Laser-induced Periodic Surface Structures Research Articles

Controlling the morphologies of femtosecond laser-induced periodic surface structure on silicon by combining deep learning with energy deposition model

As an important patterning method, femtosecond laser-induced periodic surface structure (LIPSS) has attracted widespread attention in recent years. Due to the complex physical processes involved in the femtosecond laser scanning process, it is difficult to predict the required LIPSS morphologies, which hinders rapid customization of the large-area LIPSS patterns. In this study, a structural optimization method combining data-driven deep learning with energy deposition model was proposed to control LIPSS morphologies of silicon. After femtosecond laser processing under dynamic irradiation condition, four LIPSS morphological types were well defined based on experimental results. Deep learning model was constructed to extract data features by labeling and encoding sample data. The accuracy of self-correlation validation reached 98.0% and the accuracy of cross-correlation validation reached 91.9%. Our results show that the distribution of LIPSS morphological types exhibits the dependence of energy deposition, which is the joint effect of the effective pulse number and accumulative fluence. Moreover, the period of groove (super-wavelength LIPSS) increases with the increase of energy deposition, while the period of LSFL (low spatial frequency LIPSS) slightly decreases with the increase of energy deposition. Through this structural optimization method, customization and online monitoring of large-area LIPSS patterns in the future industrial applications.

Size matters: how periodicity and depth of LIPSS influences E. coli adhesion on ferritic stainless steel

Bacterial adhesion is a serious problem in the healthcare and food industries, causing health problems and economic losses. This study evaluates the generation of Laser-Induced Periodic Surface Structures (LIPSS) with different periodicities to reduce bacterial adhesion on ferritic stainless steels. LIPSS nanostructuring evolves from hydrophilic to hydrophobic with air exposure. Fresh samples with 870 nm LIPSS show nearly 50 % reduced E. coli adhesion, but this effect diminishes in stabilized samples. This effect on bacterial adhesion can be attributed to LIPSS periodicity being like bacterial size, which would allow bacteria to accommodate and provide a suitable environment for their adhesion. Our results show that, in stable samples when the LIPSS periodicity is around 400 nm, reductions by almost 30 % are achieved and this reduction reaches 35 % when periodicity is close to 270 nm. These results indicate that smaller spatial periods prevent bacteria from fitting into valleys, so they remain on the peaks, hindering bacterial adhesion. Additionally, deeper LIPSS produced by a higher Surface Density of Applied Energy (SDAE), enhance bacteria-material interaction and, thus, increases adhesion.

Polarization and kinetics of optical emission during laser induced periodic surface structure formation on crystalline silicon

Polarization and kinetics of secondary optical emission, which occurs during the formation of laser-induced periodic surface structures (LIPSSs) on crystalline silicon by a femtosecond laser beam in air, were analyzed. It was found that the emission lines of ablated Si atoms and formed in plasma SiN molecules are unpolarized, and they decay on the nanosecond scale. The second harmonic generation (SHG) is enhanced by laser-induced surface plasmons, and the SHG polarization basically repeats the polarization of the laser. The results can be used for optical monitoring of LIPSS formation in real time, which is especially important for laser processing of large non-planar surfaces.

Sub-bandgap photo-response of black silicon fabricated by femtosecond laser irradiation under water

Here we propose a method to fabricate black Si without the need for any chalcogenide doping, accomplished by femtosecond (fs) laser irradiation in a liquid environment, aiming to fabricate the infrared detector and investigating their optoelectronic performance. Multi-scale laser-induced periodical surface structures (LIPSSs), containing micron sized grooves decorated with low spatial frequency ripples on the surface, can be clearly observed by SEM and 3D confocal microscope. The generated black Si demonstrates superior absorption capabilities across a broad wavelength range of 200-2500 nm, achieving an average absorptance of up to 71%. This represents a notable enhancement in comparison to untreated Si, which exhibits an average absorption rate of no more than 20% across the entire detectable spectrum. A metal-semiconductor-metal (MSM) type photodetector was fabricated based on this black Si, demonstrating remarkable optoelectronic properties, specifically, it attains a responsivity of 50.2 mA/W@10 V and an external quantum efficiency (EQE) of 4.02% at a wavelength of 1550 nm, significantly outperforming the unprocessed Si by more than five orders of magnitude. The great enhancement in infrared absorption as well as the optoelectronic performance can be ascribed to the synergistic effect of the multi-scale LIPSSs and the generated intermediate energy levels. On one hand, the multi-scale structures contribute to an anti-reflection and light trapping property; on the other hand, the defects levels generated through fs laser ablation process under water may narrow the band gap of the Si. The results therefore underscore the remarkable potential of black Si processed by fs laser under water for the application of photodetection, especially in the near-infrared band.

Electrical Properties of Laser Patterned Schottky Diode with ALD-Grown TiO2 Interlayer.

Heterojunction formation is the key to adjusting the electronic and optoelectronic properties of various semiconductor devices. There have been various reports on the formation and importance of semiconducting heterojunction devices based on metal oxides. Titanium dioxide (TiO2) is one of the metal oxides that has many unique properties. TiO2's importance is due to its physical and chemical properties such as large band gap, large permittivity, stability, and low leakage current density. In this context, we present the electrical properties of the metal-insulator-semiconductor (MIS) type-TiO2-based Schottky barrier diode (SBD) in the study. To create a thin layer of TiO2 on p-type silicon (p-type Si) patterned partially by the laser-induced periodic surface structure (LIPSS) technique, an atomic layer deposition (ALD) technique was used in the study. For comparison, the current-voltage (I-V) characteristics of the TiO2-based laser-patterned (LP) and nonlaser-patterned (non-LP) diodes were measured at 300 K and in the dark at ±5 V. Classical thermionic emission (TE) theory and Cheung functions were used to investigate the critical diode parameters of the diodes, including ideality factor (n), series resistance (Rs), and barrier height (Φb). The n values were obtained as 4.10 and 3.68 from the TE method and Cheung functions for the LP diode, respectively. The Φb values were found as 0.68 and 0.69 eV from the TE method and Cheung functions, respectively. According to experimental results, the laser patterning resulted in an increase in the Φb values and a decrease in the n values. After laser patterning, it was observed that the device worked effectively, and the ideality factor and barrier height values were improved. This study provides insight into the fabrication and electrical properties of TiO2-based heterojunction devices.

Optimizing Cu-alloy surface characteristics through magnetic field-enhanced fs laser treatment

We report here the effect of the magnetic field on plasma parameters and the surface structuring of the Cu alloy after fs laser irradiation. A Ti:Sapphire (800 nm, 35 fs, 1 KHz) laser is employed at various irradiances (0.011–0.117 PW/cm2) to generate plasma. A Transvers Magnetic Field (TMF) of strength 1.1 T is employed for plasma confinement. All the measurements were performed with and without TMF. The Cu plasma parameters, i.e., excitation temperature (Texc) and electron number sensity (ne), determined by laser-induced breakdown spectroscopy analysis, are higher in the presence of TMF. This magnetic field confinement of Cu plasma was studied analytically by evaluating thermal beta (βt), directional beta (βd), confinement radius (Rb), and diffusion time (td). To correlate Cu-alloy plasma parameters with surface modifications, field emission scanning electron microscope analysis is performed. It reveals the formation of low-spatial-frequency laser-induced periodic surface structures (LIPSSs) and high-spatial-frequency LIPSSs, along with agglomers and nano-rims formation. Distinct and well-defined structures are observed in the presence of a magnetic field. It is concluded that controlled surface structuring can be achieved through magnetic confinement, which enhances key plasma parameters. The technique has the potential for enhancing the fabrication of nano-gratings and field emitters, where spatial uniformity is critically important.

Convective boiling heat transfer enhancement via femtosecond laser textured inclined microfeatures

Designing efficient cooling devices is crucial in a multitude of applications. Although cooling technologies have existed for decades, there is a lack of efficient heat dissipation methods, especially for space applications where thermal management devices must dissipate heat in reduced gravity and where convective boiling heat transfer is frequently employed. In this framework, we evaluate the change in bubble production for different types of surface textures produced on 200 µm thick 316L SS foils using a femtosecond laser. The investigated textures include microscale grooves and conical holes with tilted (= 45°) geometry. In addition, we also evaluate the effect of laser-induced periodic surface structures (LIPSS), which are sub-micron scale features fabricated by fs laser processing. The fluid used is PP1, a replacement of 3M™ FC-72 in heat transfer applications.

The ablation threshold and incubation effect in picosecond laser surface treatment of CFRP

The ablation threshold of carbon fiber is a critical parameter to consider in the laser surface treatment of carbon fiber reinforced polymers (CFRP), as it is necessary to design process parameters based on the threshold fluence to ensure process quality and efficiency. The incubation effect is a key factor influencing the ablation threshold, and a more rational incubation model contributes to a more accurate prediction of the ablation threshold. The conventional incubation model describes the regular decrease of the ablation threshold with an increasing pulse number. However, experimental findings show that the time interval of pulses is also a significant factor affecting the ablation and incubation process. In this study, the conventional incubation model was improved by comprehensively considering the time interval and space interval of pulses, which shows better prediction ability of ablation threshold under different parameter sets. The temperature history of CFRP under picosecond laser irradiation was calculated to support the rationality of the optimized incubation model. In addition, the laser-induced periodic surface structures (LIPSS) and the ablation of carbon fibers and resin were investigated based on the ablation threshold.

Examining the role of magnetic fields in plasma behavior and surface evolution of a Mg alloy with varied irradiances in a femtosecond laser treatment

This paper reports the effect of a magnetic field on plasma parameters and surface structuring of the Mg alloy after laser irradiation. Femtosecond pulses of a Ti:sapphire laser system (800 nm, 35 fs, 1 KHz) are employed as the source of irradiation at various irradiances ranging from 0.011PW/cm2 to 0.117PW/cm2 to generate ablated Mg-alloy plasma. A transvers magnetic field (TMF) of strength 1.1 Tesla is employed to confine laser generated Mg plasma. All the measurements are performed with and without TMF. The two plasma parameters, i.e., excitation temperature (Texc) and electron number density (n e ) of Mg plasma, have been evaluated by laser-induced breakdown spectroscopy (LIBS) analysis. It is observed that the values of Texc and n e of laser produced plasma (LPP) of the Mg alloy are higher in the presence of a magnetic field as compared to the field free case. Both show initially an increasing trend with increasing laser irradiance and after attaining their respective maxima a decreasing trend is observed with the further increase of irradiance. The magnetic confinement validity is confirmed by analytically evaluating thermal beta (β t ), directional beta (β d ), confinement radius (R b ), and diffusion time (t d ) for LPP of the Mg alloy. To correlate the LPP parameters of the Mg alloy with surface modifications a field emission scanning electron microscope (FE-SEM) analysis is performed. It was revealed that structures like laser-induced periodic surface structures (LIPSSs), agglomerates, islands, large sized bumps, along with channels and multiple ablative layers are observed. Distinct and well-defined surface structuring is observed in the presence of TMF as compared to the field free case. It is concluded that by applying an external magnetic field during laser irradiation, controlled material surface structuring is possible for fabrication of nanogratings and field emitters where spatial uniformity is critically important.

Enhancing bonding of fresh concrete to steel through Laser Surface Texturing

In this work, we investigated the adhesion between surface-treated stainless-steel samples and a standard cement mixture. Laser surface treatments such as Direct Laser Writing (DLW) and Laser-Induced Periodic Surface Structures (LIPSS) were employed, which resulted effective to reach up to 16-fold increase of adhesion compared to the untextured. A comparison with mechanical surface treatments, i.e., sandblasting, revealed that, although an increase of adhesion was achieved probably ascribable to the higher surface roughness, such improvement in terms of bonding strength was significantly lower than the one obtained with the laser surface texturing.

Wetting Behavior Driven by Surface Morphology Changes Induced by Picosecond Laser Texturing.

The laser surface texturing (LST) technique has recently been used to enhance adhesion bond strength in various coating applications and to create structures with controlled hydrophobic or superhydrophobic surfaces. The texturing processing parameters can be adjusted to tune the surface's polarity, thereby controlling the ratio between the polar and dispersed components of the surface free energy and determining its hydrophobic character. The aim of this work is to systematically select appropriate laser and scan head parameters for high-quality surface topography of metal-based materials. A correlation between texturing parameters and wetting properties was made in view of several technological applications, i.e., for the proper growth of conformal layers onto laser-textured metal surfaces. Surface analyses, carried out by scanning electron microscopy and profilometry, reveal the presence of periodic microchannels decorated with laser-induced periodic surface structures (LIPSS) in the direction parallel to the microchannels. The water contact angle varies widely from about 20° to 100°, depending on the treated material (titanium, nickel, etc.). Nowadays, reducing the wettability transition time from hydrophilicity to hydrophobicity, while also changing environmental conditions, remains a challenge. Therefore, the characteristics of environmental dust and its influence on the properties of the picosecond laser-textured surface (e.g., chemical bonding of samples) have been studied while monitoring ambient conditions.

Surface Structuring of the CP Titanium by Ultrafast Laser Pulses

Surface structuring by ultrafast lasers is a promising technique to modify surface-related properties of materials to tailor them for specific applications. In the present study, we experimentally investigated the laser structuring of commercially pure titanium (CP Ti) using ultrafast pulses to understand the role of the laser input parameters on the development of surface morphology, optical properties, surface chemistry, and wettability behaviour. The processed surfaces were characterized by a scanning electron microscope, energy-dispersive X-ray spectroscopy (EDX), Raman microscope, optical microscope, and sessile drop method. Laser-induced periodic surface structures decorated with nanodroplets were noted to be formed on the surface of the laser-structured CP Ti. The surface roughness measurements showed that the laser-structured surfaces had nanoscale roughness values. The EDX and the Raman analyses show that laser-structured surfaces of CP Ti have a thin oxide film. Different colours on different surfaces processed by different laser parameters were observed. The wettability assessment shows that CP Ti can transition from hydrophilic–hydrophobic and vice versa depending on the environmental conditions. This study shows that laser structuring can be utilized to modify CP Ti surfaces to obtain desirable surface properties that can find potential applications in different fields.

LIPSS formation on Ni surface using mixed near-infrared laser beam harmonics

Generation of laser-induced periodic surface structures (LIPSS) is investigated with femtosecond laser pulses with 1030nm wavelength and its second and its harmonics. The second (λ2ω = 515nm) and third harmonics (λ3ω = 343nm) beams in addition to their combination were utilized to control the period of the LIPSS on a Ni surface. The resulting periodicity of the formed structures was ∼ 0.42μm, 0.22μm, and 0.33μm respectively for the second harmonic beam, third harmonic beam, and their combination. This method demonstrates a direct way to control the periodicities on a metal target by employing an individual laser beam wavelength to induce a variable range of LIPSS periods.

Hybrid periodic microstructures fabricated on chromium films by SLM-assisted nanosecond laser processing

An effective technique for preparing different hybrid periodic structures on chromium (Cr) films of different thicknesses using a 1064-nm nanosecond laser assisted by spatial light modulator (SLM) is proposed. For 1000-nm Cr films, by combining periodic modulated grating (MG) patterned by SLM with laser-induced periodic surface structures (LIPSS), regular dual-scale MG-LIPSS can be fabricated, with the morphological characteristics controlled by laser fluence, effective pulse number, and MG period. Due to diffraction effects of MG and LIPSS, MG-LIPSS patterned surfaces exhibit vivid anisotropic structural colors. In contract to MG-LIPSS, complex periodic structures consisting of MG and cracks (MGC) are fabricated on 200-nm Cr films due to the more significant thermal stress of a thinner film. Although the cracks of MGC distributed randomly, MGC show a long-range order characteristic with a certain transmittance, which can be utilized as a transmission grating with diffraction effects. These results demonstrate that optical field modulated laser machining based on SLM offers an efficient, cost-effective, and controllable method for fabricating large-area periodic structures on Cr films. Moreover, variations in film thickness can be employed to explore hybrid microstructures with specific properties, and thus utilized in diverse applications, such as optical components and anti-counterfeiting measures.

Effects of laser pulse duration on the formation dynamics of laser-induced periodic nanostructures.

Formation dynamics of laser-induced periodic surface structures (LIPSSs) on the SiC substrates were described with varying pulse numbers and pulse duration. As the number of laser pulses increases, two significant transformations become evident in the progression of structural formations. First from surface roughening with nanoparticles to LIPSS with the period that is slightly shorter than the laser wavelength. Second it turns to LIPSS with a period less than half the laser wavelength. It is found that maintaining the crystallinity is the key to changing the structures. In the cases of longer pulse width than sub-nanoseconds, no LIPSS formations are observed or LSFL does not change to HSFL because the irradiated area is poly-crystallized.

New aspects of femtosecond laser ablation of Si in water: a material perspective

We report a study of the role of material’s conductivity in determining the morphology of nanoparticles and nanostructures produced by ultrafast laser ablation of solids. Nanoparticles and textured surfaces formed by laser ablation display a wide variation in size and morphology depending on the material. In general, these qualities can be grouped as to material type, insulator, semiconductor, or metal; although each has many other different material properties that make it difficult to identify the critical material factor. In this report, we study these nanoparticle/surface structural characteristics as a function of silicon (Si) resistivity, thus honing-in on this critical parameter and its effects. The results show variations in morphology, optical, and nonlinear properties of Si nanoparticles. The yield of colloidal Si nanoparticles increased with an increase in the conductivity of Si. Laser-induced periodic surface structures formed on ablated substrates are also found to be sensitive to the initial conductivity of the material. Further, the laser ablation of Gamma-irradiated Si has been investigated to verify the influence of altered conductivity on the formation of Si nanoparticles. These observations are interpreted using the basic mechanisms of the laser ablation process in a liquid and its intricate relation with the initial density of states and thermal conductivities of the target material.

Investigating the transition from high to low spatial frequency periodic nanoripples on TiO2 (0 0 1) oriented surface upon irradiation with UV-femtosecond pulses

The formation of laser-induced periodic surface structures (LIPSS) on a TiO2 (001) oriented surface upon irradiation with UV-fs laser pulses was analyzed. Two systems of low spatial frequency LIPSS (LSFL) perpendicular to the direction of laser polarization are observed after N = 5 laser pulses. One was developed on the edges of the crater created by the first laser pulse, while the other appeared in the center of crater and spread towards the edges. When N was greater than 10, the two LIPSS systems merge. Both LSFL systems have a spatial period of approximately 200 nm and are due to the coherent superposition of surface waves scattered on inhomogeneities of a transient metallic surface. However they differ by the type of defects involved in the excitation process of surface waves. While a pronounced and localized rim triggered LSFL on the edge of the laser spot, LSFL in the central part nucleated at the shallow high spatial frequency LIPSS (HSFL), which are parallel to the laser polarization. A mechanism is proposed to explain the role of the HSFL in the formation of LSFL. Numerical simulations of the absorbed laser power density support the evolution from HSFL to LSFL as the number of laser pulses increased.

Liquid‐Assisted Laser Nanotexturing of Silicon: Onset of Hydrodynamic Processes Regulated by Laser‐Induced Periodic Surface Structures

AbstractUpon systematic studies of femtosecond‐laser processing of monocrystalline Si in oxidation‐preventing methanol, it is shown that the electromagnetic processes dominating at initial steps of the progressive morphology evolution define the onset of the hydrodynamic processes and morphology upon subsequent multi‐pulse exposure. Under promoted exposure quasi‐regular subwavelength laser‐induced periodic surface structures (LIPSSs) are justified to evolve through the template‐assisted development of the Rayleigh‐Plateau hydrodynamic instability in the molten ridges forming quasi‐regular patterns with a supra‐wavelength periodicity and preferential alignment along polarization direction of the incident light. Subsequent exposure promotes fusion‐assisted morphology rearrangement resulting in a spiky surface with random orientation, yet constant inter‐structure distance correlated with initial LIPSS periodicity. Along with the insight onto the physical picture driving the morphology evolution and supra‐wavelength nanostructure formation, this experiments also demonstrated that resulting quasi‐regular and random spiky morphology can be tailored by the intensity/polarization distribution of incident laser beam allowing on‐demand surface nanotexturing with diverse hierarchical surface morphologies exhibiting reduced reflectivity at visible and shortwave‐IR wavelengths. Finally, the practical attractiveness of the suggested approach for improving near‐IR photoresponse and expanding operation spectral range of vertical p‐n junction Si photodetector operating under room temperature and zero‐bias conditions via single‐step annealing‐free nanopatterning is highlighted.

Impact of Hybrid Electromagnetic Surface Modes on the Formation of Low Spatial Frequency LIPSS: A Universal Approach

AbstractIn this article, the impact of the excited electromagnetic surface modes in a comprehensive investigation of the formation of laser‐induced periodic surface structures (LIPSS) is analyzed. It is demonstrated that the electromagnetic origin of low spatial‐frequency LIPSS (LSFL) is the frequency detuning between propagating and localized modes due to their coupling/hybridization. The influence of the pattern profile, inhomogeneity, and material type on the coupling strength, electric‐field spatial distribution, and associated near‐field scattering are highlighted. Exploiting the potential of the approach, evidence of a universal manifestation of LSFLs is provided irrespective of the material and the authors are able to predict and validate the experimentally‐proven lower limit of LSFL periodicity (i.e., λL/2, where λL stands for the laser wavelength). Furthermore, the analysis of the electromagnetic modes predicts that the periodicity of LSFL is practically unaffected by the laser fluence, while a suppression of LSFL at high excitation levels or large number of pulses is also predicted. It is also shown that plasmonic‐active materials are not necessary for LSFL formation perpendicular to polarization. Toward these directions, an important generic metric, namely the resonance quality factor, is additionally inserted. The approach can, thus, serve as a guide for controlling laser‐induced surface topography.

Bulk ceramics of lanthanum hexaboride with enhanced spectral selectivity and photothermal efficiency for novel hybrid thermal-thermionic solar absorbers

Lanthanum hexaboride (LaB6) single-crystals are known from a long time for their excellent thermionic and field-assisted electron-emitting properties. Recently, multi-crystalline ceramics, which offer several technological advantages over single-crystals, have been studied as well, proving a low work function and Richardson constant, in addition to favorable optical properties for high-temperature solar radiation absorbers. However, to fully disclose to LaB6 ceramics the novel application of a double-function bulk thermionic emitter, enabled by the direct absorption of solar radiation, and, at the same time, thermodynamic solar absorber for Concentrating Solar Power (CSP), their optical properties should be further optimized. The ideal result, consisting in the selective increase of the solar absorptance and decrease of the thermal emittance, thus increasing both the spectral selectivity and the photothermal efficiency of the absorber, has been obtained in this work by realizing Laser-Induced Periodic Surface Structures (LIPSS) on the ceramic surface by means of femtosecond-laser treatments. LIPPS formation has been investigated as a function of the accumulated laser fluence.