Publication database

In this paper the possibility to optimize the glass dicing process by controlling the axicon-generated Bessel beam ellipticity is presented. Single-shot intra-volume modifications in soda-lime glass followed by dicing experiments of 1 mm-thick samples are performed. The Bessel beam ellipticity is essential for glass dicing process. Such beam generates intra-volume modifications with transverse crack propagation in dominant direction. Orientation of these modifications parallel to the dicing direction gives significant advantages in terms of processing speed, glass breaking force and cutting quality.

Bio-inspired surfaces are able to decrease friction with fluids and gases. The most recognizable are shark-skin-like riblet surface structures. Such bio-inspired surfaces can be formed by the laser ablation technique. In this work, bio-inspired riblet surfaces with grooves were formed using picosecond ultraviolet laser ablation on pre-heated polytetrafluoroethylene (PTFE) at various sample temperatures. The ablation of hot PTFE was found to be 30% more efficient than the conventional laser structuring at the room temperature. The friction of structured PTFE surfaces with the flowing air was investigated by using drag a measurement setup. Results show the decrease of friction force by 6% with dimensionless riblet spacing around 14–20.

Organic solid-state lasers (OSSLs) with distributed feedback (DFB) structures or distributed Bragg reflectors (DBRs) are promising for potential application in bio-sensing and hazardous materials detection. Here, the laser performances of the all-organic DFB waveguide lasers with various grating heights ranging from 0.4 to 4.7 μm were investigated. The grating structures used as the lasing cavity were fabricated using a two-photon absorption (TPA) direct laser writing (DLW) method with an SU-8 negative photoresist. The laser active layer consisted of a rhodamine 6G (R6G) laser dye and a cellulose acetate (CA) matrix. The R6G/CA solution was spin-coated onto the quartz substrate with the cavity (grating) structures to fabricate the DFB waveguide laser devices. The diffraction order of lasing ranged from m = 4 to 7. As the grating height was increased to 1.9 μm, the slope efficiency increased for all diffraction orders and the threshold decreases for each diffraction order. The dependence of the cavity (grating) length on the laser performances was investigated. The slope efficiency increased as the cavity length increased to 300 μm. The effect of the cavity (grating) position on the slope efficiency and the threshold position of the cavity (grating) was also studied. A maximum slope efficiency of 10.2% was achieved for the DFB waveguide laser device with a cavity (grating) length of 300 μm, a cavity position at 6 mm from the emission edge of the waveguide, and an aspect ratio ≈3 between the grating height of 1.74 μm and the grating width of 0.6 μm for the diffraction order m = 6 for lasing.

Light is an exceptional external stimulus for establishing precise control over the properties and functions of chemical and biological systems, which is enabled through the use of molecular photoswitches. Ideal photoswitches are operated with visible light only, show large separation of absorption bands and are functional in various solvents including water, posing an unmet challenge. Here we show a class of fully-visible-light-operated molecular photoswitches, Iminothioindoxyls (ITIs) that meet these requirements. ITIs show a band separation of over 100 nm, isomerize on picosecond time scale and thermally relax on millisecond time scale. Using a combination of advanced spectroscopic and computational techniques, we provide the rationale for the switching behavior of ITIs and the influence of structural modifications and environment, including aqueous solution, on their photochemical properties. This research paves the way for the development of improved photo-controlled systems for a wide variety of applications that require fast responsive functions.

The optical performance of high-resistivity silicon with a laser-ablated surface was studied in the transmission mode in the frequency range of 0.1-4.7 THz. A reciprocal relationship between the transmission brightness and the surface roughness was observed at discrete THz frequencies. The measured dispersion was reproduced by the THz wave scattering theory using an effective refractive index model. No significant differences between the samples processed either with psor ns-duration laser pulses in ambient air or in argon enriched atmosphere were found in the THz regime. It was demonstrated that the majority of optical losses of the silicon with the laser modified surface were due to the scattering of THz waves and not due to the absorption in silicon-compounds formed during the laser ablation.

Results of in-depth experimental analysis of the laser-assisted local copper deposition on commercial Polyamide 6 (PA 6) are presented. Pico- and nanosecond lasers were validated for surface modification of the polymer followed by silver (I) activation and finished by autocatalytic electroless copper plating on the laser-modified areas. Detailed investigations were dedicated to finding out the origin of selective metal plating, including the surface profiling and wettability dynamics, XPS analysis and electric resistance measurements of the deposited copper layer. Based on the experimental data, the mechanism of the polymer surface activation by the laser modification is proposed.

Three different experiments were performed in order to determine the mechanism of pillars formation using four-beam interference lithography. The experimental results demonstrate that pillars, fabricated in argon gas, were wider and higher compared with the pillars fabricated in nitrogen gas, low vacuum or air. It clearly indicates that the pillar bottom widening effect is not affected by the depletion of atmospheric oxygen as in all environments the fabricated pillars have a wider bottom part. Moreover, the shape of the fabricated pillars is not affecting by the back reflection from the positioning stage and by the light irradiation conditions. These results clearly indicate that the photopolymerization process is enhanced by the heat current and it determines the pillar bottom widening effect.

The marked increase in the incidence of melanoma coupled with the rapid drop in the survival rate after metastasis has promoted the investigation into improved diagnostic methods for melanoma. High-frequency ultrasound (US), optical coherence tomography (OCT), and photoacoustic imaging (PAI) are three potential modalities that can assist a dermatologist by providing extra information beyond dermoscopic features. In this study, we imaged a swine model with spontaneous melanoma using these modalities and compared the images with images of nearby healthy skin. Histology images were used for validation.

Although nanoporous Pt film has been shown to be an effective catalyst for polymer electrolyte membrane (PEM) fuel cells, the maximum power density of the cell is limited by the optimal film thickness. When the Pt film thickness exceeds the optimal value, regions with good gas transport (the side near the gas diffusion layer (GDL)) separate from regions with good proton transport (the side near the PEM), so the current density and the power density drop with increasing film thickness. Here we demonstrate that this obstacle can be overcome by laser micro-machining the GDL. The picosecond laser fabricates grooves on the GDL surface to greatly increase the effective surface area for Pt deposition, thereby reducing the local Pt film thickness. A nearly two-fold increase in the power density is achieved by using laser micro-machined periodic grooves of 20 μm depth, reaching a 0.6-V power density of 853 mW cm⁻² and a maximum power density of 1.2 W cm⁻² with a cathode Pt loading of 200 μg cm⁻². The results also indicate that further enhancement may be achieved by increasing the surface modulation depth/period ratio and by implementing a better way to fill the grooves with polymer electrolyte.

Solid-state lasers with pulse duration of 10 ps and radiation wavelength of 1064 nm were used to investigate the laser ablation efficiency dependence on processing parameters: laser fluence (pulse energy and beam spot size), beam scanning speed, pulse repetition rate, and scanned line (hatch) distance for the copper sample. Utilising a 40 W power laser, the highest ablation efficiency of 2.5 µm³/µJ and the ablation rate of 100 µm³/µs with the smallest surface roughness of 0.2 µm was obtained. Three-dimensional (3D) fabrication using a galvanometer scanner and layer-by-layer removal technique with optimal parameters defined for efficient ablation were demonstrated at a rate of 6 mm³/min. Combination of high material removal rate with excellent quality and complex 3D structure formation is in a high interest for mimicking bio-inspired surfaces, micro-mould fabrication and decorative applications.