Scanning Algorithm Optimization for Achieving Low-Roughness Surfaces Using Ultrashort Laser Pulses: A Comparative Study.

Femtosecond laser-assisted material surface modification is a rapidly growing field with numerous applications, including tribology, micromechanics, optofluidics, and medical implant treatment. For many of these applications, precise control of surface roughness after laser treatment is crucial, as it directly affects the final properties of the work surface. However, achieving low mean surface roughness values (<100 nm) is challenging due to the fundamental principles of laser light-matter interactions. The complex physical processes that occur during laser material interactions make it difficult to achieve the desired surface roughness, and only advanced scanning methods can potentially solve this issue. In our study, we analyzed laser scanning algorithms to determine the optimal method for producing surfaces with minimal roughness. We investigated how scanning parameters such as the overlap of modifications, the amount of successive line shift, and laser-scanner synchronization impact surface roughness. Using a numerical model, we obtained results that showed good agreement with experimentally acquired data. Our detailed theoretical and experimental analysis of different scanning methods can provide valuable information for the future optimization of minimal-roughness micromachining.

High-Contrast Marking of Stainless-Steel Using Bursts of Femtosecond Laser Pulses.

The marking and surface structuring of various materials is important in various industrial fields such as biomaterials, luxury goods, anti-counterfeiting, automotive and aerospace, electronics and semiconductor industries, and others. Recent advances in laser technology, such as burst-mode lasers, have opened new ways of affecting the surfaces of various materials, inducing a different appearance and/or properties of the laser-exposed areas. From earlier studies, it is known that when splitting a single pulse into multiple pulses and thus creating a quasi-MHz-GHz repetition rate regime, it is possible to increase not only the ablation efficiency but it also provides the possibility to tune the heat in-flow into the surface. Such new regimes enable the control of the surface roughness as well as the optical properties and corrosion resistance. In this work, we analyze the effect of the different burst-mode regimes for the marking of stainless-steel samples, aiming to produce high-contrast marking having different shades of black/white color (black-gray-white). Moreover, we investigate the angular dependence of the reflected light after laser treatment numerically from the measured surface morphology.

Femtosecond Laser Assisted 3D Etching Using Inorganic-Organic Etchant.

Selective laser etching (SLE) is a technique that allows the fabrication of arbitrarily shaped glass micro-objects. In this work, we show how the capabilities of this technology can be improved in terms of selectivity and etch rate by applying an etchant solution based on a Potassium Hydroxide, water, and isopropanol mixture. By varying the concentrations of these constituents, the wetting properties, as well as the chemical reaction of fused silica etching, can be changed, allowing us to achieve etching rates in modified fused silica up to 820 µm/h and selectivity up to ∼3000. This is used to produce a high aspect ratio (up to 1:1000), straight and spiral microfluidic channels which are embedded inside a volume of glass. Complex 3D glass micro-structures are also demonstrated.

Sapphire Selective Laser Etching Dependence on Radiation Wavelength and Etchant.

Transparent and high-hardness materials have become the object of wide interest due to their optical and mechanical properties; most notably, concerning technical glasses and crystals. A notable example is sapphire-one of the most rigid materials having impressive mechanical stability, high melting point and a wide transparency window reaching into the UV range, together with impressive laser-induced damage thresholds. Nonetheless, using this material for 3D micro-fabrication is not straightforward due to its brittle nature. On the microscale, selective laser etching (SLE) technology is an appropriate approach for such media. Therefore, we present our research on C-cut crystalline sapphire microprocessing by using femtosecond radiation-induced SLE. Here, we demonstrate a comparison between different wavelength radiation (1030 nm, 515 nm, 343 nm) usage for material modification and various etchants (hydrofluoric acid, sodium hydroxide, potassium hydroxide and sulphuric and phosphoric acid mixture) comparison. Due to the inability to etch crystalline sapphire, regular SLE etchants, such as hydrofluoric acid or potassium hydroxide, have limited adoption in sapphire selective laser etching. Meanwhile, a 78% sulphuric and 22% phosphoric acid mixture at 270 °C temperature is a good alternative for this process. We present the changes in the material after the separate processing steps. After comparing different processing protocols, the perspective is demonstrated for sapphire structure formation.

Optimization of selective laser etching (SLE) for glass micromechanical structure fabrication.

In this work, we show how femtosecond (fs) laser-based selective glass etching (SLE) can be used to expand capabilities in fabricating 3D structures out of a single piece of glass. First, an investigation of the etching process is performed, taking into account various laser parameters and scanning strategies. These results provide critical insights into the optimization of the process allowing to increase manufacturing throughput. Afterward, various complex 3D glass structures such as microfluidic elements embedded inside the volume of glass or channel systems with integrated functional elements are produced. A single helix spring of 1 mm diameter is also made, showing the possibility to compress it by 50%. Finally, 3D structuring capabilities are used to produce an assembly-free movable ball-joint-based chain and magnet-actuated Geneva mechanism. Due to minimized friction caused by low (down to 200 nm RMS) surface roughness of SLE-produced structures, the Geneva mechanism was shown to be capable of rotating up to 2000 RPM.

Micromachining of Transparent Biocompatible Polymers Applied in Medicine Using Bursts of Femtosecond Laser Pulses.

Biocompatible polymers are used for many different purposes (catheters, artificial heart components, dentistry products, etc.). An important field for biocompatible polymers is the production of vision implants known as intraocular lenses or custom-shape contact lenses. Typically, curved surfaces are manufactured by mechanical means such as milling, turning or lathe cutting. The 2.5 D objects/surfaces can also be manufactured by means of laser micromachining; however, due to the nature of light-matter interaction, it is difficult to produce a surface finish with surface roughness values lower than ~1 µm Ra. Therefore, laser micromachining alone can't produce the final parts with optical-grade quality. Laser machined surfaces may be polished via mechanical methods; however, the process may take up to several days, which makes the production of implants economically challenging. The aim of this study is the investigation of the polishing capabilities of rough (~1 µm Ra) hydrophilic acrylic surfaces using bursts of femtosecond laser pulses. By changing different laser parameters, it was possible to find a regime where the surface roughness can be minimized to 18 nm Ra, while the polishing of the entire part takes a matter of seconds. The produced surface demonstrates a transparent appearance and the process shows great promise towards commercial fabrication of low surface roughness custom-shape optics.

Micromachining of Invar Foils with GHz, MHz and kHz Femtosecond Burst Modes.

In this work, a burst mode laser is used for micromachining of 20 µm-250 µm thick Invar (Fe64/Ni36) foils. Holes were drilled by firing multiple pulses transversely onto the sample without moving the beam (percussion drilling). The utilized laser system generates a burst of a controllable number of pulses (at 1030 nm) with tunable pulse-to-pulse time spacing ranging from 200 ps to 16 ns. The sub-pulses within the burst have equal amplitudes and a constant duration of 300 fs that do not change regardless of the spacing in time between them. In such a way, the laser generates GHz to MHz repetition rate pulse bursts with a burst repetition rate ranging from 100 kHz to a single shot. Drilling of the material is compared with the non-burst mode of kHz repetition rate. In addition, we analyze the drilling speed and the resulting dependence of the quality of the holes on the number of pulses per burst as well as the average laser power to find the optimal micromachining parameters for percussion drilling. We demonstrate that the micromachining throughput can be of an order of magnitude higher when using the burst mode as compared to the best results of the conventional kHz case; however, excess thermal damage was also evident in some cases.

Investigation of piezoelectric ringing effects in Pockels cells based on beta barium borate crystals.

A detailed investigation of piezoelectric ringing and resonance effects in beta barium borate (BBO)-crystal-based Pockels cells in a pulse picker device is presented. Measurements mapping piezoelectric oscillation amplitude distribution in the aperture of BBO crystal are also performed. It is determined that in BBO Pockels cells, due to piezoelectric ringing and related effects, the optical contrast ratio is reduced by six to 70 times at certain (resonance) frequencies. Moreover, we demonstrate that in our pulse picker device, which uses a special sequence of control pulses (regeneration method) for the Pockels cell, at certain high-voltage control signal durations, piezoelectric oscillations can be suppressed, which significantly improves performance of the Pockels-cell-based laser pulse picker device at resonance frequencies: in such cases, the optical contrast ratio is stable and higher than 1000:1. Numerical simulation results support the experimental data.

Measurement of the phase refractive index of a photonic crystal fiber mode.

We present a new experimental technique for measurement of the refractive index of a photonic crystal fiber (PCF) fundamental mode. We demonstrate that the phase refractive index of the PCF mode can be estimated by analyzing a phase shift of interfering adjacent longitudinal laser modes of a continuous-wave laser corresponding to a shift from constructive to destructive interference. The experimental results are in very good agreement with numerically simulated phase refractive index values.

Combination of additive and subtractive laser 3D microprocessing in hybrid glass/polymer microsystems for chemical sensing applications.

We present a novel hybrid glass-polymer micromechanical sensor by combining two femtosecond laser direct writing processes: laser illumination followed by chemical etching of glass and two-photon polymerization. This incorporation of techniques demonstrates the capability of combining mechanical deformable devices made of silica with an integrated polymer structure for passive chemical sensing application. We demonstrate that such a sensor could be utilized for investigating the elastic properties of polymeric microstructures fabricated via the two-photon polymerization technique. Moreover, we show that polymeric microstructure stiffness increases when immersed in organic liquids.

Estimation of photonic crystal fiber dispersion by means of supercontinuum generation.

We present a technique for photonic crystal fiber dispersion measurement. We demonstrate that investigating supercontinuum using cross-correlation frequency resolved optical gating (XFROG) technique can be used for quantitatively characterizing dispersion and observing orthogonal polarization modes in polarization maintaining photonic crystal fibers. In addition, an XFROG trace of supercontinuum generated in a polarization maintaining photonic crystal fiber reveals complex behavior of orthogonal polarization modes that is different in normal and anomalous dispersion regions of the photonic crystal fiber.

Femtosecond laser damage resistance of oxide and mixture oxide optical coatings.

We report on the laser damage resistance of ion beam-sputtered oxide materials (Al2O3, Nb2O5, HfO2, SiO2, Ta2O5, ZrO2) and mixtures of Al2O3-SiO2, Nb2O5-SiO2, HfO2-SiO2, Ta2O5-SiO2, and ZrO2-SiO2, irradiated by single 500 fs pulses at 1030 nm. Laser-induced damage threshold (LIDT), refractive index, and bandgaps of the single-layer coatings are measured. For pure oxide materials a linear evolution of the LIDT with bandgap is observed. The results are in accordance with our simulations based on photo-ionization and avalanche-ionization. In the case of mixtures, however, deviations from the previous behaviors are evidenced. The evolution of the LIDT as a function of the refractive index is analyzed, and an empirical description of the relation between refractive index and LIDT is proposed.

Laser-induced damage of hafnia coatings as a function of pulse duration in the femtosecond to nanosecond range.

Laser-damage thresholds and morphologies of hafnia single layers exposed under femtosecond, picosecond, and nanosecond single pulses (1030/1064 nm) are reported. The samples were made with different deposition parameters in order to study how the damage behavior of the samples evolves with the pulse duration and how it is linked to the deposition process. In the femtosecond to picosecond regime, the scaling law of the laser-induced damage threshold as a function of pulse duration is in good agreement with the models of photo and avalanche ionization based on the rate equation for free electron generation. However, differences in the damage morphologies between samples are shown. No correlation between the nanosecond and femtosecond/picosecond laser-damage resistance of hafnia coatings could be established. We also report evidence of the transition in damage mechanisms for hafnia, from an ablation process linked to intrinsic properties of the material to a defect-induced process, that exists between a few picoseconds and a few tens of picoseconds.

Characterization of zirconia- and niobia-silica mixture coatings produced by ion-beam sputtering.

ZrO2-SiO2 and Nb2O5-SiO2 mixture coatings as well as those of pure zirconia (ZrO2), niobia (Nb2O5), and silica (SiO2) deposited by ion-beam sputtering were investigated. Refractive-index dispersions, bandgaps, and volumetric fractions of materials in mixed coatings were analyzed from spectrophotometric data. Optical scattering, surface roughness, nanostructure, and optical resistance were also studied. Zirconia-silica mixtures experience the transition from crystalline to amorphous phase by increasing the content of SiO2. This also results in reduced surface roughness. All niobia and silica coatings and their mixtures were amorphous. The obtained laser-induced damage thresholds in the subpicosecond range also correlates with respect to the silica content in both zirconia- and niobia-silica mixtures.

Tilted-pulse time-resolved off-axis digital holography.

In this Letter we present an improvement of time-resolved off-axis digital holography by the use of tilted femtosecond laser pulses. The pulse front tilting of the reference beam with respect to the phase front allows larger crossing angles to be used for recording of digital holograms without significant reduction of pulse interference area (typically limited by low temporal coherence of ultrashort pulses). Such approach increases the area of interference fringes, thus enabling the higher resolution of the reconstructed image as well as better separation of dc term. Temporal resolution is not deteriorated by this method, as only the reference pulse is tilted. The proposed technique was applied for direct intensity clamping observations of light filaments in water using the laser pulses of 30 fs duration.

Calculations and experimental demonstration of multi-photon absorption governing fs laser-induced damage in titania.

Laser damage phenomena are governed by a number of different effects for the respective operation modes and pulse durations. In the ultra short pulse regime the electronic structure in the dielectric coating and the substrate material set the prerequisite for the achieved laser damage threshold of an optical component. Theoretical considerations have been done to assess the impact of contributing ionization phenomena in order to find a valid description for laser-induced damage in the femtosecond (fs) domain. Subsequently, a special set of sample has been designed to verify these considerations via ISO certified laser damage testing. Examining the theoretical and experimental data reveals the importance of multi-photon absorption for the optical breakdown. For titania, the influence of multi-photon absorption has been clearly shown by a quantized wavelength characteristic of the laser damage threshold.

Time-resolved off-axis digital holography for characterization of ultrafast phenomena in water.

We present the application of time-resolved off-axis digital holography for the investigation of refractive index/transmission properties of laser-induced plasma filaments in water. Time evolution of both amplitude- and phase-contrast images of the self-focused beam in water was characterized with temporal resolution better than 50 fs. To the best of our knowledge, this is the first attempt to characterize the propagation of femtosecond laser pulse in nonlinear media using off-axis digital holography.

Stripe patterns in degenerate optical parametric oscillators.

We experimentally demonstrate the stripe (or roll) patterns in a broad-aperture degenerate optical parametric oscillator in a plane-mirror minicavity. The stabilization of stripes is achieved by seed injection at a subharmonic frequency. We measure the temporal spectra of the stripe pattern and obtain the 1/f-like noise spectra.