J.Z.P. Skolski

Modeling laser-induced periodic surface structures: Finite-difference time-domain feedback simulations

A model predicting the formation of laser-induced periodic surface structures (LIPSSs) is presented. That is, the finite-difference time domain method is used to study the interaction of electromagnetic fields with rough surfaces. In this approach, the rough surface is modified by “ablation after each laser pulse,” according to the absorbed energy profile, in order to account for inter-pulse feedback mechanisms. LIPSSs with a periodicity significantly smaller than the laser wavelength are found to “grow” either parallel or orthogonal to the laser polarization. The change in orientation and periodicity follow from the model. LIPSSs with a periodicity larger than the wavelength of the laser radiation and complex superimposed LIPSS patterns are also predicted by the model.

Modeling of Laser Induced Periodic Surface Structures

In surfaces irradiated by short laser pulses, Laser Induced Periodic Surface Structures (LIPSS) have been observed on all kind of materials for over forty years. These LIPSS, also referred to as ripples, consist of wavy surfaces with periodicity equal or smaller than the wavelength of the laser radiation. Unfortunately, the physical phenomena explaining ripple initiation, growth and transitions toward other patterns are still not fully understood. Models, explaining ripple initiation and growth, based on the laser parameters, such as the wavelength and the angle of incidence, are frequently discussed in literature. This paper presents the most promising models, their ability and limitations to predict experimental results.

Laser-induced periodic surface structures, modeling, experiments, and applications

Laser-induced periodic surface structures (LIPSSs) consist of regular wavy surface structures, or ripples, with amplitudes and periodicity in the sub-micrometer range. A summary of experimentally observed LIPSSs is presented, as well as our model explaining their possible origin. Linearly polarized continuous wave (cw) or pulsed laser light, at normal incidence, can produce LIPSSs with a periodicity close to the laser wavelength, and direction orthogonal to the polarization on the surface of the material. Ripples with a periodicity (much) smaller than the laser wavelength develop when applying laser pulses with ultra-short durations in the femtosecond and picosecond regime. The direction of these ripples is either parallel or orthogonal to the polarization direction. Finally, when applying numerous pulses, structures with periodicity larger than the laser wavelength can form, which are referred to as “grooves”. The physical origin of LIPSSs is still under debate. The strong correlation of the ripple periodicity to the laser wavelength, suggests that their formation can be explained by an electromagnetic approach. Recent results from a numerical electromagnetic model, predicting the spatially modulated absorbed laser energy, are discussed. This model can explain the origin of several characteristics of LIPSSs. Finally, applications of LIPSSs will be discussed.

Optical and electrical properties of SnO2 thin films after ultra-short pulsed laser annealing

Ultra-short pulsed laser sources, with pulse durations in the ps and fs regime, are commonly exploited for cold ablation. However, operating ultra-short pulsed laser sources at fluence levels well below the ablation threshold allows for fast and selective thermal processing. The latter is especially advantageous for the processing of thin films. A precise control of the heat affected zone, as small as tens of nanometers, depending on the material and laser conditions, can be achieved. It enables the treatment of the upper section of thin films with negligible effects on the bulk of the film and no thermal damage of sensitive substrates below. By applying picosecond laser pulses, the optical and electrical properties of 900 nm thick SnO2 films, grown by an industrial CVD process on borofloat®-glass, were modified. The treated films showed a higher transmittance of light in the visible and near infra-red range, as well as a slightly increased electrical sheet resistance. Changes in optical properties are attributed to thermal annealing, as well as to the occurrence of Laser- Induced Periodic Surface Structures (LIPSSs) superimposed on the surface of the SnO2 film. The small increase of electrical resistance is attributed to the generation of laser induced defects introduced during the fast heating-quenching cycle of the film. These results can be used to further improve the performance of SnO2-based electrodes for solar cells and/or electronic devices.

Thin film surface processing by UltraShort Laser Pulses (USLP)

In this work, we studied the feasibility of surface texturing of thin molybdenum layers on a borosilicate glass substrate with Ultra-Short Laser Pulses (USLP). Large areas of regular diffraction gratings were produced consisting of Laserinduced periodic surface structures (LIPSS). A short pulsed laser source (230 fs-10 ps) was applied using a focused Gaussian beam profile (15-30 μm). Laser parameters such as fluence, overlap (OL) and Overscans (OS), repetition frequency (100-200 kHz), wavelength (1030 nm, 515 nm and 343 nm) and polarization were varied to study the effect on periodicity, height and especially regularity of LIPSS obtained in layers of different thicknesses (150-400 nm). The aim was to produce these structures without cracking the metal layer and with as little ablation as possible. It was found that USLP are suitable to reach high power densities at the surface of the thin layers, avoiding mechanical stresses, cracking and delamination. A possible photovoltaic (PV) application could be found in texturing of thin film cells to enhance light trapping mechanisms.

Microstructural characterization of surface damage through ultra-short laser pulses

Electron back-scatter diffraction (EBSD) technique, commonly used to study the microstructural characteristics of materials, was employed for the investigation of the surface damage induced through ultra-short laser pulses. Single-crystal silicon surface was irradiated with an Ytterbium-doped YAG (Trumpf-TruMicro 5050) laser source generating laser pulses of 6.7 ps duration, a 1030 nm wavelength and linear polarization. The laser fluence level was set to values lower than the single-pulse modification threshold of the material. The laser pulses were delivered on the surface at conditions of lateral displacement, i.e. a train of laser pulses with a partial overlap (laser track). This approach made it possible to investigate the early stages of modification of the surface. Scanning electron microscope equipped with a field emission gun (Philips XL30 SEM FEG) and EDAX-TSL EBSD system was used for inspection of the surface modifications initiated with pulsed laser radiation. Depth of the generation of back-scattered electrons at different acceleration voltages of the primary beam was estimated by the use of Monte-Carlo simulation. Trajectories of primary and back-scattered electrons in a flat Si surface were generated at an angle of 74o from the surface normal, which is the angle used for the EBSD observations. High sensitivity of EBSD signal allows an estimate of the depth and intensity of the laser induced damage to the crystal lattice. It is found that the thickness of amorphous layer increases gradually with a distance from the feature center. The similarity of surface damage profiles observed at different accelerating voltages of the primary beam indicates that the damage is formed via a gradual crystal damage accumulation in subsurface layer and via the formation and growth of an amorphous layer from the surface.

Surface melting of copper by ultrashort laser pulses

The main advantage of ultrashort laser pulses in manufacturing technology is their very high removal rate of material and high quality of microstructures with the smallest dimensions at 1 μm level. The accuracy is mainly due to almost absence of thermal diffusion into bulk material. In this paper we report the investigation on polycrystalline Cu sample surface treated by 6.7 ps laser pulses with 1030 nm laser light wavelength. Scanning electron microscopy micrographs reveal the presence of jet-like structures with spherical drop-like endings, solidified spheres and many bubble bursts at even lower fluence than the threshold value for the ablation is. Within the molten material the jet-like features are due to an explosion of bubbles originated in solid-liquid-vapor transitions. In the case of below-threshold irradiation the same objects can be seen along surface scratches, dot contaminations etc., which indicate an increase of the laser light absorption on these inhomogeneitiesThe main advantage of ultrashort laser pulses in manufacturing technology is their very high removal rate of material and high quality of microstructures with the smallest dimensions at 1 μm level. The accuracy is mainly due to an almost absence of thermal diffusion into bulk material. In this paper we report the investigation on polycrystalline Cu sample surface treated by 6.7 ps laser pulses with 1030 nm laser light wavelength. Scanning electron microscopy micrographs reveal the presence of jet-like structures with spherical drop-like endings, solidified spheres and many bubble bursts at even lower fluence than the threshold value for the ablation is. Within the molten material the jet-like features are due to an explosion of bubbles originated in solid-liquid-vapor transitions. In the case of below-threshold irradiation, the same objects can be seen along surface scratches, dot contaminations etc., which indicate an increase of the laser light absorption on these inhomogeneities.

Melting of copper surface by ultrashort laser pulses

The main advantage of ultrashort laser pulses in manufacturing technology is their very high removal rate of material and high quality of microstructures with the smallest dimensions at 1 μm level. The accuracy is mainly due to almost absence of thermal diffusion into bulk material. In this paper we report the investigation on polycrystalline Cu sample surface treated by 6.7 ps laser pulses with 1030 nm laser light wavelength. Scanning electron microscopy micrographs reveal the presence of jet-like structures with spherical drop-like endings, solidified spheres and many bubble bursts at even lower fluence than the threshold value for the ablation is. Within the molten material the jet-like features are due to an explosion of bubbles originated in solid-liquid-vapor transitions. In the case of below-threshold irradiation the same objects can be seen along surface scratches, dot contaminations etc., which indicate an increase of the laser light absorption on these inhomogeneitiesThe main advantage of ultrashort laser pulses in manufacturing technology is their very high removal rate of material and high quality of microstructures with the smallest dimensions at 1 μm level. The accuracy is mainly due to an almost absence of thermal diffusion into bulk material. In this paper we report the investigation on polycrystalline Cu sample surface treated by 6.7 ps laser pulses with 1030 nm laser light wavelength. Scanning electron microscopy micrographs reveal the presence of jet-like structures with spherical drop-like endings, solidified spheres and many bubble bursts at even lower fluence than the threshold value for the ablation is. Within the molten material the jet-like features are due to an explosion of bubbles originated in solid-liquid-vapor transitions. In the case of below-threshold irradiation, the same objects can be seen along surface scratches, dot contaminations etc., which indicate an increase of the laser light absorption on these inhomogeneities.