Electronic structure and nematic phase transition in superconducting multiple-layer FeSe films grown by pulsed laser deposition method

We report comprehensive angle-resolved photoemission investigations on the electronic structure of single crystal multiple-layer FeSe films grown on CaF_2 substrate by pulsed laser deposition(PLD) method. Measurements on FeSe/CaF_2 samples with different superconducting transition temperatures T_c of 4 K, 9 K, and 14 K reveal electronic difference in their Fermi surface and band structure. Indication of the nematic phase transition is observed from temperature-dependent measurements of these samples; the nematic transition temperature is 140-160 K, much higher than ~90 K for the bulk FeSe. Potassium deposition is applied onto the surface of these samples; the nematic phase is suppressed by potassium deposition which introduces electrons to these FeSe films and causes a pronounced electronic structure change. We compared and discussed the electronic structure and superconductivity of the FeSe/CaF_2 films by PLD method with the FeSe/SrTiO_3 films by molecular beam epitaxy(MBE) method and bulk FeSe. The PLD-grown multilayer FeSe/CaF_2 is more hole-doped than that in MBE-grown multiple-layer FeSe films. Our results on FeSe/CaF_2 films by PLD method establish a link between bulk FeSe single crystal and FeSe/SrTiO_3 films by MBE method, and provide important information to understand superconductivity in FeSe-related systems.     

Laser-induced periodic surface structures on different poly-carbonate films

Laser-induced periodic surface structures (LIPSS) were generated on oriented and amorphous thick, as well as on spin-coated thin, poly-carbonate films by polarized ArF excimer laser light. The influence of the film structure and thickness on the LIPSS formation was demonstrated. Below a critical thickness of the spin-coated films the line-shaped structures transformed into droplets. This droplet formation was explained by the laser-induced melting across the whole film thickness and subsequent de-wetting on the substrate. The thickness of the layer melted by laser illumination was computed by a heat-conduction model. Very good agreement with the critical thickness for spin-coated films was found. The original polymer film structure influences the index of refraction of the thin upper layer modified by the laser treatment, as was proven by the dependence of the structure’s period on the angle of incidence both for ‘s’- and ‘p’-polarized beams. The effect of the original surface roughness – grains in thick films or holes in thin films – was studied using atomic force microscopy. It was shown that the oblique incidence of ‘s’-polarized beams results in an intensity confinement in the direction of the forward scattering and in asymmetrical interference pattern formation around these irregularities. A new, two-dimensional grating-like structure was generated on spin-coated films. These gratings might be used as a special kind of mask. Received: 10 July 2001 / Accepted: 23 July 2001 / Published online: 30 August 2001     

共掺Na~ 的Yb~(3 )∶CaF_2晶体的荧光分析与低阈值激光运转

基于975nm激发的室温下荧光光谱测量,系统地研究了在新型钠、镱共掺的氟化钙晶体中钠离子的作用机理,分析了掺钠离子浓度与激光阈值的关系,获得了具有最低激光阈值的掺杂浓度优化配比.激光实验表明对于2%Yb3 离子浓度的氟化钙,掺入3%的钠离子能够获得最低阈值的激光运转,这与理论和荧光分析完全一致.     

Effect of irradiation history on the preparation of laser induced periodic microstructure on polyimide surface

Although UV laser is proved to be an effective tool to prepare microstructure on polymer surface, laser ablation accompanied by the formation of laser induced periodic surface structure (LIPSS) limits its application in many fields. The purpose of this report is to investigate the effect of pre-irradiation in advance, using a low-fluence laser, on the LIPSS formation. The properties of pre-irradiated PI films were characterized by X-ray photoelectron spectroscopy (XPS), surface tension based on the contact angle measurements and UV-vis spectra. It was found that pre-irradiation at low fluence led to the changes in surface property such as chemical components though no LIPSS was formed. As a result, threshold of LIPSS formation on such pre-irradiated PI film decreased and fine LIPSS with deeper amplitude was obtained.     

Effect of fluence and ambient environment on the surface and structural modification of femtosecond laser irradiated Ti

Under certain conditions, ultrafast pulsed laser interaction with matter leads to the formation of self-organized conical as well as periodic surface structures(commonly reffered to as, laser induced periodic surface structures, LIPSS). The purpose of the present investigations is to explore the effect of fsec laser fluence and ambient environments(Vacuum O_2) on the formation of LIPSS and conical structures on the Ti surface. The surface morphology was investigated by scanning electron microscope(SEM). The ablation threshold with single and multiple(N = 100) shots and the existence of an incubation effect was demonstrated by SEM investigations for both the vacuum and the O_2 environment. The phase analysis and chemical composition of the exposed targets were performed by x-ray diffraction(XRD) and energy dispersive x-ray spectroscopy(EDS), respectively. SEM investigations reveal the formation of LIPSS(nano micro). FFT d-spacing calculations illustrate the dependence of periodicity on the fluence and ambient environment. The periodicity of nano-scale LIPSS is higher in the case of irradiation under vacuum conditions as compared to O_2. Furthermore, the O2 environment reduces the ablation threshold. XRD data reveal that for the O_2 environment, new phases(oxides of Ti) are formed. EDS analysis exhibits that after irradiation under vacuum conditions, the percentage of impurity element(Al) is reduced. The irradiation in the O_2 environment results in 15% atomic diffusion of oxygen.     

Improvement of the ultraviolet-proof property of silica glass fibers for ArF excimer-laser applications

The effects of preirradiation by ArF excimer laser and thermal annealing on the transmission properties of a silica fiber were investigated. The durability of the fiber when it was exposed to ArF excimer laser irradiation was improved by preirradiation at room temperature and subsequent thermal annealing at 600 degrees C.     

紫外准分子激光损伤典型光学材料的特性分析

根据已经建立的紫外准分子激光损伤典型光学材料的理论模型,研究了准分子激光对透明光学材料（石英玻璃）和非透明光学材料（K9玻璃）的损伤特性,并结合实验结果证实了理论模型的有效性。研究表明：在准分子激光对非透明光学材料辐照下,激光光斑半径越大,产生的热应力和温度越小;脉宽越小,产生的热应力越大;随着脉冲数的增加,温度和热应力都逐渐增大。值得注意的是,当重频增加至45 Hz以上时,熔融损伤阈值开始低于应力损伤阈值,这说明当重频增加到一定程度时,非透明光学材料将首先产生熔融损伤,而不再是应力损伤。在准分子激光对透明光学材料辐照下,杂质微粒的半径和掩埋深度对光学材料温度场分布有着重要影响。但当杂质半径和掩埋深度超过一定的数值时,杂质粒子的存在与表面温度并无联系。理论模型能够较好地解释石英玻璃前/后表面相同的初始损伤形貌特征。     

Photochemical welding of silica microspheres to silicone rubber by ArF excimer laser

Transparent fused silica (SiO₂) microspheres 2.5 μm in diameter were photochemically welded to transparent, flexible silicone rubber ([SiO(CH₃)₂]_n) substrate by 193 nm ArF excimer laser induced photochemical modification of silicone into silicon oxide. Single layer of silica microspheres was easily formed on an adhesive silicone rubber before laser irradiation after dropping of silica microspheres dispersed in ethanol and subsequent tape peeling. The welding rate, the percentage of welded microspheres tested by ultrasonic cleaning with ethanol, was examined by varying the single pulse fluence and irradiation time of ArF excimer laser. The welding layer underneath microsphere, silicon oxide, was also found to emit white light of strong intensity under UV light illumination.     

Periodic nanostructures self-formed on silicon and silicon carbide by femtosecond laser irradiation

Laser-induced periodic surface structures (LIPSS) were formed on Si and SiC surfaces by irradiations with femtosecond laser pulses in air. Different kinds of self-organized structures appeared on Si and SiC at laser fluences slightly higher than the damage threshold, which was measured by confocal laser scanning microscope. The characteristic spatial periodicity of every observed structure was estimated reading the peak values of the 2D Fourier transform power spectra obtained from SEM images. The evolution of the spatial periodicity was finally studied with respect to both the laser fluence and the number of laser pulses. As already observed for metals, the behavior of the spatial periodicity on laser fluence can be related to the parametric decay of laser light into surface plasma waves. Our results suggest a wide applicability of the parametric decay model on different materials, making the model a useful tool in view of different applications of LIPSS.     

超短脉冲激光诱导周期性表面结构

激光诱导周期性表面结构（Laser-induced periodic surface structures,LIPSS）具有纳米尺度的特征结构和自重复的微观尺度的排列图案,因此,LIPSS在传感器、太阳能发电、光催化等方面具有广泛的应用前景。本文首先介绍LIPSS形成过程中超快激光与物质相互作用的复杂过程,强调瞬态光学性质和表面结构变化的作用。然后综述几种具有代表性的LIPSS形成机理,并且讨论了各自的优缺点。接着介绍了LIPSS形成过程中材料的变化,主要包括材料化学成分、晶体结构和表面微观结构的变化。最后综述了LIPSS在材料表面处理、光学和机械等方面的应用。     

Excimer laser-induced diamond graphitization for high-energy nuclear applications

In this work, we have studied the structure and the morphology of a graphite layer induced on the surface of a polycrystalline thermal grade CVD diamond by focusing a pulsed excimer laser operating at KrF (wavelength 248 nm) and ArF (wavelength 193 nm) mixtures. By micro-Raman and photoluminescence spectroscopies, as well as scanning electron microscopy, we reported the synthesis of a turbostratic t-graphite layer after irradiation with ArF laser. By contrast, irradiating with a KrF laser beam, we obtained a disordered graphite layer with 10 laser shots, while 200 consecutive laser pulses resulted in target ablation.     

Cleaning with water decomposed products obtained by laser irradiation

ArF excimer laser irradiation can decompose water, and decomposed products contain highly reactive substrates, such as radicals. We propose cleaning using pure water with the aid of water decomposed products obtained by ArF excimer laser irradiation. In this study, the oxidation potential of decomposed products was estimated in metal etching. Then, cleaning of cutting oil was examined. The focal point of the lens used was set at the water surface. Specimens were aligned parallel to the laser beam, so that only decomposed products affected contaminants. As a result, decomposed products could not etch nickel and copper plates, but only zinc plates. Cutting oil was cleaned after 18 000 irradiation pulses. The range of the region cleaned was 5 mm around the focal point.     

Temporal extension of stable glow discharges in fluorine-based excimer laser gas mixtures by the addition of xenon

The effect of addition of xenon on the long term homogeneity of discharges in F₂and ArF excimer laser gas mixtures was investigated in a small-volume discharge chamber. The gas mixture in the discharge chamber was preionized by X-rays. A special electrical excitation circuit containing a pulse forming line provided a long, square-shaped current pumping pulse of a predetermined duration to the discharge electrodes. The initiation and the development of the discharge was monitored via its fluorescence signal with an intensified CCD camera. We found that adding Xe up to partial pressures of 0.53 mbar extended the homogeneous phase of the discharge from 80 ns to approximately 200 ns in He/F₂as well as in He/Ar/F₂and Ne/Ar/F₂excimer laser gas mixtures. Monitoring of the ArF and XeF spontaneous emission signals showed that the formation of ArF excimers remained unaffected by the addition of xenon (up to 1.3 mbar) to the laser gas mixture.     

Low-temperature fabrication of silicon nitride films by ArF excimer laser irradiation

The low-temperature fabrication of silicon nitride films by ArF excimer laser irradiation has been studied. Two fabrication methods are presented. One is photoenhanced direct nitridation of a silicon surface with NH₃ for very thin gate insulators, and the other is photo-enhanced deposition of silicon nitride films with Si₂H₆ and NH₃ gases for stable passivation films. The ArF excimer laser irradiation dissociates the NH3 gas producing NH and NH₂ radicals which proved effective in instigating the nitridation reaction. The quality of both films has been much improved and the growth temperature has been lowered by using laser irradiation. These photo-enhanced processes seem to be promising ULSI techniques because they do not depend on high temperatures and are free from possible reactor contamination.     

Surface modification of a polymer film by cryogenic laser ablation of organosilicon compounds

An organosilicon compound, hexamethylcyclotrisilazane (HMCTS), was photolyzed in a solid film at 84 K with an ArF excimer laser. A polymeric film was prepared as a result of polymerization and/or crosslinking using a reactive species such as 1-(methylsilyl)methanimine, generated by photolysis. At room temperature in air, the film changed into a polymer having siloxane units. However, fragments generated by laser irradiation of HMCTS in the frozen film made a similar polymer film having a siloxane structure on a PVA film in air. The polymeric film with siloxane units prepared by the cryogenic laser ablation method displayed hydrophobic properties. Received: 23 May 2001 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Periodic surface structures on crystalline silicon created by 532 nm picosecond Nd:YAG laser pulses

Creation of laser-induced morphology features, particularly laser-induced periodic surface structures (LIPSS), by a 532 nm picosecond Nd:YAG laser on crystalline silicon is reported. The LIPSS, often termed ripples, were produced at average laser irradiation fluences of 0.7, 1.6, and 7.9 J cm⁻². Two types of ripples were registered: micro-ripples (at micrometer scale) in the form of straight parallel lines extending over the entire irradiated spot, and nano-ripples (at nanometer scale), apparently concentric, registered only at the rim of the spot, with the periodicity dependent on laser fluence. There are indications that the parallel ripples are a consequence of the partial periodicity contained in the diffraction modulated laser beam, and the nano-ripples are very likely frozen capillary waves. The damage threshold fluence was estimated at 0.6 J cm⁻².     

Excimer ArF laser with an output energy of 1.3 J at 2.0% efficiency on the He:Ar:F2 mixture

In this paper it is shown that to achieve a maximum efficiency and high output energy of an ArF (193 nm) excimer laser, one should use optimal pump intensity. It has been shown experimentally that the optimal pump intensity for an ArF excimer laser with the mixture of He:Ar:F₂ has a value of 4.5–5.0 MW/cm³. The results of an experimental study of the pump and active medium parameters effect on the efficiency and output energy of the ArF excimer laser on the mixture of He:Ar:F₂ are presented. To provide high pump intensity of an active medium, the excitation scheme of the LC-inverter type has been used where the current return conductor inductance had been increased from 30 to 80 nH. This allows the pump to achieve levels of intensity above 5.0 MW/cm³. By using the pump intensity of 5.0 MW/cm³ in an active medium of He:Ar:F₂–79.7:20:0.3 at total pressure of 2.4 atm, we are the first to obtain the output energy of 1.3 J at the total efficiency of 2.0%. The pulse duration (FWHM) was 15±1 ns and the peak pulse power was 85 MW. PACS 42.55.Lt; 42.60.Lh     

Laser-based techniques for living cell pattern formation

In the production of biosensors or artificial tissues a basic step is the immobilization of living cells along the required pattern. In this paper the ability of some promising laser-based methods to influence the interaction between cells and various surfaces is presented. In the first set of experiments laser-induced patterned photochemical modification of polymer foils was used to achieve guided adherence and growth of cells to the modified areas: (a) Polytetrafluoroethylene was irradiated with ArF excimer laser (λ=193 nm, FWHM=20 ns, F=9 mJ/cm²) in presence of triethylene–tetramine liquid photoreagent; (b) a thin carbon layer was produced by KrF excimer laser (λ=248 nm, FWHM=30 ns, F=35 mJ/cm²) irradiation on polyimide surface to influence the cell adherence. It was found that the incorporation of amine groups in the PTFE polymer chain instead of the fluorine atoms can both promote and prevent the adherence of living cells (depending on the applied cell types) on the treated surfaces, while the laser generated carbon layer on polyimide surface did not effectively improve adherence. Our attempts to influence the cell adherence by morphological modifications created by ArF laser irradiation onto polyethylene–terephtalate surface showed a surface–roughness dependence. This method was effective only when the Ra roughness parameter of the developed structure did not exceed the 0.1 micrometer value. Pulsed laser deposition with femtosecond KrF excimer lasers (F=2.2 J/cm²) was effectively used to deposit structured thin films from biomaterials (endothelial cell growth supplement and collagen embedded in starch matrix) to promote the adherence and growth of cells. These results present evidence that some surface can be successfully altered to induce guided cell growth.     

Nitric oxide flow tagging in unseeded air

A scheme for molecular tagging velocimetry is presented that can be used in air flows without any kind of seeding. The method is based on the local and instantaneous creation of nitric oxide (NO) molecules from N(2) and O(2) in the waist region of a focused ArF excimer laser beam. This NO distribution is advected by the flow and can be visualized any time later by laser-induced fluorescence in the gamma bands. The creation of NO is confirmed by use of an excitation spectrum. Two examples of the application of the new scheme for air-flow velocimetry are given in which single laser pulses are used for creation and visualization of NO.     

Laser-induced surface modification of polystyrene

The modification induced in polystyrene (PS) by the ArF excimer laser radiation has been investigated. Various numbers of the laser pulses of the energies below the material ablation threshold were applied. Changes in the chemical composition of the PS surface layer were studied by the X-ray photoelectron spectroscopy (XPS). Analysis of the morphological changes in the polymer surface layer was performed via the atomic force microscopy (AFM). The contact angles of test liquids (water and diiodomethane) were measured with use of a goniometer while the surface energy (SE) was calculated by the Owens-Wendt method. It was found that the surface energy change was mainly affected by surface roughness caused by the laser radiation and that surface oxidation had not considerably contributed to this change. The increase in the SE was mostly due to its disperse component.