Fs Laser Irradiation Research Articles

Ageing of commercial austenitic, martensitic and duplex stainless steel surfaces after formation of laser-induced periodic surface structures by fs-laser irradiation

The time-dependent wetting behavior was investigated for commercially available austenitic, martensitic and duplex stainless steel surfaces after femtosecond (fs) laser irradiation. For this purpose, highly regular LIPSS with almost identical geometric properties were generated on the stainless steels in an air environment by near-infrared fs-laser processing (λ = 1025 nm, τ = 300 fs, frep = 100 kHz, v = 0.67 m/s, Δx = 6 μm, F = 1.1 J/cm2) and compared to the polished initial surfaces as reference. The wetting with distilled water was evaluated as a function of aging time by contact angle measurements using the sessile drop method. The laser-induced morphological, topographical and chemical alteration of the surface was characterized by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, glow-discharge optical emission spectroscopy as well as high-resolution imaging and scanning transmission electron microscopy. Despite an almost identical LIPSS-based topography alteration, the stainless steels revealed very different ageing behavior. For permanently used samples, it was shown that the ageing effect is less pronounced than for samples that were stored in ambient air following fs-laser irradiation until the wetting measurement. The results obtained make an important contribution to understanding and controlling the ageing effect of metal surfaces and thus facilitate the transfer of LIPSS-based functional surface engineering and design to industrial processes.

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.

Exploring fs-laser irradiation damage subthreshold behavior of dielectric mirrors via electrical measurements

Abstract With ultrafast laser systems reaching presently 10 PW peak power or operating at high repetition rates, research towards ensuring the long-term, trouble-free performance of all laser-exposed optical components is critical. Our work is focused on providing insight into the optical material behavior at fluences below the standardized laser-induced damage threshold (LIDT) value by implementing a simultaneous dual analysis of surface emitted particles using a Langmuir probe (LP) and the target current (TC). ${\mathrm{HfO}}_2$ and ${\mathrm{ZrO}}_2$ thin films deposited on fused silica substrates by pulsed laser deposition at various ${\mathrm{O}}_2$ pressures for defect and stoichiometry control were irradiated by Gaussian, ultrashort laser pulses (800 nm, 10 Hz, 70 fs) in a wide range of fluences. Both TC and LP collected signals were in good agreement with the existing theoretical description of laser–matter interaction at an ultrashort time scale. Our approach for an in situ LIDT monitoring system provides measurable signals for below-threshold irradiation conditions that indicate the endurance limit of the optical surfaces in the single-shot energy scanning mode. The LIDT value extracted from the LP-TC system is in line with the multipulse statistical analysis done with ISO 21254-2:2011(E). The implementation of the LP and TC as on-shot diagnostic tools for optical components will have a significant impact on the reliability of next-generation ultrafast and high-power laser systems.

Humidity‐Induced Reversible Crystallization of Laser‐Printing Perovskite Quantum Dots in Glass

AbstractThe complex and high‐precision patterning of perovskite quantum dots (PeQDs) is of vital importance for exploring their new functionalities and device applications. In this work, a strategy based on the combination femtosecond (fs) laser‐irradiation and humidity‐control is reported to construct patterns of cyan CsPb(Cl/Br)3 PeQDs in a transparent glass medium. Benefiting from their ionic crystal feature and low formation energy, CsPb(Cl/Br)3 PeQDs can be locally crystallized and decomposed via fs laser irradiation without further heat‐treatment. More notably, it is found that a water molecule is able to affect the growth of CsPb(Cl/Br)3 PeQDs in glass. By modulation of relative humidity in air, the selectively decomposed perovskite structure is spontaneously regenerated, and the highly emissive CsPb(Cl/Br)3 PeQDs can be in situ manufactured in the confined glass region. This allows for reversible luminescence from the same patterns of laser printing‐erasing‐recovering, and the process can be repeated for multiple cycles without destruction of optical performance and robustness. The results provide a flexible method to develop new encryption/decryption technology for information security and anti‐counterfeiting.

Creation of a Periodic Domain Structure in MgOLN by Femtosecond Laser Irradiation

The systematic imaging of the damaged tracks and domain patterns created in the MgOLN plates by one-step fs-laser irradiation at different depths was carried out. It is shown that the domains in the bulk have a spindle-like shape and start to grow in the Z− direction from the track ends. The domain shape changes from a spindle-like one with charged walls to a hexagonal prism with neutral walls after the domain reaches the polar surface. The length of the domains located in the bulk increases linearly with the pulse energy. The hexagonal domain shape at the surface is typical for the crystals of the lithium niobate family. The obtained effects have been considered in terms of the kinetic approach. After irradiation, the domains appear in the vicinity of the track ends with maximum electric field strength and grow under the action of a spatially nonuniform pyroelectric field. The key role of the pyroelectric field is confirmed by the creation of new domains at the surface without correlation with the position of the focusing point located at the vicinity of the surface. The 3D domain pattern was produced, which represented four layers of the regular matrices consisting of elongated domains about 100 μm in length.

Femtosecond laser-induced crystallization in Er/Yb:BaTiO3 perovskite films: Effects on the optical and electrical properties

Femtosecond laser-induced crystallization in Er3+/Yb3+:BaTiO3 (BTEY) perovskite films has sparked significant research interest in materials science and photonics, driven by the need for innovative techniques to control material properties and exploit their potential. Our study delves into this burgeoning area by demonstrating the controlled crystallization of amorphous BTEY thin films under femtosecond laser (fs-laser) irradiation at 1030 nm. The threshold fluence to achieve laser processing was determined as approximately 1.1 J/cm2 at 1 kHz, as well as the irradiation conditions for crystallization (pulse energy, repetition rate, and scanning speed), whose background mechanism is discussed. Our Raman spectroscopy analysis confirmed crystallization when irradiation was carried out at 1 kHz repetition rate, while no evidence of the crystalline phase of BTEY emerged at 1 MHz irradiation. Moreover, our results reveal a remarkable increase in luminescence enhancement and surface potential (∼65 mV) in the film. These outcomes represent a significant breakthrough with relevant implications for advanced photonic devices like modulators and detectors. By harnessing fs-laser pulses for BTEY crystallization, our findings offer new avenues for practical device applications.

Formation of laser-induced periodic surface structures on Zr-based bulk metallic glasses with different chemical composition

Bulk metallic glasses (BMG) are amorphous metal alloys known for their unique physical and mechanical properties. In the present study, the formation of femtosecond (fs) laser-induced periodic surface structures (LIPSS) on the Zr-based BMGs Zr46Cu46Al8, Zr61Cu25Al12Ti2, Zr52.5Cu17.9Al10Ni14.6Ti5 (Vit105) and Zr57Cu15.4Al10Ni12.6Nb5 (Vit106) was investigated as a function of their different chemical composition. For this purpose, LIPSS were generated on the sample surfaces in an air environment by fs-laser irradiation (λ = 1025 nm, τ = 300 fs, frep = 100 kHz). The surface topography was characterized by scanning electron microscopy and atomic force microscopy. Moreover, the impact of LIPSS formation on the structure and chemical surface composition was analyzed before and after fs-laser irradiation by X-ray diffraction and X-ray photoelectron spectroscopy as well as by transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. Despite the different chemical composition of the investigated BMGs, the fs-laser irradiation resulted in almost similar properties of the generated LIPSS patterns. In the case of Zr61Cu25Al12Ti2, Vit105 and Vit106, the surface analysis revealed the preservation of the amorphous state of the materials during fs-laser irradiation. The study demonstrated the presence of a native oxide layer on all pristine BMGs. In addition, fs-laser irradiation results in the formation of laser-induced oxide layers of larger thickness consisting of an amorphous ZrAlCu-oxide. The precise laser-structuring of BMG surfaces on the nanoscale provides a versatile alternative to thermoplastic forming of BMG surfaces and is of particular interest for the engineering of functional material surfaces.

SiC Measurements of Electron Energy by fs Laser Irradiation of Thin Foils.

SiC detectors based on a Schottky junction represent useful devices to characterize fast laser-generated plasmas. High-intensity fs lasers have been used to irradiate thin foils and to characterize the produced accelerated electrons and ions in the target normal sheath acceleration (TNSA) regime, detecting their emission in the forward direction and at different angles with respect to the normal to the target surface. The electrons' energies have been measured using relativistic relationships applied to their velocity measured by SiC detectors in the time-of-flight (TOF) approach. In view of their high energy resolution, high energy gap, low leakage current, and high response velocity, SiC detectors reveal UV and X-rays, electrons, and ions emitted from the generated laser plasma. The electron and ion emissions can be characterized by energy through the measure of the particle velocities with a limitation at electron relativistic energies since they proceed at a velocity near that of the speed of light and overlap the plasma photon detection. The crucial discrimination between electrons and protons, which are the fastest ions emitted from the plasma, can be well resolved using SiC diodes. Such detectors enable the monitoring of the high ion acceleration obtained using high laser contrast and the absence of ion acceleration using low laser contrast, as presented and discussed.

Femtosecond Laser-Induced Nano-Joining of Volatile Tellurium Nanotube Memristor.

Nanowire/nanotube memristor devices provide great potential for random-access high-density resistance storage. However, fabricating high-quality and stable memristors is still challenging. This paper reports multileveled resistance states of tellurium (Te) nanotube based on the clean-room free femtosecond laser nano-joining method. The temperature for the entire fabrication process was maintained below 190 °C. A femtosecond laser joining technique was used to form nanowire memristor units with enhanced properties. Femtosecond (fs) laser-irradiated silver-tellurium nanotube-silver structures resulted in plasmonic-enhanced optical joining with minimal local thermal effects. This produced a junction between the Te nanotube and the silver film substrate with enhanced electrical contacts. Noticeable changes in memristor behavior were observed after fs laser irradiation. Capacitor-coupled multilevel memristor behavior was observed. Compared to previous metal oxide nanowire-based memristors, the reported Te nanotube memristor system displayed a nearly two-order stronger current response. The research displays that the multileveled resistance state is rewritable with a negative bias.

Localized nitrogen-vacancy centers generated by low-repetition rate fs-laser pulses

Among hundreds of impurities and defects in diamond, the nitrogen-vacancy (NV) center is one of the most interesting to be used as a platform for quantum technologies and nanosensing. Traditionally, synthetic diamond is irradiated with high-energy electrons or nitrogen ions to generate these color-centers. For precise positioning of the NV centers, fs-laser irradiation has been proposed as an alternative approach to produce spatially localized NV centers in diamond. However, most of the studies reported so far used high-repetition rate fs-laser systems. Here, we studied the influence of the irradiation conditions on the generation of NV$^-$. Specifically, we varied pulse fluence, laser focusing, and the number of pulses upon irradiation with 150 fs pulses at 775 nm from a Ti:sapphire laser amplifier operating at 1 kHz repetition rate. Optically Detected Magnetic Resonance (ODMR) was used to investigate the produced NV centers, revealing a sizeable zero-field splitting in the spectra and indicating the conditions in which the lattice strain produced in the ablation process may be deleterious for quantum information applications.

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

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.

Comparison of Dopant Incorporation and Near-Infrared Photoresponse for Se-Doped Silicon Fabricated by fs Laser and ps Laser Irradiation

Se-doped silicon films were fabricated by femtosecond (fs) laser and picosecond (ps) laser irradiating Si–Se bilayer film-coated silicon. The surface morphology, impurity distribution, crystal phase, and near-infrared photocurrent response of fs-laser-processed and ps-laser-processed Si are compared. With the same number of laser pulse irradiation, fs laser induces quasi-ordered micron-size columnar structures with some deeper gullies, and ps laser induces irregular nanoscale spherical particles with some cavities. Compared with the fs-laser-produced Se-doped layer, ps laser irradiation produces a Se-doped layer with better crystallinity and higher doping concentration, resulting in a higher photocurrent response for picosecond laser-processed Si in the near-infrared band. The changes brought about by ps laser processing facilitate the application of ultrafast laser-processed chalcogen-doped silicon for silicon-based integrated circuits.

Photo-induced structural and optical changes of CsPbBr3 perovskite nanocrystals in glasses

Cesium lead halide perovskite nanocrystals have particular optoelectronic properties and important applications in various fields. Embedding cesium lead halide perovskite nanocrystals into glass matrices have improved their thermal and chemical stabilities. However, these cesium lead halide perovskite nanocrystals in glasses are still sensitive to intense light irradiation, limiting their wide applications. In this work, fs laser irradiation on the structural and optical properties of cesium lead bromide (CsPbBr3) perovskite nanocrystals (PNCs) is investigated. It turns out that fs laser irradiation does not change the structural skeletons and band gap energies of CsPbBr3 PNCs in glasses, but induces surface Br vacancies. In-situ transient absorption spectral analysis reveals one <10 ps trapping process of photo-generated carriers after fs laser irradiation, and confirms that Br vacancies are induced by Br migration driven by photo-generated carriers. In addition, CsPbBr3 PNCs with high PL QY in glasses through Br passivation can be more easily damaged by fs laser, making it difficult to achieve stable stimulated emission towards laser application. These results shine light on the effect of halogen contents on the photo-stability of cesium lead halide PNCs and the regulation of cesium lead halide PNCs embedded glasses for photo-electronic applications.

Femtosecond Laser Direct Writing of Antireflection Microstructures on the Front and Back Sides of a GaSe Crystal

The development of antireflection coatings is crucially important to improve the performance of various photonic devices, for example, to increase the efficiency of harmonic generators based on high-refractive index crystals with significant Fresnel losses. A promising technique for the reducing of radiation reflection is to change the refractive index by fabrication of antireflection microstructures (ARM) on the surface. This paper presents the results of ARM direct writing on the surfaces of a nonlinear GaSe crystal (of ε modification, according to Raman and photoluminescence spectroscopy data) using fs laser radiation and a multiples approach. An increase in transmission from 65% to 80% for an ARM fabricated on one side of the crystal and up to 94% for ARMs fabricated on both sides is demonstrated. The increase in transmission with the increasing pulse energy, as well as with an increase in the number of pulses used for the formation of a single crater, is shown. The experimental results of ARM transmission of GaSe are in qualitative agreement with the simulation results based on the measured profiles and morphology of the ARM structures.

Persistent Photogenerated State Attained by Femtosecond Laser Irradiation of Thin Td-MoTe2

Laser excitation has emerged as a means to expose hidden states of matter and promote phase transitions on demand. Such laser induced transformations are often rendered possible owing to the delivery of spatially and/or temporally manipulated light, carrying energy quanta well above the thermal background. Here, we report time-resolved broadband femtosecond (fs) transient absorption measurements on thin flakes of Weyl semimetal candidate $T_d$-MoTe$_2$ subjected to various levels and schemes of fs-photoexcitation. Our results reveal that impulsive fs-laser irradiation alters the interlayer behavior of the low temperature $T_d$ phase as evidenced by the persistent disappearance of its characteristic coherent $^1$A$_1$ $=$ 13 cm$^{-1}$ shear phonon mode. We found that this structural transformation withstands thermal annealing up to 500 K, although it can be reverted to the 1$T$\' phase by fs-laser treatment at room temperature. Our work opens the door to reversible optical control of topological properties.

Research on Monocrystalline Silicon Micro-Nano Structures Irradiated by Femtosecond Laser.

Femtosecond (fs) laser processing has received great attention for preparing novel micro-nano structures and functional materials. However, the induction mechanism of the micro-nano structures induced by fs lasers still needs to be explored. In this work, the laser-induced periodic surface structure (LIPSS) of monocrystalline silicon (Si) under fs laser irradiation is investigated. Three different layers named amorphous silicon (a-Si) layer, transition layer, and unaffected Si layer are observed after laser irradiation. The a-Si layer on the surface is generated by the resolidification of melting materials. The unaffected Si layer is not affected by laser irradiation and maintains the initial atomic structure. The transition layer consisting of a-Si and unaffected Si layers was observed under the irradiated subsurface. The phase transition mechanism of Si irradiated by fs laser is “amorphous transition”, with the absence of other crystal structures. A numerical model is established to describe the fs laser-Si interaction to characterize the electronic (lattice) dynamics of the LIPSS formation. The obtained results contribute to the understanding of fs laser processing of Si at the atomic scale as well as broaden the application prospects of fs laser for treating other semiconductor materials.

Propagation dynamics of the solid–liquid interface in Ge upon ns and fs laser irradiation

Monitoring the laser-induced melting and solidification dynamics of Ge upon laser irradiation is an enormous challenge due to the short penetration depth of its liquid phase. In this work, real-time pump-probe experiments in combination with finite element calculations have been employed to investigate the melting and solidification dynamics of germanium upon ns and fs laser pulse irradiation (λ = 800 nm). Excellent agreement between experiments and simulations allowed us to indirectly determine additional time- and depth-dependent information about the transformation dynamics of germanium, including the thickness evolution of the molten layer, as well as its melting and solidification velocities for the two pulse durations for different fluences. Our results reveal considerable differences in the maximum thickness of the molten Ge superficial layers at sub-ablative fluences for ns and fs pulses, respectively. Maximum melt-in velocities of 39 m s−1 were obtained for ns pulses at high fluences, compared to non-thermal melting of a thin layer within 300 fs for fs pulses already at moderate fluences. Maximum solidification velocities were found to be 16 m s−1 for ns pulses, and up to 55 m s−1 for fs pulses. Weak signs of amorphization were observed for fs excitation, suggesting that the lower limit of solidification velocities for a complete amorphization is above 55 m s−1. In addition, we show high precision measurements of the melt-in velocities over the first 20 nm by means of fs microscopy with sub-ps temporal resolution. Here, differences of the melt-in process of several orders of magnitude were observed, ranging from virtually instantaneous melting within less than 2 ps even for a moderate peak fluence up to 200 ps for fluences close to the melting threshold.

A low-damage copper removal process by femtosecond laser for integrated circuits

To develop a low-damage, low-cost, high-precision copper (Cu) removal process is conducive to promoting the development of integrated circuits in the direction of three-dimensional integration. In this study, a Cu film removal process without damage to the underlying substrate for integrated circuits by femtosecond (fs) laser was proposed. Theoretically, the feasibility of removing Cu film without damaging the substrate by fs-laser was demonstrated by studying the laser energy absorption and coupling, and temperature transient response of Cu/SiC bilayer material under fs-laser irradiation. Experimentally, fs-lasers were used to remove Cu films on SiC, SiO2, and Si3N4 substrates commonly used in integrated circuits. The experimental results show that the fs-laser with suitable fluence can effectively remove the Cu film without damaging the substrate, which is due to the nonlinear absorption of laser fluence by Cu film and the thermal resistance at the metal-nonmetal interface. Moreover, compared with the high-fluence fs-laser, the low-fluence multi-pulse fs-laser is more suitable for the effective removal of Cu film without damage to the substrate. Finally, the application of fs-laser in the preparation of an on-chip filter demonstrates the great potential of fs-laser in the fields of ICs and electronic device processing.

Laser-Induced Graphitization of Diamond Under 30 fs Laser Pulse Irradiation.

The degree of laser-induced graphitization from a sp3-bonded to a sp2-bonded carbon fraction in a single crystal chemical vapor deposited (CVD) diamond under varying fluence of an ultrashort pulsed laser (30 fs, 800 nm, 1 kHz) irradiation has been studied. The tetrahedral CVD sp3 phase is found to transition to primarily an sp2 aromatic crystalline graphitic fraction below the critical fluence of 3.9 J/cm2, above which predominantly an amorphous carbon is formed. A fractional increase of fluence from 3.3 to 3.9 J/cm2 (∼20%) results in a substantially (∼3-fold) increased depth of the sp2 graphitized areas owing to the nonlinear interactions associated with a fs laser irradiation. Additionally, formation of a C═O carbonyl group is observed below the critical threshold fluence; the C═O cleavage occurrs gradually with the increase of irradiation fluence of 30 fs laser light. The implications for these findings on enhancement of fs driven processing of diamonds are discussed.