Influence of focusing depth on the microfabrication of waveguides inside silica glass by femtosecond laser direct writing

We studied the influence of focusing depth on the index change threshold and damage threshold of silica glass irradiated by a focused 120 fs laser beam. Both thresholds increased with the focusing depth. The aspect ratio of the waveguide cross section can be selected by changing the focusing depth. A 5 mm long waveguide was written at the depth of 2100 μm, which was single mode at 632.8 nm and exhibited propagation loss of 0.56 dB/cm. The refractive index change was calculated to be ∼2.47×10^-3. The influence of the focusing depth should be considered in multi-layer devices as shown in the fabrication of a 3×3 waveguide array. PACS 42.62.-b; 42.82.Et; 81.05.Kf     

Effect of beam diameter on the propagation of intense femtosecond laser pulses

This paper describes the effects observed during the propagation of intense femtosecond laser pulses in air following the modification of the laser beam diameter with a pair of convex–concave lenses placed/mounted in a telescopic configuration. We observed that by reducing the diameter of the beam the detected back-scattered nitrogen fluorescence from the filaments becomes more stable on a shot-to-shot basis while, for a larger beam size, greater fluctuations are observed that are not correlated to shot-to-shot fluctuations in the laser pulse energy. This result leads to a new method to control the fluorescence signal which can be very important in remote-sensing applications. PACS 42.65.Jx; 42.68.Ay; 42.68.Wt     

Spectral hole burning and fluorescence in femtosecond laser induced Sm-doped glasses

The femtosecond laser was used to irradiate sol-gel derived Sm³⁺-doped Al₂O₃-SiO₂ glasses, in which the Sm³⁺ was reduced into Sm²⁺ ions. The fluorescence line narrowing was applied to investigate the coordination sphere of the Sm²⁺ ion. The spectral hole burning was performed on ⁷F₀→⁵D₀ transition of the Sm²⁺. The depth and width of the burnt holes were ∼27% and ∼4 cm⁻¹ FWHM at 7 K, respectively. Hole spectra were stable up to room temperature. The hole-burning efficiency was superior to that of Sm²⁺ in H₂ treated glasses and comparable to that in X-ray in terms of hole-burning dynamics.     

飞秒激光烧蚀硅表面产生微纳结构过程中激光脉冲数目的影响

为了研究飞秒激光脉冲数目与硅表面形貌之间的关系,在相同的SF6气体氛围下,改变照射硅表面的飞秒激光脉冲数,发现在飞秒激光照射下由硅表面形成的微型锥状尖峰的高度与飞秒激光脉冲数呈现一种非线性关系.通过对该关系的研究有利于找出在制造具有较高吸收效率的高微型锥状尖峰的"黑硅"的实验条件,有利于基于"黑硅"材料的光电器件转化效...     

掺Er光纤飞秒激光器中电光晶体对激光器参数的影响

由于受增益介质上能级寿命的影响,掺Er光纤光梳的梳齿线宽一般在百kHz量级.为了实现光梳梳齿线宽的压窄,一种有效的方法是在激光器中增加快速响应的电光晶体,使光纤光梳的伺服锁定带宽提高到百kHz以上,为光纤光梳的快速伺服锁定提供反馈机构.这其中,高品质的飞秒激光器是核心.基于此,本文主要研究了掺Er光纤飞秒激光器中电光晶...     

Influence of laser parameters and material properties on micro drilling with femtosecond laser pulses

We report experimental results of drilling micro holes in Al, Mo, Ti, Cu, Ag, Au, and brass thin metal foils in air using 60-femtosecond Ti:sapphire laser pulses. The influence of laser parameters and material properties on drilling processes at the sub-10-micron scale has been examined. Comparison of hole drilling using 60-fs pulses with that using 50-ps and 10-ns pulses at an energy fluence of ~15 J/cm² indicates that a significant amount of melting is present in both ps and ns regimes, whereas with fs pulses, fewer and much smaller droplets from the molten phase are observed.     

偏振对飞秒激光辐照LiF晶体的影响

飞秒激光聚焦到LiF晶体内部, 晶体的加工形貌随偏振改变. 实验表明, 偏振方向平行于<110> 晶向时, 加工起点到表面的距离是<100>偏振下的1.08 倍; 而<110>偏振下加工终点到表面的距离是<100> 偏振下的1.01 倍. 为了解释加工形貌的偏振依赖, 建立了逆韧致辐射、雪崩电离和无辐射跃迁的模型, 首先, 价带电子通过强场电离和雪崩电离, 从激光中吸收能量跃迁到导带, 该过程用电子密度演化方程和傍轴非线性薛定谔方程描述, 求解方程得到导带电子密度; 其次, 导带电子通过无辐射跃迁过程释放能量给晶格, 由能量守恒计算出晶格温度沿激光传播方向的分布; 最后, 晶格温度超过熔点以上的区域被加工. 模拟结果显示, <110>偏振下加工起点到表面的距离是<100> 偏振下的1.03倍, 而<110>偏振下加工终点到表面的距离是<100>偏振下的0.981 倍, 与实验结果基本一致. 虽然Z扫描技术测量的非线性折射率随偏振方向变化, 但是非线性折射率的变化趋势与实验结果相反. 模拟和实验证明逆韧致辐射导致加工形貌随偏振变化.     

Enhanced near field mediated nanohole fabrication on silicon substrate by femtosecond laser pulse

Investigation of the process of nanohole formation on silicon surface mediated with near electromagnetic field enhancement in vicinity of gold particles is described. Gold nanospheres with diameters of 40, 80 and 200 nm are used. Irradiation of the samples with laser pulse at fluences below the ablation threshold for native Si surface, results in a nanosized surface modification. The nanostructure formation is investigated for the fundamental (λ = 800 nm, 100 fs) and the second harmonic (λ = 400 nm, 250 fs) of the laser radiation generated by ultrashort Ti:sapphire laser system. The near electric field distribution is analyzed by an Finite Difference Time Domain (FDTD) simulation code. The properties of the produced morphological changes on the Si surface are found to depend strongly on the polarization and the wavelength of the laser irradiation. When the laser pulse is linearly polarized the produced nanohole shape is elongated in the E-direction of the polarization. The shape of the hole becomes symmetrical when the laser radiation is circularly polarized. The size of the ablated holes depends on the size of the gold particles, as the smallest holes are produced with the smallest particles. The variation of the laser fluence and the particle size gives possibility of fabricating structures with lateral dimensions ranging from 200 nm to below 40 nm. Explanation of the obtained results is given on the basis simulations of the near field properties using FDTD model and Mie's theory.     

Molecular depth profiling and imaging using cluster ion beams with femtosecond laser postionization

The emergence of cluster ion sources as viable SIMS probes has opened new possibilities for detection of neutral molecules by laser postionization. Previous studies have shown that with atomic bombardment multiphoton ionization using high-power femtosecond pulses leads to photofragmentation. The large amount of photofragmentation can be mostly attributed to high amounts of internal energy imparted to the sputtered molecules during the desorption process. Several pieces of preliminary data suggest that molecules subjected to cluster beam bombardment are desorbed with lower internal energies than those subjected to atomic beam bombardment. Lower energy molecules may then be less likely to photodissociate creating less photofragments in the laser postionization spectra. Here we present data taken from coronene films prepared by physical vapor deposition comparing a 40 keV ion source with a 20 keV Au⁺ ion source, which supports this hypothesis. Furthermore, the depth profiling capabilities of cluster beams may be combined with laser postionization to obtain molecular depth profiles by monitoring the neutral flux. In addition, imaging and depth profiling may be combined with atomic force microscopy (AFM) to provide three-dimensional molecular images.     

Fabrication of large-area hole arrays using high-efficiency two-grating interference system and femtosecond laser ablation

We present a novel method to fabricate hole arrays by forming a four-beam interference pattern with two gratings. In this method a femtosecond laser beam is split into four and collected to interfere using two cascaded diffractive gratings. One benefit of this grating pair is that it is achromatic, because of the geometry of the grating pair, and therefore it is suitable for femtosecond ablation. Grating pairs were designed and fabricated for a standard Ti:sapphire femtosecond laser, with 800-nm central wavelength, so that the interference pattern generates holes with less than 1-μm diameter. Holes with this size diffract with a colorful visual appearance in the visible wavelength range and therefore these structures are suitable for security, authentication and decorative marking. We show that this method is suitable for fast ablation of hole arrays in both silicon and steel.     

Generation of debris in the femtosecond laser machining of a silicon substrate

A mechanism of debris accumulation around the hole created on a silicon substrate with a femtosecond laser is experimentally studied. The hole created by the laser pulses has an edge consisting of pillared structures. However, no solid or liquid materials with similar dimensions as the pillared structures were ejected from the hole as the laser machining was performed in vacuum or in the presence of various gases. When the target was placed in a gas under 100 Torr, a thick layer of cotton-like material formed around the hole. Laser irradiation of a vertically placed target resulted in re-solidified material that formed after dripping from the hole edge, showing that substrate was partially melted. The surface morphology features suggest that the debris was formed as a result of aggregation of small particles such as atoms or atomic clusters on the substrate surface instead of large-scale droplets or fragments, which are commonly observed with nanosecond lasers. PACS 78.67.Bf; 81.15.Fg; 81.16.Mk; 81.20.Wk; 81.65.Cf     

Influence of polarization on the hole formation with picosecond laser

Linear polarization effects on laser drilling were investigated by the use of a picosecond laser system. It was observed that the exit shapes of stainless steel were placed perpendicular to the direction of polarization. According to theoretical analysis, the main factor that determines the exit hole shape is the difference between R _s and R _p. When this gap is large enough, the bulges on the exit hole will be easily observed. Inversely, when the gap is small, bulges can be barely spotted on the exit hole. This is further confirmed by comparative experiment and calculation conducted on a copper sheet.     

The influence of substrate temperature on femtosecond laser micro-processing of silicon,stainless steel and glass

We report the influence of substrate temperature on femtosecond laser ablation of silicon, stainless steel, and glass. Remarkable decrease in surface roughness was observed under high substrate temperature for silicon and stainless steel. While the ablation efficiency of glass as a typical wide band-gap material is scarcely altered at 900 K, the efficiency for stainless steel as a conductor apparently increased about 20% accompanied to the elevation of substrate temperature from 300 to 900 K. Silicon wafer results in slight increase of the ablation efficiency with decreasing the ablation threshold. Considering that the melting temperature of glass is much lower than those of silicon and steel, the observations from this work suggests that the material ablation caused by the ultrafast laser irradiation could not be explained in term of only laser-induced thermal excitation.     

Heating process and damage threshold analysis of Au film coated on Cu substrate for femtosecond laser

The heating processes of a two-layer film assembly of Au padded with Cu irradiated by femtosecond laser pulse are studied using a two-temperature model. It is found that the chosen substantially influence the energy transport, and consequently the temperature variation, and thermal equilibrium time. At the same laser fluence, the different thickness of gold film leads to a change of gold surface temperature. By choosing the thickness of the gold layer in the two-layer film assemblies, the damage threshold of the gold film can be maximized. The results can be used to optimize the damage threshold of gold coating optical components.     

Surface enhanced Raman scattering silica substrate fast fabrication by femtosecond laser pulses

We report the fabrication of surface enhanced Raman spectroscopy (SERS) fused silica glass substrates using fast femtosecond-laser (fs-laser) scan, followed by silver chemical plating. A cross-section enhancement factor (EF) of 2.5×10⁶, evaluated by Rhodamine 6G (10⁻⁷ M solution), was obtained. The Raman mapping indicated a good uniformity over the fs-laser scanned area. The dimension and pattern of the SERS activated region can be conveniently controlled by laser 2D scanning, potentially enabling integration of SERS into a high-order optical–chemical analysis system on a glass chip.     

Formation of nanostructures by an intense femtosecond pulse laser irradiating Au film on sapphire substrate

We investigate the morphology change of Au film on sapphire substrate by irradiating with a 1 kHz femtosecond pulse laser. Under observation of a scanning electron microscope, a textured nanostructure was formed in the exposed area on Au film due to laser ablation and subsequent stress relaxation. This process was strongly determined by the laser intensity profile and the dynamics of molten liquid. With the increasing of laser pulses number, the Au film was broken down and then a few polarization-dependent nanoripples arranged in the same direction appeared on the sapphire surface, which may result from a spatial modulation of energy due to the interference between the incident light and the excited surface plasmon polaritons. In addition, we used an energy dispersive spectrometer to analyze the chemical composition of nanoripples on the surface and in the ablated crater, respectively. The changes of O and Al elements implied that a complicated chemical transformation participated in the nanoripples formation process. We believe that present results are very useful for the analysis of the interaction between femtosecond laser and solids, especially the film material.     

Influence of femtosecond laser marking on the corrosion resistance of stainless steels

Marking is of prime importance in the field of biomaterials to allow the identification of surgical tools as well as prostheses. Nowadays, marking is often achieved by means of laser beam, which may modify the characteristics of the treated surfaces. The use of laser devices delivering nanosecond pulses is known to induce dramatic corrosion degradations during sterilization or decontamination processes of the biomaterials. The aim of the present study is to investigate the ability of femtosecond (pulse duration in the 10⁻¹⁵ s range) laser treatments to avoid preferential corrosion processes of the marked areas, in order to extend the durability and the reliability of biomaterials. Experiments have been performed on martensitic Z30C13 and austenitic 316L stainless steels. Electrochemical measurements (cyclic polarization curves) were carried out to determine the passive state of samples before and after engraving, their corrosion rate and their susceptibility to localized corrosion. Further protracted immersion tests were also carried out to evaluate the natural long-term degradation of engraved parts. The electrochemical behavior is then explained on the basis of surface characterizations. Femtosecond laser marking is shown to provide an electrochemical ennoblement. Moreover, the chemical composition is not affected so that the passive character of both stainless steels is maintained, even improved if we consider the susceptibility to localized corrosion.     

Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses

By moving silica glass in a preprogrammed structure, we directly produced three-dimensional holes with femtosecond laser pulses in single step. When distilled water was introduced into a hole drilled from the rear surface of the glass, the effects of blocking and redeposition of ablated material were greatly reduced and the aspect ratio of the depth of the hole was increased. Straight holes of 4-mu;m diameter were more than 200 microm deep. Three-dimensional channels can be micromachined inside transparent materials by use of this method, as we have demonstrated by drilling a square-wave-shaped hole inside silica glass.     

High-resolution monitoring of the hole depth during ultrafast laser ablation drilling by diode laser self-mixing interferometry

We demonstrate that diode laser self-mixing interferometry can be exploited to instantaneously measure the ablation front displacement and the laser ablation rate during ultrafast microdrilling of metals. The proof of concept was obtained using a 50-μm-thick stainless steel plate as the target, a 120?ps/110?kHz microchip fiber laser as the machining source, and an 823?nm diode laser with an integrated photodiode as the probe. The time dependence of the hole penetration depth was measured with a 0.41?μm resolution.