Second harmonic generation by femtosecond Yb-doped fiber laser source based on PPKTP waveguide fabricated by femtosecond laser direct writing

The frequency doubling of femtosecond pulses from an Yb-doped fiber laser source was demonstrated in a PPKTP waveguide fabricated by femtosecond laser direct writing. The PPKTP waveguide contains a fixed period of 8.9 μm and the feomtosecond fundamental pulses have a central wavelength of 1044 nm. A maximum SHG power of 406 mW was produced, yielding a conversion efficiency of 5.6%. Numerical simulations were carried out to investigate the property of frequency doubling for femtosecond pulses. The results show that the SHG process proceeds even the quasi-phase-matching (QPM) condition is not well satisfied, which is significantly different from that of “long” pulses or CW light and is accorded with the experimental results.     

Influence of scanning velocity on femtosecond laser direct writing lines on FOTURAN glass

Lines are induced on the surface of a photosensitive (FOTURAN) glass by focused femtosecond laser transverse writing with scanning velocity in a wide range of 40- 1800μm/s. The formed lines are analyzed using scanning electron microscope (SEM) and optical microscope (OM). It is observed that three distinct morphologies of lines are produced depending on the scanning velocity. Lines written in low velocity level (40 - 100 μm/s) and high velocity level (1000 - 1800 μm/s) are uniform and regular, while those written in moderate velocity level (150 - 600 μm/s) are rough. The influence of scanning velocity is explained based on different pulses overlapping or cumulative dose of laser exposure in irradiated area. Fabrication of shallow groove on the surface is also demonstrated.     

Characteristics of filament induced Dammann gratings fabricated using femtosecond laser

To establish optimal processing conditions for direct write fabrication of diffractive optical elements such as gratings, waveguides, lenses, we have investigated the effect of process parameters such as scan speed, numerical aperture (NA) of objective lens, pulse energy on the characteristics of the filament induced inside fused silica with a femtosecond Ti:sapphire laser. The optimum process parameters were used to fabricate a number of Dammann gratings, 6×6 array, having different thicknesses and number of layers. The performance of these optical elements was evaluated by measuring their diffraction efficiencies. All gratings fabricated were strongly birefringent, the zero order spot with high intensity was not separated from the spot array, and the intensity distribution of 6×6 spot array exhibited some degree of nonuniformity. The single layer Dammann grating fabricated with a thickness of 80 μm attained a maximum diffraction efficiency of 38.8%.     

Soda-lime glass microlens arrays fabricated by laser: Comparison between a nanosecond and a femtosecond IR pulsed laser

We present the manufacturing of microlens arrays on soda-lime glass substrates by using two different IR pulsed lasers: a nanosecond Nd:YVO₄ laser (1064 nm) and a femtosecond laser based on Ytterbium crystal technology (1030 nm). In both cases, the fabrication technique consists of the combination of a direct-write laser process, followed by a post-thermal treatment assisted by a CO₂ laser. Through the analysis of the morphological characteristics of the generated microlenses, the different physical mechanisms involved in the glass ablation process with a nanosecond and a femtosecond laser are studied. In addition, by analyzing the optical features of the microlenses, a better result in terms of the homogeneity and quality of the spot focuses are observed for those microlenses fabricated with the Nd:YVO₄ nanosecond laser. Microlens arrays with a diameter of 80 and 90 µm were fabricated.     

Flexible gratings fabricated in polymeric plate using femtosecond laser irradiation

Flexible gratings embedded in poly-dimethlysiloxane (PDMS) were fabricated using femtosecond laser pulses. Photo-induced gratings in a flexible PDMS plate were directly written by a high-intensity femtosecond (130 fs) Ti: Sapphire laser (λ_p=800 nm). Refractive index modifications with 4 μm diameters were photo-induced after irradiation of the femtosecond pulses with peak intensities of more than 1×10¹¹ W/cm². The graded refractive index profile was fabricated to be symmetric around the center of the focal point. The diffraction efficiency of the grating samples is measured by an He-Ne laser. The maximum value of refractive index change (Δn) in the laser-modified regions was estimated to be approximately 3.17×10⁻³.     

Superhydrophobic surfaces fabricated by microstructuring of stainless steel using a femtosecond laser

Fabrication of superhydrophobic surfaces induced by femtosecond laser is a research hotspot of superhydrophobic surface studies nowadays. We present a simple and easily-controlled method for fabricating stainless steel-based superhydrophobic surfaces. The method consists of microstructuring stainless steel surfaces by irradiating samples with femtosecond laser pulses and silanizing the surfaces. By low laser fluence, we fabricated typical laser-induced periodic surface structures (LIPSS) on the submicron level. The apparent contact angle (CA) on the surface is 150.3°. With laser fluence increasing, we fabricated periodic ripples and periodic cone-shaped spikes on the micron scale, both covered with LIPSS. The stainless steel-based surfaces with micro- and submicron double-scale structure have higher apparent CAs. On the surface of double-scale structure, the maximal apparent CA is 166.3° and at the same time, the sliding angle (SA) is 4.2°.     

飞秒激光作用下的硅表面微结构及发光特性

 采用近红外飞秒激光辐照浸泡在硫酸溶液中的N型单晶硅片,激光波长800 nm,脉宽200 fs,重频1 kHz,平均功率为100 mW,而硫酸溶液的质量分数分别选择为0.1%和1%。辐照后硅表面呈直径为5~8 mm,高度15 mm的柱型结构。分析其荧光特性,并通过比较硅材料表面微结构与激光光源、扫描参数、硅片背景环境的关系,确定最佳辐照条件为激光扫描速度750 mm/s,扫描间距5 mm/s。最终在厚度0.5 mm、直径26 mm的硅片上获得10 mm×10 mm的方形扫描区域,荧光光谱显示激光扫描后的区域在700 nm附近有很强的荧光发射。分析结果表明飞秒激光扫描改变了样品的表面微结构尺寸,增大了吸收面积,扩展了荧光激发波长,有效提高了样品的吸收效率和荧光发光相对强度(超过扫描前发光相对强度的2倍),荧光发射谱的变化是由量子限制效应和表面态模型共同作用的结果。     

Three-dimensional microfluidic channel with arbitrary length and configuration fabricated inside glass by femtosecond laser direct writing

We demonstrate, for the first time to the best of our knowledge, fabrication of three-dimensional microfluidic channels with arbitrary lengths and configurations inside glass by femtosecond laser direct writing. The main fabrication process includes two steps: (1) direct formation of hollow microchannels in a porous glass substrate immersed in water by femtosecond laser ablation and (2) postannealing of the glass substrate at ～1150°C by which the porous glass can be consolidated. We show that a square-wavelike channel with a total length of ～1.4 cm and a diameter of ～64 μm can be easily produced ～250 μm beneath the glass surface.     

Generation of bubbles in glass by a femtosecond laser

Permanent microscale bubbles with varied size and number density are induced in borosilicate glasses by adjusting the focusing depth (FD) of a tightly focused femtosecond laser. With continuously increasing of the focusing depth, the average size of generated bubbles experiences an increase-decrease process. However, the number density of generated bubbles experiences an opposite changing process compared to the change of the size. The possible mechanism for the bubble generation and changing with the focusing depth has been discussed.     

Self-assembled volume vortex grating induced by femtosecond laser pulses in glass

We presented a microfabrication process for optical volume vortex grating inside glass by femtosecond laser pulses. The self-trapped filament of femtosecond laser pulses can induce hundreds μm-long region refractive-index changes in glass. We realized the restructured optical vortex beams using a collimated He–Ne laser beam. The maximum first-order diffraction efficiency was about 19.6%. The volume vortex grating structure fabricated in glass is polarization dependent.     

Optical waveguide writing inside Foturan glass with femtosecond laser pulses

We investigated the femtosecond laser writing of optical waveguides inside Foturan glass at various pulse energies and focusing depths. An optimal waveguide fabricated solely by femtosecond laser irradiation showed a refractive index modulation of ∼1.7×10^-3 and a minimum transmission loss of ∼0.80 dB/cm. This type of waveguide had lower transmission loss than those fabricated by a hybrid process of femtosecond laser exposure and following thermal treatment. An optical splitter was also fabricated at high pulse energy. PACS 42.65.Re; 42.82.Et; 42.70.Gi     

飞秒激光在光敏玻璃内制作微孔

用20倍显微物镜将波长为775 nm的飞秒激光聚焦在光敏玻璃（FOTURAN）内部,通过纵向写制模式由表面以下500 μm曝光至表面,并结合热处理和在浓度8%的氢氟酸超声溶液中腐蚀50 min,在FOTURAN内部制作了直径为几十μm的微孔.利用光学和扫描电子显微镜分析发现微孔具有圆形横截面和清晰边缘,目前得到的深宽比大约为7.通过在宽范围内改变入射激光能流（2.3~36.2 J/cm2）和写制速度（100~1 000 μm/s）,研究了这两项飞秒激光入射参量对制作微孔的影响.发现写制速度对制作微孔直径影响较小,而利用相对低的入射激光能流曝光可得到较大深宽比的微孔,并且在此情形下制作微孔的横截面更圆,璧面光滑度更高,并分析了原因.     

Single- and multi-scan femtosecond laser writing for selective chemical etching of cross section patternable glass micro-channels

We report on the fabrication of three dimensional micro-fluidic channels in fused silica glass using a combination of femtosecond laser writing and hydrofluoric acid wet etching to flexibly create various cross-sectional profiles of highly uniform shape and smooth vertical walls. The laser power, polarization, focusing depth, scanning angle and scanning speed were systematically studied with single- and multi-scan configurations to assess optimum micro-channel formation including etch rate, surface roughness, and stress-induced crack formation. We introduce the formation of vertical access-ports that extend the buried channel formation to unlimited length without tapering or distortion of the channel cross-sectional shape.     

3D integration of microcomponents in a single glass chip by femtosecond laser direct writing for biochemical analysis

3D integration of microcomponents in a single glass chip by femtosecond laser direct writing followed by post annealing and successive wet etching is described for application to biochemical analysis. Integration of microfluidics and microoptics realized some functional microdevices like a μ-fluidic dye laser and a biosensor. As one of practical applications, we demonstrate inspection of living microorganisms using the microchip with 3D microfluidic structures fabricated by the present technique.     

Fabrication of microchannels in glass using focused femtosecond laser radiation and selective chemical etching

We use the combination of femtosecond laser dielectric modification and selective chemical etching to fabricate high-quality microchannels in glass. The photoinduced modification morphology has been studied in fused silica and in borosilicate glass BK7, using ultra-high spatial resolution techniques of selective chemical etching followed by atomic force or scanning electron microscopy. The analysis shows that the high differential etch rate inside the modified regions, is determined by the presence of polarization-dependent self-ordered periodic nanocracks or nanoporous structures. We also investigate the optimum irradiation conditions needed to produce high-aspect ratio microchannels with small symmetric cross-sections and smooth walls. PACS 42.62.-b; 42.65.Re; 81.05.Kf; 87.80.Mj     

Fabrication of microstructures in Foturan glass using infrared femtosecond laser pulses and chemical etching

In this study, a method for the fabrication of microstructures on the surface and inside Foturan glass by femtosecond laser-induced modification was developed. This technique was followed by heat treatment to crystallize the modified area, and the specimen was then placed in an 8% HF acid solution for chemical etching. The fabricated microstructures were observed using scanning electron microscopy (SEM). The results demonstrated that the etching time is an important parameter in the fabrication of microstructures on Foturan glass. An example of a tapered U-shaped microchannel with a minimized neck diameter of about 5 μm at the central point for cell detection is presented.     

Surface treatment of CFRP composites using femtosecond laser radiation

In the present work, we investigate the surface treatment of carbon fiber-reinforced polymer (CFRP) composites by laser ablation with femtosecond laser radiation. For this purpose, unidirectional carbon fiber-reinforced epoxy matrix composites were treated with femtosecond laser pulses of 1024 nm wavelength and 550 fs duration. Laser tracks were inscribed on the material surface using pulse energies and scanning speeds in the range 0.1–0.5 mJ and 0.1–5 mm/s, respectively. The morphology of the laser treated surfaces was investigated by field emission scanning electron microscopy. We show that, by using the appropriate processing parameters, a selective removal of the epoxy resin can be achieved, leaving the carbon fibers exposed. In addition, sub-micron laser induced periodic surface structures (LIPSS) are created on the carbon fibers surface, which may be potentially beneficial for the improvement of the fiber to matrix adhesion in adhesive bonds between CFRP parts.     

Laser-active colour centres with functional periodic structures in LiF fabricated by two interfering femtosecond laser pulses

We report a new technique to fabricate both laser-active F₂ and F₃⁺ colour centres in lithium fluoride and permanent periodic gratings with fringe spacings as fine as sub-micron size simultaneously by two interfering infrared femtosecond (fs) laser pulses. In particular, the optical properties of such colour centres produced by a single fs laser pulse are compared with those created by damage from radiation such as X-rays. Moreover, the present technique is applied to the first production of three-dimensional active channel waveguide and a pulsed distributed-feedback (DFB) laser at around 700 nm in LiF containing F₂ colour centres with fine-pitched micro-grating structures.     

Fabrication of glass micro-prisms using ultra-fast laser pulses with chemical etching process

In this study, a new process of glass micro-prism structures is investigated by an ultra-fast laser irradiation with chemical etching process. The ultra-fast laser is employed by an all-in-one femtosecond laser (FS-laser) system with the amplifier as an excitation source for patterning the structures. Here, the center wavelength of laser is frequency-doubled to 517 nm. Besides, the repetition rate and pulse width of laser are 100 kHz and 350 fs, respectively. First, the embedded gratings of glass with different pitches can be fabricated using a FS-laser process. Afterwards, the glass samples are placed in the hydrofluoric acid (HF) solution for 15 min to develop structures. Finally, the results of this study demonstrated that the V-cut micro-prisms are successfully formed by controlling etching concentration between intrinsic glass material and modified areas.     

Sub-micron scale patterning using femtosecond laser and self-assembled monolayers interaction

Standard positive photoresist techniques were adapted to generate sub-micron scale patterns of gold substrate using self-assembled monolayers (SAMs) and femtosecond laser. Self-assembled monolayers formed by the adsorption of alkanethiols onto gold substrate are employed as very thin photoresists. The process underlying photopatterning of SAMs on gold is well-known at the phenomenological level. Alkanethiolates formed by the adsorption of alkanethiols are oxidized on exposure to UV light in the presence of air to alkylsulfonates. Specifically, it is known that deep UV light of wavelength less than 200 nm is necessary for oxidation to occur. In this study, solid state femtosecond laser of wavelength 800 nm is applied for photolithography. The results show that ultrafast laser of near infrared (NIR) range wavelength can replace deep UV laser source for photopatterning using thin organic films. The essential basis of our approach is the photochemical excitation of specific reactions in a particular functional group (in this case a thiolate sulfur atom) distributed with monolayer coverage on a solid surface. Femtosecond laser photolithography could be applied to fabricate the patterning of surface chemical structure and the creation of three-dimensional nanostructures by combination with suitable etching methods.