Femtosecond Laser Pulse Irradiation Research Articles

Ultrafast postprocessing of yttrium aluminum garnet ceramics via femtosecond pulse laser irradiation

We studied rapid densification and homogenization of sintered yttrium aluminum garnet (YAG) ceramics via sintering followed by femtosecond pulse laser (800 nm) irradiation. We formed localized, high-density microstructures within a YAG ceramic matrix. The laser irradiated regions were characterized using scanning electron microscopy, elemental mapping, and x-ray diffraction. Our results show that phase separation of YAG into YAlO3 and Al2O3 can be obtained in situ within stoichiometric YAG. With an optimum laser power and processing conditions, this process can be used for producing dense YAG ceramics.

Reversible Photochemical Control of Doping Levels in Supported Graphene

Controlling the type and density of charge carriers in graphene is vital for a wide range of applications of this material in electronics and optoelectronics. To date, chemical doping and electrostatic gating have served as the two most established means to manipulate the carrier density in graphene. Although highly effective, these two approaches require sophisticated graphene growth or complex device fabrication processes to achieve both the desired nature and the doping densities with generally limited dynamic tunability and spatial control. Here, we report a convenient and tunable optical approach to tune the steady-state carrier density and Fermi energy in graphene by photochemically controlling the concentration of adsorbed molecular O2, a p-dopant in graphene, using femtosecond pulsed laser irradiation in the UV range. As an all-optical approach, it allows spatial control over doping levels. Combined terahertz (THz) spectroscopy and electrical device measurements reveal that the Fermi level in laser-illuminated graphene can be controllably and reversibly tuned between p- and n-type in a large range (over ∼600 meV from −420 to +180 meV) by readily tuning the peak intensity and the duration of the laser irradiation treatment. Furthermore, we demonstrate that our photochemical approach for doping of graphene allows one to optically write doping structure with spatial control. Given the ease, effectiveness, and simplicity of the method, this photochemical doping mechanism offers a simple, reversible approach to control the steady-state electronic and optical properties of graphene.

The influence of the thermal diffusivity of the substrates on fabrication of metal nanostructures by femtosecond laser irradiation

We demonstrate the fabrication of nanowire gratings by irradiation of femtosecond laser pulses to platinum thin films on various substrates: silicon carbide, aluminum nitride, silicon and fused silica. Scanning electron microscopy showed that many cracks were formed on the nanowire surfaces formed on silicon carbide, aluminum nitride and silicon substrates, while few cracks were formed on a fused silica substrate. Elemental analysis by energy-dispersive X-ray spectroscopy indicated that melting or evaporation of the platinum thin film could hardly occur in the case of the silicon carbide and aluminum nitride substrates. Calculated results by two temperature models revealed that the lattice temperature within the platinum thin films after laser irradiation depends on the thermal diffusivity of the substrates, which could influence the melting phase existence time. From the experimental and the calculated results, melting of the metal thin film could have influenced metal nanowire grating formation.

Periodic surface structures on Ni–Fe film induced by a single femtosecond laser pulse with diffraction rings

This Letter reports the formation of periodic surface structures on Ni–Fe film irradiated by a single femtosecond laser pulse. A concave lens with a focus length of −150 mm is placed in front of an objective (100×, NA=0.9), which transforms the Gaussian laser field into a ring distribution by the Fresnel diffraction. Periodic ripples form on the ablation area after the irradiation of a single femtosecond laser pulse, which depends on the laser polarization and laser fluence. We propose that the ring structure of the laser field leads to a similar transient distribution of the permittivity on the sample surface, which further launches the surface plasmon polaritons. The interaction of the incident laser with surface plasmon polaritons dominates the formation of periodic surface structures.

Microfiber Bragg grating for temperature and strain sensing applications

Fiber Bragg grating is inscribed on microfiber with femtosecond laser pulses irradiation. The microfiber is fabricated by stretching a section of single mode fiber over a flame. Periodic grooves are carved on the microfiber by the laser as have been observed experimentally. The microfiber Bragg grating is demonstrated for temperature and strain sensing, and the strain sensitivity is improved with decreased diameters of the microfibers.

Field emission study from an array of hierarchical micro protrusions on stainless steel surface generated by femtosecond pulsed laser irradiation

This paper reports our results on femtosecond (fs) pulsed laser induced surface micro/nano structuring of stainless steel 304 (SS 304) samples and their characterization in terms of surface morphology, formed material phases on laser irradiation and field emission studies. Our investigations reveal that nearly uniform and dense array of hierarchical micro-protrusions (density: ∼5.6×105 protrusions/cm2) is formed upon laser treatment. Typical tip diameters of the generated protrusions are in the range of 2–5μm and these protrusions are covered with submicron sized features. Grazing incidence X-ray diffraction (GIXRD) analysis of the laser irradiated sample surface has shown formation mainly of iron oxides and cementite (Fe3C) phases in the treated region. These laser micro-structured samples have shown good field emission properties such as low turn on field (∼4.1V/μm), high macroscopic field enhancement factor (1830) and stable field emission current under ultra high vacuum conditions.

Study of textured ZnS irradiated by femtosecond laser pulses

A micro-nanostructured Zinc Sulphide (ZnS) surface has been obtained by the irradiation of femtosecond laser pulses on polycrystalline CVD (Chemical Vapor Deposition) ZnS materials. The evolution of the ZnS microstructure has been clearly observed with an increasing number of deposition pulses. The distributed state of micro-nanostructures on the ZnS surface has been directly linked to the Gaussian beam distribution of the laser energy. Moreover, a uniform textured surface with large-area nanograting patterns has been obtained by a high-speed line-scanning technique using an appropriate laser fluence. The optical properties of the surface-textured ZnS in a wide spectral region have been measured by Fourier transform infrared and ultraviolet and visible transmission spectroscopy. When compared with the ZnS base, the transmittance of the textured ZnS surface covered with nanogratings decreases at infrared wavelengths.

Chemical surface modification of graphene oxide by femtosecond laser pulse irradiation in aqueous suspensions

Reduction of graphene oxide (GO) by femtosecond laser pulse irradiation of an aqueous suspension was studied. Different laser parameters such as laser fluence and irradiation time were scanned to obtain the optimum reduced graphene oxide (rGO) with fewer defect sites and lower electrical resistivity. The fabricated rGO samples were characterized using several techniques such as X-ray diffraction, UV–Visible absorption spectrometry, Raman spectroscopy, X-ray photoelectron spectroscopy, and others. The XRD profiles of rGO revealed that the interplanar spacing between carbon layers significantly decreased to 3.51 A, which is close to that of pristine graphite. Furthermore, the intensity ratio of D and G bands of rGO measured by Raman spectroscopy was more than 20 % smaller than that of GO, indicating the enhancement of sp2 domains. It is noted that the defect sites and the disorder carbon double bond networks on the basal graphene plane were relatively decreased after reduction. In addition, the electrical resistivity of rGO significantly decreased to 3.3 Ω·cm under the optimum condition. From these results, femtosecond laser can be used as a suitable tool for GO reduction because it is a simple, controllable, and flexible method for getting highly reduced graphene oxide.

Laser-triggered release of encapsulated molecules from polylactic-co-glycolic acid microcapsules.

The controlled release of encapsulated molecules from a microcapsule is a promising method of targeted drug delivery. Laser-triggered methods for the release of encapsulated molecules have the advantage of spatial and temporal controllability. In this study, we demonstrated the release of encapsulated molecules from biodegradable polymer-based microcapsules using near-infrared femtosecond laser pulses. The polylactic-co-glycolic acid microcapsules encapsulating fluorescein isothiocyanate-dextran molecules were fabricated using a dual-coaxial nozzle system. Irradiation of femtosecond laser pulses enhanced the release of the molecules from the microcapsules, which was accompanied by a decrease in the residual ratio of the microcapsules. The laser-induced modification of the surface of the shell of the microcapsules indicated the potential for sustained release as well as burst release.

Crystallization of amorphous hydrogenated silicon (a-Si:H) films under irradiation with femtosecond laser pulses

Crystallization of thin films of amorphous hydrogenated silicon under the irradiation of femtosecond laser pulses has been studied. It was found that the crystallization has a clearly pronounced threshold nature and depends on the laser emission wavelength. As shown the best results are achieved in crystallization at the laser wavelength range of 740–760 nm.

Enhanced photoacoustics from gold nano-colloidal suspensions under femtosecond laser excitation.

Enhanced photoacoustic (PA) intensity from gold nanosphere and nanorod colloidal suspensions in water under tightly-focused femtosecond pulsed laser irradiation was systematically investigated. PA signal amplitudes were measured by ultrasound transducers at frequencies of 5, 10, and 25 MHz. The experimental results revealed a linear-dependence of the relative photoacoustic amplitude on the laser power and the mechanism was attributed to non-radiative relaxation dynamics of surface plasmon oscillations. When gold nanorod with longitudinal absorption/extinction peak at 800 nm coincides with the wavelength of femtosecond laser pulses, the most efficient PA signal is generated. Laser excitation was kept within a thermal stability region of gold nanoparticles, i.e., colloidal suspension can be continuously reused for PA generation.

Influence of femtosecond pulsed laser irradiation on bismuth germanium oxide single crystal properties

High quality bismuth germanium oxide single crystals were irradiated by a femtosecond pulsed laser beam of increasing power. Analyses performed on irradiated and unirradiated samples showed significant changes in transmittance, transmission spectra, sample color, Raman spectra, X-ray diffraction (XRD) pattern, Verdet constant, magneto-optical property, and absorption coefficient. After irradiation, the transmission spectra values increased whereas anisotropy detected in the transmission spectra of unirradiated samples disappeared. The change of color caused by irradiation was noticeable to the naked eye. The XRD measurements confirmed structural changes induced by laser irradiation, i.e., the laser-beam-incident side of the sample became almost amorphous, whereas the side opposite to the incident can be indexed to the Bi12GeO20 compound. Irradiation caused increase of Raman spectra peaks with the exception of crystal peaks of type E, which disappeared. The femtosecond pulsed laser irradiation can be used to improve bismuth germanium oxide single crystal optical properties.

Crystallization of Ge2Sb2Te5 thin films by nano- and femtosecond single laser pulse irradiation.

The amorphous to crystalline phase transformation of Ge2Sb2Te5 (GST) films by UV nanosecond (ns) and femtosecond (fs) single laser pulse irradiation at the same wavelength is compared. Detailed structural information about the phase transformation is collected by x-ray diffraction and high resolution transmission electron microscopy (TEM). The threshold fluences to induce crystallization are determined for both pulse lengths. A large difference between ns and fs pulse irradiation was found regarding the grain size distribution and morphology of the crystallized films. For fs single pulse irradiated GST thin films, columnar grains with a diameter of 20 to 60 nm were obtained as evidenced by cross-sectional TEM analysis. The local atomic arrangement was investigated by high-resolution Cs-corrected scanning TEM. Neither tetrahedral nor off-octahedral positions of Ge-atoms could be observed in the largely defect-free grains. A high optical reflectivity contrast (~25%) between amorphous and completely crystallized GST films was achieved by fs laser irradiation induced at fluences between 13 and 16 mJ/cm2 and by ns laser irradiation induced at fluences between 67 and 130 mJ/cm2. Finally, the fluence dependent increase of the reflectivity is discussed in terms of each photon involved into the crystallization process for ns and fs pulses, respectively.

Immobilized Multifunctional Polymersomes on Solid Surfaces: Infrared Light-Induced Selective Photochemical Reactions, pH Responsive Behavior, and Probing Mechanical Properties under Liquid Phase.

Fixing polymersomes onto surfaces is in high demand not only for the characterization with advanced microscopy techniques but also for designing specific compartments in microsystem devices in the scope of nanobiotechnology. For this purpose, this study reports the immobilization of multifunctional, responsive, and photo-cross-linked polymersomes on solid substrates by utilizing strong adamantane-β-cyclodextrin host-guest interactions. To reduce nonspecific binding and retain better spherical shape, the level of attractive forces acting on the immobilized polymersomes was tuned through poly(ethylene glycol) passivation as well as decreased β-cyclodextrin content on the corresponding substrates. One significant feature of this system is the pH responsivity of the polymersomes which has been demonstrated by swelling of the immobilized vesicles at acidic condition through in situ AFM measurements. Also, light responsivity has been provided by introducing nitroveratryloxycarbonyl (NVOC) protected amine molecules as photocleavable groups to the polymersome surface before immobilization. The subsequent low-energy femtosecond pulsed laser irradiation resulted in the cleavage of NVOC groups on immobilized polymersomes which in turn led to free amino groups as an additional functionality. The freed amines were further conjugated with a fluorescent dye having an activated ester that illustrates the concept of bio/chemo recognition for a potential binding of biological compounds. In addition to the responsive nature, the mechanical stability of the analyzed polymersomes was supported by computing Young's modulus and bending modulus of the membrane through force curves obtained by atomic force microscopy measurements. Overall, polymersomes with a robust and pH-swellable membrane combined with effective light responsive behavior are promising tools to design smart and stable compartments on surfaces for the development of microsystem devices such as chemo/biosensors.

Structural Changes Induced in Grapevine (Vitis vinifera L.) DNA by Femtosecond IR Laser Pulses: A Surface-Enhanced Raman Spectroscopic Study.

In this work, surface-enhanced Raman spectra of ten genomic DNAs extracted from leaf tissues of different grapevine (Vitis vinifera L.) varieties, respectively, are analyzed in the wavenumber range 300–1800 cm−1. Furthermore, structural changes induced in grapevine genomic nucleic acids upon femtosecond (170 fs) infrared (IR) laser pulse irradiation (λ = 1100 nm) are discussed in detail for seven genomic DNAs, respectively. Surface-enhanced Raman spectroscopy (SERS) signatures, vibrational band assignments and structural characterization of genomic DNAs are reported for each case. As a general observation, the wavenumber range between 1500 and 1660 cm−1 of the spectra seems to be modified upon laser treatment. This finding could reflect changes in the base-stacking interactions in DNA. Spectral shifts are mainly attributed to purines (dA, dG) and deoxyribose. Pyrimidine residues seem to be less affected by IR femtosecond laser pulse irradiation. Furthermore, changes in the conformational properties of nucleic acid segments are observed after laser treatment. We have found that DNA isolated from Feteasca Neagra grapevine leaf tissues is the most structurally-responsive system to the femtosecond IR laser irradiation process. In addition, using unbiased computational resources by means of principal component analysis (PCA), eight different grapevine varieties were discriminated.

First-principles simulations for excitation of currents in linear carbon chains under femtosecond laser pulse irradiation

We discuss the response of linear carbon chains to a femtosecond laser pulse. The influence of the chain structure on the excited currents was taken into account. The simulation results showed that excited currents were reversible when laser intensity was relatively weak. An unexpected decrease of current amplitude was found for a laser pulse with a wavelength of 200 nm, which may indicate the transient break of pi bonds in linear carbon chains. For chains with more than 10 atoms, the excited current oscillated with increasing amplitude during simulation time, which was due to the formation of plasma-like resonance.

Induction of neuronal activation by femtosecond-pulsed laser irradiation and its potential application for amyloid-β-induced toxicity assessment.

Manipulating neural activity is crucial for studying the neural connectivity and the pathophysiology of neurodegenerative disease. Among various techniques for neural activation, direct optical stimulation method with femtosecond-pulsed laser is simple and can be specifically applied on a single neuron. Brief irradiation of femtosecond laser pulses on a neuron elevates intracellular calcium, and it propagates to adjacent neurons. However, the mechanisms of laser-induced neural activation are still unclear. In this report, we have elucidated the mechanism of laser-induced neural activation which could be mediated by superoxide, specifically blocked by diphenyleneiodonium chloride, and depletion in intracellular calcium storage. Furthermore, we also showed that the propagation of calcium initiated by laser stimulation is dependent on the presence of extracellular calcium as well as electrical and chemical synapses. We verified the applicability of such mechanism for the assessment of neuronal functionality, by measuring calcium elevation, intracellular calcium propagation, ROS increase, and performing cell death assay in vehicle and Aβ-treated neurons. This work suggests promising applications of the potential for implementing such laser-induced neural activation for rapid and reliable drug screening.

Sub-micron magnetic patterns and local variations of adhesion force induced in non-ferromagnetic amorphous steel by femtosecond pulsed laser irradiation

Periodic ripple and nanoripple patterns are formed at the surface of amorphous steel after femtosecond pulsed laser irradiation (FSPLI). Formation of such ripples is accompanied with the emergence of a surface ferromagnetic behavior which is not initially present in the non-irradiated amorphous steel. The occurrence of ferromagnetic properties is associated with the laser-induced devitrification of the glassy structure to form ferromagnetic (α-Fe and Fe3C) and ferrimagnetic [(Fe,Mn)3O4 and Fe2CrO4] phases located in the ripples. The generation of magnetic structures by FSPLI turns out to be one of the fastest ways to induce magnetic patterning without the need of any shadow mask. Furthermore, local variations of the adhesion force, wettability and nanomechanical properties are also observed and compared to those of the as-cast amorphous alloy. These effects are of interest for applications (e.g., biological, magnetic recording, etc.) where both ferromagnetism and tribological/adhesion properties act synergistically to optimize material performance.

Effects of optical absorbance with ablation characteristics in femtosecond laser irradiation of carbon reinforced Al2O3 composites

The thresholds and depths of ablation of carbon [carbon nanotubes (CNTs) or graphene nanoplatelets (GNPs)] reinforced Al2O3 composites were compared and correlated with those of monolithic Al2O3 using femtosecond pulsed laser irradiation (λ = 1027 nm, τP = 380 fs). These composites were processed using a spark plasma sintering method (Ts = 1500°C, τs = 10 min and P = 40 MPa) through which highly densified (more than 97%) composites were fabricated. The optical absorbance of each composite improved in the wavelength range from ultraviolet to near infrared. Compared with the corresponding values for monolithic Al2O3, the ablation thresholds of CNT/Al2O3 and GNP/Al2O3 were 50 and 75% lower respectively, and their ablation depths were two to three times deeper.

Tribological performance of sub-100-nm femtosecond laser-induced periodic surface structures on titanium

Sub-100-nm laser-induced periodic surface structures (LIPSS) were processed on bulk titanium (Ti) surfaces by femtosecond laser pulse irradiation in air (30fs pulse duration, 790nm wavelength). The laser peak fluence, the spatial spot overlap, and the number of overscans were optimized in a sample-scanning geometry in order to obtain large surface areas (5mm×5mm) covered homogeneously by the LIPSS. The laser-processed regions were characterized by optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM). The friction coefficient of the nanostructured surfaces was tested during 1000 cycles under reciprocal sliding conditions (1Hz, 1.0N normal load) against a 10-mm diameter ball of hardened 100Cr6 steel, both in paraffin oil and in engine oil used as lubricants. Subsequently, the corresponding wear tracks were qualified by OM, SEM, and energy dispersive X-ray analyses (EDX). The results of the tribological tests are discussed and compared to that obtained for near wavelength-sized fs-LIPSS, processed under somewhat different irradiation conditions. Some constraints for a beneficial effect of LIPSS on the tribological performance are provided.