Laser-induced Periodic Surface Structures Research Articles

Higher Suitability of NbMoTaW over Its Elemental Metals for Laser Induced Periodic Surface Structure/Particle-Aggregate UV-to-MIR Ultrabroadband Absorber

The peculiarity and higher suitability of NbMoTaW over its composed elemental metals for femtosecond laser induced LIPSS/particle-aggregate UV-to-MIR ultrabroadband absorber are firstly revealed. The performance of ultrabroadband absorbers is adjustable by changing the density of NbMoTaW particle aggregates on laser induced periodic surface structures (LIPSS) with increasing repetition rate from 0.5 to 1 MHz and laser power from 1 to 12 W till a whole coverage at 12 W and 1 MHz, under which condition only lower-performance microstripes consisting of particle-aggregate and LIPSS (on Mo, Ta, and W) or macropore/LIPSS micro/nanostructures (on Nb) are achievable on elemental metals. NbMoTaW-LIPSS decorated by a few particle aggregates is more suitable than LIPSS itself to be used as selective solar absorber, indicating the capacity of deposited particle aggregates to modulate absorbance selectivity. It is deduced that NbMoTaW-RHEA should possess a lower melting temperature than W and a high electron-phonon coupling constant, facilitating nano-structuring/nano-synthesis.

Optimised diamond to graphite conversion via a metastable sp1-bonded carbon chain formation under an ultra-short femtosecond (30 fs) laser irradiation

An application of an ultra-short femto-second (fs) pulsed laser (30 fs, 800 nm, 1 kHz) radiation under a simultaneously varied laser fluence (0.6, 1.2 and 1.8 J/cm2) and number of pulses (2000–32,000 pulses per irradiation site) to process diamond has been investigated to determine an optimal conversion of the diamond's tetrahedral sp3 phase into an aromatic sp2 phase with a high degree of crystallinity and without a formation of an sp2 olefinic fraction, thermal damage or an irradiation-induced surface cracking. The highest sp2 aromatic crystallinity and the most significant sp3-to-sp2 conversion efficiency in diamond was attained at 1.2 J/cm2 with 32,000 pulses. The sp3-to-sp2 phase transition was marked by a formation of a metastable carbon in the sp1 hybridisation (carbyne), evidenced by the characteristic Raman mode at ca. 2065 cm−1. Inclusions of covalently bound carbonyl (-CO) at ca. 1705 cm−1 were also observed on diamond under lower irradiation doses. Both the sp1 and carbonyl contributions resolved gradually as irradiation dose increased. Well-defined laser-induced periodic surface structures (LIPSS) (aka. nano-ripples) with a high spatial frequency distribution and periodicity of around 100 nm were observed forming in the irradiated regions on diamond owing to the interaction of the laser pulse with the surface plasmons; an ordered and homogeneous formation of LIPSS structures has been observed at 1.2 J/cm2 with 8000 pulses. The impact and implications of these findings on the design and development of future ‘all-carbon’ devices by means of a direct non-ablative fs laser processing of diamond are discussed.

Formation of picosecond laser-induced periodic surface structures on steel for knee arthroplasty prosthetics

The formation of laser-induced periodic surface structures (LIPSS) on mirror-polished 304-grade stainless steel sheets with dimensions 25 mm × 25 mm × 0.8 mm upon irradiation with picosecond laser pulses in air and water environments at different wavelengths, number of pulses, and laser energy densities was investigated. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the LIPSS. Tunable periodicity of the LIPSS was observed in both media at different wavelengths and fluence. Fluence was shown to be the main formation parameter of LIPSS; however, the medium was also demonstrated to play an important role. Our results show that LIPSS can be successfully generated on stainless steel in a single-step process and that they can be easily modified by adjusting laser parameters.

Deep learning-based optical authentication using the structural coloration of metals with femtosecond laser-induced periodic surface structures.

Structurally colored materials present potential technological applications including anticounterfeiting tags for authentication due to the ability to controllably manipulate colors through nanostructuring. Yet, no applications of deep learning algorithms, known to discover meaningful structures in data with far-reaching optimization capabilities, to such optical authentication applications involving low-spatial-frequency laser-induced periodic surface structures (LSFLs) have been demonstrated to date. In this work, by fine-tuning one of the lightweight convolutional neural networks, MobileNetV1, we investigate the optical authentication capabilities of the structurally colorized images on metal surfaces fabricated by controlling the orientation of femtosecond LSFLs. We show that the structural color variations due to a broad range of the illumination incident angles combined with both the controlled orientations of LSFLs and differences in features captured in the image make this system suitable for deep learning-based optical authentication.

Laser ablation of RB-SiC composite by femtosecond laser irradiation

Reaction-bonded silicon carbide composites (RB-SiC), consisting of SiC grains and Si matrix, have a complex ablation mechanism as two-phase composites under femtosecond laser irradiation. The ablation threshold of RB-SiC was calculated, which decreases (from 1.01 J/cm2 to 0.56 J/cm2) with increasing pulse number N and finally stabilized. At low fluence (F/Fth ≤ 5) and a low number of pulses, photochemical ablation dominates. For a higher number of pulses and high fluence (5 <F/Fth ≤ 50), photothermal ablation dominates with photochemical ablation. The results show that structures such as pits, convex, and holes exist on the surface of Crater I, while Laser-induced Periodic Surface Structures (LIPSS), and condensation structures exist on the surface of Crater II. In both types of ablation craters, the SiC grain surface produces the Low Spatial Frequency LIPSS (LSFL) with a period of about 900 nm and the High Spatial Frequency LIPSS (HSFL) with a period of about 300 nm. Based on the surface morphology, microstructure, and physical phase analysis of the ablation craters, a preliminary model of the removal mechanism of the ablation craters by femtosecond laser irradiation was established. These findings are expected to provide theoretical and experimental support for the precision processing of RB-SiC by femtosecond laser.

Laser-induced periodic surface structures as optical resonators for organic thin-film distributed feedback lasers

Laser-induced periodic surface structures (LIPSS) have received considerable attention due to their potential for micro-and nanostructuring and surface functionalization of various materials. We present a novel application of LIPSS on a glass substrate as distributed feedback (DFB) laser resonators, capable of providing sufficient positive optical feedback to achieve lasing in thin-film waveguides with organic small molecules as a gain medium. The direct femtosecond micromachining allows for easy variation of the periodicity across a broad range of values, including those required to reach 1st Bragg order DFB operation. We investigate several small molecule organic thin-film systems and observe lasing in strong accordance with the underlying periodicities of all photonic structures involved. These results demonstrate the effectiveness of LIPSS as DFB laser resonators and suggest that they could facilitate the integration of organic thin-film media-based lasers and other photonic devices into various integrated photonic systems.

Heat accumulation effects in femtosecond laser-induced subwavelength periodic surface structures on silicon

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for femtosecond laser-induced subwavelength periodic surface structures on silicon. Femtosecond laser micromachining is no longer a nonthermal process, as long as the repetition rate reaches up to 100 kHz due to heat accumulation. Moreover, a higher repetition rate generates much better defined ripple structures on the silicon surface, based on the fact that accumulated heat raises lattice temperature to the melting point of silicon (1687 K), with more intense surface plasmons excited simultaneously. Comparison of the surface morphology on repetition rate and on the overlapping rate confirms that repetition rate and pulse overlapping rate are two competing factors that are responsible for the period of ripple structures. Ripple period drifts longer because of a higher repetition rate due to increasing electron density; however, the period of laser structured surface is significantly reduced with the pulse overlapping rate. The Maxwell–Garnett effect is confirmed to account for the ripple period-decreasing trend with the pulse overlapping rate.

A process chain for the mass production of nanopatterned bactericidal plastic parts

Surface nanopatterning of plastic parts is a promising solution to enable the mass production of bactericidal applications without using antibiotics. In this work, an innovative process chain for efficiently manufacturing bactericidal parts was developed and validated. It is based on a particular type of laser-induced periodic surface structuring of injection molds that can be effectively replicated onto size-tunable bactericidal nanopatterns. Their polyvalent bactericidal performance directly correlates with the surface feature parameters Shh, Spd, and Spc, showing that higher, denser, and sharper spikes are more effective in killing bacteria by perforating their membrane.

Periodic transparent nanowires in ITO film fabricated via femtosecond laser direct writing

This paper reports the fabrication of regular large-area laser-induced periodic surface structures (LIPSSs) in indium tin oxide (ITO) films via femtosecond laser direct writing focused by a cylindrical lens. The regular LIPSSs exhibited good properties as nanowires, with a resistivity almost equal to that of the initial ITO film. By changing the laser fluence, the nanowire resistances could be tuned from 15 to 73 kΩ/mm with a consistency of ±10%. Furthermore, the average transmittance of the ITO films with regular LIPSSs in the range of 1200–2000 nm was improved from 21% to 60%. The regular LIPSS is promising for transparent electrodes of nano-optoelectronic devices—particularly in the near-infrared band.

Generation mechanisms of laser-induced periodic nanostructures on surfaces of microgrooves

Functional surfaces with hierarchical micro-/nanostructures play a vital role in improving performances in emerging fields over the past decades. In this work, we propose a hybrid manufacturing method for efficiently fabricating hierarchical nanotextured microgrooves by combining fly cutting and femtosecond laser nanopatterning. Nanostructures, including nanogratings and nanoholes, can be uniformly generated in a V-shaped groove by controlling laser polarization. We experimentally study the influence of the laser parameters (laser fluence and effective pulse number) to investigate the morphological evolution of nanostructures in the V-shaped groove under laser irradiation. By theoretical calculations and analyses, we find that the interference between the incident and reflected laser is helpful for ablating the nanogratings. In addition, this interference can also couple with the laser-SPPs (surface plasmon polaritons) interference and form nanoholes. The results enrich the diversity of achievable laser-induced periodic surface structures on complex surfaces and provide a new way to efficiently fabricate large-area hierarchical micro-/nanostructures at low cost.

Scan direction of circularly polarized laser beam determines the orientation of laser-induced periodic surface structures (LIPSSs) on silicon

Upon irradiation of a silicon surface with circularly polarized green nanosecond laser pulses, the formation of linear periodic nanostructures is observed. Due to the lack of inherent directional anisotropy by the laser polarization, no 1D-laser-induced periodic surface structures (LIPSSs) formation is expected. The orientation of the formed surface modulation depends on the laser scan direction. Silicon wafers, which are often used in LIPSS studies, are commonly considered inert substrates. This assumption needs to be reconsidered. Our finding is not explained by the current LIPSS theories.

Influence of film thickness and substrate roughness on the formation of laser induced periodic surface structures in poly(ethylene terephthalate) films deposited over gold substrates

A study of the formation of Laser Induced Periodic Surface Structures (LIPSS) using near-infrared femtosecond pulsed laser irradiation on poly(ethylene terephthalate) (PET) films deposited over gold substrates has been carried out. We report the influence of the gold substrate roughness and the PET film thickness on LIPSS formation and analyze it in terms of the features of the electric field distribution obtained by computer simulations using COMSOLTM. We obtain LIPSS with periods close to the irradiation wavelength as long as the aforementioned substrate and film parameters remain below certain threshold values, in particular for polymer thicknesses below 200 nm and substrate roughness of few nm. However, experiments show the impossibility of LIPSS formation for rough substrates as well as thick films above these threshold values. In our numerical simulations, we notice the generation of Surface Plasmon Polariton (SPP) in the film-substrate interface that gives rise to a periodical field pattern on the surface of the thin film. This periodicity is broken for a certain level of substrate roughness or film thickness. Moreover, the evolution of the period of the SPP as the substrate roughness and film thickness change for given laser parameters is qualitatively in good agreement with the experimental LIPSS period (below but close to the irradiation laser wavelength). In conclusion, the experimental findings are explained by the formation and behavior of SPP in the thin film-substrate interface. On these grounds, we propose that, for our case of study, this SPP formation and the subsequent inhomogeneous rise in temperature induced by the periodic field on the surface of the sample is the leading mechanism contributing to LIPSS formation.

Gold nanoparticles coated LIPSS on GaAs for trace detection of RDX and Tetryl

Femtosecond laser-induced periodic surface structures (LIPSS) on GaAs were accomplished using ablation in the air by varying the input pulse energy (5–100 μJ). A single laser scan line was drawn at each energy, and sub-wavelength LIPSS were observed within each line. Their morphology was thoroughly analysed, and their formation mechanisms were discussed with appropriate theoretical support (SIPE-Drude model). These GaAs-LIPSS have good quality over large ablation areas, and the periodicity (663±16 nm) was nearly the same for all energies. Further, a set of GaAs-LIPSS was produced within an area of 2 × 2 mm2 utilizing an optimized energy of 5 μJ. Photoluminescence and Raman studies were performed to understand the laser-induced damage on these structures. Subsequently, these samples were coated with a 25 nm Au layer and annealed at 400 °C in ambience. The resulting plasmonic Au nanoparticles decorated GaAs-LIPSS were used as SERS substrates for detecting dye (MB) and explosive (RDX, Tetryl) molecules. Enhancement factors of ∼105 and ∼104 (lowest detected concentrations of 10−9 M and 10−5 M) were recorded for dye and explosive molecules, respectively. We believe that these results will aid in further developing GaAs-LIPSS with smaller feature sizes for improved sensing and optoelectronic applications.

Rapid Fabrication of Wavelength-Scale Micropores on Metal by Femtosecond MHz Burst Bessel Beam Ablation.

The preparation of the wavelength-scale micropores on metallic surfaces is limited by the high opacity of metal. At present, most micropores reported in the literature are more than 20 µm in diameter, which is not only large in size, but renders them inefficient for processing so that it is difficult to meet the needs of some special fields, such as aerospace, biotechnology, and so on. In this paper, the rapid laser fabrications of the wavelength-scale micropores on various metallic surfaces are achieved through femtosecond MHz burst Bessel beam ablation. Taking advantage of the long-depth focal field of the Bessel beam, high-density micropores with a diameter of 1.3 µm and a depth of 10.5 µm are prepared on metal by MHz burst accumulation; in addition, the rapid fabrication of 2000 micropores can be achieved in 1 s. The guidelines and experimental results illustrate that the formations of the wavelength-scale porous structures are the result of the co-action of the laser-induced periodic surface structure (LIPSS) effect and Bessel beam interference. Porous metal can be used to store lubricant and form a lubricating layer on the metallic surface, thus endowing the metal resistance to various liquids' adhesion. The microporous formation process on metal provides a new physical insight for the rapid preparation of wavelength-scale metallic micropores, and promotes the application of porous metal in the fields of catalysis, gas adsorption, structural templates, and bio-transportation fields.

Crystallinity in periodic nanostructure surface on Si substrates induced by near- and mid-infrared femtosecond laser irradiation

Laser-induced periodic surface structure (LIPSS), which has a period smaller than the laser wavelength, is expected to become a potential technique for fine surface processing. We report the microscopic and macroscopic observations of the crystallinity of LIPSSs, where the characteristics such as defects generation and residual strain were analyzed, respectively. The LIPSSs were formed on a Si substrate using two different femtosecond pulses from Ti:Sapphire laser with near-infrared wavelength (0.8 μm) and free-electron laser (FEL) with mid-infrared wavelength (11.4 μm). The photon energies of the former and latter lasers used here are higher and lower than the Si bandgap energies, respectively. These LIPSSs exhibit different crystalline states, where LIPSS induced by Ti:Sapphire laser show residual strain while having a stable crystallinity; in contrast, FEL-LIPSS generates defects without residual strain. This multiple analysis (microscopic and macroscopic observations) provides such previously-unknown structural characteristics with high spatial resolution. To obtain LIPSS with suitable properties and characteristics based on each application it is paramount to identify the laser sources that can achieve such properties. Therefore, identifying the structural information of the LIPSS generated by each specific laser is of great importance.

Femtosecond laser-induced periodic surface structures on diamond-like nanocomposite films

We study the formation of laser-induced periodic surface structures (LIPSS) on diamond-like nanocomposite (DLN) a-C:H:Si:O films and titanium-doped DLN films during femtosecond (fs) laser ablation processing with linearly-polarized beams of IR and visible fs-lasers (wavelengths 1030 nm and 515 nm, pulse duration 320 fs, pulse repetition rates 100 kHz-2 MHz, scanning beam velocity 0.04–0.4 m/s). The studies are focused on (i) comparison of high spatial frequency LIPSS (HSFL) and low spatial frequency LIPSS (LSFL) formed on DLN and Ti-DLN films by IR fs-laser processing, (ii) effects of the pulse repetition rate on the parameters of LIPSS formed on the DLN and Ti-DLN films, (iii) Raman spectroscopy analysis of the LIPSS-structured films with application for ultrathin surface graphitization, and (iv) relationship between the fs-laser-induced surface graphitization and LIPSS formation on the films. A variety of the HSFL and LSFL have been produced on the surface of DLN and Ti-DLN films, with all the LIPSS being oriented perpendicular to the beam polarization direction. The HSFL periods are varied from ~80 to 240 nm and the LSFL periods are varied from 355 to 840 nm, depending on the fs-laser irradiation conditions (wavelength, fluence, pulse repetition rate) and films properties. Various plasmonic effects such as the superposition of the HSFL and LSFL and emergence of very unusual sinusoid-like structures on the DLN and Ti-DLN films are presented and discussed. • LIPSS fabrication on DLN and Ti-DLN films with IR and visible femtosecond lasers • Effect of surface graphitization on HSFL and LSFL formation thresholds • Raman analysis of ultrathin surface graphitization on LIPSS-structured DLN films • Influence of pulse repetition rate on fs-laser ablation and LIPSS parameters • Plasmonic effects in the fs-LIPSS formation on diamond-like nanocomposite films

Physicochemical Modifications on Thin Films of Poly(Ethylene Terephthalate) and Its Nanocomposite with Expanded Graphite Nanostructured by Ultraviolet and Infrared Femtosecond Laser Irradiation

In this work, the formation of laser-induced periodic surface structures (LIPSS) on the surfaces of thin films of poly(ethylene terephthalate) (PET) and PET reinforced with expanded graphite (EG) was studied. Laser irradiation was carried out by ultraviolet (265 nm) and near-infrared (795 nm) femtosecond laser pulses, and LIPSS were formed in both materials. In all cases, LIPSS had a period close to the irradiation wavelength and were formed parallel to the polarization of the laser beam, although, in the case of UV irradiation, differences in the formation range were observed due to the different thermal properties of the neat polymer in comparison to the composite. To monitor the modification of the physicochemical properties of the surfaces after irradiation as a function of the laser wavelength and of the presence of the filler, different techniques were used. Contact angle measurements were carried out using different reference liquids to measure the wettability and the solid surface free energies. The initially hydrophilic surfaces became more hydrophilic after ultraviolet irradiation, while they evolved to become hydrophobic under near-infrared laser irradiation. The values of the surface free energy components showed changes after nanostructuring, mainly in the polar component. Additionally, for UV-irradiated surfaces, adhesion, determined by the colloidal probe technique, increased, while, for NIR irradiation, adhesion decreased. Finally, nanomechanical properties were measured by the PeakForce Quantitative Nanomechanical Mapping method, obtaining maps of elastic modulus, adhesion, and deformation. The results showed an increase in the elastic modulus in the PET/EG, confirming the reinforcing action of the EG in the polymer matrix. Additionally, an increase in the elastic modulus was observed after LIPSS formation.

Fabrication of Anti-Reflection Coatings on GaSe Crystal Surfaces by Laser-Induced Periodic Surface Structuring

Fabrication of Anti-Reflection Coatings on GaSe Crystal Surfaces by Laser-Induced Periodic Surface Structuring

One-Step Femtosecond Laser Irradiation of Single-Crystal Silicon: Evolution of Micro-Nano Structures and Damage Investigation

As a pillar material of semiconductors, single-crystal silicon (Si) has attracted much attention due to its excellent electronic properties. Meanwhile, femtosecond laser processing as a convenient fabrication method for miniaturized structures provides an effective way to improve the performance of Si-based devices. However, the final-state analysis of femtosecond laser-induced Si micro-nano structures remains to be explored. In this study, the evolution of the micro-nano structures and the variation of surface and subsurface damage with pulse number were studied. Based on the atomic structure, the amorphous layer, dislocation layer, and unaffected layer were found. The amorphous layer resulted from the re-solidification of melted materials. The dislocation layer consisted of dislocations and lattice distortion. The unaffected layer retained the initial lattice structure and was not affected by laser irradiation. The high temperature and stress induced by multiple pulses led to the stacking faults and nano-twins in the subsurface layer. The evolution of the Si structure was essentially a transition process from a dislocation layer to a high-density defect layer and a transition process from nanotips to laser-induced periodic surface structures. During this process, dislocations dominated the femtosecond laser-induced microstructural deformation, affecting the nano-hardness of the irradiated surface. As a result, the nano-hardness of Si structures fabricated using femtosecond laser processing was maximally enhanced by 9.54%. This work provides new information about ultrafast laser-induced surface and subsurface damage, which is significant for the efficient and high-precision fabrication of Si-based functional micro-nano devices.

Nanoscale patterning of metallic surfaces with laser patterned tools using a nanoimprinting approach

In this study, we demonstrate that metallic substrates with a nanocrystalline grain size can be structured down to the micro- and nanometre range during a room temperature nanoimprinting process. WC-Co hard metal dies are patterned by Direct Laser Interference Patterning (DLIP), generating an array of separated asperities with a height of 4.7 µm and a spacing of 11.3 µm. Additionally, Laser Induced Periodic Surface Structures (LIPSS) with a spacing of 500 nm are formed as a hierarchical substructure. The patterned dies can be used to deform metallic substrates due to their high hardness. For structure replication, the WC-Co tool is pressed onto a nanocrystalline CuZn30 model alloy at room temperature. The pattern transfer process from the hard metallic tool to the nanocrystalline alloy is discussed in detail: At low contact pressures, the asperities on the tools act as single contact points, leading to the formation of separated dimples with a depth of up to 0.8 µm in the nanocrystalline alloy. At high contact pressures, the tool pattern, including the LIPSS is almost completely transferred to the substrate. The formed asperities protrude out of the surface, with a height to width aspect ratio of 0.4. It is thereby demonstrated, that plastic flow processes during the imprinting of nanocrystalline alloys allow for the replication of the DLIP tool pattern, even down to nano-scaled LIPSS. The imprinted array of surface asperities exhibits hydrophobic properties, due to a combined topographical and chemical influence on the wetting behaviour.