Unipolar resistive switching in high-resistivity Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> junctions

We report the first demonstration of laser forward transfer using a real-time reconfigurable mask based on a spatial light modulator. The ability to dynamically change the projected beam shape and size of a coherent light source, in this case a 355-nm pulsed UV laser, represents a significant technological advancement in laser direct-write processing. The application of laser transfer techniques with adaptive control of the laser beam pattern is unique and represents a paradigm shift in non-lithographic processing. This work describes how the size and shape of an incident laser beam can be dynamically controlled in real time with the use of a digital micromirror device (DMD), resulting in laser-printed functional nanomaterials with geometries identical to those of the projected beam. For applications requiring additive non-lithographic techniques, this novel combination, which relies on the laser forward transfer of variable, structured voxels, represents a dramatic improvement in the capabilities and throughput of laser direct-write processes.     

铷原子热蒸气中强非线性效应产生激光模式图样的实验研究

研究了具有高斯横向分布的连续激光束单次通过铷原子蒸气后,在近共振附近铷原子蒸气中,由强的非线性克尔效应导致激光分裂成细丝的现象,并且这些细丝的衍射图样在远场通过相干叠加,可以形成具有规则结构的斑图模式.实验上研究了输入光功率,铷泡温度和抽运激光频率相对于⁸⁵Rb原子D₂线的不同失谐位置等因素对斑图模式的影响.由于铷原子的超精细能级结构,在铷原子蒸气中同时存在与三阶非线性效应相关的四波混频现象,通过扫描探测光的频率同时观察到具有斯托克斯和反斯托克斯光子的拉曼增益现象. 关键词： 铷原子蒸气 克尔效应 自聚焦 斑图     

Interference of transverse rings in multifilamentation of powerful femtosecond laser pulses in air

We observe multiple filaments and interference of their ring structures in the propagation of 14 mJ, 45 fs infrared laser pulse in air. We suggest a simple physical model describing the formation and the interference of rings as the result of superposition of the background field of the whole beam and the fields that diverge from the filaments due to the defocusing in the laser-produced plasma. The size and the number of maxima in the interference pattern depend on the position of the filament formation along the direction of propagation. The simulated picture of the ring structure interference is in qualitative agreement with the one obtained from the experiment.     

Control of multiple filamentation in air

In this Letter we provide what is believed to be the first experimental evidence of suppression of the number of filaments for high-intensity laser pulses propagating in air by beam astigmatism. We also show that the number, pattern, and spatial stability of the filaments can be controlled by varying the angle that a focusing lens makes with the axial direction of propagation. This new methodology can be useful for applications involving atmospheric propagation, such as remote sensing.     

Interference pattern generation and simulation in the single beam of a white light continuum

We demonstrate the interference phenomenon in the White Light Continuum(WLC) generated by a single femtosecond laser beam. Different kinds of spatial interference patterns of the WLC generation under various conditions were investigated. The spatial patterns were attributed to interference between the filaments in the WLC generated by the fundamental laser beam yielding the diffraction effect from spatial confinement. Simulations of different patterns were performed. By comparing the results of simulation with those of experiments,the distances of several micrometers between the neighboring filaments can be derived,which agree with the literature values from direct measurements.     

脉冲激光引起铜膜镜面的环形损伤波纹研究

 用波长1.06μm、半高宽10ns的脉冲Nd:YAG激光辐照铜膜镜面，在激光辐照区，用光学显微镜观察到有规律的环形波纹状损伤图案，波纹平均周期约几十μm。通过对光路系统分析，认为样品前的小孔光阑对激光产生了菲涅尔衍射，使得在样品表面光强分布变成周期性环状分布。在极短的相互作用时间内，热扩散很小，损伤图案依赖于光强分布。并依据实验参数，用柯林斯公式对样品表面的光强分布进行了计算，所得光强分布的周期与损伤波纹的周期基本一致。     

The morphology of pulsed laser-induced damage on the surface of a copper film mirror

A copper film mirror was irradiated using a pulsed Nd:YAG laser (1.06 μm,10 ns) through an aperture. After laser irradiation, the damaged spot was analyzed using an optical microscope and periodic damage rings were found at the laser irradiation spot, the average spacing of these rings being about 40–50 μm. The damage ring could be observed in the periphery of the laser spot under lower laser energy densities. As the laser fluence increased, the periodic damage rings grew to cover the whole laser spot. Under higher laser energy density, the whole film was damaged and the rings disappeared. A Fresnel diffraction model is used to explain the appearance of these rings. The laser beam is diffracted by the aperture in the optic path, the laser intensity distribution at the copper film mirror becomes periodic rings and damage ripple pattern forms instantly with the laser ablation. The laser intensity distribution at the mirror surface is calculated using Collins formula, with which the period of the damage rings and other phenomena can be explained.     

Generation of a variable linear array of phase-coherent supercontinuum sources

We investigate the coherence properties of a linear array of white-light sources produced in bulk media by ultrashort laser pulses. The array is generated out of the spatial interference pattern between two laser pump pulses, so that the number of supercontinuum sources and their separations can be easily manipulated by varying the geometry of the laser beam interaction. We find that all the secondary white-light sources which arise from the generation of filaments in the optical medium are well phase-locked and are thus able to generate stable and high-visibility multiple-beam interference patterns in the far-field. Observations are compared to the results of a simple model which takes into account a clamping of the peak laser intensity inside the filaments and includes intensity-dependent phase shifts among the different sources. PACS 42.65.Jx; 42.65.Ky; 42.65.Re     

Refractive index change mechanisms in femtosecond laser written ceramic Nd:YAG waveguides: micro-spectroscopy experiments and beam propagation calculations

The effect that femtosecond laser filamentation has on the refractive index of Nd:YAG ceramics, and which leads to the formation of waveguide lasers, has been studied by micro-spectroscopy imaging, beam propagation experiments and calculations. From the analysis of the Nd³⁺ luminescence and Raman images, two main types of laser induced modifications have been found to contribute to the refractive-index change: (i) a lattice defect contribution localized along the self-focusing volume of the laser pulses, in which lattice damage causes a refractive-index decrease, and (ii) a lattice strain-field contribution around and inside the filaments, in which the pressure-driven variation of the inter-atomic distances causes refractive-index variations. Scanning near-field optical-transmission and end-coupling experiments, in combination with beam propagation calculations, have been used to quantitatively determine the corresponding contribution of each effect to the refractive-index field of double-filament waveguides. Results indicate that the strain-field induced refractive-index increment is the main mechanism leading to waveguiding, whereas the damage-induced refractive-index reduction at filaments leads to a stronger mode confinement.     

Two-dimensional angular filtering by volume Bragg gratings in photothermorefractive glass

We propose and perform two-dimensional angular filtering on the basis of volume Bragg gratings (VBGs) in a series connection for cleaning up a laser beam. The dependence between angular selectivity and VBG parameters is analyzed with the coupled wave theory. Near-field modulation, contrast ratio, and power spectral density were used to evaluate the effect of filtering in both the spatial and frequency domain. The results from our initial experiments showed that the near-field modulation and contrast ratio of laser beam were remarkably improved.     

Two-dimensional organization of a large number of stationary optical filaments by adaptive wave front control

We present an adaptive technique for the formation of multiple co-propagating and stationary filaments in a gaseous medium. Wavefront shaping of the initial beam is performed using a deformable mirror to achieve a complete two-dimensional control of the multi-spot intensity pattern in the laser focus. The spatial organization of these intensity spots yields reliable formation of up to five stable and stationary filaments providing a test bed for fundamental studies on multiple filamentation. PACS 42.65.Jx; 52.38.Hb; 42.65.Sf     

Angular laser cleaning for effective removal of particles. from a solid surface

Laser removal of small particles from a metal surface is carried out by changing the incident angle of the laser beam. It has been found that a dramatic improvement of cleaning efficiency in terms of area and energy is observed when using the laser at glancing angle of incidence as compared to perpendicular. Furthermore substrate damage is greatly reduced and probably eliminated at glancing angles. The process mechanism is discussed by considering the adhesion and the laser-induced cleaning forces for different incident angles. It is shown that there are different laser–matter interactions operating. Received: 25 April 2000 / Accepted: 9 May 2000 / Published online: 5 October 2000     

离子束清洗在激光薄膜中的应用

介绍了在激光薄膜中End Hall型离子源离子束清洗的应用。通过实验验证了基片的二次污染和离子束的清洗效果,观测了离子束清洗前后基片的表面形貌变化。研究了用离子束清洗基片时对薄膜抗激光损伤阈值的作用。分析了用离子束清洗基片时其基片表面的性质,如清洁度、表面能、接触角、表面形貌的变化机理。指出了杂质微粒的去除和附着力的增加是如何使薄膜抗激光损伤阈值显著提高的。     

A novel optical technique for the estimation of porosity in porous silicon thin films

Porous silicon (por-Si) can be produced when silicon single crystals are submerged in fluoride solutions and irradiated with laser light. The shape of the por-Si film is determined by the laser beam intensity profile. When laser light is reflected from a Gaussian-shaped film, a divergent beam, which exhibits ring patterns, is observed. The rings are formed by a combination of optical interference and Fresnel diffraction. The size of the pattern is determined by the shape and depth of the film interfaces as well as the index of refraction of the film. The index of the film is determined by the porosity and the index of the fluid that fills the pores. We explore the application of measurements of the reflected beam patterns to the determination of porosity for por-Si thin films. We report the first direct estimation of the porosity of photochemically produced porous silicon. Porosities of 70-95% are found for n-type Si(111) etched in 48% HF with 633-nm illumination. Having demonstrated the success of this technique, we discuss improvements and extensions that can be made.     

强激光圆对称超高斯光束的自聚焦环

 通过数值求解非线性薛定谔方程,详细描述了圆对称超高斯光束在Nd:glass介质中传输形成的自聚焦环。初始光束受到扰动后,环就会分裂形成强烈的细光束,显示出自聚焦环的不稳定性,细光束具有很大的峰值光强,将对介质造成极大的危害。     

相干彩虹的形成机制

用一束白光激光聚焦照射液体(水、丙酮、无水乙醇、汽水等)或固体(冰、有色玻璃、塑料、彩色蜡等),出现了多级的彩色干涉环,即相干彩虹.高强度白光局部地加热了液体(固体),改变了它的密度(以及折射性质),从而导致光程差的出现,不同波长的光都发生干渉,形成了彩色的干涉环.有色玻璃在反射模式下也出现了相干彩虹,此时的干涉完全来自于玻璃表面轮廓的变化,并且无参数拟合的结果定量地符合观测到的干涉图案.     

超声清洗与激光预处理对减反膜阈值的影响

 研究了超声清洗和激光预处理两种后处理手段对减反膜的损伤特性的影响。采用电子束蒸发技术制备了1 064 nm减反膜,利用超声清洗及激光预处理的方法分别对样品进行处理,并对处理前后的样品分别进行激光损伤阈值测试及破斑深度测量。结果表明：处理后减反膜的损伤阈值均有所提升,但激光预处理的阈值增强效果更加明显;超声清洗前后的破斑深度没有大的变化,而激光预处理后的破斑深度比处理前浅得多;原因在于超声清洗只能去除表面杂质,激光预处理可减少和抑制膜层内较深处的缺陷。     

Measuring the topological charge of optical vortices with an axicon

We analyzed the spatial spectrum of the diffraction intensity pattern of an ideal Bessel beam and found an implicit rule that the number of the bright rings in the spatial spectrum is equal to the topological charge of the Bessel beam. The radius of the bright and dark rings has some relation with the topological charge and can be determined accurately. It provides us with a new way for measuring the topological charge of an optical vortex through its diffraction intensity pattern after an axicon. The results of simulation coincide with the theory.     

Towards a control of multiple filamentation by spatial regularization of a high-power femtosecond laser pulse

We propose a new method for controlling randomly generated multiple filaments during the propagation of femtosecond laser pulses in optical media. The method is based on introducing a periodic amplitude modulation of the transverse beam profile. It is shown both experimentally and numerically that the introduction of a periodic mesh into a propagation path of a femtosecond near-infrared laser pulse leads to a deterministic spatial distribution of multiple filaments in the presence of initial random fluctuations. As a result, the number of filaments is increased as compared to the random case. PACS 42.65.Jx; 42.25.Bs; 42.60.Jf     

Highly nonlinear fundamental mechanisms of excitation and coloring of wide-gap crystals by intense femtosecond laser pulses

Analysis of the spatial distribution of the color centers formed in wide-gap LiF and MgF₂ crystals in a laser beam channel has shown that these centers are formed in numerous longitudinal filaments into which a laser beam splits when propagating in a medium. The luminescence of the produced color centers is photoluminescence, which is excited by the supercontinuum radiation in the filaments.