Sub-micron ablation of metallic thin film by femtosecond pulse laser

The ability to machine very small features in a material has a wide range of applications in industry. We ablated holes into thin film of 100 nm thickness made from various metals by femtosecond pulsed laser ablation. Using a Ti:Sapphire laser which supplies a laser pulse of 150 fs duration at central spectrum wavelength of 400 nm, we have produced a series sub-micron holes, whose diameters are less than 200 nm with a focused laser spot of 1.7 μm. We found that the material damage threshold has a great influence on the quality of the produced features. Experimental results shows that the heat-affected zone and the degree of being affected reduce with the increase of threshold value.     

Sub-micron patterning of solid materials with ultraviolet femtosecond pulses

The treatment of solid materials using subpicosecond laser pulses at 248 nm is studied. The potential of the combination of femtosecond pulse durations with ultraviolet wavelengths for precise material processing is demonstrated. Imaging techniques combined with diffractive optical elements are exploited for the fabrication of nanometer-scale features on practically all classes of materials including metals, semiconductors, and dielectrics. Two-dimensional periodic surface structures with excellent quality and reproducibility over large areas with feature sizes down to the 100 nm range are reached .An extension of this technique to transparent solids to reach defect sizes of a few hundreds of nanometers inside various materials is also possible. As a technical highlight, efficient drilling of a great number of holes with diameters below 500 nm through metal foils is demonstrated.Irradiation is performed using a UV laser system based on a Ti:Sapphire-Excimer hybrid device generating 300 fs pulses at 248 nm. Operation at 300 Hz (resulting in 10 W average output power) allows high speed machining, opening up new possibilities in industrial applications. PACS 42.15.Eq; 41.85.Ct; 52.38.Mf; 81.16.Rf; 81.65.Cf     

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.     

Er:Yb-doped waveguide laser fabricated by femtosecond laser pulses

Laser action is demonstrated in a 20-mm-long waveguide fabricated on an Er:Yb-doped phosphate glass by femtosecond laser pulses. An output power of 1.7 mW with approximately 300 mW of pump power coupled into the waveguide is obtained at 1533.5 nm. Waveguides are manufactured with the 520-nm radiation from a frequency-doubled, diode-pumped, cavity-dumped Yb:glass laser operating at a 166-KHz repetition rate, with a 300-fs pulse duration.     

Embedded birefringent computer-generated holograms fabricated by femtosecond laser pulses

Birefringent computer-generated holograms are fabricated in bulk fused silica by tight focusing of infrared femtosecond laser pulses. The polarization properties of the elliptically polarized diffracted light are in excellent agreement with the theoretical model. We experimentally demonstrate that for such birefringent structures the signal-to-noise ratio increases by approximately 9 dB when polarization filtering is used to suppress the undiffracted beam.     

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.     

Large-area surface-enhanced Raman scattering-active substrates fabricated by femtosecond laser ablation

A rapid and simple approach to fabricate large-area surface-enhanced Raman scattering-active(SERS-active) substrates is reported.The substrates are fabricated by using femtosecond laser(fs-laser) direct writing on Silicon wafers,followed by thin-film coating of metal such as gold.The substrates are demonstrated to exhibit signal homogeneity and good enhancement ability for SERS.The maximum enhancement factor(EF) up to 3×10 7 of such SERS substrates for rhodamine 6G(R6G) at 785 nm excitation wavelength was measured.This technique could demonstrate a functional microchip with SERS capability of signal homogeneity,high sensitivity and chemical stability.     

Microlens arrays of high-refractive-index glass fabricated by femtosecond laser lithography

Microlens arrays of high-refractive-index glass GeO₂-SiO₂ were fabricated by femtosecond laser lithography assisted micromachining. GeO₂-SiO₂ thin glass films, which were deposited by plasma-enhanced chemical vapor deposition, have a refractive index of 1.4902 and exhibit high transparency at wavelengths longer than 320 nm. Using a femtosecond laser, three-dimensional patterns were written inside resists on GeO₂-SiO₂ films, and then the patterns were transferred to the underlying films by CHF₃ and O₂ plasma treatments. This combined process enabled us to obtain uniform microlens structures with a diameter of 38 μm. The heights of the transferred lenses were approximately one-quarter the height of the resist patterns, due to differences in the plasma etching rates between GeO₂-SiO₂ and the resist. The lens surfaces were smooth. When 632.8-nm-wavelength He-Ne laser light was normally coupled to the lenses, focal spots with a diameter of 3.0 μm were uniformly observed. The combined process was effective in fabricating three-dimensional surfaces of inorganic optical materials.     

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.     

Thermal process of silica glass microchannels fabricated by femtosecond laser ablation

In order to improve the morphology of microchannels fabricated by femtosecond laser ablation, the thermal process was introduced into the post-treatment processing. It was found that the thermal process cannot only decrease the roughness but also the width and depth of the microchannel. The change rates of width, depth, and roughness of the microchannel increase with processing temperature. When we prolong the time of constant temperature, the change rate of the width decreases at the beginning, and then it tends to be stable. However,the change rates of depth and roughness increase, and then they tend to be stable. In this Letter, we discuss the reasons of the above phenomena.     

Birefringent Fresnel zone plates in silica fabricated by femtosecond laser machining

We demonstrate maskless, single-step fabrication of strongly birefringent Fresnel zone plates by focusing of femtosecond laser pulses deep within silica substrates. The process allows us to produce alternate zone rings directly by inducing a local refractive-index modification of the order of n~10(-2) . The embedded zone plates shown in this Letter exhibit efficiencies that vary by as much as a factor of ~6 for orthogonal polarizations. Focal lengths of primary and secondary foci are shown to compare well with theory.     

Forming mechanisms and wettability of double-scale structures fabricated by femtosecond laser

Constructing double-scale structures on material surface is helpful for achieving its superhydrophobicity. Therefore, it is quite necessary to investigate the forming mechanisms of double-scale structures on material surface. In the study, the forming mechanisms of various structures induced by femtosecond laser on the K9 glass surface are discussed, such as crater, ripples, and cones. The surface with double-scale structures is fabricated by femtosecond laser. On the surface, the apparent contact angle and sliding angle of a droplet are 152.3±1.5° and 4.6±0.8°, respectively. Lastly, through theoretical and experimental analyses, we demonstrate that the double-scale structures are effective for realizing superhydrophobicity of material surface and restricting the wetting transition from Cassie mode to Wenzel mode.     

Liquidly process in femtosecond laser processing

Nano-sized water-crown like structure in array was firstly generated on metallic thin film by interfering femtosecond laser processing. We named the structure as “nanocrown”. Ridges are standing on the edge of each ablated hole. The shapes of ridges are spike, nano-waterdrop and bead on column. The radius of the top of a spike was just 7 nm, which is far smaller than that of nanobump generated in the previous work. The self-rising in liquidly process result in the generation of mesoscopic nanostructure with the size between nanohorn or nanotube and micron structures processed by machining or lithography. This is a new surface modification technique in top-down technology.     

Stable superhydrophobic surface with hierarchical mesh-porous structure fabricated by a femtosecond laser

Inspired by the lotus leaf, a new superhydrophobic surface with hierarchical mesh-porous structure is fabricated by femtosecond laser irradiation on silicon. The fabricated surface shows a superhydrophobic character with water contact angle being found to reach up to 158^°±1^° and sliding angle of 4^°±0.5^°. The superhydrophobicity is stable even if the PH of solution changes from 1 to 14. And the surface also exhibits excellent self-cleaning effect and bouncing behavior, implying that the adhesion of the surface is extremely low. This work will enhance further understanding of the wettability of a solid surface with special surface morphology.     

Wavelength division with three-dimensional couplers fabricated by filamentation of femtosecond laser pulses

Refractive-index changes can be induced by filamentation of 800-nm, 1-kHz femtosecond laser pulses in silica glass. Two-dimensional translation of a 40-microm-long filament leads to the formation of a curved waveguide because of bending by the previously induced refractive-index change. The fabrication of 2-mm directional couplers to split the coupled beam into 1:1 at a wavelength of 632.8 nm is demonstrated. The realization of three-dimensional directional couplers and wavelength division in the output from the couplers is also demonstrated.     

Few-layer graphene film fabricated by femtosecond pulse laser deposition without catalytic layers

The films of few-layer graphene are formed through laser exfoliation of a highly ordered pyrolytic graphite(HOPG), without a catalytic layer for the growth process. The femtosecond(fs) laser exfoliation process is investigated at different laser fluences and substrate temperature. For fs laser exfoliation of HOPG, the few-layer graphene is obtained at 473 K under an optimal laser fluence. The formation of few-layer graphene is explained by removal of intact graphite sheets occurred by an optimal laser fluence ablation. The new insights may facilitate the controllable synthesis of large area few-layer graphene.     

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°.     

Fiber inline interferometric refractive index sensors fabricated by femtosecond laser and fusion splicing

A fiber inline interferometric refractive index （RI） sensor consisting of a microchannel and a fiber taper is proposed in this letter. The microchannel is fabricated by combining femtosecond laser micromachining and arc fusion splicing. No subsequent chemical etching process is needed. Three sensors with microchannel widths of 4, 8, and 10 μm are prepared. The sensitivity in the RI range from 1.33 to 1.35 is up to -361.29 nm/RIU at the microchannel width of 8 μm. The sensitivity is -20 times greater than that of the paired taper-based MZI sensors and long period fiber grating pair MZI sensors.     

基于高重复频率飞秒激光直写技术制作衍射光栅的研究

基于由光子晶体光纤飞秒激光器产生的高重复频率飞秒激光搭建了飞秒激光微加工系统,利用LabVIEW编写程序精密控制三维移动平台的移动路径和高速快门,在加工功率为1.5W、移动速率为1mm/s时,利用直写技术在无胶铬版上得到了微米量级的任意形状的平面衍射光栅。利用CCD相机对光栅制备过程进行实时监视,保证了光栅的加工质量。制备的光栅在显微镜下观察,条纹边缘清晰、线纹粗细均匀。使用He-Ne激光照射加工的光栅,可以获得清晰、稳定、与理论符合很好的夫琅和费(Frauhofer)衍射图样,显示了所加工的光栅具有良好的光学性能。实验结果也表明了光子晶体光纤飞秒激光在光栅制作过程中所具备的良好的加工性能。