Spatial Frequency Laser-induced Periodic Surface Research Articles

Low spatial frequency laser-induced periodic surface structures in fused silica inscribed by widely tunable femtosecond laser pulses

The formation and evolution of laser-induced periodic surface structures in fused silica under irradiation of widely tunable (in the 1–3 upmum range) linearly polarized femtosecond (200 fs) pulses was studied experimentally. The structures were inscribed in high fluence regime (exceeding the surface ablation threshold for a single pulse) and characterized by using scanning electron microscopy and two dimensional Fourier transform. The results revealed rapid (after irradiation with a few successive pulses) formation of periodic laser-induced periodic surface structures aligned parallel to laser polarization, whose period increases with increasing the inscription wavelength, obeying the lambda /n law. With further increase of number of pulses, the generated structures gradually reorganize into laser polarization-independent low spatial frequency annular structures associated with formation of the damage crater, which fully established after irradiation with a few tens of successive laser pulses. This particular evolution scenario was observed over the entire wavelength tuning range of incident pulses.

Formation mechanism of high spatial frequency laser-induced periodic surface structures and experimental support

High spatial frequency laser-induced periodic surface structures (high-spatial frequency LIPSSs; HSFLs), being beyond the optical diffraction limit, have attracted great attention due to their outstanding physical characteristics and potential for use in various fields. However, the formation mechanism of HSFL is still controversial, which severely limits their further applications. In this work, a novel numerical model with experimental support are proposed to explain the formation of HSFL through the existence of subsurface plasmon polaritons (Sub-SPPs) generated at the subsurface of the material. According to the results, the period of HSFL is determined by the competition between the propagation loss of Sub-SPP and the penetration loss of fs-laser pulses. Given this competition, the HSFL formation can be investigated through the optical method by introducing a typical dielectric constant and thereby avoiding the discussion of complex electronic behavior. The numerical simulation results are in good agreement with the experimental data acquired on various materials, thus evidencing the applicability of this model. This work provides a clear physical picture of HSFL formation, allowing one to develop state-of-the-art surface morphology controlling methods for potential manufacturing applications.

Conical third harmonic generation from volume nanogratings induced by filamentation of femtosecond pulses in transparent bulk materials

We report on observations of conical third harmonic emission that emerges during supercontinuum generation produced by self-focusing and filamentation of high (20-200 kHz) repetition rate 180 fs, 1035 nm pulses from an amplified Yb:KGW laser in various nonlinear crystals and glasses: YAG, sapphire, YLF, LiF, CaF2, MgF2, LiSAF, fused silica and BK-7 glass. We show that conical third harmonic generation is a phase-matched four-wave mixing process, where noncollinear phase matching is achieved by means of reciprocal lattice vector, inversely proportional to the period of nanograting, which is inscribed by femtosecond filament in the volume of nonlinear material. The existence of a particular period required to phase match conical third harmonic generation was indirectly verified by investigations of periodicity features of high and low spatial frequency laser-induced periodic surface structures, in which matter is reorganized in a similar fashion.

Wavelength and decay time tuning of intraband transitions in low spatial frequency laser-induced periodic surface structures

We present the study of the alteration of wavelength and decay time of intraband transitions in the laser-induced periodic surface structures (LIPSS) of silver formed by varying the incident laser peak fluence. The studies are performed using a femtosecond laser (λ0 = 800 nm) based ultrafast transient absorption spectroscopy (UFTS)setup. Using the same laser, an array (~2 mm size) of laser-induced periodic surface structures were obtained in a single-step fabrication process in the open air without translating the sample. Each unit cell of the array comprises low spatial frequency LIPSS (LSFL) near the center with nanoparticles/clusters around the edges. As obtained from 2D Fast Fourier Transform, the overall periodicity(Λ) of LSFL is within 0.714λ0–0.723λ0 range. The nanoparticles/clusters show a random size distribution that varies from 50 nm to 300 nm. In UFTS measurement, the LIPSS was excited by a 3.3 eV laser. The SPP/SPR dip in the data is an indication of intraband transition or hot electron generation. The effect of change in structuring due to varying laser fluence on these intraband transitions is also analyzed. It is observed that there is a non-linear redshift (~16 nm) in the SPR peak wavelength with a significant enhancement in the relaxation times of the hot electrons due to localized annealing of shallow defects. Therefore, a large tunability in wavelength of SPP/SPR and lifetime of hot electrons was achieved by varying the laser fluence. The study highlights the suitability of LIPSS in applications requiring efficient energy transfer, enhanced photocatalytic action, and tunable sensors.

Femtosecond laser shockwave peening ablation in liquids for hierarchical micro/nanostructuring of brittle silicon and its biological application

This paper presents a new technique, termed femtosecond laser shock peening ablation in liquids (fs-LSPAL), which can realize simultaneous crack micro/nanomanufacturing and hierarchical micro/nanolaser ablation, giving rise to the formation of diverse multiscale hierarchical structures, such as macroporous ratcheted structures and en échelon microfringes decorated with parabolic nanoripples. Through analysis of surface morphologies, many phenomena have been confirmed to take place during fs-LSPAL, including en échelon cracks, nanostriation, ripple densification, crack branching, and selective formation of high spatial frequency laser-induced periodic surface structures of 100–200 nm in period. At a high laser power of 700 mW, fs-LSPAL at scanning speeds of 0.2 mm s−1 and 1 mm s−1 enables the generation of height-fluctuated and height-homogeneous hierarchical structures, respectively. The height-fluctuated structures can be used to induce ‘colony’ aggregates of embryonic EB3 stem cells. At 200 mW, fs-LSPAL at 1 mm s−1 is capable of producing homogeneous tilt macroporous structures with cracked structures interleaved among them, which are the synergistic effects of bubble-induced light refraction/reflection ablation and cracks. As shown in this paper, the conventional laser ablation technique integrated with its self-driven unconventional cracking under extreme conditions expands the horizons of extreme manufacturing and offers more opportunities for complex surface structuring, which can potentially be used for biological applications.

Evolution of microstructures on stainless steel induced by ultra-short pulsed laser ablation

Ultra-short pulsed laser ablation of stainless steel is accompanied by the evolution of different microstructures. Depending on the fluence, accumulated energy and number of laser passes cones from impurities, laser induced periodic surface structures, cone-like protrusion (CLP), and thermal bumps evolve at the surface. These often unwanted morphologies can be induced or inhibited by carefully choosing the strategy and laser parameters. The investigated range reveals a small processing window for defined 515 nm sub 1 ps ablation leading to low surface roughness using circular polarization. Hitherto, the origin and dependencies of CLP are still not well understood and for the first time a precursor ripple structure reported. These precursor ripples reveal supra-wavelength periodicity with about $$2\,\upmu \hbox {m}$$ spacing and evolve earliest after the second layer of ablation. Potentially, low spatial frequency laser-induced periodic surface structure generated with the first laser pass with pulse and hatch overlap are the root cause of CLP evolution. Moreover, the CLP growth is grain orientation and strongly polarization state dependent. Preferentially, CLP start to evolve at the {110} planes of the face-centered cubic crystals of the inspected austenitic stainless steel and linear polarized laser radiation revealing a 1:1 aspect ratio of $$10\,\upmu \hbox {m}$$ . A nanoindentation study at the interface near region on cross-sections reveals robust mechanical properties of this CLP structure.

Underwater persistent bubble-assisted femtosecond laser ablation for hierarchical micro/nanostructuring

In this study, we demonstrate a technique termed underwater persistent bubble assisted femtosecond laser ablation in liquids (UPB-fs-LAL) that can greatly expand the boundaries of surface micro/nanostructuring through laser ablation because of its capability to create concentric circular macrostructures with millimeter-scale tails on silicon substrates. Long-tailed macrostructures are composed of layered fan-shaped (central angles of 45°–141°) hierarchical micro/nanostructures, which are produced by fan-shaped beams refracted at the mobile bubble interface (≥50° light tilt, referred to as the vertical incident direction) during UPB-fs-LAL line-by-line scanning. Marangoni flow generated during UPB-fs-LAL induces bubble movements. Fast scanning (e.g. 1 mm s−1) allows a long bubble movement (as long as 2 mm), while slow scanning (e.g. 0.1 mm s−1) prevents bubble movements. When persistent bubbles grow considerably (e.g. hundreds of microns in diameter) due to incubation effects, they become sticky and can cause both gas-phase and liquid-phase laser ablation in the central and peripheral regions of the persistent bubbles. This generates low/high/ultrahigh spatial frequency laser-induced periodic surface structures (LSFLs/HSFLs/UHSFLs) with periods of 550–900, 100–200, 40–100 nm, which produce complex hierarchical surface structures. A period of 40 nm, less than 1/25th of the laser wavelength (1030 nm), is the finest laser-induced periodic surface structures (LIPSS) ever created on silicon. The NIR-MIR reflectance/transmittance of fan-shaped hierarchical structures obtained by UPB-fs-LAL at a small line interval (5 μm versus 10 μm) is extremely low, due to both their extremely high light trapping capacity and absorbance characteristics, which are results of the structures’ additional layers and much finer HSFLs. In the absence of persistent bubbles, only grooves covered with HSFLs with periods larger than 100 nm are produced, illustrating the unique attenuation abilities of laser properties (e.g. repetition rate, energy, incident angle, etc) by persistent bubbles with different curvatures. This research represents a straightforward and cost-effective approach to diversifying the achievable hierarchical micro/nanostructures for a multitude of applications.

One-step fabrication of bi- and quad-directional femtosecond laser-induced periodic surface structures on metal with a depolarizer

With femtosecond (fs) laser pulse irradiation, a simple but efficient technique to fabricate bi- and quad-directional low spatial frequency laser-induced periodic surface structures (BD- and QD-LSFLs) is suggested on metals. By inserting a quartz wedge achromatic depolarizer prior to the focusing optics, we change the polarization of fs laser pulses periodically in one dimension within the laser spot, and find that this method leads to the creation of BD- and QD-LSFLs by simply raster scanning fs laser pulses on Ni. It is also demonstrated that the interval of orientation change of BD-LSFLs can be manipulated with defocus.

Surface morphology and straight crack generation of ultrafast laser irradiated β-Ga2O3

Single crystal (010) β-Ga2O3 was irradiated by a Ti:sapphire ultrafast laser (150 fs pulse width) with varying fluences and a number of pulses in air ambient. Femtosecond laser-induced damage threshold of β-Ga2O3 is reported. Single pulse exposure results in surface morphological changes above a threshold laser fluence of 1.11 J/cm2. Laser-induced straight cracks aligned to the [001] crystallographic direction are observed in the laser irradiated regions, which are believed to be caused by laser-induced thermal stress, due to the unique low thermal conductivity and anisotropy associated with β-Ga2O3. Multiple pulse irradiation below the single pulse damage threshold fluence exhibited the formation of high spatial frequency laser-induced periodic surface structures. Electron backscattering diffraction and Raman spectroscopy suggested that there was no apparent phase transition of the irradiated β-Ga2O3 material for either single pulse or multiple pulse irradiation. This work serves as a starting point to further understanding the material properties of β-Ga2O3 and to unlock the potential for ultrafast laser material processing of β-Ga2O3.

Surface Plasmon Polariton Triggered Generation of 1D-Low Spatial Frequency LIPSS on Fused Silica

We report on the generation of low spatial frequency laser-induced periodic surface structures along straight lines on fused silica by spatially scanning the laser parallel to its polarization direction. The influence of the applied laser fluence and the scanning speed on the periodic surface structures is investigated. The parameter study shows that periodic structures appear in a limited parameter regime of combined fluence and scan speed with periodicities smaller than the laser wavelength. Most strikingly, we observe a perpendicular orientation of the self-assembled periodic structures to the electrical field of the laser, notably a previously unreported result for this dielectric material. This behavior is explained taking into account calculations of surface plasmon polaritons including a Drude model for free carrier excitation within silica by femtosecond laser irradiation.

Manipulation of multiple periodic surface structures on metals induced by femtosecond lasers

Under femtosecond laser pulse irradiation, the selective control of structural periods of low spatial frequency laser-induced periodic surface structures (LSFLs) is investigated on metals at large incident beam angles. As increasing the number of irradiating pulses, we produce three types of LSFLs, small-, dual-, and large-scale LSFLs, and find two kinds of structural period variations, the sequential structural variations from small- to dual- to large-scale LSFLs and the nonsequential variation directly from small- to large-scale LSFLs by controlling the laser fluence. Using the efficacy factor with the help of the Maxwell-Garnett theory, our study also shows that all three types of LSFLs can be understood by the interference between the incident beam and surface plasmon polaritons at the air-nanostructure composite and metal interface rather than the air-metal interface.

Two-Dimensional Low Spatial Frequency Laser-Induced Periodic Surface Structuring of Sapphire

Two-Dimensional Low Spatial Frequency Laser-Induced Periodic Surface Structuring of Sapphire

50-nanometer femtosecond pulse laser induced periodic surface structures on nitrogen-doped diamond

Ultrashort pulse laser interaction with nitrogen-doped type I-b diamond was investigated with near-infrared femtosecond laser pulses at repetition rates of 5kHz and 11MHz. High spatial frequency laser-induced periodic surface structures with periodicities of 50 and 200nm were generated perpendicular to the polarization of the electric field. The novel 50nm features are found to be due to a phase transition from crystalline diamond to a graphitic phase in contrast to the 200nm laser-induced periodic surface structures consisting still of crystalline material.

Morphology control of laser-induced periodic surface structure on the surface of nickel by femtosecond laser

An interesting transition between low spatial frequency laser-induced periodic surface structure (LIPSS) and high spatial frequency LIPSS (HSFL) on the surface of nickel is revealed by changing the scanning speed and the laser fluence. The experimental results show the proportion of HSFL area in the overall LIPSS (i.e., K) presents a quasi-parabola function trend with the polarization orientation under a femtosecond (fs) laser single-pulse train. Moreover, an obvious fluctuation dependence of K on the pulse delay is observed under a fs laser dual-pulse train. The peak value of the fluctuation is found to be determined by the polarization orientation of the dual-pulse train.

Cellular reactions toward nanostructured silicon surfaces created by laser ablation

Silicon wafers were structured with a femtosecond laser on the cm2 scale with high spatial frequency laser-induced periodic surface structures. These areas are characterized by regular parallel ripples with a period of the order of 100 nm. The particular ripple spacing is determined by the illumination wavelength of the tunable femtosecond laser. The cellular reaction to the structured silicon wafers and to the same materials, coated with calcium phosphate nanoparticles by electrophoretic deposition, was studied using L929 fibroblasts, human mesenchymal stem cells, and epithelial cells. The cells adhered uniformly to structured and unprocessed areas after seeding but significantly preferred the unstructured silicon after 48 h. This behavior disappeared after coating the structured surface with calcium phosphate nanoparticles.