Femtosecond Laser Polishing Research Articles

Realizing the High Efficiency of Type‐II Superlattice Infrared Sensors Integrated Wire‐Grid Polarizer via Femtosecond Laser Polishing

AbstractA comprehensive strategy to enhance the polarization performance of mid‐wave infrared photodetectors (PDs) is developed and implemented by integrating wire‐grid polarizers (WGPs) using nanoimprint lithography and femtosecond laser (FSL) polishing. This combined approach offers significant advantages, including large‐area fabrication capabilities, practical device integration, and improved polarization characteristics. By addressing optical losses, the primary factor contributing to polarization degradation through the thermal effects of FSL polishing, substantial improvements are achieved in surface roughness and grain boundary reduction on the WGP, resulting in remarkable performance enhancements. As a result, the extinction ratio of the integrated WGP InAs/GaSb type‐II superlattice PD achieves an impressive value of up to 1044. This approach holds promising potential for advancing polarization‐based imaging and measurement systems to new heights.

Parameter Optimization of RB-SiC Polishing by Femtosecond Laser

Reaction-boned silicon carbide (RB-SiC) is considered a new material for large lightweight ground-based space telescopes due to its high specific stiffness, low thermal deformation, and excellent optical quality. The excellent mechanical properties of RB-SiC result in the low efficiency of traditional polishing and mechanical polishing. In this paper, a polishing method for RB-SiC based on a femtosecond laser is proposed to improve surface quality. A theoretical heat conduction model was established in the process of femtosecond laser irradiation of SiC. We analyzed the ablation type and calculated the single-pulse ablation threshold of SiC, which verified the feasibility of femtosecond laser polishing. Further, the effects of polishing parameters on the polished surface quality were analyzed by a series of experiments, and the optimal parameters were selected. It was observed to improve polishing efficiency and can replace the intermediate steps of traditional mechanical polishing.

Femtosecond laser polishing of additively manufactured parts at grazing incidence

Surface polishing is usually a requisite for making additively manufactured (AM) parts ready for practical applications. Due to its advantages of being flexible and noncontact, laser polishing has attracted increasing research interest. In this research, femtosecond (fs) laser polishing was established to post-process both the top surfaces and sidewalls of AM parts. The challenge to remove three levels of roughness (i.e., the initial surface roughness of the AM parts, the undulations newly introduced during fs laser polishing, and the micro-nanoscale surface features induced by fs laser irradiation) was identified and addressed. Mirror surfaces with Sa < 200 nm were achieved on stainless steel parts printed with an initial roughness >20 µm. Both parallel- and perpendicular-incidence were investigated for the polishing, with the former verified to be more effective in eliminating the initial roughness of the AM parts, due to the elongated focal intensity profile of a Gaussian beam irradiated on the AM part surfaces. The challenge of forming three-zone surfaces during the parallel-incidence was further addressed through a grazing-incidence polishing approach, and uniform smooth surfaces were realized. Fine-tuning the laser power enabled controlling the submicron surface features formed under fs laser irradiation, which determined the final achievable surface roughness.

Femtosecond laser polishing with high pulse frequency for improving performance of specialised aerospace material systems: MCrAlY coatings in thermal barrier coating system

MCrAlY coatings are widely used as bond coats in thermal barrier coating system to protect thermally loaded structures from oxidation during high-temperature service. Surface roughness (Sa) and microstructures are essential for high-temperature oxidation resistance of MCrAlY coatings. Despite the successful use of laser polishing with long pulse durations for surface modification of alloys, inevitable excessive ablation of the material limits its application to smoothing of MCrAlY coatings. In this study, a novel polishing technique based on remelting was developed via femtosecond laser polishing with high pulse frequency (FLPHPF). Different single pulse fluence and scanning trajectories were performed, and mechanisms of melt-condensation and polishing effect induced via FLPHPF were investigated. Microstructural observations revealed that FLPHPF treatment could reduce Sa, refine grains, introduce dislocations, and change elemental distributions. The restructured and polished remelted layer contributed to a thinner, continuous, and uniform α-Al2O3 scale during the oxidation process, which verifies that FLPHPF technique is an effective and feasible method to improve the high-temperature oxidation resistance of MCrAlY coatings. Based on this research, a laser polishing technique with low ablation and controlled thermal accumulation was proposed. Moreover, FLPHPF broadens the application prospects of ultrashort pulse lasers and provides an alternative processing strategy for the surface modification of ultra-thin fine structures.

Effects of incidence angle and optimization in femtosecond laser polishing of C/SiC composites

In this article, C/SiC ceramic matrix composites (CMC) were polished by femtosecond laser. The effects of different laser incidence angles on the machined surface were studied in detail, and the process parameters were optimized. The results have indicated that different laser incident angles influence the machined surface of CMC. With the increase of laser incident angle, the defects and oxidation degree decrease significantly, the carbon (C) content gradually decreases, silica (Si) content gradually increases, and oxygen (O) content is almost unchanged. Under an incident angle of 70°, the surface carbon fiber along the axial direction was neatly cut, and the bump structure was eliminated. The laser's incident angle greatly influences the corrugated structure of the carbon fiber section. The response surface method establishes the quadratic response model of four parameters and surface roughness. The small-scale optimization method predicted the optimal expected value as follows: single pulse energy is 71.53 μJ, defocus amount is −0.13 mm, incident angle is 20.61°, Y-axis spot overlap rate 77.50%, and the optimal surface roughness (Ra) is predicted as 1.03 μm. The experimental results show that the average surface roughness of 0.92 μm is within the 95% confidence interval mean (95% CI Mean), indicating that the experimental results are reliable. It is 51.98% lower than the original surface roughness value of 1.92 μm. The optimal surface roughness value is 0.71 μm, 63.02% lower than the original surface roughness value.

Influence of Pulse Energy and Defocus Amount on the Mechanism and Surface Characteristics of Femtosecond Laser Polishing of SiC Ceramics.

SiC ceramics have excellent comprehensive properties and are typical hard and brittle materials that are difficult to process and are widely used in many fields. Laser polishing technology has developed into a new surface processing technology, and femtosecond laser polishing has become an important method for the precision machining of hard and brittle materials. In this paper, SiC ceramics were ablated and polished by infrared femtosecond laser, the laser ablation threshold of SiC ceramics was calculated and the influence of pulse energy and defocus amount on the surface morphology, surface roughness, polishing depth and oxidation degree of femtosecond laser polishing of SiC ceramics were investigated. The results show that when the laser repetition frequency f = 175 kHz, wavelength λ = 1064 nm and ablation time t = 9 s, the laser ablation threshold of SiC ceramics is 0.355 J/cm2. With the increase in pulse energy, the surface roughness first decreased and then increased, and the polishing depth showed an overall upward trend. The change of defocus amount will lead to the change of the laser spot diameter. With the increase of the defocus amount, the laser spot irradiated on the workpiece surface becomes larger, and the laser energy density decreases, which results in the decrease of the laser ablation ability and polishing depth and the increase of the polished surface roughness. Periodic nano-ripple structures appeared on the laser-induced surface. Through Energy Dispersive Spectrometer (EDS) elemental analysis, it was found that there was an oxidation phenomenon in SiC ceramics polished by femtosecond laser in an air environment, and the change of pulse energy and defocus amount had insignificant effects on the degree of oxidation.

Femtosecond Laser Polishing of Additively Manufactured Parts at Grazing Incidence

Femtosecond Laser Polishing of Additively Manufactured Parts at Grazing Incidence

Femtosecond Laser Polishing of Additively Manufactured Parts at Grazing Incidence

Femtosecond Laser Polishing of Additively Manufactured Parts at Grazing Incidence

Influence of Pulse Energy and Scanning Speed of Femtosecond Laser on Surface Roughness of SiC Ceramics

Laser polishing, as a novel surface processing technology, has been successfully employed for improving the surface smoothness and geometric accuracy of components for metals, glasses, fibers and ceramics. However, continuous wave and microsecond and nanosecond lasers leads to some undesirable defects of the polished surface, such as heat affected zone, debris redeposition and thermal stress induced microcracks, especially for hard brittle materials. To solve this issue, this article investigated femtosecond laser polishing of SiC ceramics. Influence of pulse energy and scanning speed on surface roughness and surface morphologies was studied. In addition, the relationship between laser parameters and surface roughness and removal depth of SiC ceramics has been revealed. The experimental results indicated that the polished surface without big voids and spalling was obtained through optimizing femtosecond laser parameters.

One-step fabrication of fine surfaces via femtosecond laser on sliced SiC

A one-step, facile approach of femtosecond laser polishing for achieving fine surface in sliced SiC is reported. The effect of wavelength and pulse number on formation mechanism of surface morphology and composition are studied firstly. On the basis of relationship between removal depth and defect depth, variation on surface profile investigating, fine surface is obtained by fine-tuning processing parameters. Regular grooves formed by wire saw on subsurface is eliminated completely but no damage generates. Not only that, curves of indentation indicate that quality on subsurface is improved. The improvement both in surface and subsurface benefits from small thermal effect and high machining accuracy of femtosecond laser. Homogeneous structure/composition on subsurface after polishing has higher stable friction coefficient，which will be beneficial to the subsequent ultra-precision polishing.

A STUDY OF FEMTOSECOND LASER POLISHING OF CVD NANOPOLYCRYSTALLINE DIAMOND FILMS

Femtosecond (fs) laser ablation has been recognized as an effective and promising technique for high-precision processing of natural and synthesized diamond. In this work, a study of femtosecond laser polishing for nanopolycrystalline diamond (NCD) films by chemical vapor deposition (CVD) is reported. The laser irradiation is induced by 200-fs laser pulses with a repetition frequency of 50[Formula: see text]MHz, and various laser fluences are employed to investigate their polishing effectiveness. The results show that the optimal laser fluence is 0.7[Formula: see text]J/cm2, at which the nanodiamond grains on top of the cauliflower-like clusters of NCD films can be ablated. With such laser fluence, the mean surface roughness of NCD films reduces from 73.84[Formula: see text]nm to 31.88[Formula: see text]nm, which presents a 57% reduction. Nevertheless, when the laser fluence rises beyond 0.7[Formula: see text]J/cm2, large amount of amorphous carbon (a-C) balls and porous lava-like morphology would come into being, resulting in severe degradation of the NCD surface.

Femtosecond laser polishing of SiC/SiC composites: Effect of incident angle on surface topography and oxidation

In this article, a new ultrafast laser polishing method for SiC/SiC composites was reported and the effects of different incident angles were discussed in detail. The results indicated that the incident angle had a great influence on the polishing of SiC/SiC composites. Under the normal incident condition, the surface roughness was barely improved and the laser-induced defects even degraded the process quality. With the increasing incident angle, surface oxidation and defects dramatically decreased. At 80 degrees, the surface convex structure is almost eliminated and the protruding fiber was neatly cut off. Furthermore, a theoretical model of laser polishing for SiC/SiC composites is used to analyze the laser propagation and power density distribution in different incident angles. The model successfully predicted the laser energy resonance enhancement and illustrated the formation mechanism of laser-induced defects and nanoripples. Meanwhile, the result shows that the laser energy concentrated on the convex fiber of composites as the incident angle increased, which explained that the SiC/SiC composites polishing quality was significantly improved under the inclined incident condition.

Investigation on the underwater femtosecond laser polishing SiC ceramic

Owing to high hardness and high brittleness of SiC ceramic, it is difficult to machine into structural ceramic components with desired surface quality and dimensional accuracy. In this paper, a method of polishing SiC ceramic by underwater femtosecond laser was proposed. The influence of laser frequency and pulse energy on surface morphologies, 3 D profiles, depth and surface roughness of polished SiC ceramics during underwater femtosecond laser processing was investigated in detail. The experimental results indicated that the removal depth during underwater polishing increased and then decreased with the increase of laser frequency and pulse energy. The surface roughness increased as laser frequency increased. The surface roughness decreased and then increased with the increase of pulse energy. Through laser parameter optimization, a high quality polished surface without cracks, pits and debris redopesition was obtained under laser frequency of 40 kHz and pulse energy of 40 μJ. At this point, the average removal depth of 32.19 μm and the average surface roughness of 0.72 μm were obtained. The proposed method in this paper significantly improve surface finishment and higher dimensional accuracy of the machined SiC ceramic components.

An experimental investigation of scan trajectory into the underwater femtosecond laser polishing SiC ceramic

Silicon carbide (SiC) ceramics, due to their excellent mechanical properties and oxidation resistance at elevated temperature, have a widespread application in microelectronics and aerospace. However, the machining of SiC ceramics into the desired structural ceramic components presents a challenge because of their high hardness and high brittleness. In this paper, a novel method of underwater femtosecond laser polishing SiC ceramics was proposed. We investigated the influence of scan trajectory, such as scanning pitch and polishing mode, and femtosecond laser pulse energy on surface roughness and polishing depth during underwater polishing processing. The experimental results indicated that the polishing depth decreased with the increasing scanning pitch, and then tending to saturate. A deep cavity was obtained under the condition of high laser pulse energy and small scanning pitch during polishing. However, the obtained surface quality was rough at this condition, which could be used for rough polishing operation because the high material removal rate is the primary concern. In addition, the cross-scanning polishing mode achieved the better surface quality without cracks and pits than the transverse polishing mode, but the polishing efficiency was low. In addition, under appropriate machining conditions, the smooth polished surface with surface roughness 0.76 µm was obtained by using 15 µm scanning pitch together with high laser pulse energy and cross-scanning polishing mode.

Enhancing Perovskite Solar Cell Performance through Femtosecond Laser Polishing

Nonradiative recombination loss is a key process that determines the performance of perovskite solar cells, and how to control it is significant for the research and development of perovskites. Generally, traditional chemical modification/passivation methods are complicated and prone to secondary contamination. Herein, femtosecond (fs) laser polishing as a promising technique is demonstrated to ameliorate the surface of perovskite films, reduce nonradiative recombination loss, and improve solar cell performance. The high‐intensity fs laser pulses can remove around 20 nm‐thick perovskite top layer through an ionization process, help to decrease the grain boundary density, and enlarge the grain size of perovskite films after recrystallization. It is believed that fs laser polishing is a time‐effective and highly precise technique that is suitable for large‐scale device production, thus will trigger more applications in optoelectronics.

Femtosecond laser polishing of germanium [Invited

Freeform optics can reduce the cost, weight, and size of advanced imaging systems, but it is challenging to manufacture the complex rotationally asymmetric surfaces to optical tolerances. To address the need for disruptive, high-precision sub-aperture forming and finishing techniques for freeform optics, we investigate an alternative, non-contact polishing methodology using femtosecond lasers, combining modeling, experiments, and demonstrations. Femtosecond-laser-based polishing of germanium was investigated using an experimentally-validated two-temperature model of laser/germanium interaction to guide the understanding and selection of laser parameters to achieve near-nonthermal ablation for polishing and figuring. For the first time to our knowledge, model-guided femtosecond laser polishing of germanium was successfully demonstrated, achieving precision material removal while maintaining single-digit nanometer optical surface quality. The demonstrated femtosecond-laser-based polishing technique lays the foundation for semiconductor optics polishing/fabrication using femtosecond lasers and opens a viable path for high-precision, complex sub-aperture optical polishing tasks on various materials.

Simulation and experiment of femtosecond laser polishing quartz material

ABSTRACTFemtosecond laser is ideal tool for micro-machining of hard and brittle materials, such as quartz material. The basis of experiments on femtosecond laser polishing quartz shown that, when other process parameters were the same, the greater the scanning offset distance, the more obvious ravines in polished surface, the more rough bottom surface. Using MATLAB construct a mathematical model to explain the formation mechanism of roughness. Modeling indicated the parameter of femtosecond polishing should maximize the use of smaller laser energy, recycling roughness mathematical model to select the appropriate offset distance.