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.     

In-bulk and surface structuring of sapphire by femtosecond pulses

The actual space-time dependent intensity distribution of a tightly focused (numerical aperture NA = 1.35) Gaussian femtosecond pulse is modeled inside dielectric material. Such focusing is typically used for recording with sub-wavelength resolution inside dielectrics. The multi-pulse structuring inside the bulk and on the surface of sapphire are demonstrated. Formation of nano-cracks and nano-crystals is revealed inside the crystalline sapphire. Ripple formation on the surface is discussed in terms of the efficacy map calculated by theory given in ref. [J.E. Sipe, J.F. Young, J.S. Preston, H.M. van Driel, Laser-induced periodic surface structure. I. Theory, Phys. Rev. B 27 (2) (1983) 1141-1154.].     

Sub-wavelength surface structuring of NiTi alloy by femtosecond laser pulses

Generation of self-organized sub-wavelength surface structures on a nickel–titanium alloy plate by femtosecond laser pulses is investigated experimentally through line-scribing experiments in air. It is found that Bragg-like relief gratings, with the orientation perpendicular to the laser polarization, are formed over the entire laser-scribed regions. The average period is measured as 630±30 nm. Distinctive features of these novel surface structures include nanoparticle-covered grating ridges and the maintainable spatial period regardless of incidence angles. With different laser parameters and sample scan speeds, sub-wavelength grating structures can evolve into cellular-like nanotextures. Optimal conditions for forming these surface structures are determined in terms of laser energy and scan speed. Elementary analyses of the structured surfaces by X-ray diffraction and photoelectron spectroscopy reveal that both the crystal structures and the chemical elements can remain in their original states, but the surface grains are refined and the atomic percentages are varied after femtosecond laser treatments.     

Laser-induced thermal stresses on steel surface

In laser heat treatment of steels, a thin surface layer of austenite forms during heating and subsequent phase change process in the cooling period. However, thermal stress develops due to high-temperature gradient attainment in the surface vicinity which in turn results in microcrack development at the surface. The present study is carried out to compute the temperature profiles due to step input pulse laser radiation and determine the resulting thermal stresses. The study is extended to include three-step input pulses having the same energy content. This provides the comparison for the influence of the pulse length on the resulting thermal stresses. To validate the theoretical predictions, an experiment is conducted to irradiate the AISI 4142 steel surface by an Nd–YAG laser. Microphotography and EDS analysis of the heated regions are carried out. It is found that considerable thermal stress is eveloped at the workpiece surface due to attainment of high-temperature gradient in this region. In addition, microcracks are observed at the surface of the irradiated spot.     

Laser-induced surface metallization of ceramic PLZT

Hot pressed, optically transparent ferroelectric ceramics of lanthanum modified lead zirconate titanate (PLZT) have been locally reduced and metallized by means of ultraviolet Kr⁺ laser irradiation in a hydrogen atmosphere. The technique allows maskless single-step metallic patterning of the material.On leave from Cietvielu Fizikas Institüts, Latvijas Valsts Universitäte, Riga, Latvian SSR, USSR     

Laser-induced resonance shifts of single molecules self-coupled by a metallic surface

The spectral properties of single molecules placed near a metallic surface are investigated at low temperatures. Because of the high quality factor of the optical resonance, a laser-induced shift of the molecular lines is evidenced for the first time. The shift dependence on the laser excitation intensity and on the dephasing rate of the transition dipole is studied. A simple theoretical model of a laser-driven molecule self-coupled by a mirror is developed to qualitatively interpret the observations.     

熔石英表面铜颗粒污染激光诱导损伤

利用SAGA-S激光器输出的355 nm波长激光,研究了熔石英表面铜颗粒污染的激光损伤规律。采用磁控溅射的方式在洁净熔石英表面制备不同尺寸的颗粒状污染物,用1-on-1,10-on-1,20-on-1的方式测试经污染后的熔石英基底的损伤阈值,并采用光学显微镜观察损伤形貌、CCD在线测量损伤斑尺寸。结果表明:污染后的熔石英基底的损伤主要发生在后表面,而且以热烧蚀和热应力为主,基底的损伤阈值与熔石英前表面污染颗粒尺寸呈负指数关系,随后表面污染物颗粒尺寸的增大呈略微下降。前表面颗粒污染物诱导损伤斑尺寸为颗粒污染物尺寸的4倍,后表面颗粒污染物引起的损伤斑尺寸约为颗粒污染物尺寸的2倍。并绘出损伤斑尺寸与颗粒尺寸、辐照方式之间的关系。     

Laser-induced formation and surface processing of High-Temperature superconductors

The use of lasers in the formation and surface processing of high-temperature superconductors (HTS) is reviewed. Presently, thin film fabrication by reactive laser sputtering, and surface patterning by laser-induced reduction/metallization and ablation are the most promising applications. The great majority of the investigations have been performed for Y-Ba-Cu-O.     

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。     

CO2 Laser-induced plasma formation on a copper surface covered by dielectric particles

The results of an experimental study of TEA CO₂ laser plasma formation in front of a copper surface covered by crystallites of sodium chloride are reported. It is found that the threshold laser-beam intensities for plasma formation depend upon the mean surface concentration of sodium chloride. The results obtained are explained within the frame of a thermal model of surface-plasma formation.     

Laser-induced quantum adsorption of atoms on a surface

A method of the quantum adsorption of atoms on a surface is proposed and experimentally implemented. The loading of atoms into a surface potential well (adsorption) occurs due to the loss of kinetic energy in the process of the inelastic collision of two laser-excited atoms. This scheme is implemented for Rb atoms adsorbed on the surface of a YAG crystal. The possibility of producing microstructures of arbitrary shape that consist of atoms localized on the dielectric surface is also demonstrated.     

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     

PECVD技术制备单层光学薄膜抗激光损伤特性研究

采用PECVD技术在BK7玻璃基底上沉积了不同厚度的单层SiO2（折射率为1.46）和SiNx（折射率为1.84）光学薄膜,并对这2种膜层进行抗激光损伤阈值（LIDT）测试,分析讨论了PECVD技术制备的单层光学薄膜与抗激光损伤特性之间的关系。实验结果表明:PECVD技术制备的单层SiO2薄膜有较高的LIDT,薄膜光学厚度在o/4~o/2之间时,在光学厚度为350 nm时,LIDT有最小值21.7 J/cm2,光学厚度为433 nm时,LIDT有最大值27.9 J/cm2。SiNx薄膜的LIDT随着光学厚度增加而减小,在光学厚度为o/4时,LIDT有最大值29.3 J/cm2,光学厚度为o/2时,LIDT有最小值4.9 J/cm2。     

Laser-induced breakdown of soda-lime glass microspheres using Nd:YAG laser

The Nd:YAG laser-induced breakdown of 20 μm glass microspheres was investigated using time-resolved optical shadow and Schlieren images. Time-resolved imaging showed the location of the initial breakdown and the shockwave motion over its first 400 μm of expansion. Measured shockwave velocities were in the range of 1–10 km/s and showed a linear dependency on laser fluence within 30 ns.     

Laser-induced breakdown threshold and fracture versus the nanostructure of two-phase glass

The radiation hardness and mechanical strength of single-and two-phase glasses are studied for the case when nanosecond laser pulses (λ= 1.06 μm, τ_0.5 ≈ 12.5 ns) are focused inside the material. Laser interferometry is applied to measure the displacement of the free surface, find optical breakage thresholds, and carry out the fractographic analysis of damaged regions. It is shown that breakdown channels and damage regions develop in a nonlinear manner according to optical breakdown mechanisms, changing each other with an increase in the laser energy. The strength of the two-phase glass is found to be more than four times that of the single-phase glass, although their elastic properties differ insignificantly. Such a considerable difference in the hardness of these materials with chemically similar constitutents is attributed to the presence of the double-lattice nanometer-scale structure of the two-phase glass.     

Laser-induced graphitization on a diamond (111) surface

We report an atomistic simulation study of laser-induced graphitization on the diamond (111) surface. Our simulation results show that the diamond to graphite transition occurs along different pathways depending on the length of the laser pulse being used. Under nanosecond or longer laser pulses, graphitization propagates vertically into bulk layers, leading to the formation of diamond-graphite interfaces after the laser treatment. By contrast, with femtosecond (0.2-0.5 ps) laser pulses, graphitization of the surface occurs layer by layer, resulting in a clean diamond surface after the ablation. This atomistic picture provides an explanation of recent experimental observations.     

UBK7玻璃后表面缺陷诱导体内激光损伤

对UBK7玻璃在波长为1.064靘的短脉冲激光作用下产生的损伤进行了研究。通过对体损伤的形貌的相衬显微镜观测,发现体内和后表面炸裂点间的丝状损伤,从而提出后表面缺陷导致体损伤的理论解释。同时分析了表面损伤和体损伤的机理,对后表面损伤阈值低于前表面的原因作了讨论。     

纳秒强激光诱导K9玻璃表面损伤实验

 使用脉宽约10 ns的Nd:YAG激光器,研究低加工缺陷条件下K9光学玻璃在单激光脉冲作用下的损伤形貌。利用软边光阑加长程衍射的方法实现光斑整形,利用微分干涉光学显微镜和扫描电镜对样品前后表面的损伤形貌进行观察和成像,对比研究了K9光学玻璃前后表面的损伤形貌,分析了损伤机制。研究结果显示：在红外纳秒激光辐照下,加工缺陷是K9光学玻璃产生初始损伤的主要诱因;前表面的损伤主要表现为微坑及高温等离子体产生的冲蚀变色和表面微裂纹;后表面出现了均匀的亚波长周期性光栅结构,这种周期性结构是后表面损伤增长的主要诱因。     

Laser-induced oxidation of the Si(111) surface

Pulsed laser induced oxidation of clean Si(111) surfaces has been studied by Auger electron spectroscopy and electron energy loss spectroscopy. The short duration time of the pulse has allowed a precise investigation of the first stages of the oxidation. About 1–2 oxide monolayers first grow in less than 10 s. Their stoichiometry evolves from SiO_x towards SiO₂ with increasing beam energy densities. Once this superficial layer has formed, no evolution is seen with further irradiation, suggesting that oxygen diffusion during the pulse duration cannot sustain the oxide growth.