Cross-sectional morphological profiles of ripples on Si, SiC, and HOPG

The cross-sectional profiles of ripple structures on silicon (Si), silicon carbide (SiC), and highly oriented pyrolytic graphite (HOPG) were studied by direct observation. The ripple structures were cut by an ion beam, and their cross sections were observed by scanning electron microscopy. The results showed that the cross sections of coarse ripples on Si and SiC have a convex shape with narrower valleys, whereas those of HOPG have sharp ridges and wide wings with a poorer aspect ratio. This difference may arise from the difference in material phase conversion processes induced by femtosecond laser irradiation. The cross-sectional profiles of fine ripples on SiC and HOPG, which give useful information on the ripple formation process, are also discussed.     

Biocompatibility of nanoactuators: stem cell growth on laser-generated nickel–titanium shape memory alloy nanoparticles

Nanoactuators made from nanoparticulate NiTi shape memory alloy show potential in the mechanical stimulation of bone tissue formation from stem cells. We demonstrate the fabrication of Ni, Ti, and NiTi shape memory alloy nanoparticles and their biocompatibility to human adipose-derived stem cells. The stoichiometry and phase transformation property of the bulk alloy is preserved during attrition by femtosecond laser ablation in liquid, giving access to colloidal nanoactuators. No adverse effect on cell growth and attachment is observed in proliferation assay and environmental electron scanning microscopy, making this material attractive for mechanical stimulation of stem cells.     

Analysis of plasma characteristics and conductive mechanism of laser assisted pulsed arc welding

This study aims to investigate the arc plasma shape and the spectral characteristics during the laser assisted pulsed arc welding process. The arc plasma shape was synchronously observed using a high speed camera, and the emission spectrum of plasma was obtained by spectrometer. The well-known Boltzmann plot method and Stark broadening were used to calculate the electron temperature and density respectively. The conductive mechanism of arc ignition in laser assisted arc hybrid welding was investigated, and it was found that the plasma current moved to the arc anode under the action of electric field. Thus, a significant parabolic channel was formed between the keyhole and the wire tip. This channel became the main method of energy transformation between the arc and the molten pool. The calculation results of plasma resistivity show that the laser plasma has low resistivity as the starting point of conductive channel formation. When the laser pulse duration increases, the intensity of the plasma radiation spectrum and the plasma electron density will increase, and the electron temperature will decrease.     

Fabrication of three-dimensional microfluidic channels in glass by femtosecond pulses

Fabrication of three-dimensional microfluidic channels in glasses by water-assisted ablation with femtosecond laser pulses was investigated. The experimental results showed that formation of the photoinduced microchannels by femtosecond pulses depended on the incident laser power and the scanning speed. For the same scanning speed, the shape of cross-section of channels changed from ellipse to circle with increasing the laser power. Under the optimum condition of laser processing, we fabricated two layers of microfluidic channels with diameter of about 8 μm inside glass. The distance between two layers of microchannels was about 20 μm.     

Influence of colloidal particle transfer on the quality of self-assembling colloidal photonic crystal under confined condition

The relationship between colloidal particle transfer and the quality of colloidal photonic crystal(CPC) is investigated by comparing colloidal particle self-assembling under the vertical channel(VC) and horizontal channel(HC) conditions.Both the theoretical analyses and the experimental measurements indicate that crystal quality depends on the stability of mass transfer.For the VC,colloidal particle transfer takes place in a stable laminar flow,which is conducive to forming high-quality crystal.In contrast,it happens in an unstable turbulent flow for the HC.Crystals with cracks and an uneven surface formed under the HC condition can be seen from the images of a field emission scanning electron microscope(SEM) and a three-dimensional(3D) laser scanning microscope(LSM),respectively.     

Surface Shape Measurement by Wavelength Scanning Interferometry Using an Electronically Tuned Ti:sapphire Laser

We developed a high speed and high resolution surface shape measurement system based on wavelength scanning interferometry with an electronically tuned Ti:sapphire laser. This laser emits pulses and the wavelength of each pulse can be tuned arbitrarily within 680 nm and 1056 nm. We also designed a high speed multiport CCD camera as a detector. This camera is synchronized with wavelength scanning of the laser at the frequency of 250 Hz. We could measure the object shape with a height resolution of 3.05 μm at the tuning range from 740.0 nm to 842.3 nm. By simple parabolic curve fitting to the Fourier peak of interference signals from each CCD pixel, the height resolution has been improved to the order of submicrometers.     

高精度基准平面建立方法分析

基准平面的确立,是进行表面参数评定的基础;建立理想的基准平面,在几何量形位公差检测及相关工程测量方面具有重要作用.为了建立高精度的激光扫描基准平面,对扫描平面形成过程中光束的传播进行了详细分析;根据光学矢量反射定律,推导出了扫描误差的理论公式;在此基础上揭示了误差补偿的基本原理,导出了用于补偿扫描机构产生的扫描误差的理论公式;提出了据此准则进行设计的扫描机构的模型.分析表明,用激光及其扫描装置建立光学基准面时,扫描误差是不可避免的,这种误差不加补偿,则最终将引入基准光学平面影响基准精度,进而降低参数的评定精度;借助于所推导的误差补偿公式,是可以补偿这种误差的,这对于建立高精度的扫描基准平面具有理论指导意义.     

Experimental observation and numerical simulation of guided modes in Nd:YLiF4 channel waveguides produced by carbon ion implantation

We report on, to our knowledge the first time, the channel waveguide formation in Nd:YLiF₄ laser crystal produced by 6 MeV carbon ion implantation. The guided modes are observed by using an end‐face arrangement. We construct the two‐dimensional (2D) refractive index profile of the channel waveguide cross section, which is based on the related planar waveguide index distribution as well as the rectangular shape of the waveguide cross sections. The modal intensity distribution is numerically calculated by using the beam propagation method according to the reconstructed index profile, which shows a reasonable agreement with the experimental result. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Plasma waveguide formation in predissociated clustering gases

We report on the use of a novel technique to create a plasma waveguide suitable for guiding high-intensity laser pulses in underdense plasmas. A narrow channel of a clustering gas is dissociated with a low-intensity prepulse. This prepulse is followed by a high-intensity, focused laser pulse. The high absorption of the clusters surrounding the dissociated atomic channel causes the remaining annulus of clusters to become highly ionized, leaving low-density plasma in the center. We have interferometrically probed the formation of this channel with picosecond laser pulses.     

Nonuniform composition profile in In0.5Ga0.5As alloy quantum dots

We use cross-sectional scanning tunneling microscopy to examine the shape and composition distribution of In0.5Ga0.5As quantum dots (QDs) formed by capping heteroepitaxial islands. The QDs have a truncated pyramid shape. The composition appears highly nonuniform, with an In-rich core having an inverted-triangle shape. Thus the electronic properties will be drastically altered, relative to the uniform composition generally assumed in device modeling. Theoretical analysis of the QD growth suggests a simple explanation for the unexpected shape of the In-rich core.     

Fast manipulation of laser localized structures in a monolithic vertical cavity with saturable absorber

We show the spontaneous and controlled formation of bistable and localized laser spots in the transverse section of a monolithic vertical cavity laser with a saturable absorber. Successive incoherent writing and erasure is obtained up to 80 MHz repetition rate with a 60 ps localized excitation. We also show the formation of clusters of laser localized states. All these observations are in good qualitative agreement with existing models.     

干涉法诊断由纳秒激光诱导产生的大气等离子体的电子密度

通过改进的马赫-曾德尔干涉仪获得了高质量的Nd:YAG激光诱导大气等离子体干涉条纹图.利用快速傅里叶变换（FFT）分析法恢复了干涉图波面,通过Abel逆变换进行密度反演,重建了不同时刻激光等离子体电子密度的三维分布,并得到了激光等离子体膨胀速度与延迟时间的关系.结果显示,纳秒激光诱导大气击穿形成的等离子体具有等离子体通道结构,等离子体膨胀速度的迅速衰减,对等离子体通道的塌陷起到了促进作用,等离子体形状的离心率在大约48 ns时达到最大值,然后开始向圆形演变. 关键词： 激光等离子体电子密度 干涉测量 Abel逆变换     

Self-focusing, channel formation, and high-energy ion generation in interaction of an intense short laser pulse with a He jet

Using interferometry, we investigate the dynamics of interaction of a relativistically intense 4-TW, 400-fs laser pulse with a He gas jet. We observe a stable plasma channel 1 mm long and less than 30 microm in diameter, with a radial gradient of electron density approximately 5 x 10(22) cm(-4) and with an on-axis electron density approximately ten times less than its maximum value of 8 x 10(19) cm(-3). A high radial velocity of the surrounding gas ionization of approximately 3.8 x 10(8) cm/s has been observed after the channel formation, and it is attributed to the fast ions expelled from the laser channel and propagating radially outward. We developed a kinetic model which describes the plasma channel formation and the subsequent ambient gas excitation and ionization. Comparing the model predictions with the interferometric data, we reconstructed the axial profile of laser channel and on-axis laser intensity. The estimated maximum energy of accelerated ions is about 500 keV, and the total energy of the fast ions is 5% of the laser pulse energy.     

Structure formation in excimer laser ablation of stretched poly(ethylene therephthalate) (PET): the influence of scanning ablation

Multiple pulse laser ablation of stretched PET is performed with an ArF excimer laser (193 nm) in order to produce micro channels. The surface structure remaining after "scanning ablation", in which the sample is moved during irradiation, is compared to the known results upon "static ablation". We observed that the debris contribution is enhanced upon scanning ablation, which has a major impact for the channels used in micro-fluidic applications. A more fundamental change of the channel floor structure, as seen by SEM, occurs at high fluences and short mask lengths. The channel floor structure originates from the structure on the irradiated ramp at the end of the channel. The "scanning structure" appears only if the irradiated end ramp forms an angle higher than 10.55(ǂ.15)° with the non-irradiated sample surface. The angle of light incidence is not responsible for the structure changes, in agreement with literature reports. Other possible mechanisms are briefly discussed.     

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. I. Experimental results

Using a plasma channel produced by an ultrashort laser pulse, we have studied the laser triggering and guiding of a positive leader from the tip of a 2-m vertical rod standing on the bottom plane of a 7-m plane-plane gap. The purpose of this setup was to reproduce in the laboratory the electric field conditions leading to the onset of a positive upward leader from a ground rod as a downward negative leader is approaching during a thunderstorm, in order to demonstrate the working principle of a possible future laser lightning rod. The leader triggering properties of the laser-created plasma channel have been studied as a function of the synchronization of the laser pulse with the voltage impulse applied to the gap. We show that the laser pulse reduces the inception voltage of the leader compared to its normal value and that the laser plasma channel guides the propagation of the upward leader at a velocity ten times higher than that of an ordinary leader, with a significantly lower charge per unit length. We show that laser guiding of the leader significantly reduces the breakdown voltage of the gap and that the effect of the laser channel at the end of a lightning rod can be compared quite favorably with the effect of an additional metal rod of the same length.     

Self-organized nanostructure by a femtosecond laser on silicon

In this paper, ripple structure and hole arrays were investigated. Techniques based on laser scanning microscopes for the processing of periodic structures on silicon with a femtosecond laser (800 nm, 1 kHz, 130 fs) had been used. Ripples and any holes were obtained after laser irradiation in air with S and P polarization. The effect of the scanning speed (v) and the lens with different numerical apertures (NA) on the laser-induced surface topography were studied. We found that the femtosecond laser produces periodic ripples on the silicon surface of the submicron level, the width of line has a tight relationship to the NA and scanning speed. Finally, we process the microapparatus on silicon, for which the wide channel was 800 nm.     

Dual-axes confocal microscopy with post-objective scanning and low-coherence heterodyne detection

We present a dual-axes confocal microscope that employs postobjective scanning and low-coherence heterodyne detection to collect vertical cross-sectional images from biological tissue with high axial resolution, reduced noise from scattered light, deep tissue penetration, and a large dynamic range. This architecture can be scaled down to millimeter dimensions with microelectromechanical systems technology for performance of in vivo optical biopsy.     

A plasma channel scheme for the interaction of an intense femtosecond laser pulse with a deuterium cluster jet

We propose a plasma channel scheme to obtain an improved table-top laser driven fusion neutron yield as a result of explosions of large deuterium clusters irradiated by an intense laser pulse. A cylindrical plasma channel is created by two moderate intensity laser prepulses at the edge of a deuterium cluster jet along which an intense main laser pulse propagates several nanoseconds later. With the aid of this plasma channel, the main laser pulse will be allowed to deposit its energy into the central region of the deuterium gas jet where the cluster sizes are larger and the atomic density is higher. The plasma channel formation and its impact on the deuterium ion energy spectrum and the consequent fusion neutron yield have been investigated. The calculated results show that a remarkable increase of the table-top laser driven fusion neutron yield would be expected.     

强激光在窄通道中传播的理论研究

在考虑相对论和有质动力非线性以及全局电量守衡的前提下,分析了强激光在冷等离子体窄通道中稳定传播的情况。采用较为简化的二维理论模型,给出了描述激光和通道横向结构的解,对不同通道宽度、通道密度、激光强度和电磁模式等进行了讨论,分析了其对激光在等离子体通道中传播的影响。分析发现,在存在预通道的情况下,当等离子体通道的密度大于临界密度很多时(例如20倍临界密度),即使是在激光波长量级的通道中,激光仍然可以传播。通道越宽,等离子体密度越小;激光强度越大越容易传播。在同样的通道和传输情况下,TE0模传输所需要的激光强度比TE1模要小。     

Formation of plasma channels in the interaction of a nanosecond laser pulse at moderate intensities with helium gas jets

We report on a detailed study of channel formation in the interaction of a nanosecond laser pulse with a He gas jet. A complete set of diagnostics is used in order to characterize the plasma precisely. The evolution of the plasma radius and of the electron density and temperature are measured by Thomson scattering, Schlieren imaging, and Mach-Zehnder interferometry. In gas jets, one observes the formation of a channel with a deep density depletion on axis. Because of ionization-induced defocusing which increases the size of the focal spot and decreases the maximum laser intensity, no channel is observed in the case of a gas-filled chamber. The results obtained in various gas-jet and laser conditions show that the channel radius, as well as the density along the propagation axis, can be adjusted by changing the laser energy and gas-jet pressure. This is a crucial issue when one wants to adapt the channel parameters in order to guide a subsequent high-intensity laser pulse. The experimental results and their comparison with one-dimensional (1D) and two-dimensional hydrodynamic simulations show that the main mechanism for channel formation is the hydrodynamic evolution behind a supersonic electron heat wave propagating radially in the plasma. It is also shown from 2D simulations that a fraction of the long pulse can be self-guided in the channel it creates. The preliminary results and analyses on this subject have been published before [V. Malka et al., Phys. Rev. Lett. 79, 2979 (1997)].