Synchronization of chaos in unidirectionally and bidirectionally coupled multiple time delay laser diodes with electro-optical feedback

We report on chaos synchronization in both unidirectionally and bidirectionally coupled multiple time delay laser diodes with electro-optical feedback. We derive existence and sufficient stability conditions for the synchronization regimes. We calculate the Lyapunov exponents, information dimension, and Kolmogorov-Sinai entropy for a single and double delay time lasers to demonstrate that multiple time delay laser system can offer higher complexity than a single time delay laser. We demonstrate that in coupled multiple time delay lasers additional feedback(s) can play a stabilizing role. We compare the synchronization quality for closed loop and open loop receiver laser configurations and find better synchronization quality for partially open loop receiver (when the receiver laser has only one feedback loop), than the open loop receiver configuration (when the receiver contains no feedback loops). We also study the effect of the feedback phase on the correlation coefficient between the interacting laser systems. Analytical results are fully supported by numerical simulations.     

Study on the microstructure and wear resistance of the composite coatings fabricated on Ti-6Al-4V under different processing conditions

Composite coatings mainly containing titanium carbides and borides were produced by laser surface alloying of Ti-6Al-4V with graphite and boron mixed powders. The test results show that the coatings have higher hardness (1600-1700 HV_0.1) and are more resistant to wear than the as-received sample. Laser scanning speed and the content of alloying elements (weight ratio of graphite to boron) have an effect on both the microstructure and the wear resistance of the coatings. TEM results show that strip titanium carbides and borides grow alternately and thus restrain the formation of coarse needle-like TiB and dendritic TiC crystals produced by laser alloying of titanium alloys with boron and graphite separately.     

高等离子体密度区域的光子加速现象

本文研究了激光尾场实验中的透射光谱。和标准尾场实验不同的是,我们采用了更高的等离子体密度。实验中仍然观察到了明显的频谱加宽与频谱分裂,这是光子加速的显著标志。为解释该现象,我们做了二维粒子模拟。模拟结果证明在该高密度等离子体中, 光子加速仍然可以发生。     

Advanced design of periodical architectures in bulk metals by means of Laser Interference Metallurgy

Patterning of high-resolution features on large-area metallic substrates has been performed by means of the Laser Interference Metallurgy method. Due to the intensity distribution of the interference pattern, this technique allows to locally and periodically heat the material surface to temperatures higher than the melting point with a long-range order. In this study, commercial stainless steel, copper and aluminum substrates were irradiated using single pulses of a nanosecond Nd:YAG laser with two and three laser-beam configurations operating at 355 nm of wavelength. Thermal simulations have been performed by finite element method and compared to the experiments. The results indicate that the structuring is produced by a surface tension driven mechanism induced by the thermal gradient. Moreover, metals with short thermal diffusion lengths present very homogeneous structures and the structure depth that can be achieved at relatively high laser fluences during single-pulse experiments is on the order of the diffusion length.     

Strain Rate Sensitivities of Face-Centred-Cubic Metals Using Molecular Dynamics Simulation

We use dislocation theory and molecular dynamics （MD） simulations to investigate the effect of atom properties on the macroscopic strain rate sensitivity of f cc metals. A method to analyse such effect is proposed. The stress dependence of dislocation velocity is identified as the key of such study and is obtained via 2-D MD simulations on the motion of an individual dislocation in an fcc metal. Combining the simulation results with Orowan＇s relationship, it is concluded that strain rate sensitivities of fcc metals are mainly dependent on their atomic mass rather than the interatomic potential. The order of strain rate sensitivities of five fcc metals obtained by analysing is consistent with the experimental results available.     

Thermal modelling of laser welding and related processes: a literature review

The main emphasis of this review is on thermal modelling and prediction of laser welding in metals. However as similar techniques are employed to model conventional welding processes such as arc, resistance and friction, as well as related processes such as alloying, cladding and surface hardening, part of this review is given over to the modelling of these processes where appropriate. The time frame of the review is up to the year 2002.     

Laser Nitriding of Iron,Stainless Steel,and Plain Carbon Steel Investigated by Mössbauer Spectroscopy

Nitriding is a common method for improving the hardness, mechanical properties, wear and corrosion resistance of metals. Laser nitriding of metals is an efficient process, where the irradiation of surfaces in air or nitrogen atmospheres with short laser pulses leads to a fast take-up of nitrogen into the irradiated surfaces. This process has been extensively investigated for pure iron, but usually, no tools or functional parts are made of pure iron. Mainly steel or cast iron is used as a base material. Therefore, when looking for technical applicability, also the influence of alloying elements on the laser nitriding process is of great interest. Besides the pure iron various carbon steels and an austenitic stainless steel were studied in laser nitriding experiments in order to investigate the influence of the material itself. Here, the process is investigated via Conversion Electron and X-ray Mössbauer Spectroscopy (CEMS and CXMS), Resonant Nuclear Reaction Analysis (RNRA), and X-Ray Diffraction (XRD). It appears that carbon steels are even better suited for the laser nitriding process than pure iron, and the laser nitriding also works efficiently for the stainless steel which is normally difficult to be nitrided.     

Supercontinuum Gneneration and Modes Analysis in Secondary Cores of a Hollow-Core Photonic Crystal Fiber

We demonstrate the generation of supercontinuum spectra in three secondary cores of a hollow-core photonic crystal fiber pumped by femtosecond laser pulses, respectively. The supercontinuum spectra are mainly a result of the soliton self-frequency shift and an amplification of dispersive wave at visible wavelengths. Detailed mode simulations show that with the increasing core length, the modes transfer from ＂double-points＂ to ＂single-point＂ since the pump laser is more easily coupled into the two side cores of a secondary core when the core length is small. The simulation results also explain why the experimental observed far-field beam patterns of first two secondary cores are different.     

Deep drilling of metals by femtosecond laser pulses

Results of recent investigations on deep drilling of metals by femtosecond laser pulses are reported. At high laser fluences, well above the ablation threshold, femtosecond lasers can drill deep, high-quality holes in metals without any post-processing or special gas environment. It is shown that for high-quality drilling of metals, the following processes are important: (1) laser-induced optical breakdown of air containing metal vapor and small metal particles (debris) generated by multi-pulse femtosecond laser ablation, (2) transformation of laser pulses into light filaments, and (3) low-fluence finishing. Received: 15 November 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/2788-100, E-mail: ch@lzh.de     

Generation of mixed metallic nanoparticles from immiscible metals by spark discharge

Using a spark discharge system, we synthesized Ag-Cu, Pt–Au and Cu-W mixed particles a few nanometers in size. These combinations have miscibility gaps in the bulk form. The microsecond sparks between electrodes consisting of the respective materials, form a vapour cloud. Very fast quenching of the mixed vapour results in the formation of nanoparticles. To investigate the morphology, size, composition and structure of the particles, TEM, XRD analyses and EDS elemental mapping were performed on the samples. The average compositions were measured by ICP and the specific surface areas were determined by the BET. Our method produces Ag-Cu and Au–Pt mixed crystalline phases that do not exist in macroscopic samples. For Cu-W, alloying is not observed, and the metals are mixed on a scale of about 1 nm.     

Ring opening reaction of 1,3-cyclohexadiene studied by semiclassical dynamics simulation

A semiclassical dynamics simulation study is reported for the ring opening reaction of 1,3-cyclohexadiene (CHD) triggered by a femtosecond-scale laser pulse. The results clearly demonstrate that, following the excitation by the laser pulse, the ring opening occurs at ∼110 fs and the molecule decays to the ground electronic state at ∼210 fs due to non-adiabatic transition of electrons from LUMO to HOMO orbitals. Isomerization of the product of the ring opening reaction, 1,3,5-hexatriene (HT), to various stable isomers are also well demonstrated by the simulations.     

Process monitoring of powder pre-paste laser surface alloying

Instead of the continuous powder delivery method using a powder feeder for thick layer laser cladding, pre-pasting of the alloying powder on the substrate is a widely used method to supply the coating powders into the melt pool for LSA. A method to monitor the process of laser surface alloying based on the infrared emission from the melt pool using infrared photodiodes was developed. The technique is solely aimed at the process of laser surface alloying using pre-paste metal powder on the substrate surface prior to laser melting. This monitoring technique is able to distinguish the existence or the absence of the pre-paste powder and the consistency of the laser surface alloying process. The technique is of low cost and is simple to implement into the process.     

Influence of initial micro-porosity of target on material ejection under nanosecond laser pulses

Heating and explosive destruction of the near surface volume of metal with initial micro-porosity under nanosecond laser pulse is simulated. It is shown that presence of initial micro-porosity may influence and modify the ejection mechanisms from melt and from solid phase. Thermal and gas dynamic processes in a gas bubble growing in laser irradiated melt are analysed. Experiments show rather different surface morphology defined by micro-explosions and melt ejection for different metals. Possible explosion of micro-pores and solid particles release under nanosecond laser irradiation is analysed for metals with high melting point. It is shown that the stress attains the values that are close to the damage threshold.     

Efficient omnidirectional couplers achieved by anisotropic photonic crystal waveguides

Photonic crystals (PCs) have many potential applications because of their ability to control light-wave propagation. We have investigated omnidirectional couplers in two-dimensional anisotropic PC structures. The anisotropic PC coupler composed of an anisotropic-dielectric cylinder in air is studied by solving Maxwell's equations using the plane wave expansion method and finite-difference time-domain method. The photonic band structures are found to exhibit absolute bandgaps for the square lattices. Numerical simulations show that the incident light-waves at both transverse electric and transverse magnetic modes have efficient coupling in anisotropic PC coupler with square lattices. The guided modes and coupling length are analyzed by considering various line defect anisotropic PC waveguides and interaction regions of couplers. Such a mechanism of omnidirectional coupling should open up a new application for designing components in photonic integrated circuits.     

Theoretical investigation of laser pulse width dependence in a thermal confinement regime

Previous molecular dynamics (MD) simulations of ultraviolet (UV) laser ablation demonstrate the distinct dependence of material ejection on laser fluence and laser pulse duration. In this paper, we examine the pulse width dependence when the laser pulse widths are appropriate for the thermal confinement regime. We perform MD simulations of laser ablation with a laser pulse duration of 1 ns and compare with a pulse width of 150 ps as in previous simulations. The simulations confirm that the pulse width in thermal confinement regime does not dramatically influence the molecular ejection mechanism. The simulations reveal differentiations, however, in plume composition and the ablation threshold value. PACS 02.70.Ns; 61.80.Az; 79.20.Ap     

Magneto-optical trap of metastable helium-3 atoms

A technique for vortex creation in trapped Bose–Einstein condensates is suggested: vortices can be excited at the edge of a condensate and guided to the center by a laser beam moving along a spiral trajectory. Numerical simulations demonstrate the suggested technique. Parameter ranges for the method are given. Computer animations illustrate the dynamics of the guided vortices. Received: 3 December 1999 / Revised version: 16 March 1999 / Published online: 6 September 2000     

Correlation spectroscopy as a tool for detecting losses of ligand elements in laser welding of aluminium alloys

The plasma plume induced during laser welding of metals is a mixture of metal vapour, coming from the vaporised weld pool surface and shielding gas. The influence of the shielding gas on the welded joints quality is not yet well understood and very few investigations, to the best of our knowledge, were addressed to study its role in case of welding of aluminium–magnesium alloys. In this paper we present a study of the dynamics of plasma plume produced in laser welding of 5xxx aluminium alloys by means of correlation spectroscopy. By our results we can correlate the influence of the welding speed, in case of ineffective gas shielding, to the loss of alloying elements. Finally, the results obtained are consistent with the EDX analysis performed in post-processing on the welded joints.     

Surface cleaning of metals by pulsed-laser irradiation in air

The surface-cleaning effect of metals was investigated using KrF-excimer-laser irradiation of metal surfaces in air. The laser-induced cleaning of copper, stainless steel and aluminum surfaces was studied. It is found that laser cleaning is an effective cleaning process for metals even if the metal surfaces are heavily contaminated. It is also found that short wavelength and pulse duration are necessary for laser surface-cleaning. The energy density of the laser pulse is an important parameter in the cleaning process. Low energy density results in a cleaner surface but a larger pulse number is required, whereas high energy density can achieve higher cleaning efficiency but the temperature rise can cause surface oxidation and secondary contamination. In contrast to the KrF-excimer-laser, the pulsed CO₂ laser is not effective in surface-cleaning. The mechanisms of laser cleaning may include laser photodecomposition, laser ablation and surface vibration due to the impact of the laser pulse. Laser cleaning provides a new dry process to clean different substrate surfaces and can replace the conventional wet cleaning processes such as ultrasonic cleaning with CFC and other organic solvents.     

Ab initio description of the first stages of laser-induced ultra-fast nonthermal melting of InSb

Using first principles, all-electron calculations and dynamical simulations we study the behavior of solid InSb immediately after intense femtosecond excitation. First, we determine the laser-excited potential energy surfaces with high accuracy for different electronic temperatures (corresponding to different laser fluences). Then, we demonstrate that, although most phonon modes become only slightly softened even for high electron temperatures, the transverse acoustic modes at the boundary of the Brillouin zone undergo dramatic changes and become unstable. This is the origin of nonthermal melting. Based on these results, the dynamics during the first hundreds of femtoseconds after laser excitation can be unambiguously elucidated. Our results are in agreement with recent experiments and support the predictions made by Stampfli and Bennemann for silicon.     

Laser cooling of fermions to the superfluid transition temperature

Numerical simulations for realistic experimental parameters demonstrate that laser cooling on the attractive side of the Feshbach resonance can drive fermions much below the superfluid transition. For the assumed set of experimental parameters the transition takes place at 0.35T(F), and laser cooling can drive the system down to at least 0.085T(F) in a time of a few seconds. Superfluid growth is self-consistently included in simulations.