Carbon dendrite formation induced by pulsed laser irradiation

Porous carbon dendrite has been prepared by irradiating graphite targets with 532 nm, 10 ns laser pulses. The prepared samples were characterized by means of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The measurement results show that carbon dendrite structures with cluster diameter of 10 nm were obtained on the irradiated target surface in an inert gas atmosphere. The evolution of target surface morphology induced by different laser intensities was investigated. The formation mechanism of the dendrite structure has been discussed in detail. The laser intensity plays an important role in the formation of the nanostructures and there exists an optimum intensity to prepare the carbon dendrite.     

Microstructure evolution on the surface of stainless steel by Nd:YAG pulsed laser irradiation

New structure phenomena with the grain sizes of 60 nm to 1 μm would be expected on the stainless steel surface by Nd:YAG pulsed laser irradiation. Nano-structures with various shapes and sizes were formed mainly during the solidification and most shapes of particles were diversiform according to different distances from the center of the spot. The morphologies were of equiaxed nano-particles and faceted hexagons. The surface re-solidification velocities have been estimated according to the numeral simulation of the thermal conditions. It was proved by the XRD that they were mainly consisted of γ-Fe and manganese oxides. The XPS results confirmed the EDS results that on the surface the alloy elemental composition in the outermost layer were rich in Mn and poor in Fe and in reverse in the center of the laser spot. Through observation of morphologies grown on the laser irradiated surfaces, direct evidence of growth mode transition from a continuous form to a lateral form was provided in one laser spot.     

Crown-like structure development on titanium exposed to multipulse Nd:YAG laser irradiation

We report micrometre-sized crown-like structure growth on a Ti surface by multipulse Nd:YAG (λ=1.064 μm,τ=170 ns) laser irradiation in air at atmospheric pressure. The irradiation was performed at 8×10⁷ W/cm² laser-pulse intensity, below the ablation threshold. A ring-shape structure develops in the centre of the irradiation spot after the action of five laser pulses. The further increase of the laser-pulse number leads gradually to a crown-like structure, which has, for 150 pulses, a height of 120–140 μm above the non-irradiated Ti surface. The forming crater’s depth does not exceed the height of the grown structure. In the neighbouring zone, after the action of 25 laser pulses, microcracks of the oxide surface layer develop. With the next pulses this leads to the formation of a surface microrelief. The crown-like-structure growth is originated by molten material movement attributed to the laser-induced plasma-recoil pressure. Received: 6 June 2001 / Accepted: 6 January 2002 / Published online: 26 March 2002     

Polarization resolved laser induced breakdown spectroscopy by single shot nanosecond pulsed Nd:YAG laser

Single shot nanosecond polarization resolved laser induced breakdown spectroscopy is introduced to spectrally analyse solid sample, e.g. copper. It is shown that continuum background emission of plasma generated by such laser pulse is much strongly polarized than discrete line radiation. By using the PRLIBS technique, continuum background emission is effectively suppressed. Furthermore, it is illustrated that polarization state of continuum emission in copper sample is mostly in vertical direction and, also, does not depend on laser beam polarization.     

The influence of laser power and repetition rate on oxygen and nitrogen insertion into titanium using pulsed Nd:YAG laser irradiation

Oxygen and nitrogen insertion in a titanium substrate is performed in air using a Q-switched Nd-YAG laser. This process modifies the surface by the formation of specific layers on the substrate. These layers show different properties, largely influenced by the insertion of elements in the layers. The treatment conditions, especially the laser parameters (fluence and repetition rate), must be known and controlled. Using nuclear analysis, we demonstrate that oxygen insertion is mainly influenced by repetition rate, and that nitrogen insertion is controlled by laser fluence. The physical phenomena involved in the oxygen and nitrogen insertion are discussed.     

Comparison of surface films formed on titanium by pulsed Nd:YAG laser irradiation at different powers and wavelengths in nitrogen atmosphere

The nitridation of titanium (Ti) caused by a Q-switched Nd:YAG laser under nitrogen gas atmosphere was investigated in situ using X-ray photoelectron spectroscopy (XPS). A laser having a wavelength of 1064 nm and 532 nm (SHG mode) was irradiated on a titanium substrate in an atmosphere-controlled chamber, and the substrate was then transported to an XPS analysis chamber without exposing it to air. The characteristics of the surface layer strongly depend on the laser power. When the power is relatively low, a titanium dioxide layer containing a small amount of nitrogen is formed on the substrate. Laser irradiation beyond a certain laser power is required to obtain a stoichiometric titanium nitride (TiN) layer. A TiN layer and an oxynitride layer with a TiO_xN_y-like structure are formed as the topmost and the lower surface layer, respectively, when the laser power exceeds this threshold value. The threshold laser power strongly depends on the wavelength of the laser, and this threshold value for the 532-nm laser is quite lower than that for the 1064-nm laser.     

The effects of pulsed Nd:YAG laser irradiation on surface energy of copper

The effect of laser surface treatment on the surface energy of copper plate was investigated in terms of the surface microstructure analysis and theoretical computation in this paper. The surfaces of the copper plates were treated by Nd:YAG pulsed laser with different powers. The microstructures of the treated copper plates were analyzed by optical microscopy and X-ray diffraction (XRD), and the wetting experiment was performed to evaluate the variation of surface energy. The results showed that the surface microstructure and the corresponding surface energy of copper, changed with the variation of the laser power. The experimental results further explained by XRD results and theoretical calculation, demonstrated that the surface energy changed when the crystal structure in the surface layer was re-oriented in a preferred orientation after laser irradiation.     

Surface hardening of titanium by pulsed Nd:YAG laser irradiation at 1064- and 532-nm wavelengths in nitrogen atmosphere

The surface hardness of titanium modified by laser irradiation at different wavelengths in nitrogen atmosphere was investigated. Further, surface characteristics such as morphology, chemical state, and chemical composition in the depth direction were also studied. The size and depth of the craters observed in the laser-irradiated spots increased monotonically with an increase in the laser power. Furthermore, the crater formed by the 532-nm laser was deeper than that formed by the 1064-nm laser for the same laser power. Laser power beyond a certain threshold value was required to obtain a titanium nitride layer. When the laser power exceeds the threshold value, a titanium nitride layer of a few tens of nanometers in thickness was formed on the substrate, whereas a titanium oxide layer containing small amounts of nitrogen was formed when the laser power is below the threshold value. Thus, it was shown that laser irradiation using appropriate laser parameters can successfully harden a titanium substrate, and the actual hardness of the titanium nitride layer, measured by nanoindentation, was approximately five times that of an untreated titanium surface.     

Surface treatment of titanium by Nd:YAG laser irradiation in the presence of nitrogen

This work presents the surface treatment of commercial titanium alloy by means of a Nd:YAG (1.064 7m) laser in the presence of nitrogen. The treated surface was characterised by using scanning electron microscopy, atomic force microscopy, X-ray diffraction, secondary ion mass spectrometry, UV-visible spectrophotometry and microindentation. Experimental results show the formation of a nitrogen-rich layer, 500 nm thick, with a surface morphology characterised by the presence of polygonal structures which suggest that melting occurred under the action of the laser. The ' phase of titanium nitride was identified, in addition to a nitrogen in !-titanium solid solution. The reflectance spectrum of the yellow-golden sample is similar to that of deposited titanium nitride thin films. The hardness of the treated surface was measured by microindentation and found to be 14 GPa, a value three times higher than that of the titanium alloy. In scratch tests the nitrided layer detached at a load of 0.9 N.     

Biocompatible film deposition by using Nd:YAG pulsed laser

A Nd:YAG laser operating at the fundamental wavelength (1064 nm) and at the second harmonic (532 nm), with 9 ns pulse duration, 100–900 mJ pulse energy, and 30 Hz repetition rate mode, was employed to ablate in vacuum (10^?6 mbar) biomaterial targets and to deposit thin films on substrate backings. Titanium target was ablated at the fundamental frequency and deposited on near-Si substrates. The ablation yield increases with the laser fluence and at 40 J/cm ² the ablation yield for titanium is 1.2×10¹⁶ atoms/pulse. Thin film of titanium was deposited on silicon substrates placed at different distance and angles with respect to the target and analysed with different surface techniques (optical microscopy, scanning electron spectrosopy (SEM), and surface profile).

Hydroxyapatite (HA) target was ablated to the second harmonic and thin films were deposited on Ti and Si substrates. The ablation yield at a laser fluence of 10 J/cm ² is about 5×10¹⁴ HA molecules/pulse. Thin film of HA, deposited on silicon substrates placed at different distance and angles with respect to the target, was analysed with different surface techniques (optical microscopy, SEM, and Raman spectroscopy).

Metallic films show high uniformity and absence of grains, whereas the bio-ceramic film shows a large grain size distribution. Both films found special application in the field of biomaterial coverage.     

Plasma splashing from Al and Cu materials induced by an Nd:YAG pulsed laser

Plasma splashing from Al and Cu target materials and the growth of thin films on Cu and Al, respectively, has been studied using a Q-switched Nd:YAG laser with a 1064-nm, 80-mJ, 8-ns pulse width as the source of ablation. The target kept rotating and the substrate, Cu for Al and vice versa, was placed at an angle of 15° with respect to the beam axis. During the laser-matter interaction, the targets absorbed thermal energy within the thermal region depth of 4.7 (1.1) nm, yielding an ablated skin depth of 6.7 (4.2) nm. The surface morphology of the exposed targets was studied by analyzing SEM micrographs obtained using a ZEISS SUPRA 35 VP. The obtained results are explained on the basis of different sputtering/ablation mechanisms. Comparatively severe damage forming a bigger crater is seen on the Al target surface in contrast to the crater on the Cu surface. This observation is correlated with the blustering effect and/or debris formation. Energy dispersive spectroscopy (EDX) of the substrates yielded the deposition of micrometric grain-size particles.     

用倍频晶体KTP实现Nd：YAG激光锁模的研究

报道用倍频晶体KTP实现Nd:YAG激光锁模的系统研究,给出了该锁模激光器输出脉宽的表达式,并和实验测量值相比较,计算值跟实验结果符合较好.文中还分析了各种参数对该锁模激光输出性能的影响.     

Experimental study of electro-optical-switched pulsed Nd:YAG laser

We report the specification of a compact and stable side diode-pumped Q-switched pulsed Nd:YAG laser. We experimentally study and compare the performance of the pulsed Nd:YAG laser in the free-running and Q-switched modes at different pulse repetition rates from 1 Hz to 100 Hz. The laser output energy is stabilized by using a special configuration of the optical resonator. In this laser, an unsymmetrical concave–concave resonator is used and this structure helps the mode volume to be nearly fixed when the pulse repetition rate is increased. According to the experimental results in the Q-switched operation, the laser output energy is nearly constant around 70 m J with an FWHM pulse width of 7 ns at100 Hz. The optical-to-optical conversion efficiency in the Q-switched regime is 17.5%.     

Morphological and structural characterizations of different oxides formed on the stainless steel by Nd:YAG pulsed laser irradiation

The effects of laser surface irradiation on microstructure of AISI 304 stainless steel were investigated. The stainless steel surface was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). XRD patterns indicated that different oxides, such as chromium oxides and manganese oxides were fabricated successfully on the surface of the stainless steel by Nd:YAG pulsed laser irradiation. The effects of the growth conditions such as the laser power density on the morphologies of the different oxides have been investigated. From the FESEM, EDS (energy-dispersive spectrum) and TEM observations, the oxides with triangle-like, quadrangular and hexagonal morphologies have been fabricated successfully. The XPS was used to verify the formed oxides which had been detected by the XRD patterns. It was considered that laser power density had a critical role in the formation of different oxides.     

Formation of nano-crystalline and amorphous phases on the surface of stainless steel by Nd:YAG pulsed laser irradiation

Thin surface layers consisting of nano-crystalline and amorphous phases on the surface of stainless steel have been attained under the Nd:YAG pulsed laser irradiation. The phases and microstructures were investigated by X-ray diffraction (XRD) and high resolution transmission electron microscope (HRTEM). The phase compositions of the surface determined by XRD were α-Fe (ferrite) and γ-Fe (austenite) or only γ-Fe in the near surface region on the bases of the different laser power densities. The nano-crystalline grains with sizes of 4-100 nm could result from high cooling rate and crystallization in amorphous region by homogeneous and heterogeneous nucleation. The formation of the amorphous phase was attributed to the higher cooling rates.     

Radiation effects and pulsed laser deposition of titanium by Nd:Yag laser

A study of Ti laser irradiation and thin film deposition produced by an Nd:Yag pulsed laser is presented. The laser pulse, 9?ns width, has a power density of the order of 10¹⁰?W/cm². The titanium etching rate is of the order of 1?µg/pulse, it increases with the laser fluence and shows a threshold value at about 30?J/cm² laser fluence. The angular distribution of ejected atoms (neutrals and ions) is peaked along the normal of the target surface. At high fluence, the fractional ionization of the plasma produced by the laser is of the order of 10%. Time-of-flight measurements demonstrate that the titanium ions, at high laser fluence, may reach kinetic energies of about 1?keV. Obtained results can be employed to produce energetic titanium ions, to produce coverage of thin films of titanium and to realize high adherent titanium-substrate interfaces. The obtained results can be employed to produce energetic titanium ions, to produce a coverage of thin titanium films on polymers, and to realize highly adherent titanium–substrate interfaces.     

SnO2 thin films grown by pulsed Nd:YAG laser deposition

SnO₂ thin films have been deposited on glass substrates by pulsed Nd:YAG laser at different oxygen pressures, and the effects of oxygen pressure on the physical properties of SnO₂ films have been investigated. The films were deposited at substrate temperature of 500°C in oxygen partial pressure between 5.0 and 125 mTorr. The thin films deposited between 5.0 to 50 mTorr showed evidence of diffraction peaks, but increasing the oxygen pressure up to 100 mTorr, three diffraction peaks (110), (101) and (211) were observed containing the SnO₂ tetragonal structure. The electrical resistivity was very sensitive to the oxygen pressure. At 100 mTorr the films showed electrical resistivity of 4×10⁻² Ω cm, free carrier density of 1.03×10¹⁹ cm⁻³, mobility of 10.26 cm² V⁻¹ s⁻¹ with average visible transmittance of ∼87%, and optical band gap of 3.6 eV.     

高功率高光束质量脉冲Nd: YAG激光器

 以实现高功率、高光束质量的脉冲激光输出为目的,对非对称平-凹谐振腔的结构进行了理论分析。设计出了高功率、高光束质量非对称放置的平-凹谐振腔、双氙灯泵浦的脉冲Nd: YAG激光器。当占空比为9%时,实现输出激光平均功率近480 W,光束参数积优于12.7 mm·mrad,电光转化效率近4%,与理论分析吻合,可用芯径300 μm的光纤传输,不稳定性优于±1%。加工实验证明有较好的质量：切割材料为不锈钢,厚度为3 mm时、切割速度为0.6 m/min和厚度为1.5 mm、切割速度为1.2 m/min时,两种情况下所得切缝宽度均为250 μm,且切割上下沿光滑。     

Experimental study of electro-optical Q-switched pulsed Nd:YAG laser

We report the specification of a compact and stable side diode-pumped Q-switched pulsed Nd:YAG laser. We experimentally study and compare the performance of the pulsed Nd:YAG laser in the free-running and Q-switched modes at different pulse repetition rates from 1 Hz to 100 Hz. The laser output energy is stabilized by using a special configuration of the optical resonator. In this laser, an unsymmetrical concave-concave resonator is used and this structure helps the mode volume to be nearly fixed when the pulse repetition rate is increased. According to the experimental results in the Q-switched operation, the laser output energy is nearly constant around 70 mJ with an FWHM pulse width of 7 ns at 100 Hz. The optical-to-optical conversion efficiency in the Q-switched regime is 17.5%.