants. This combination of distinct material processing techniques enables rapid fabrication of diverse functional micro- featured and integrated devices. Hopefully, the proposed approach will find numerous applications in the field of ms, microfluidics, microoptics and many others.

Thermal and optical properties of sol-gel and SU-8 resists

We report on a combined differential scanning calorimetric (DSC) and Raman scattering study of thermal polymerization of sol-gel organic-inorganic SZ2080 and SU-8 resists. In SZ2080, endothermic peak at 95°C signify drying of the resist and justifies the required pre-bake at around 100°C for 1-2 h for the best performance during femtosecond (fs-)direct laser writing. A strong exothermic peak at 140°C (under 2 K/min heating rate) completes polymerization of the resist. It is revealed that 1wt% of photoinitiators change Raman scattering intensity of SZ2080 and can contribute efficiently to heating and cross-linking of photo-polymers. In the case of SU-8, a 65°C DSC feature related to solvent evaporation was observed. The strongest changes in Raman spectrum occurs at a narrow 895 cm-1 band which is linked to polymerization. Raman scattering taken during DSC revealed spectral changes following the polymerization; an applicability of this method for monitoring photopolymerization induced by ultra-fast laser sources and feasibility of a laser-modulated calorimetry is discussed.

Analysis of the Micromachining Process of Dielectric and Metallic Substrates Immersed in Water with Femtosecond Pulses

Micromachining of 1 mm thick dielectric and metallic substrates was conducted using femtosecond pulse generated filaments in water. Several hundred microjoule energy pulses were focused within a water layer covering the samples. Within this water layer, non-linear self-action mechanisms transform the beam, which enables higher quality and throughput micromachining results compared to focusing in air. Evidence of beam transformation into multiple light filaments is presented along with theoretical modeling results. In addition, multiparametric optimization of the fabrication process was performed using statistical methods and certain acquired dependencies are further explained and tested using laser shadowgraphy. We demonstrate that this micromachining process exhibits complicated dynamics within the water layer, which are influenced by the chosen parameters.

Rapid microfabrication of transparent materials using a filamented beam of the IR femtosecond laser

Glass drilling and welding applications realized with the help of femtosecond lasers attract industrial attention , however, desired tasks may require systems employing high numerical aperture (NA) focusing conditions, low repetition rate lasers and complex fast motion translation stages. Due to the sensitivity of such systems, slight instabilities in parameter values can lead to crack formations, severe fabrication rate decrement and poor quality overall results. A microfabrication system lacking the stated disadvantages was constructed and demonstrated in this report. An f-theta lens was used in combination with a galvanometric scanner, in addition, a water pumping system that enables formation of water films of variable thickness in real time on the samples. Water acts as a medium for filament formation, which in turn decreases the focal spot diameter and increases fluence and axial focal length . This article demonstrates the application of a femtosecond (280fs) laser towards two different micromachining techniques: rapid cutting and welding of transparent materials. Filament formation in water gives rise to strong ablation at the surface of the sample, moreover, the water, surrounding the ablated area, adds increased cooling and protection from cracking. The constructed microfabrication system is capable of drilling holes in thick soda-lime and hardened glasses. The fabrication time varies depending on the diameter of the hole and spans from a few to several hundred seconds. Moreover, complex-shape fabrication was demonstrated. Filament formation at the interface of two glass samples was also used for welding applications. By varying repetition rate, scanning speed and focal position optimal conditions for strong glass welding via filamentation were determined.