Femtosecond Laser Pulse Irradiation Research Articles

Confocal Raman imaging of femtosecond laser induced microstructures in germanate glasses

Localized microstructures were induced in a germanate glass by irradiation with a focused 800 nm, 250 kHz femtosecond laser pulses. Compared with unirradiated glass, an increase in intensity and a frequency shift of the Raman scattering bands have been observed in a ring-shaped region inside the laser-modified structure. After annealing the sample near the glass transition temperature, the ring-shaped contrast due to enhanced Raman scattering faded away. We propose that the increase in Raman scattering band intensity may be due to local compression of the glass induced by the femtosecond laser pulses irradiation.

Pulsed-laser creation and characterization of giant plasma membrane vesicles from cells

Femtosecond-pulsed laser irradiation was found to initiate giant plasma membrane vesicle (GPMV) formation on individual cells. Laser-induced GPMV formation resulted from intracellular cavitation and did not require the addition of chemical stressors to the cellular environment. The viscosity, structure, and contents of laser-induced GPMVs were measured with fluorescence microscopy and single-particle tracking. These GPMVs exhibit the following properties: (1) GPMVs grow fastest immediately after laser irradiation; (2) GPMVs contain barriers to free diffusion of incorporated fluorescent beads; (3) materials from both the cytoplasm and surrounding media flow into the growing GPMVs; (4) the GPMVs are surrounded by phospholipids, including phosphatidylserine; (5) F-actin is incorporated into the vesicles; and (6) caspase activity is not essential for GPMV formation. The effective viscosity of 65 nm polystyrene nanoparticles within GPMVs ranged from 32 to 434 cP. The nanoparticle diffusion was commonly affected by relatively large, macromolecular structures within the bleb.

Fabrication of platinum particles by intense, femtosecond laser pulse irradiation of aqueous solution

Platinum nano-particles were directly fabricated through molecular dissociation process induced by tightly focused femtosecond laser pluses in hydrogen hexachloroplatinate (IV) hexahydrate (H 2PtCl 6·6H 2O) aqueous solution. After irradiation, it was found that UV–visible absorption spectrum of the solution was changed and a large number of small platinum particles were synthesized. The variation of absorbance in the spectrum and the yield the particle synthesis depended on the concentration of the solution. The most effective condition for particle synthesis was 0.1 wt%. Black precipitates of platinum particles were observed in the bottom of the vessel after irradiation in some cases. The dispersibility of the particles was greatly improved by the addition of polyvinylpyrrolidone (PVP) as a dispersant, and no precipitates were found in the solution with 1.0 × 10 −2 wt% PVP. Consequently, highly monodispersed platinum nano-particles with the mean size of about 3 nm were successfully fabricated.

Single-mode grating reflection in all-solid photonic bandgap fibers inscribed by use of femtosecond laser pulse irradiation through a phase mask

Single-mode Bragg grating reflection has been achieved in all-solid photonic bandgap fibers by use of femtosecond laser pulse irradiation through a phase mask. The grating created is confined to the all-silica fiber core region and exhibits higher thermal stability than other type I femtosecond-pulse-induced gratings.

Ultrathin amorphization of single-crystal silicon by ultraviolet femtosecond laser pulse irradiation

The mechanisms of amorphization for crystalline Si (c-Si) induced by ultraviolet femtosecond laser irradiation are described in this paper. The wavelength of the laser pulse was 267 nm, which is the third harmonics of a Ti:sapphire laser. We performed a laser scanning microscopy and a transmission electron microscopy for surface and structural analysis and imaging pump-probe measurements to investigate the dynamics of the process. From the analyses, we confirmed that the thickness of the amorphized layer was quite uniform and there is no lattice defect under the amorphized section. The thickness of the amorphous Si (a-Si) layer was 7 nm and the threshold fluence of the amorphization was 44 mJ/cm2. From the Imaging Pump-Probe measurement it was revealed that the melting time is less than 1 ns and ultra high speed melting and re-solidification process was occurred. The melting depth estimated by the Imaging Pump-Probe measurement was 7 nm. The melted portion completely corresponded to the amorphized section.

Modeling energy dependence of the inner-shell x-ray emission produced by femtosecond-pulse laser irradiation of xenon clusters

We employ the Los Alamos suite of atomic physics codes to model the inner-shell x-ray emission spectrum of xenon and compare results with those obtained via high-resolution x-ray spectroscopy of xenon clusters irradiated by 30fs Ti:Sapphire laser pulses. We find that the commonly employed configuration-average approximation breaks down and significant spin-orbit splitting necessitates a detailed level accounting. We reproduce an interesting spectral trend for a series of experimental spectra taken with varying pulse energy for fixed pulse duration. To simulate the experimental measurements at increasing beam energies, we find that spectral modeling requires an increased hot electron fraction, but decreased atomic density and bulk electron temperature. We believe these latter conditions to be a result of partial cluster destruction due to the increased energy in the laser prepulse.

Modeling of nonlinear responses in BK7 glass under irradiation of femtosecond laser pulses

In this paper, the nonlinear response in BK7 glass sample is investigated under irradiation of 200 fs laser pulses at 800 nm wavelength by using of transmission measurement method. Nonlinear transmission is observed for incident pulse energies above 37μJ. A theoretical model based on simultaneously absorption of one, two and three-photon absorption is presented. The contribution of free-carriers absorption is also included on the model. The differential equation is solved analytically. The experimental results and predictions of theoretical model are in good agreement. The values of three-photon absorption coefficient and free-carriers absorption cross-section are reported.

Ultrashort Pulse Manipulation of ZnO Nanowire Growth

A procedure of femtosecond pulse laser irradiation was incorporated into the synthesis of zinc oxide (ZnO) nanowires in aqueous solutions to investigate the photo-initiated heterogeneous nucleation induced by the irradiation and the associated nanowire growth. Elongated ZnO nanowires with smooth planes and end tips were successfully grown following the irradiation process and subsequent hydrothermal treatments in a catalyst-free environment, compared to aggregated flower-like nanostructures with porous and rough surfaces, grown from homogeneous nucleation without laser irradiation. Studies using femtosecond laser systems at 1 kHz and 250 kHz repetition rates show that the pulse energy is critical in the heterogeneous nucleation process for the growth of ZnO nanowires. A minimum threshold pulse energy, 200 microJ/pulse for the 1 kHz system and 2.4 microJ/pulse for 250 kHz, is observed beyond which well-defined and individually separated nanowires were grown. Thermal effect caused by the 250 kHz repetition rate provides a counter-balance to the low pulse energy required for the growth process. XRD analysis of the nanowires reveals a hexagonal structure while photoluminescence shows emission at about 385 nm. The overall results show that the pulse energy is critical for heterogeneous nucleation while the irradiation duration affects the density of nucleation, which together with the hydrothermal treatment temperature influence the growth rate and thus the morphology of the nanowires.

Study of spectral and annealing properties of fiber Bragg gratings written in H_2-free and H_2- loaded fibers by use of femtosecond laser pulses

The spectral and annealing properties of a series of fiber Bragg gratings (FBGs) written in both H(2)-loaded and H(2)-free fibers by use of 800nm femtosecond laser pulse irradiation and created through a phase mask, have been investigated. It is found that type II FBGs inscribed in H(2)- loaded fibers exhibit superior spectral quality when compared with those written in H(2)-free fibers. Isochronal annealing tests shows that type II FBGs written in H(2)-free fibers have the highest thermal stability, followed (in order of stability) by H(2)-loaded type II, H(2)-free type I and then H(2)-loaded type I FBGs. The thermal stability of the H(2)-loaded type II FBGs can effectively be increased by using a high temperature pre-annealing treatment. After the treatment, type II FBGs written into both H(2)-free and H(2)-loaded fibers can sustain long-term annealing (for more than 12 hours) at temperatures of more than 1000 masculineC while their high reflectivities can still be maintained. This demonstrates the real potential of the FBGs developed and investigated in this work to be used as the ideal sensing elements for a series of high temperature measurement applications.

Temperature distribution and modification mechanism inside glass with heat accumulation during 250kHz irradiation of femtosecond laser pulses

Heat accumulation by high repetition rate femtosecond laser irradiation inside glass generates a much larger modification than that by a single pulse. In this study, we determined the temperature distribution due to heat accumulation and the characteristic temperature for heat modification inside a soda lime glass by analyzing the relationship between the radius of modification and glass temperature. The validity of the analysis was confirmed by reproducing the modification due to two-beam irradiation. The determined characteristic temperature suggested that the temperature distribution and the spatial dependence of the stress relaxation are important in the mechanism of heat modification.

Synthesis and two-photon properties of a multipolar chromophore containing indenofluorenyl units

A new multipolar fluorophore derived from triphenylamine as the core with diphenylaminoindenofluorenyl moieties incorporated at the peripheral positions has been synthesized and experimentally shown to possess strong two-photon absorptivities in near-IR region and intense upconverted visible emission under the irradiation of femtosecond laser pulses.

New Two-Photon Activated Photodynamic Therapy Sensitizers Induce Xenograft Tumor Regressions after Near-IR Laser Treatment through the Body of the Host Mouse

The aim of this study was to show that novel photodynamic therapy (PDT) sensitizers can be activated by two-photon absorption in the near-IR region of the spectrum and to show, for the first time, that such activation can lead to tumor regressions at significant tissue depth. These experiments also evaluated effects of high-energy femtosecond pulsed laser irradiation on normal tissues and characterized the response of xenograft tumors to our PDT protocols. Human small cell lung cancer (NCI-H69), non-small cell lung cancer (A549), and breast cancer (MDA-MB-231) xenografts were induced in SCID mice. Irradiation of sensitized tumors was undertaken through the bodies of tumor-bearing mice to give a treatment depth of 2 cm. Posttreatment tumor regressions and histopathology were carried out to determine the nature of the response to these new PDT agents. Microarray expression profiles were conducted to assess the similarity of responses to single and two-photon activated PDT. Regressions of all tumor types tested were seen. Histopathology was consistent with known PDT effects, and no, or minimal, changes were noted in irradiated normal tissues. Cluster analysis of microarray expression profiling showed reproducible changes in transcripts associated with apoptosis, stress, oxygen transport, and gene regulation. These new PDT sensitizers can be used at a depth of 2 cm to produce excellent xenograft regressions. The tumor response was consistent with known responses to single-photon activated PDT. Experiments in larger animals are warranted to determine the maximal achievable depth of treatment.

Sparse-exposure technique in holographic two-photon polymerization

Holographic two-photon polymerization is based on a high-speed, low-loss parallel laser irradiation technique inside photosensitive materials using a computer-generated hologram displayed on a liquid crystal spatial light modulator. We demonstrated a sparse exposure technique combining parallel exposure and scanning exposure to improve the fabrication throughput and to achieve simultaneous fabrication of linear structures with different widths. We also demonstrated fabrication of space-variant structures by changing a CGH, as well as parallel fabrication of voxel structures with single femtosecond laser pulse irradiation.

Three-Dimensional Residue-Free Volume Removal inside Sapphire by High-Temperature Etching after Irradiation of Femtosecond Laser Pulses

We applied the femtosecond laser-assisted etching technique, that is, irradiation of focused femtosecond laser pulses followed by selective chemical etching, to volume removal inside sapphire. At room temperature, volume etching only slightly advanced while residue remained inside the volume. By increasing the etching temperature, complete volume etching without residue was achieved. Complete etching was, however, accompanied by undesirable phenomena of surface pits or cracks, which are expected to be excluded through further improvement of processing.

Evaluation of femtosecond laser pulse irradiation of ancient parchment

In this work, femtosecond (fs) laser irradiation of ancient parchment is evaluated as a novel cleaning technique that, in comparison with nanosecond (ns) irradiation, could considerably reduce the thermal and chemical damage to the substrate. To investigate the interaction of ultrashort laser pulses with parchment, two historical specimens were exposed to Ti:Sapphire laser using the fundamental and second harmonic emission (795 and 398 nm, 120 fs pulses). The ablation threshold fluences were determined resulting in values of 1.1 and 0.15 J/cm 2 at 795 and 398 nm respectively. A study of the effects of the fs laser–parchment interaction as a function of fluence and wavelength was carried out and the results were compared with those obtained for Nd:YAG laser irradiation (532 and 355 nm, pulse duration 6 ns). FT-Raman spectra show the disappearance of the amorphous carbon bands upon fs treatment, indicative of the removal of the carbonaceous contamination of the parchment. At the same time the parchment structural bands remain unaffected. Spectrofluorimetric measurements reveal an enhancement of photodegradation products in the substrate. Results are discussed in terms of the comparison between expected mechanisms operating in the fs and ns irradiation regimes.

Ion exchange in glass using femtosecond laser irradiation

We explain the occurrence of ion exchange and an index profile around the focal point inside a commercial crown glass formed by femtosecond laser irradiation. The index profile in the photoinduced area has a ring-shaped pattern, which indicates that local densification occurred in the glass. An irregular surface reflecting the density distribution is formed around the focal point by dry etching process using a focused ion beam. By the irradiation of femtosecond laser pulses, the effect of ion exchange between the focal point and the surrounding area is also observed in the area in which local densification occurred.

Multilayer optical memory using femtosecond-laser induced fluorescence in rare-earth ion doped glass

We report three-dimensional fluorescent memory by recording optical bits with irradiation of femtosecond laser pulses at 800 nm and by reading photoluminescence change in Eu3+ doped glass. We produced multi-layered micro-bit patterns and read the blue emission from the 405 and 325 nm excitations due to permanent reduction of Eu3+ to Eu2+ in sodium borate glass by scanning the irradiated region in multilayers.

Site-selective formation of Si nanocrystal in SiO2 by femtosecond laser irradiation and Al deoxidization effects

We have developed a robust method for fabricating Si nanoregions in silica glass using femtosecond laser processing. We attained a vivid formation of silicon-rich nanoregions site-selectively generated in SiO2 by irradiation of femtosecond laser pulses to the interface of a SiO2 substrate and deposited aluminum (Al) thin film, where the Al element acts as a gettering site for O atoms. Growth of high-density Si nanocrystals and amorphous Si was observed by transmission electron microscopy in the region that was multiply irradiated with the femtosecond laser. Furthermore, local annealing with a cw laser enhances the Si nanocrystal growth, which was determined by micro-Raman measurements.

Femtosecond laser irradiation of metal and thermal oxide layers on silicon: studies utilising cross-sectional transmission electron microscopy

We present the results of 800 and 400 nm wavelength, femtosecond laser pulse irradiation of a sample consisting of a metal film on thermally-grown oxide on silicon. On selected sites, cross-sectional transmission electron microscopy was performed to provide information on sub-surface changes not observable with surface scanning electron microscopy. A range of pulse energies in single-pulse irradiation exists for which the metal film was removed but the oxide was not appreciably thinned. For a sufficiently high pulse energy within this range, substantial defects were observed in the underlying silicon. Five infrared pulses of a relatively high fluence created significant defects, as well as producing polycrystalline material on top of the original oxide and metal. We discuss various factors which may play a role in the formation of the observed features.

Amorphization dynamics of Ge2Sb2Te5 films upon nano- and femtosecond laser pulse irradiation

Phase transformations of crystalline Ge2Sb2Te5 films upon pulsed laser irradiation have been studied using in situ reflectivity measurements with temporal resolution. Two different configurations allowed point probing with nanosecond temporal resolution and imaging with subpicosecond temporal and micrometer spatial resolution. The role of the pulse duration and laser fluence on the dynamics of the phase change and the degree of amorphization is discussed. Several advantageous features of femtosecond compared to nanosecond laser-induced amorphization are identified. Moreover, a high-resolution study of the amorphization dynamics reveals the onset of amorphization at moderate fluences to occur within ∼100 ps after arrival of the laser pulse. At high fluences, amorphization occurs after ∼430 ps and the molten phase is characterized by an anomalously low reflectivity value, indicative of a state of extreme supercooling.