Femtosecond Laser Irradiation Research Articles

Studying the viability and growth kinetics of vancomycin-resistant Enterococcus faecalis V583 following femtosecond laser irradiation (420–465 nm)

Enterococcus faecalis is among the most resistant bacteria found in infected root canals. The demand for cutting-edge disinfection methods has rekindled research on photoinactivation with visible light. This study investigated the bactericidal activity of femtosecond laser irradiation against vancomycin-resistant Enterococcus faecalis V583 (VRE). The effect of parameters such as wavelength and energy density on the viability and growth kinetics of VRE was studied to design an optimized laser-based antimicrobial photoinactivation approach without any prior addition of exogenous photosensitizers. The most effective wavelengths were 430 nm and 435 nm at a fluence of 1000 J/cm2, causing a nearly 2-log reduction (98.6% and 98.3% inhibition, respectively) in viable bacterial counts. The colony-forming units and growth rate of the laser-treated cultures were progressively decreased as energy density or light dose increased at 445 nm but reached a limit at 1250 J/cm2. At a higher fluence of 2000 J/cm2, the efficacy was reduced due to a photobleaching phenomenon. Our results highlight the importance of optimizing laser exposure parameters, such as wavelength and fluence, in bacterial photoinactivation experiments. To our knowledge, this is the first study to report an optimized wavelength for the inactivation of VRE using visible femtosecond laser light.

Stain-free enucleation of mouse and human oocytes with a 1033nm femtosecond laser.

Preparation of a recipient cytoplast by oocyte enucleation is an essential task for animal cloning and assisted reproductive technologies in humans. The femtosecond laser is a precise and low-invasive tool for oocyte enucleation, and it should be an appropriate alternative to traditional enucleation by a microneedle aspiration. However, until recently, the laser enucleation was performed only with applying a fluorescent dye. This work is aimed to (1)achieve femtosecond laser oocyte enucleation without applying a fluorescent dye and (2)to study the effect of laser destruction of chromosomes on the structure and dynamics of the spindle. We applied polarized light microscopy for spindle visualization and performed stain-free mouse and human oocyte enucleation with a 1033nm femtosecond laser. Also, we studied transformation of a spindle after metaphase plate elimination by a confocal microscopy. We demonstrated a fundamental possibility of inactivating the metaphase plate in mouse and human oocytes by 1033nm femtosecond laser radiation without applying a fluorescent dye. Irradiation of the spindle area, visualized by polarized light microscopy, resulted in partly or complete metaphase plate destruction but avoided the microtubules impairment. After the metaphase plate elimination, the spindle reorganized, however, it was not a complete depolymerization. This method of recipient cytoplast preparation is expected to be useful for animal cloning and assisted reproductive technologies.

Femtosecond pulse train-facilitated periodic nanostructuring on TiN films via laser-oxidation

Laser-induced periodic surface structure is a universal phenomenon observed when solid materials are irradiated with pulsed lasers. Typically, these structures are generated using Gaussian pulses. Here, we investigate the formation of periodic nanostructures on titanium nitride thin films using two different laser sources: Gaussian pulses and femtosecond pulse trains generated through a Fabry-Perot cavity. Our findings indicate that the formation of self-organized nanostructures on the thin films can occur through either laser-induced oxidation or laser ablation processes, depending on laser fluence, scanning speed, and pulse duration. Interestingly, when utilizing femtosecond pulse trains, we observe that the range of laser fluence thresholds for oxidative nanostructure formation is significantly wider compared to Gaussian pulses. To comprehend the underlying mechanisms driving these observations, we employ a two-temperature model to calculate the ultrafast dynamics of hot electrons when exposed to laser irradiation. Our results reveal that the initial sub-pulse generates a substantial population of hot electrons, exciting surface plasmons and resulting in the creation of a periodic energy distribution. Before these hot electrons can dissipate, subsequent sub-pulses reach the thin film, re-exciting a large number of hot electrons. Consequently, the periodic energy distribution persists for an extended duration, creating more favorable conditions for the formation of periodic oxidative nanostructures when using pulse trains. These findings contribute to a comprehensive understanding of the mechanisms involved in the formation of periodic nanostructures under femtosecond laser irradiation.

Femtosecond Laser Nanomachining of High‐Aspect‐Ratio Channels in Bulk Fused Silica

AbstractThee‐dimensional nano‐structuring of bulk fused silica glass remains a challenging issue due to limitations of traditional fabrication methods. In the present work the fabrication of hollow mm‐length channels is demonstrated in bulk fused silica by femtosecond laser irradiation followed by chemical etching, achieving feature sizes of 300 nm. The tight focusing and threshold energy conditions allow to reveal unexplored fabrication regimes. The robustness and repeatability of the process are demonstrated through the fabrication of fused silica diffraction gratings, made of parallel hollow channels with variable pitches even in the sub‐wavelength regime.

Photochemistry of Bismuth- and Silver-Containing Glasses under Femtosecond Laser Irradiation: Energy Transfers and 3D-localized Background-Free Near-Infrared Fluorescence Emission

Photochemistry of Bismuth- and Silver-Containing Glasses under Femtosecond Laser Irradiation: Energy Transfers and 3D-localized Background-Free Near-Infrared Fluorescence Emission

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.

Controlled photothermal ablative processing of commercial polymers minimizing undesired thermal effects under high frequency femtosecond laser irradiation

The response of three commercial polymers (poly(vinyl chloride) (PVC), poly(ethylene terephthalate) (PET) and polypropylene (PP)) with different thermal properties under high repetition rates (1 kHz-1 MHz) with femtosecond (450 fs) multi-pulse laser irradiation at λ = 515 nm (1.4 J/cm2) is reported resulting in a complete study with controlling the ablation depth and minimizing collateral thermal effects. Tunable ablation depth is achieved accurately by varying the repetition rate at a constant fluence. The results are compared to a photothermal model that aims at explaining the heat accumulation effect of successive pulses as a function of the repetition rate and predicts three different heat regimes (non-cumulative, cumulative and saturation). The threshold frequencies for each regime can be estimated from the model, providing control for selecting frequency values and thermal regimes. Thermal analyses are performed to characterize the materials, concluding that thermal parameters are vital for selecting optimal materials and laser processing parameters.

High throughput clogging free microfluidic particle filter by femtosecond laser micromachining.

In recent decades, driven by the needs of industry and medicine, researchers have been investigating how to remove carefully from the main flow microscopic particles or clusters of them. Among all the approaches proposed, crossflow filtration is one of the most attractive as it provides a non-destructive, label-free and in-flow sorting method. In general, the separation performance shows capture and separation efficiencies ranging from 70% up to 100%. However, the maximum flow rate achievable (µL/min) is still orders of magnitude away from those suitable for clinical or industrial applications mainly due to the low stiffness of the materials typically used. In this work, we propose an innovative hydrodynamic-crossflow hybrid filter geometry, buried in a fused silica substrate by means of the femtosecond laser irradiation followed by chemical etching technique. The material high stiffness combined with the accuracy of our manufacturing technique allows the 3D fabrication of non-deformable channels with higher aspect ratio posts, while keeping the overall device dimensions compact. The filter performance has been validated through experiments with both Newtonian (water-based solution of microbeads) and non-Newtonian fluids (blood), achieving separation efficiencies of up to 94% and large particles recovery rates of 100%, even at very high flow rates (mL/h).

Ultra-corrosion-resistant aluminum alloys achieved by femtosecond laser complete remediation of microarc oxidation defects

Microarc oxidation (MAO) is a widely employed technology for the corrosion protection of metal surfaces due to the formation of thick-layer ceramic substances. However, its practical effectiveness is strictly limited by the inevitable concomitant generation of numerous microscale pores/cracks on the oxidation surface to directly expose the substrate materials. As concerning this problem, we here introduced a novel method of the femtosecond laser irradiation to completely eliminate the above MAO defects on aluminum alloys, which consequently modulates the passivation layer into the dense and the superhydrophobic. Meanwhile, with the deep insights into the phase composition variations of the oxide layer, we can find that the laser treatments enable the development of both γ-Al2O3 and α-Al2O3 nanocrystallines with embedment in the amorphous substances to significantly improve the chemical stability of materials. Such laser modifications eventually result in the ultra-corrosion-resistant surface of aluminum alloys, with the measured corrosion current density of 1.46 × 10-13 A/cm2 as well as the impedance value over 112 GΩ∙cm2, about two orders of magnitude enhancement in the anticorrosion compared to the initial MAO sample.

Unveiling second harmonic generation from femtosecond-laser microstructured Nd:YAG crystal

In this work we apply second harmonic microscopy to the analysis of damage tracks inscribed by femtosecond laser irradiation in a Nd:YAG crystal. While second harmonic generation is not expected in the bulk of this centrosymmetric material, the 2D and 3D images obtained via second harmonic microscopy show that the induced micro-modification of the crystal structure leads to a localized generation of the nonlinear signal. The nature of this modification and its dependence on irradiation and detection parameters is discussed. These findings demonstrate the capability of second harmonic microscopy for the morphological analysis of written structures in Nd:YAG and open the door for the design and fabrication of new nonlinear structures to be integrated in novel photonic devices.

Modifying single-crystal silicon and trimming silicon microring devices by femtosecond laser irradiation

Single-crystal silicon (c-Si) is a vital component of photonic devices and has obvious advantages. Moreover, femtosecond-pulsed laser interactions with matter have been widely applied in micro/nanoscale processing. In this paper, we report the modification mechanisms of c-Si induced by a femtosecond laser (350 fs, 520 nm) at different pulse fluences, along with the mechanism of this technique to trim the phase error of c-Si-based devices. In this study, several distinct types of final micro/nanostructures, such as amorphization and ablation, were analyzed. The near-surface morphology was characterized using optical microscopy, scanning electron microscopy, and atomic force microscopy. The main physical modification processes were further analyzed using a two-temperature model. By employing Raman spectroscopy, we demonstrated that a higher laser fluence significantly contributes to the formation of more amorphous silicon components. The thickness of the amorphous layer was almost uniform (approximately 30 nm) at different induced fluences, as determined using transmission electron microscopy. From the ellipsometry measurements, we demonstrated that the refractive index increases for amorphization while the ablation decreases. In addition, we investigated the ability of the femtosecond laser to modify the effective index of c-Si microring waveguides by either amorphization or ablation. Both blue and red shifts of resonance spectra were achieved in the microring devices, resulting in double-direction trimming. Our results provide further insight into the femtosecond laser modification mechanism of c-Si and may be a practical method for dealing with the fabrication errors of c-Si-based photonic devices.

Structured Filamentation of High-Power Femtosecond Laser Radiation Modulated by Amplitude Mesh Masks

Structured Filamentation of High-Power Femtosecond Laser Radiation Modulated by Amplitude Mesh Masks

Development of a novel signal restoration algorithm to eliminate the distorted components in space charge measurement excited by femtosecond laser irradiation

AbstractFor the space charge measurement method with optical excitation for electrical insulation, the distortion of the excitation laser on the measurement signal and the accumulation of space charge have not been discussed before. Both experiment and simulation results show that three types of distorted components introduced by optical excitation can lead to distortion of the upper interface signal. Further, the factors causing distortion of the measured signal and the formation mechanism are analysed. A virtual electrode potential compensation method and a distorted component extraction method are proposed. Meanwhile, an attenuation restoration and charge conversion correction factor is introduced, leading to a novel signal processing algorithm adapted to space charge optical measurement methods. Compared with the traditional algorithm, the novel algorithm has an error of less than 1% in the calculation of charge and electric field strength, with an increase in accuracy of 47.7% and 9.81%, respectively. By applying the novel restoration algorithm, the comparison shows that the space charge accumulation does not change significantly under laser irradiation, verifying the reliability of the optical excitation. The given algorithm can provide a theoretical and technical foundation for the realisation of the optical measurement method for space charge.

Self-assembly plasmonic gold nanoribbons on few-layer PtSe2 under femtosecond laser irradiation

Functional two-dimensional (2D) materials have been extensively explored for a wide range of applications such as energy generation, low-power computing, and biosensing. In this work, we present an approach involving the integration of six-layer platinum diselenide (PtSe2) as an interlayer between the thin gold film and SiO2 substrate to induce spontaneous formation of plasmonic nanostructures (nanoribbons) on the upper gold film (∼8 nm) under 1030 nm femtosecond laser irradiation. The formation of periodic nanostructures is attributed to the periodic energy deposition that occurs in the PtSe2 layers under intense femtosecond laser pulses. Notably, the self-assembled gold nanostructures exhibit a distinctive polarization-dependent plasmonic response in the near-infrared spectral region and could be directly fabricated in a centimeter scale within several minutes. This straightforward method for self-assembling plasmonic nanostructures using layered materials may expand the utility of functional 2D materials and advance the cost-effective and large-area fabrication of plasmonic thin-film nanostructures in a simplified manner.

Photocell-Based Optofluidic Device for Clogging-Free Cell Transit Time Measurements.

Measuring the transit time of a cell forced through a bottleneck is one of the most widely used techniques for the study of cell deformability in flow. It in turn provides an accessible and rapid way of obtaining crucial information regarding cell physiology. Many techniques are currently being investigated to reliably retrieve this time, but their translation to diagnostic-oriented devices is often hampered by their complexity, lack of robustness, and the bulky external equipment required. Herein, we demonstrate the benefits of coupling microfluidics with an optical method, like photocells, to measure the transit time. We exploit the femtosecond laser irradiation followed by chemical etching (FLICE) fabrication technique to build a monolithic 3D device capable of detecting cells flowing through a 3D non-deformable constriction which is fully buried in a fused silica substrate. We validated our chip by measuring the transit times of pristine breast cancer cells (MCF-7) and MCF-7 cells treated with Latrunculin A, a drug typically used to increase their deformability. A difference in transit times can be assessed without the need for complex external instrumentation and/or demanding computational efforts. The high throughput (4000-10,000 cells/min), ease of use, and clogging-free operation of our device bring this approach much closer to real scenarios.

Size reduction and water dispersibility improvement of hexagonal boron nitride particles by femtosecond laser irradiation in water

Controlling the size and surface state of inorganic particles, which strongly influence their dispersibility in solvents, is important for diverse applications. Intense femtosecond laser pulses can induce plasma formation in material–dispersed solvents and interact with both materials and solvents. In this study, femtosecond laser pulses are employed to modify hexagonal boron nitride (hBN) particles dispersed in water, with the aim of evaluating the effects of the femtosecond laser process on the size reduction and surface modification of hBN particles. Shadowgraph imaging reveals the formation of the reactive environment in hBN–dispersed water, resulting from the ionization of water molecules which leads to the generation of OH radicals. Evaluation of the hBN particle sizes suggests an overall reduction from 160 to 110 nm after 60 min of irradiation and the generation of nanodots between 5 and 10 nm in size. In addition, it is confirmed that the number of particles with higher zeta potentials increases after the samples are laser-irradiated, suggesting a change in the surface state. Consequently, the duration of hBN particle dispersion in water is significantly increased, with an improvement of at least one order of magnitude, for the laser-irradiated samples. This study presents a demonstration of the formation of a reaction field that affects hBN particles in size and dispersibility in water.

Femtosecond Laser Irradiation to Zirconia Prior to Calcium Phosphate Coating Enhances Osteointegration of Zirconia in Rabbits.

Calcium phosphate (CaP) coating of zirconia and zirconia-based implants is challenging, due to their chemical instability and susceptibility to thermal and mechanical impacts. A 3 mol% yttrium-stabilized tetragonal zirconia polycrystal was subjected to femtosecond laser (FsL) irradiation to form micro- and submicron surface architectures, prior to CaP coating using pulsed laser deposition (PLD) and low-temperature solution processing. Untreated zirconia, CaP-coated zirconia, and FsL-irradiated and CaP-coated zirconia were implanted in proximal tibial metaphyses of male Japanese white rabbits for four weeks. Radiographical analysis, push-out test, alizarin red staining, and histomorphometric analysis demonstrated a much improved bone-bonding ability of FsL-irradiated and CaP-coated zirconia over CaP-coated zirconia without FsL irradiation and untreated zirconia. The failure strength of the FsL-irradiated and CaP-coated zirconia in the push-out test was 6.2-13.1-times higher than that of the CaP-coated zirconia without FsL irradiation and untreated zirconia. Moreover, the adhesion strength between the bone and FsL-irradiated and CaP-coated zirconia was as high as that inducing host bone fracture in the push-out tests. The increased bone-bonding ability was attributed to the micro-/submicron surface architectures that enhanced osteoblastic differentiation and mechanical interlocking, leading to improved osteointegration. FsL irradiation followed by CaP coating could be useful for improving the osteointegration of cement-less zirconia-based joints and zirconia dental implants.

Effect of dopant concentration on femtosecond pulsed laser irradiation of yttria-stabilized zirconia for generating nanopores

Yttria-stabilized zirconia (YSZ) is a fine ceramics material that has been applied to dental implants and mechanical components. As YSZ is a hard, brittle, and chemically stable material, that undergoes phase transformation during mechanical loading, it is difficult to fabricate nanostructures using mechanical processing methods. In this study, a surface structuring method using a femtosecond pulsed laser was proposed, which is effective in suppressing the mechanical loading-induced phase transformation. As the material properties of YSZ vary with the concentration of yttria dopant, polycrystalline zirconia samples with different yttria concentrations (2, 3, 5, and 8 mol%) were used to investigate the effect of yttria concentrations on laser processing characteristics for nanostructuring. At all concentrations, nanopore generation in the surface grain was achieved by laser irradiation near the ablation threshold. In addition, there was no significant change in the crystal structure before and after laser irradiation, and no damage to the bulk occurred. At 2 mol%, intergranular cracks were generated around the pores. By increasing the dopant level to 5 mol%, the lens effect of the crystal grains became stronger due to enhanced grain growth, and the diameter and number of pores increased. In addition, pores formed preferentially on smaller grains at 5 mol%. However, for YSZ with 8 mol%, the pore formation phenomenon was different from that in the lower dopant level, as the area near the grain boundary was preferentially ablated, and microcracks formation and exfoliation of the surface also occurred. These results contribute to a deeper understanding of the laser ablation characteristics of zirconia with different dopant levels and to the development of surface structuring methods for fine ceramics materials.

Gigahertz electromagnetic pulse emission from femtosecond relativistic laser-irradiated solid targets.

The interactions between high-intensity laser and matter produce particle flux and electromagnetic radiation over a wide energy range. The generation of extremely intense transient fields in the radio frequency-microwave regime has been observed in femtosecond-to-nanosecond laser pulses with 1011-1020-W/cm2 intensity on both conductive and dielectric targets. These fields typically cause saturation and damage to electronic equipment inside and near an experimental chamber; nevertheless, they can also be effectively used as diagnostic tools. Accordingly, the characterization of electromagnetic pulses (EMPs) is extremely important and currently a popular topic for present and future laser facilities intended for laser-matter interaction. The picosecond and sub-picosecond laser pulses are considerably shorter than the characteristic electron discharge time (∼0.1 ns) and can be efficient in generating GHz EMPs. The EMP characterization study of femtosecond laser-driven solid targets is currently mainly in the order of 100 mJ laser energy, in this study, the EMP generated by intense (Joule class) femtosecond laser irradiation of solid targets has been measured as a function of laser energy, laser pulse duration, focal spot size, and target materials. And a maximum electric field of the EMP reaching up to 105 V/m was measured. Analyses of experimental results confirm a direct correlation between measured EMP energy and laser parameters in the ultrashort pulse duration regime. The EMP signals generated by femtosecond laser irradiation of solid targets mainly originate from the return current inside the target after hot electron excitation. Numerical simulations of EMP are performed according to the target charging model, which agree well with the experimental results.

Revealing a spontaneous two-stage process in femtosecond laser-induced fully-oxidized nanostructures

Femtosecond laser interactions with matter facilitate the creation of diverse structures on material surfaces, including laser induced periodic surface structures (LIPSS), ablation nanoparticles, sub-diffraction nanogrooves, and plasmonic nanostructures. In this study, we reveal a unique nanostructure on silicon induced by femtosecond laser irradiation, distinguished by its pea-shaped morphology. At the center of this nanostructure, a nano-slit is oriented perpendicular to the laser polarization. The nano-slit penetrates the pea-shaped nanostructure and an underlying layer with a depth of approximately 600 nm. Energy dispersive X-ray spectroscopy (EDS) analysis confirms that the nanostructure and the underlying layer consist entirely of silicon oxide. A spontaneous two-stage oxidation process is proposed to explain the formation mechanism of fully-oxidized nanostructures. The presence of nano-slits endows the oxide nanostructures with polarization sensitivity, which shows the potential in applications such as optical encryption and data storage. In addition, the spontaneous two-stage oxidation process allows for the maskless fabrication of oxide nanowires with a precision down to hundreds of nanometers, providing the flexibility to produce arbitrary nanostructures. This capability opens up new possibilities for manipulating light beams and enabling on-chip nano-glass printing.