Femtosecond Laser Interaction Research Articles

Magnetic nanoparticles produced by femtosecond laser pulse: Magnetic force microscopy study

Magnetic nanoparticles (NPs) were deposited on Si substrates by interaction of femtosecond laser with thin films of Ni and Fe. Magnetic Force Microscopy (MFM) was used to determine the magnetic properties and size distribution of the particles. Ni nanoparticles were relatively uniform in size with an average diameter of about 15 nm and average magnetic moment of 3.4 × 10–20 A m2. Fe nanoparticles were less uniform and particles size ranged from 14 nm to 36 nm in diameter, with an average magnetic moment of 6.5 × 10–20 A m2. Due to high impact energy both Ni and Fe nanoparticles were deformed upon impact resulting in a shape of oblate spheroid. The NPs shape change due to the impact was analyzed thermodynamically and was correlated to the magnetic moment measured for the different NPs type. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of diamond-like carbon by fs laser irradiation of organic liquids

The high intensities generated in femtosecond (fs) laser interactions offers the possibility of novel formation routes for diamond and diamond-like carbon materials. Coulomb explosion, a common phenomenon in intensive fs irradiation, has recently been shown to lead to a direct graphite–diamond phase transition on the surface of graphite. In this paper we report the results of fs irradiation of a variety of liquid organic compounds at intensities in the Coulomb explosion regime. The products of laser-induced chemistry under these conditions have been studied using visual/surface enhanced Raman and transmission electron microscopy (TEM). Surface enhanced Raman spectra/TEM show that an intermediate diamond phase, trans-polyacetylene chains and amorphous carbon are present after fs irradiation of liquid alkanes. We also find that the diamond component can be enhanced by irradiation in the presence of certain transition metals; however the origin of this effect is still uncertain. Diamond films deposited in this way are found to exhibit a nano-assembled structure involving individual nanodiamonds extending over an area of about 1 cm 2. This process represents a wet chemical method for room temperature formation of diamond-like carbon films

飞秒激光形成的半导体低维结构与发光

飞秒激光形成的半导体低维结构与发光

Deposit of glass fragments during femtosecond laser penetrating keratoplasty

To evaluate femtosecond laser interaction with the applanation lens during pre-programmed penetrating keratoplasty corneal cuts. Three different-shaped penetrating keratoplasty dissections were performed on edematous corneas from bank eyes using a clinical femtosecond laser system (Intralase FS60) with energies higher than 2 microJ, and the "depth into glass" parameter at 50 microm, which is defined as the length over which the laser interacts with the glass of the applanation cone in contact with the cornea. Additional full-thickness corneal incisions were obtained with an experimental laser source with technical characteristics similar to the clinical laser. Following cutting, tissue sections were examined by optical microscopy (OM), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). After the procedure, the cones were examined by optical and scanning electron microscopy (SEM). A control was obtained by repeating the procedures and stopping the laser at the cornea-lens interface. OM and TEM analysis of the tissue showed the presence of solid particles of a maximum dimension of 1.5 mum on the epithelium and the anterior stroma, regardless of the laser system used to cut. The EELS technique revealed their composition as silicon dioxide. We believe that the fragments originate from the applanation cone, which is machined by the laser interacting with the glass in contact with cornea. This is consistent with the structures observed on the lens by OM and SEM. Radial and circumferential tracks on the surface of the lens are visible, corresponding to the laser path in penetrating keratoplasty protocols. No particles were found in the control samples. When performing penetrating keratoplasty corneal cuts by infra-red femtosecond laser, the applanation lens in contact with the cornea is machined by the laser depending on the system parameters. As a consequence, microscopic glass fragments are created, which may remain in the tissue. This unwanted effect can be avoided by stopping the procedure at the lens-cornea interface.

Femtosecond pulse laser interactions with thin silicon films and crater formation considering optical phonons and wave interference

The extensive numerical study is conducted to examine interaction of femtosecond pulse laser with thin silicon film structure by using the two-temperature model (2TM) and the three-temperature model (3TM), on the basis of the relaxation time approximation to Boltzmann’s transport equation. The present study investigates mainly transient behaviors for carrier number density, electron temperatures, and phonon temperatures, and it compares the results of 3TM with those of 2TM for energy transfer characteristics. In particular, the one-dimensional model is extended to two-dimensional model for the direct estimation of laser-induced crater formation. In addition, the thin film optics and the electromagnetic theory are used to simulate the propagation of laser energy absorption in stratified media with variation of complex refractive index. It is found that quite different tendencies are observed for femtosecond pulse lasers between 2TM and 3TM. For the 3TM, there is clearly nonequilibrium between optical phonons and acoustic phonons when the femtosecond lasers are irradiated, whereas as the laser pulse becomes longer, two different phonons are thermally in equilibrium state. The crater depths and shapes are significantly affected by film thickness, laser fluence, and pulse duration because of the wave interference effect as well as the difference in energy relaxation times. Moreover, the optical properties are found to depend on laser absorption and laser pulses. This tendency which is successfully estimated by the two-dimensional 3TM takes place in thin film structures differently unlike the bulk silicon.

飞秒激光与固体靶相互作用中背表面的渡越辐射

飞秒激光与固体靶相互作用中背表面的渡越辐射

Energetic electrons produced in the interaction of a kiloHertz femtosecond laser with tantalum targets

The electrons produced in the interaction of a high repetition rate laser with a thick tantalum target generate a continuous distribution of photons via the bremsstrahlung process occurring mainly in the target. The photon energy distributions, between 50 and 500 keV, are unfolded to obtain the true X-ray energy distributions. These distributions are used in a Monte Carlo simulation to infer the initial electron energy distributions. These properties have been studied at laser intensities ranging from 3 × 1015 to 6 × 1016 W cm− 2. The electron energy distributions are different above ≃2 × 1016 W cm− 2 as compared to lower intensities. This is evidence for a different laser plasma interaction regime.

Material decomposition mechanisms in femtosecond laser interactions with metals

A numerical hydrodynamic study of femtosecond laser ablation is presented. A detailed analysis of material decomposition is performed using a thermodynamically complete equation of state with separate stable and metastable phase states and phase boundaries. The lifetime of the metastable liquid state is estimated based on the classical theory of homogeneous nucleation. In addition, mechanical fragmentation of the target material is controlled based on available criteria. As a result, several ablation mechanisms are observed. A major fraction of the ablated material, however, is found to originate from the metastable liquid region, which is decomposed either thermally in the vicinity of the critical point into a liquid-gas-mixture or mechanically at high strain rate and negative pressure into liquid droplets and chunks. The calculation results explain available experimental findings.

A comparison of energy distribution of hot electrons from the front and the rear sides of targets during the interaction of femtosecond laser with foil targets

The energy distribution of hot electrons from the front and the rear sides of targets have been measured during the interaction of femtosecond laser with the foil target. The results show that the peaks of energy spectrum of hot electrons from the front and the rear sides of targets are located at 430 keV and 175 keV, respectively. Fitted with Maxwell distribution, the temperatures of hot electrons from the front of targets are 218 keV and 425 keV. Obviously, the energy spectrum of hot electrons from the rear of targets is “softened”, which is due to Debye sheath electric field and self-magnetic field produced by hot electrons transported in the target, and has temperatures of 96 keV and 347 keV, which are lower than those from the front of targets.

Effects of atomic number Z on the energy distribution of hot electrons generated by femtosecond laser interaction with metallic targets

The effects of atomic number Z on the energy distribution of hot electrons generated by the interaction of 60fs, 130mJ, 800nm, and 7×1017W/cm2 laser pulses with metallic targets have been studied experimentally. The results show that the number and the effective temperature of hot electrons increase with the atomic number Z of metallic targets, and the temperature of hot electrons are in the range of 190–230keV, which is consistent with a scaling law of hot electrons temperature.

Absolute energy distribution of hard x rays produced in the interaction of a kilohertz femtosecond laser with tantalum targets

Previous reports have indicated the anomalous excitation rate for the 6.2keV nuclear level of Ta181 in a plasma produced with a femtosecond laser. A detailed characterization of the electrons and x-ray sources produced in such a plasma is required to interpret these results. In a preliminary work, the continuous energy distribution of hard x rays (10–500keV) produced in the interaction of a kilohertz femtosecond laser beam with a tantalum solid target is investigated in the 3×1015–6×1016W∕cm2 range of intensity. A sodium iodide detector with appropriate shielding is used. Strong collimation and absorption filters are used to avoid the pileup of photons in the detector. The response function of this setup is calculated with the GEANT3 simulation code. We demonstrate the necessity to quantify the Compton scattered events in the raw spectra in order to restore the absolute x-ray energy distribution.

Gas-cluster targets for femtosecond laser interaction: Modeling and optimization

A novel mathematical model for the investigations of a cluster formation process in a gas jet is presented, which enables us to obtain the detailed description of the spatial and temporal distributions of all cluster target parameters. In this model, a cluster target is considered as a two-phase medium, consisting of the continuous gas phase and the discrete condensed phase (clusters). The detailed nozzle geometry is also taken into account in this model. In order to confirm the advantage of the present model over a conventional model, a considerable amount of numerical computations has been carried out and the results are compared with the data obtained from Hagena’s theory [Rev. Sci. Instrum. 63, 2374 (1992)]. Based on the developed modeling, a three-staged nozzle, which cannot be modeled using the conventional model, is designed for the purpose of producing a sufficient amount of micron-sized clusters. The generation of unprecedented amount of keV x rays from the laser-cluster interaction experiments with this nozzle and their accurate intensity dependences on various experimental parameters support the adequateness of the nozzle design.

Relativistic Electron Acceleration in a Wake Field Generatedby the Intense Femtosecond Laser Interaction with a Mixture Jet ofDeuteriumClusters and Molecules

High energy electron acceleration in a wake field generated in theintense ultrashort (30 fs) laser pulse cluster gas jet interaction isexperimentally demonstrated. Relativistic electrons with energy of60 MeV were observed. These high energy electrons split into twobeams due to the relativistic self-focusing of the laser.

Role of a native oxide on femtosecond laser interaction with silicon (100) near the damage threshold

Si (100) with and without a 14–25Å thick native oxide was laser machined at grazing incidence using a Ti:sapphire femtosecond pulsed laser under ultrahigh vacuum conditions. The resulting damage feature size and morphology indicate that the presence or absence of the native oxide significantly affects the mechanism for femtosecond laser-induced damage. We propose that a fluence-dependent modification of the oxide by the incident laser pulse must be considered when studying femtosecond laser damage of Si (100) with a native oxide. Data are also presented that are consistent with a dose-dependent phase transformation in the amorphous oxide. The implications of the native oxide, including relative damage thresholds of the underlying Si (100) and the role of the oxide in damage morphology are addressed.

Numerical simulation of femtosecond laser interaction with silicon

A numerical model for simulating femtosecond laser interaction with silicon is presented. This model simulates laser beam propagation by directly solving Maxwell’s equations using the finite-difference time-domain method, so that complete electromagnetic nature of a laser beam is simulated accurately. Accurate computation of laser beam passage ensures realistic heat generation pattern in the target material. The two-temperature model has been adopted to simulate the energy transport of electrons and lattices. This study reveals interesting results regarding thermal energy generation and complex laser beam passage in the silicon target. The temperature histories of electrons and lattices are presented.

Two‐Photon Photopolymerization and 3D Lithographic Microfabrication

AbstractFor Abstract see ChemInform Abstract in Full Text.

Double-peak emission of hot electrons generated by femtosecond laser interaction with solid targets

Double-peak emission of hot electrons has been observed in the interaction of a 60-fs, 125-mJ, 800-nm, p-polarized laser pulse with Al targets. One peak in the specular direction is due to the reflected laser light, which excites a plasma wave to accelerate electrons. The other peak, which is more consistent with theories of Phys. Plasmas 6, 2855 (1999)] and Phys. Rev. Lett 82, 743 (1999)], is produced by the resonance absorption at approximately 15 degrees from the target normal.

Evidence for Rescattering in Intense, Femtosecond Laser Interactions with a Negative Ion

It is now well established that energetic electron emission, nonsequential ionization, and high harmonic generation, produced during the interaction of intense, femtosecond laser pulses with atoms (and atomic positive ions), can be explained by invoking rescattering of the active electron in the laser field, the so-called rescattering mechanism. In contrast for negative ions, the role of rescattering has not been established experimentally. By irradiating F- ions with ultrashort laser pulses, F+ ion yields as a function of intensity for both linearly and circularly polarized light have been measured. We find that, at intensities well below saturation for F+ production by sequential ionization, there is a small but significant enhancement in the yield for the case of linearly polarized light, providing the first clear experimental evidence for the existence of the rescattering mechanism in negative ions.

Dynamics of femtosecond laser interactions with dielectrics

Femtosecond laser pulses appear as an emerging and promising tool for processing wide bandgap dielectric materials for a variety of applications. This article aims to provide an overview of recent progress in understanding the fundamental physics of femtosecond laser interactions with dielectrics that may have the potential for innovative materials applications. The focus of the overview is the dynamics of femtosecond laser-excited carriers and the propagation of femtosecond laser pulses inside dielectric materials.

Sub-micron scale patterning using femtosecond laser and self-assembled monolayers interaction

Standard positive photoresist techniques were adapted to generate sub-micron scale patterns of gold substrate using self-assembled monolayers (SAMs) and femtosecond laser. Self-assembled monolayers formed by the adsorption of alkanethiols onto gold substrate are employed as very thin photoresists. The process underlying photopatterning of SAMs on gold is well-known at the phenomenological level. Alkanethiolates formed by the adsorption of alkanethiols are oxidized on exposure to UV light in the presence of air to alkylsulfonates. Specifically, it is known that deep UV light of wavelength less than 200 nm is necessary for oxidation to occur. In this study, solid state femtosecond laser of wavelength 800 nm is applied for photolithography. The results show that ultrafast laser of near infrared (NIR) range wavelength can replace deep UV laser source for photopatterning using thin organic films. The essential basis of our approach is the photochemical excitation of specific reactions in a particular functional group (in this case a thiolate sulfur atom) distributed with monolayer coverage on a solid surface. Femtosecond laser photolithography could be applied to fabricate the patterning of surface chemical structure and the creation of three-dimensional nanostructures by combination with suitable etching methods.