Limitations of Structural Insight into Ultrafast Melting of Solid Materials with X-ray Diffraction Imaging

In this work, we analyze the application of X-ray diffraction imaging techniques to follow ultrafast structural transitions in solid materials using the example of an X-ray pump–X-ray probe experiment with a single-crystal silicon performed at a Linac Coherent Light Source. Due to the spatially non-uniform profile of the X-ray beam, the diffractive signal recorded in this experiment included contributions from crystal parts experiencing different fluences from the peak fluence down to zero. With our theoretical model, we could identify specific processes contributing to the silicon melting in those crystal regions, i.e., the non-thermal and thermal melting whose occurrences depended on the locally absorbed X-ray doses. We then constructed the total volume-integrated signal by summing up the coherent signal contributions (amplitudes) from the various crystal regions and found that this significantly differed from the signals obtained for a few selected uniform fluence values, including the peak fluence. This shows that the diffraction imaging signal obtained for a structurally damaged material after an impact of a non-uniform X-ray pump pulse cannot be always interpreted as the material’s response to a pulse of a specific (e.g., peak) fluence as it is sometimes believed. This observation has to be taken into account in planning and interpreting future experiments investigating structural changes in materials with X-ray diffraction imaging.

Fabrication of a Functional Relief on the Surface of a Polyvinyl Chloride Film by Nanosecond Laser Microtexturing-=SUP=-*-=/SUP=-

Modification of the surface of polyvinyl chloride (PVC) deposited on a copper foil under the influence of nanosecond laser radiation with different peaks fluence was studied by using infrared spectroscopy of attenuated total reflection and optical microscopy. The dependences of the wetting angle of the texture surface with water on the peak fluence of laser radiation were measured. The obtained various relief textures demonstrate a sharp drop in water contact angles from (~ 50o) to super hydrophilic (~ 15o), which allows us to design and fabricate micro-scale elements of laboratory devices for the tasks of bactericidal treatment, collection and removal of liquids, as well as their separation and purification. A stable increase in the contact angles of water with the surface of the textures to hydrophobic (~ up to 140o) observed over time indicates degradation of the polymer coating.

Influence of Bulk Temperature on Laser-Induced Periodic Surface Structures on Polycarbonate

In this paper, the influence of the bulk temperature (BT) of Polycarbonate (PC) on the occurrence and growth of Laser-induced Periodic Surface Structures (LIPSS) is studied. Ultrashort UV laser pulses with various laser peak fluence levels F 0 and various numbers of overscans ( N OS ) were applied on the surface of pre-heated Polycarbonate at different bulk temperatures. Increased BT leads to a stronger absorption of laser energy by the Polycarbonate. For N OS < 1000 High Spatial Frequency LIPSS (HSFL), Low Spatial Frequency LIPSS perpendicular (LSFL-I) and parallel (LSFL-II) to the laser polarization were only observed on the rim of the ablated tracks on the surface but not in the center of the tracks. For N OS ≥ 1000 , it was found that when pre-heating the polymer to a BT close its glass transition temperature ( T g ), the laser fluence to achieve similar LIPSS as when processed at room temperature decreases by a factor of two. LSFL types I and II were obtained on PC at a BT close to T g and their periods and amplitudes were similar to typical values found in the literature. To the best of the author’s knowledge, it is the first time both LSFL types developed simultaneously and consistently on the same sample under equal laser processing parameters. The evolution of LIPSS from HSFL, over LSFL-II to LSFL I, is described, depending on laser peak fluence levels, number of pulses processing the spot and bulk temperature.

Effects of Laser Fluence and Pulse Overlap on Machining of Microchannels in Alumina Ceramics Using an Nd:YAG Laser

The quality of micro-features in various technologies is mostly affected by the choice of the micro-fabrication technique, which in turn results in several limitations with regard to materials, productivity, and cost. Laser beam micro-machining has a distinct edge over other non-traditional methods in terms of material choices, precision, shape complexity, and surface integrity. This study investigates the effect of laser fluence and pulse overlap while developing microchannels in alumina ceramic using an neodymium-doped yttrium aluminum garnet (Nd:YAG) laser. Microchannels 200 µm wide with different depths were machined using different laser peak fluence and pulse overlap (percentage of overlap between successive laser pulses) values. It was found that high pulse overlaps and fluences should be avoided as they give rise to V-shaped microchannels i.e., 100% bottom width errors. The optimal peak fluence range was found to be around 125–130 J/cm2 corresponding to 3–5 µm depth per scan. In addition, channels fabricated with moderate pulse overlap were found to be of good quality compared to low pulse overlaps.

Femtosecond laser materials processing of a-Si:H below the ablation threshold

ABSTRACTLaser processing of thin-film silicon is a promising approach for the realization of polycrystalline silicon for large area electronics and solar cell applications. In this study we investigate the material modification of amorphous hydrogenated silicon (a-Si:H) with different hydrogen content (30%, 13% and <1%) by means of femtosecond (fs) laser pulses. Depending on the peak fluence applied, hydrogen diffusion/effusion, layer crystallization or material ablation can be achieved. Despite the low absorption coefficient of a-Si:H at the center wavelength of an amplified Titanium Sapphire laser at 790 nm a high local energy deposition close to the surface of the a-Si:H layer is observed, which can be attributed to a nonlinear absorption process.