Femtosecond Laser Pulse Irradiation Research Articles

Precipitation of silver particles by femtosecond laser pulses inside silver ion doped glass

We report on photosensitivities in silver ion doped glasses. We succeeded in precipitation control of silver particles without heat treatment by the irradiation of femtosecond laser pulses. The silver particles can be precipitated in a given location. Therefore it is possible to process a three-dimensional structure. This result should be applicable to functional optical devices.

Ion generation in a low-density plastic foam by interaction with intense femtosecond laser pulses.

Energetic proton generation in low-density plastic (C5H10) foam by intense femtosecond laser pulse irradiation has been studied experimentally and numerically. Plastic foam was successfully produced by a sol-gel method, achieving an average density of 10 mg/cm(3). The foam target was irradiated by 100 fs pulses of a laser intensity 1 x 10(18) W/cm(2). A plateau structure extending up to 200 keV was observed in the energy distribution of protons generated from the foam target, with the plateau shape well explained by Coulomb explosion of lamella in the foam. The laser-foam interaction and ion generation were studied qualitatively by two-dimensional particle-in-cell simulations, which indicated that energetic protons are mainly generated by the Coulomb explosion. From the results, the efficiency of energetic ion generation in a low-density foam target by Coulomb explosion is expected to be higher than in a gas-cluster target.

Color center production by femtosecond pulse laser irradiation in LiF crystals

We report the production of color centers in LiF single crystals by ultrashort high intensity laser pulses (60fs, 10 GW). An intensity threshold for color centers creation of 2 TW/cm(2) was determined, which is slightly smaller than the continuum generation threshold. We could identify a large amount of F centers that gave rise to aggregates such as F(2), F(2) (+) and F(3) (+). The proposed mechanism of formation is based on multiphoton excitation that also produce short lived F(2) (+) centers. It is also shown that it is possible to write tracks in the LiF crystals with dimensional control.

Periodic Bell-shaped Upheaval Structure on Surface of Polycarbonate by Irradiation of Femtosecond Laser Pulse

A periodic bell-shaped upheaval structure was formed on the surface of polycarbonate (PC) by line irradiation of a near-infrared (NIR) femtosecond laser pulse onto PC bulks. The upheaval structures were controlled by the irradiation depth and irradiation energy in PC. For example, the increase in irradiation energy at a constant target depth (50 µm) induced the discontinuous change in structures: a small periodic bell-shaped upheaval structure, an intermediate continuous band-shaped structure without a clear upheaval structure and a final large periodic bell-shaped upheaval structure. The size of the bell-shaped upheaval structure varied from approximately 15 µm to 45 µm in diameter of a basal plane upon changing the irradiation conditions. Using the cross-sectional images of the irradiated PC bulk samples observed using an optical microscope, a scanning electron microscope (SEM) and a differential interference laser scanning microscope, we confirmed the position of cracks and voids, and the structure with refractive index modulation formed in PC bulk. The mechanism of the formation of the upheaval structures in PC was investigated by changing the irradiation conditions. We proposed a mechanism of the formation of the upheaval structure in PC on the basis of multiphoton absorption, anisotropic thermal gradient, growth of microcracks by microexplosion at the focused point and movement of the grown cracks toward the surface along the anisotropic thermal gradient.

Femtosecond electron diffraction for direct measurement of ultrafast atomic motions

We have developed a femtosecond electron diffraction system capable of directly measuring the complete transient structures with atomic level detail and on 400-fs time scale in solid materials. Additionally, a diffraction image with significant signal-to-noise ratio to reveal the long-range order can be obtained with a single electron pulse of 700 fs in duration. A direct observation of ultrafast lattice expansion following the irradiation of femtosecond pulsed laser of Ag film has been demonstrated.

Characteristics of superconducting flux flow transistors under irradiation of femtosecond laser pulses

We have fabricated superconducting flux flow transistors with YBCO thin films and investigated the vortex flow characteristics under the irradiation of femtosecond laser pulses. The vortex flow velocity was calculated by the magnetic field dependence of the flow voltage. The obtained velocity without laser irradiation is 0.08–1.37 × 10 4 m/s for the bias current I B=0.4–0.8 mA. The flow voltage increased gradually when the irradiated laser power was up to 10 mW, however, it increased drastically when the laser power was more than 10 mW. The maximum velocity of 6.3 × 10 4 m/s was obtained at the laser power of 15 mW. The result indicates that the laser pulses excite the vortices and accelerate the vortex velocity.

Structures induced in polysilane and thin polysilane layer coated polymer films by irradiation of femto-second laser pulse

We investigated the structures induced in polysilane and polysilane coated thermoplastic bis-phenol A type epoxy (Epoxy) and poly(methyl methacrylate) (PMMA) films by irradiation of near-infrared (NIR) femto-second laser pulse. Long stripe-shaped structures with approximately 200–400 μm length was induced in polysilane bulk. Thin polysilane layer coated Epoxy and PMMA films showed photo-sensitive effect on the formation of stripe-shaped structures in PMMA and Epoxy, whereas no structures were induced in uncoated PMMA and Epoxy films. An ensemble of stripe-shaped structures worked as a diffraction grating. This photo-sensitive effect has potential application to grating structures in optical polymeric fiber (OPF).

Imaging femtosecond laser-induced electronic excitation in glass

While substantial progress has been achieved in understanding laser ablation on the nanosecond and picosecond time scales, it remains a considerable challenge to elucidate the underlying mechanisms during femtosecond laser material interactions. We present experimental observations of electronic excitation inside a wide band gap glass during single femtosecond laser pulse (100 fs, 800 nm) irradiation. Using a femtosecond time-resolved imaging technique, we measured the evolution of a laser-induced electronic plasma inside the glass and calculated the electron number density to be on the order of 1019 cm−3.

Structure induced by irradiation of femtosecond laser pulse in dyed polymeric materials

AbstractWe investigated the structures induced by an irradiation of a near‐infrared (NIR) femtosecond laser pulse in dye‐doped polymeric materials {poly(methyl methacrylate) (PMMA), thermoplastic epoxy resin (Epoxy), and a block copolymer of methyl methacrylate and ethyl acrylate‐butyl acrylate [p(MMA/EA‐BA) block copolymer]}. Dyes used were classified into two types—type 1 with absorption at 400 nm and type 2 with no absorption at 400 nm. The 400‐nm wavelength corresponds to the two‐photon absorption region by the irradiated NIR laser pulse at 800 nm. Type 1 dye‐doped PMMA and p(MMA/EA‐BA) block copolymer showed a peculiar dye additive effect for the structures induced by the line irradiation of a NIR femtosecond laser pulse. On the contrary, dye‐doped Epoxy did not exhibit a dye additive effect. The different results among PMMA, p(MMA/EA‐BA) block copolymer, and Epoxy matrix polymers are supposed to be related to the difference of electron‐acceptor properties. The mechanism of this type 1 dye‐additive‐effect phenomenon for PMMA and p(MMA/EA‐BA) block copolymer is discussed on the basis of two‐photon absorption of type 1 dye at 400 nm by the irradiation of a femtosecond laser pulse with 800 nm wavelength and the dissipation of the absorbed energy to the polymer matrix among various transition processes. Dyes with a low‐fluorescence quantum yield favored the formation of thicker grating structures. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2800–2806, 2002

Two-Photon Photochromism of an Organic Material for Holographic Recording

We report the two-photon-induced photoisomerization of 3-[1-(1,2-dimethyl-1H-indol-3yl)-ethylidene]-4-isopropylidene-dihydrofuran-2,5-dione (1), a photochromic compound with Imax ) 385 nm, using 775-nm femtosecond pulsed laser irradiation. The resulting photoisomer had Imax ) 582 nm. The kinetic rate constant for the two-photon-induced electrocyclic isomerization reaction was measured at two different intensities (two different powers), showing a quadratic dependence with respect to the pump intensity. Results of pump-probe solution phase experiments and guest/host polymer thin film interferometric imaging studies are reported. A two-photon absorption molecular cross section U2 ) 10.3 10 -48 cm 4 ‚s/photon was measured using Z-scan, further supporting a two-photon-induced isomerization process. Two-photon-induced interferometric recording in a fulgide-containing polymer film was demonstrated.

Femtosecond laser induced crystallization and permanent relief grating structures in amorphous inorganic (In2O3+1 wt % TiO2) films

This letter presents an investigation of crystalline relief grating structures induced by irradiation of near-infrared femtosecond laser pulses on an amorphous inorganic (In2O3+1 wt % TiO2) film. The shapes of crystallized relief structures were sensitive to the scanning rate and the focused point height of irradiation, and the optimized irradiation condition gave cone-shaped cross section structures. Selective wet etching on unirradiated amorphous regions using a 3% hydrochloric acid solution could make sharper relief grating structures of crystalline regions. Diffraction efficiency of the relief grating structures with Au coating was measured, and it was confirmed that first-order diffraction, efficiencies were approximately 40% and 20% for etched and nonetched samples, respectively.

Motion of bubble in solid by femtosecond laser pulses

We report that irradiation of femtosecond laser pulses moves a microscopic bubble inside crystalline calcium fluoride and amorphous silica glass. In situ observation revealed that the bubble moves against the direction of propagation of laser pulses as far as 2 microns. We also demonstrate the lateral movement of a void along the axis perpendicular to the beam propagation axis by shifting the laser focus.

Diffraction Measurement of Grating Structure Induced by Irradiation of Femtosecond Laser Pulse in Acrylate Block Copolymers

Diffraction measurement was carried out for the evaluation of grating structures induced by irradiation of near-infrared (NIR) femtosecond laser pulse in block copolymers of methyl methacrylate and (ethyl acrylate and butyl acrylate) {p-(MMA/EA-BA) block copolymer}. Line irradiation, performed by scanning these copolymer bulks with a laser spot, produced the aggregated deposits of sub-micron scale. Stripe-shaped grating structures of approximately 3–5 µm width and approximately 30–220 µm thickness were induced at an interval of 7 to 15 µm. The total number of gratings was between 10 and 50. The grating structure obtained was evaluated by means of diffraction measurement. Raman–Nath type and Bragg type transmission diffraction patterns with diffraction efficiencies of approximately 1–13% and Q values of approximately 0.1–9 were observed depending on the grating interval and thickness. Refractive index changes evaluated using the Bessel function were approximately 0.20-3.0×10-3. A Bragg type diffraction pattern was observed in the sample with a Q value of approximately 9 and a refractive index change of 0.30×10-3 was obtained by optimization using the Robust design (Taguchi Method). Relative grating intensity linearly increased with increasing photo-crosslinker content in the acrylate block copolymer. Multiple scanning of line irradiation increased the refractive index from 0.30×10-3 for a single scan to 1.1×10-3 for three scans.

Ultrafast dynamics induced by femtosecond laser pulse irradiation of atom-adsorbed metal surfaces

We present a theory of time-resolved two-photon photoemission (time-resolved 2PPE, TR2PPE) spectroscopy on metal surfaces on which an atom (adparticle) is adsorbed, and study the effect of the adparticle motion induced by femtosecond laser pulse irradiation on the TR2PPE spectra. The TR2PPE spectrum is calculated by nonequilibrium perturbation theory treating the adparticle motion within the trajectory approximation. First the laser pulse irradiation causes excitation of an electron from a bulk state into a localized state on the adparticle, which results in the change of the potential energy curve for the adparticle motion, then the adparticle begins to move. Numerical results show that the effect of the adparticle motion is manifested in the motion of the peak position in the 2PPE energy spectrum as a function of the pump–probe delay time. The motion of the peak position in the long delay time region mainly provides information on the potential energy (kinetic energy of the adparticle) whereas the dynamical change of the energy level of the localized electron state due to the adparticle motion can modify the peak structure.

X-ray induced reduction of rare earth ion doped in Na2O-Al2O3-B2O3 glasses.

5Na2O-10Al2O3-85B2O3 glasses doped with 0.05 mol% Sm2O3 or 0.05 mol% Eu2O3 turn to orange after irradiation by focussed femtosecond pulsed laser or x-ray irradiation. To know the mechanism of photo-induced coloring, the glasses were studied by Sm or Eu L(III) XANES before and after x-ray or UV irradiation. XANES in both edges showed that a part of trivalent ion converted to divalent ion upon the irradiation of x-ray. These facts reveal that photoreduction of rare earth ion is the mechanism of photo-induced coloring. However no photoreduction upon UV irradiation occurred in both glasses.

Surface microstructuring of aluminum alloy 2024 using femtosecond excimer laser irradiation

Surface microstructuring and modification of aluminum alloy 2024-T3 were investigated using a femtosecond pulse laser irradiation. The surface micrographs of the scanning electron microscopy were characterized as a function of incident laser fluence. The surface features ranging from submicrometer to micrometers were developed through the variation in laser fluence and laser pulse count. Results indicated that surface patterns were uniquely constructed in a controlled use of pulsed laser parameters for the ablation. Two ablation regimes were found in the logarithmic dependence of the ablated depth on laser fluence and two components were attributed to the optical and energy penetration. A thermal dynamic model was adopted to analyze the ablation processes, and the theoretical analysis agreed well with the experimental results.

Morphological Changes of Silver Nanoparticle Distributions in Glass Induced by Ultrashort Laser Pulses

Persistent form modifications of silver nanoparticles in glass and the resulting dichroitic color changes induced by irradiation of a single intense femtosecond laser pulse were studied by means of high-resolution electron microscopy and spatially resolved optical transmission spectroscopy. Electron microscopy reveals a variety of morphological changes of the silver particles, in particular irregularly shaped particles surrounded by halolike structures consisting of very small silver clusters. The spectral analysis suggests that at lower intensities preferably desorption of silver into the glass matrix occurs producing isotropic spectral changes, while at higher intensities anisotropic spectra caused by particle deformation are observed. An analysis of the effects produced by different laser wavelengths allows one to correlate these findings with the variations of particle sizes depending on their penetration depth.

Surface texturing of aluminum alloy 2024-T3 via femto- and nanosecond pulse excimer laser irradiation

Surface texturing effects of aluminum alloy 2024-T3 using femtosecond and nanosecond pulse laser irradiation were studied. The micrographs of the scanning electron microscopy (SEM) were characterized as a function of incident laser fluence. Results indicated that the surface features, ranging from nano to microdimension, can be developed through variation in laser fluence intensities. Two ablation regimes in the logarithmic fluence dependence of the ablated depth for the 500-fs-pulse irradiation were observed. The theoretical analysis for ablation processes is in good agreement with the experimental results.

432 水の蒸発凍結現象に及ぼす極短パルスレーザー照射の影響

In this study, the effect of femtosecond very-short laser irradiation on the evaporation-freezing behavior of water droplet was experimentally examined under abrupt evacuation condition. In order to improve the performance of a novel transport-storage system for cold, which utilizes the evaporation-freezing phenomenon of water, it is desirable to reduce the supercooling of water by adding freezing promoters into the water. However, the evaporation characteristics of water might be affected by the freezing promoters, since some kinds of the promoters may strengthen the hydrogen bondings among water molecules. In this study, the effect of the "strength" of hydrogen bondings on the evaporation-freezing characteristics of water was therefore experimentally examined by using the femtosecond very-short pulse laser irradiation. Wavelength of the irradiated laser was about 780 nm, which corresponds to the third-overtone of an O-H stretching vibration of water, and thus the irradiation must weaken the hydrogen bondings among the water molecules. From the results, it was confirmed that the femtosecond laser irradiation results in increase in cooling rate, reduction of freezing time, and decreasing the freezing temperature. These results suggest that the freezing promoters added into water results in reduction of cooling rate of water due to evaporation, and that the performance of the cold transport-storage system is not always improved by adding such kind of freezing promoters.

Femtosecond Laser-Induced Disorder of the (1×1)-Relaxed GaAs(110) Surface

Surface physical changes (disorder and/or damage) due to femtosecond pulsed laser irradiation are observed in a new regime of extremely low laser fluence ( $<{10}^{\ensuremath{-}3}$ of the single shot damage fluence) and large number of laser shots ( $>{10}^{10}$). No bulk damage was detected. The surface disorder is reversible by thermal annealing at 580 \ifmmode^\circ\else\textdegree\fi{}C. We propose a new mechanism for femtosecond laser-induced surface physical reactions. Disorder is caused by a surface dangling bond hole-induced lattice instability which lowers the barrier to disorder to the order of ${k}_{B}T$. This occurs during high density local fluctuations even though the mean hole density is far lower than that needed for global instability.