Femtosecond Laser Research Articles

Toxicological problems of tattoo removal: characterization of femtosecond laser-induced fragments of Pigment Green 7 and Green Concentrate tattoo ink.

Femtosecond lasers represent a novel tool for tattoo removal as sources that can be operated at high power, potentially leading to different removal pathways and products. Consequently, the potential toxicity of its application also needs to be evaluated. In this framework, we present a comparative study of Ti:Sapphire femtosecond laser irradiation, as a function of laser power and exposure time, on water dispersions of Pigment Green 7 (PG7) and the green tattoo ink Green Concentrate (GC), which contains PG7 as its coloring agent. The treated samples were subsequently analyzed via UV‒Vis spectroscopy, gas chromatography‒mass spectrometry (GC‒MS), SEM imaging and associated statistical analysis. We found that, on average, the discoloration efficacy of femtosecond laser treatment was comparable to that of nanosecond lasers as were the decomposition products. In fact, two primary types of fragments are produced, both of which are potentially harmful, resulting either from the decomposition of chlorinated phthalocyanine (i.e., PG7) or from the active chlorination of naphthalene impurities. However, the outcomes for the PG7 and GC treatments differed significantly from each other from several points of view. The spectral intensity patterns of GC and PG7 were distinct, depending on the treatment conditions, and showed linearity with power only in the case of GC. Additionally, the relative ratios of the fragment products differed significantly, with the production rate showing a linear dependence on power only in the case of GC and no discernible trend for PG7. Shape and size distribution of the generated particles were highly dependent on the type of sample. Femtosecond laser irradiation of GCs primarily produces nanoparticles with a homogeneous size distribution, which are typically considered nontoxic. Large aggregates also formed, exhibiting a regular shape. In contrast, PG7 yielded rods and needles with aspect ratios similar to those of toxic fibers.

1:4 resonance, Arnold tongues and Marotto's chaos of a discrete reduced Lorenz system

In this paper, firstly, asymptotically embedding the fourth iterate of the normal form of a discrete reduced Lorenz system into a flow, we present rigorously the bifurcation sequence as the parameter varies around the 1:4 strong resonance point, from which an Arnold tongue corresponding to a rotation number 1/4 is obtained. Then, by the theory of normal form, we give theoretically the Arnold tongues in the weak resonances such that the system possesses two periodic orbits on the stable invariant closed curve generated from the Neimark–Sacker bifurcation. Furthermore, we obtain rigorously the parameter conditions under which the system has the chaotic behaviour in Marotto's sense. Finally, by numerical simulations, not only our results are verified, but also some new and interesting dynamical behaviours are discovered.

Anion photoelectron velocity-map imaging using a tunable laser at a 100kHz repetition rate.

We present velocity-map imaging (VMI) of photoelectrons detached from anions using an optical parametric amplifier operating at a repetition rate as high as 100kHz. The light source generates femtosecond (fs) laser pulses tunable from near-infrared to ultraviolet (310-2600nm), which interact synchronously with mass-selected anion bunches. We demonstrate this technique by measuring two-dimensional projections of photoelectrons ejected from silver trimer anions, Ag3-, across a photon energy range from 2.43 to 4.00eV (509-310nm), with an average power of 50-300 mW. This high-repetition-rate VMI setup allows rapid data acquisition of photoelectron spectra and laboratory-frame photoelectron angular distributions of anions at various photon energies, facilitating investigation of their electronic and geometric structures. Taking advantage of the fs pulses, this approach will also enable time-resolved photoelectron imaging for tracking electronic and nuclear dynamics of anions with high efficiency.

Defect-assisted reversible phase transition in mono- and few-layer ReS2

2D transition metal dichalcogenide (TMD) materials have attracted interest due to their remarkable excitonic, optical, electrical, and mechanical properties, which are dependent on their crystal structure. Consequently, controlling the crystal structure of these materials is essential for fine-tuning their performance, e.g., linear and nonlinear optical, as well as charge transport properties. While various phase-switching TMD materials are available, their transitions are often irreversible. Here, we investigate the mechanism of a light-induced reversible phase transition in mono- and bilayer rhenium disulfide (ReS2). Our observations, based on transmission electron microscopy, nonlinear spectroscopy, and density functional theory, reveal a transition from the ground T″ (double-distorted T) to the metastable H′ (distorted H) phase under femtosecond laser irradiation or influence of highly-energetic electrons. We show that the formation of sulfur vacancies facilitates this phenomenon. Our findings pave the way toward manipulating the crystal structure of ReS2 and possibly its heterostructures.

Simulation and Experimental Study of the Single-Pulse Femtosecond Laser Ablation Morphology of GaN Films

Gallium nitride (GaN) exhibits distinctive physical and chemical properties that render it indispensable in a multitude of electronic and optoelectronic devices. Given that GaN is a typical hard and brittle material that is difficult to machine, femtosecond laser technology provides an effective and convenient tool for processing such materials. However, GaN undergoes complex physical and chemical changes during high-power ablation, which poses a challenge to high-precision processing with controllable geometry. In this study, the quantitative relationship between the parameters of a single-pulse femtosecond laser and GaN ablation morphology was investigated using isotherm distribution. A multiphysics model using COMSOL Multiphysics® was developed to generate the isothermal distributions. Experiments were conducted on the femtosecond laser ablation of GaN at various single-pulse energies, and the resulting ablation morphologies were compared with the predictions from the multiphysics model. The comparison demonstrated that the calculated isotherm distribution accurately predicted not only the ablation diameter and depth but also the crater shape across a broad range of laser fluences. The predicted errors of the ablation diameters and depths were within 4.71% and 10.9%, respectively. The root mean square error (RMSE) and coefficient of determination (R2) were employed to evaluate the prediction errors associated with the crater shapes, which fell within the range of 0.018–0.032 μm and 0.77–0.91, respectively. This study can provide an important reference for utilizing femtosecond lasers in the precise ablation of GaN to achieve desired geometries.

3D Femtosecond Laser Beam Deflection for High-Precision Fabrication and Modulation of Individual Voxelated PCM Meta-Atoms.

Optical metasurfaces have found widespread applications in the field of optoelectronic devices. However, achieving dynamic and flexible control over metasurface functionalities, while also developing simplified fabrication methods for metasurfaces, continues to pose a significant challenge. Here, the study introduces a PCM-only metasurface that exclusively consists of voxel units crafted from different phases of phase-change materials. Micro-nano regions, with varying phase states, are directly utilized as resonant elements and embedded in the material, forming the metasurface voxel meta-atoms. By manipulating the morphology, size, and arrangement of these meta-atoms, the study achieves both the fabrication and modulation of the PCM-only metasurface. Additionally, a 3D high-precision voxelated beam deflection method based on femtosecond laser phase modulation is introduced to streamline the fabrication and modulation of PCM-only metasurface. By using a spatial light modulator to load the blazed grating phase manipulation beam for precise deflection, high-precision deflection processing of individual meta-atom on metasurfaces can be achieved, with a deflection accuracy of up to 200nm. By loading Fresnel phase manipulation beams to move along the z-axis, perfect modulation of PCM sub-meta-atoms can be achieved. The 3D femtosecond laser beam deflection technology will bring many potential application opportunities in the fields of optoelectronic device fabrication and functional control.

High-power ultraflat broadband supercontinuum generation in fluorotellurite fiber pumped by a dual-wavelength femtosecond laser

In this Letter, we report an ultraflat high-power supercontinuum (SC) based on a low-loss short-length fluorotellurite fiber. A novel high-peak power dual-Raman soliton femtosecond laser is used as a pump source, which effectively extends the mid-infrared SC spectral range and enhances the flatness of the SC. Finally, we obtained a 10.4 W SC laser source with a spectral range from 1.8 to 4.2 µm in a 34 µm fluorotellurite fiber; the 5 dB bandwidth of the source completely covers 1.9–4.05 µm. To the best of our knowledge, this is the first report of a high-power SC generated in a fluorotellurite fiber with a flat spectral edge of >4 µm. This result highlights the fluorotellurite fiber’s spectral broadening capabilities under high-power conditions, demonstrating performance on par with that of ZBLAN fibers.

Evaluation of safe exposure time for two-photon microscopy imaging of acrylic-painted mock-ups

Abstract Nonlinear optical microscopies are widely used in the biological and biomedical fields, as they are non-invasive techniques that permit the safe structural and morphological characterisation of cells and tissues. They are increasingly being used in the Cultural Heritage field because of their ability to overcome some limits of the well-established optical techniques. However, since nonlinear optical microscopies use pulsed laser sources with high peak power, their application in Cultural Heritage raises concerns due to artworks' unique, priceless, and delicate nature. In this paper, we present a new method for evaluating the photo-induced damage when using a near-infrared femtosecond pulsed laser to perform two-photon excited fluorescence imaging of acrylic-painted mock-ups. In particular, we explore herein several irradiation conditions, varying the exposure time and excitation power, in order to provide useful experimental indications for safely imaging acrylic paints with two-photon microscopy.

Influence of Photoemission Geometry on Timing and Efficiency in 4D Ultrafast Electron Microscopy.

Broader adoption of 4D ultrafast electron microscopy (UEM) for the study of chemical, materials, and quantum systems is being driven by development of new instruments as well as continuous improvement and characterization of existing technologies. Perhaps owing to the still-high barrier to entry, the full range of capabilities of laser-driven 4D UEM instruments has yet to be established, particularly when operated at extremely low beam currents (~fA). Accordingly, with an eye on beam stability, we have conducted particle tracing simulations of unconventional off-axis photoemission geometries in a UEM equipped with a thermionic-emission gun. Specifically, we have explored the impact of experimentally adjustable parameters on the time-of-flight (TOF), the collection efficiency (CE), and the temporal width of ultrashort photoelectron packets. The adjustable parameters include the Wehnelt aperture diameter (DW), the cathode set-back position (Ztip), and the position of the femtosecond laser on the Wehnelt aperture surface relative to the optic axis (Rphoto). Notable findings include significant sensitivity of TOF to DW and Ztip, as well as non-intuitive responses of CE and temporal width to varying Rphoto. As a means to improve accessibility, practical implications and recommendations are emphasized wherever possible.

Ultrafast Laser-Induced Spin Dynamics in All-Semiconductor Ferromagnetic CrSBr-Phosphorene Heterostructures.

Ultrashort laser pulses are extensively used for efficient manipulation of interfacial spin injection in two-dimensional van der Waals (vdW) heterostructures. However, physical processes accompanying the photoinduced spin transfer dynamics on the all-semiconductor ferromagnetic vdW heterostructure remain largely unexplored. Here, we present a computational investigation of the femtosecond laser pulse induced purely electron-mediated spin transfer dynamics at a time scale of less than 50 fs in a vdW heterostructure. The latter is composed of two semiconducting monolayers, namely, a ferromagnetic material CrSBr and a nonmagnetic phosphorene, and is denoted as CrSBr-P. We observe an ultrafast spin injection from the Cr atoms to the P atoms in a few femtoseconds by both optically induced and interfacial atom-mediated spin transfer effects. We also show that the demagnetization and spin transfer in the ferromagnetic-nonmagnetic CrSBr-P vdW heterostructure can be sensitively manipulated by laser pulses with different fluences. Our study offers a microscopic understanding of spin dynamics in these vdW heterostructures aiming toward their potential spintronic applications, which rely on optically controlled spin transfer processes.

Decoupling Carrier Dynamics and Energy Transport in Ultrafast Near-Field Nanoscopy.

Ultrafast near-field optical nanoscopy has emerged as a powerful platform to characterize low-dimensional materials. While analytical and numerical models have been established to account for photoexcited carrier dynamics, quantitative evaluation of the associated pulsed laser heating remains elusive. Here, we decouple the photocarrier density and temperature increase in near-field nanoscopy by integrating the two-temperature model (TTM) with finite-difference time-domain (FDTD) simulations. These results reveal that the electron-phonon coupling in a silicon film after femtosecond laser excitation is most pronounced within approximately 3 ps─substantially shorter than the photocarrier decay time scale at tens of picoseconds. Moreover, the coupled TTM-FDTD method indicates that ultrafast laser heating can cause up to a 14% variation in the near-field signal at a 220 μJ/cm2 pump pulse fluence. Our numerical results are further validated by transient experiments, highlighting the potential of this method for investigations of carrier and thermal phenomena in emerging nanomaterials and nanodevices.

Optical Precise Ablation of Targeted Individual Neurons In Vivo.

Targeted cell ablation is a powerful strategy for investigating the function of individual neurons within neuronal networks. Multiphoton ablation technology by a tightly focused femtosecond laser, with its significant advantages of noninvasiveness, high efficiency, and single-cell resolution, has been widely used in the study of neuroscience. However, the firing activity of the ablated neuron and its impact on the surrounding neurons and entire neuronal ensembles are still unclear. In this study, we describe the depolarization process of targeted neuron ablation by a femtosecond laser based on a standard two-photon microscope in vitro and in vivo. The photoporation damages the cell membrane, depolarizes the membrane potential, and thus disables the neuron's ability to fire action potentials. The dysfunctional neuron after laser ablation affects both the responses of surrounding neighbors and the functions of ensemble neurons in vivo. Although abnormal Ca2+ responses in spatially surrounding neurons are observed, the damage effect is confined to the focal volume. The function of the neuronal ensembles that associate with a specific visual stimulation is not influenced by the ablation of an individual member of the ensemble, indicating the redundancy of the ensemble organization. This study thus provides an insight into the targeted neuron ablation as well as the role of an individual neuron in an ensemble.

Hybrid Femtosecond Laser 3D Processing Technology for Rapid Integration of Functional Optical Devices on Fibers

Hybrid Femtosecond Laser 3D Processing Technology for Rapid Integration of Functional Optical Devices on Fibers

‘Ship-in-a-Bottle’ Integration of pH-Sensitive 3D Proteinaceous Meshes into Microfluidic Channels

Microfluidic sensors incorporated onto chips allow sensor miniaturization and high-throughput analyses for point-of-care or non-clinical analytical tools. Three-dimensional (3D) printing based on femtosecond laser direct writing (fs-LDW) is useful for creating 3D microstructures with high spatial resolution because the structures are printed in 3D space along a designated laser light path. High-performance biochips can be fabricated using the ‘ship-in-a-bottle’ integration technique, in which functional microcomponents or biomimetic structures are embedded inside closed microchannels using fs-LDW. Solutions containing protein biomacromolecules as a precursor can be used to fabricate microstructures that retain their native protein functions. Here, we demonstrate the ship-in-a-bottle integration of pure 3D proteinaceous microstructures that exhibit pH sensitivity. We fabricated proteinaceous mesh structures with gap sizes of 10 and 5 μm. The sizes of these gaps changed when exposed to physiological buffers ranging from pH of 4 to 10. The size of the gaps in the mesh can be shrunk and expanded repeatedly by changing the pH of the surrounding buffer. Fs-LDW enables the construction of microscopic proteinaceous meshes that exhibit dynamic functions such as pH sensing and might find applications for filtering particles in microfluidic channels.

Femtosecond laser two-step rotary drilling method for SiCf/SiC composites

Femtosecond laser two-step rotary drilling method for SiCf/SiC composites

Micropattern of Core-shell Ag@MCS/PEGDA Nanoparticles Fabricated by Femtosecond Laser Maskless Optical Projection Lithography

Micropattern of Core-shell Ag@MCS/PEGDA Nanoparticles Fabricated by Femtosecond Laser Maskless Optical Projection Lithography

Aqueous Proteomic and Metabolomic Profiles in Low-Energy vs High-Energy Femtosecond Laser-Assisted Cataract Surgery.

To investigate the aqueous proteomics and metabolomics in low-energy and high-energy femtosecond laser-assisted cataract surgery (FLACS). In this prospective observational study, 72 patients were randomized to 3 groups: low-energy FLACS, high-energy FLACS, and conventional phacoemulsification (controls). Aqueous was collected after femtosecond laser treatment or at the beginning of surgery (controls). Proteomic analysis was conducted using a data-independent acquisition method, whereas aqueous metabolomics were analyzed with liquid chromatography-tandem mass spectrometry. Bioinformatics analyses were performed to integrate the results of proteomics and metabolomics. Compared with low-energy FLACS, significantly elevated aqueous hemoglobin subunit beta, G protein subunit beta, carbonic anhydrase 1, and asymmetric dimethylarginine were observed in high-energy FLACS, suggesting significantly greater oxidative stress, inflammation, immunity, metabolism, and mitochondrial fatty acids oxidation. Compared with controls, significantly increased aqueous proteins and metabolites related to immune and inflammation (beta-crystallin B1, hemoglobin subunit beta, putrescine, and spermine) and oxidative stress (heat shock proteins, peroxiredoxins, and long-chain acylcarnitines) were observed in FLACS. Joint pathway analysis revealed nicotinate/nicotinamide metabolism and riboflavin metabolism were significantly overexpressed in high-energy FLACS compared with low-energy FLACS, whereas the pentose phosphate pathway and glycolysis were the most significant pathways when comparing FLACS with controls. FLACS induced higher immunological and inflammatory responses, oxidative stress reactions, and mitochondrial fatty acid oxidative stress compared with controls. These differential effects were more pronounced when a higher laser energy was used.

MoTe2 Photodetector for Integrated Lithium Niobate Photonics.

The integration of a photodetector that converts optical signals into electrical signals is essential for scalable integrated lithium niobate photonics. Two-dimensional materials provide a potential high-efficiency on-chip detection capability. Here, we demonstrate an efficient on-chip photodetector based on a few layers of MoTe2 on a thin film lithium niobate waveguide and integrate it with a microresonator operating in an optical telecommunication band. The lithium-niobate-on-insulator waveguides and micro-ring resonator are fabricated using the femtosecond laser photolithography-assisted chemical-mechanical etching method. The lithium niobate waveguide-integrated MoTe2 presents an absorption coefficient of 72% and a transmission loss of 0.27 dB µm-1 at 1550 nm. The on-chip photodetector exhibits a responsivity of 1 mA W-1 at a bias voltage of 20 V, a low dark current of 1.6 nA, and a photo-dark current ratio of 108 W-1. Due to effective waveguide coupling and interaction with MoTe2, the generated photocurrent is approximately 160 times higher than that of free-space light irradiation. Furthermore, we demonstrate a wavelength-selective photonic device by integrating the photodetector and micro-ring resonator with a quality factor of 104 on the same chip, suggesting potential applications in the field of on-chip spectrometers and biosensors.

Fiber Optic Micro-Hole Salinity Sensor Based on Femtosecond Laser Processing.

This study presents a novel reflective fiber Fabry-Perot (F-P) salinity sensor. The sensor employs a femtosecond laser to fabricate an open liquid cavity, facilitating the unobstructed ingress and egress of the liquid, thereby enabling the direct involvement of the liquid in light transmission. Variations in the refractive index of the liquid induce corresponding changes in the effective refractive index of the optical path, which subsequently influences the output spectrum. The dimensions and quality of the optical fiber are meticulously regulated through a combination of femtosecond laser cutting and chemical polishing, significantly enhancing the mechanical strength and sensitivity of the sensor's overall structure. Experimental results indicate that the sensor achieves salinity sensitivity of 0.288 nm/% within a salinity range of 0% to 25%. Furthermore, the temperature sensitivity is measured at a minimal 0.015 nm/°C, allowing us to neglect temperature effects. The device is characterized by its compact size, straightforward structure, high mechanical robustness, ease of production, and excellent reproducibility. It demonstrates considerable potential for sensing applications in the domains of biomedicine and chemical engineering.

Enhanced two-temperature model and its application in comprehensive analysis of femtosecond laser melting of gold, copper and their alloys.

Extensive research on ultrashort laser-induced melting of noble metals like Au, Ag and Cu is available. However, studies on laser energy deposition and thermal damage of their alloys, which are currently attracting interest for energy harvesting and storage devices, are limited. This study investigates the melting damage threshold (DT) of three intermetallic alloys of Au and Cu (Au3Cu, AuCu and AuCu3) subjected to single-pulse femtosecond laser irradiation, comparing them with their constituent metals. This is accomplished by extending an earlier-developed two-temperature model (TTM)-based code with several improvements, including precise modeling of temperature-dependent optical properties and ballistic electron transport. The dynamic optical model inclusive of ballistic effects is demonstrated to reproduce the experimental DT of pure metals with minimal variation and is therefore adopted for further investigation. Our simulations reveal that the alloy films have significantly lower incipient and complete melting thresholds compared to the pure metals due to their low thermal conductivity and high electron-phonon coupling strength. Theoretical studies on varying the thickness of metal and alloy films unveil the usual trend of a rapid increase in DT up to a certain thickness, followed by a saturation region. This universal DT profile is elucidated by proposing a first-of-its-kind analytical function. Excellent agreement between the coefficients of the function with optical and electron diffusion parameters derived from the comprehensive theory proposed here reinforces the robustness of the model. The novelty of this study also lies in introducing the concept of a critical film thickness for which the entire film attains the melting temperature at its complete melting threshold.