Micromachining of a thin film by laser ablation using femtosecond laser with masks

A gold thin film was machined by laser ablation using a femtosecond laser with mask patterns in the shape of lines and numbers. The patterns were successfully transferred with proper focusing and laser fluence. The optimal femtosecond laser fluence to keep the line width was about 5.2 mJ/cm² on the mask, and 99 mJ/cm² on the film. The processing resolution was 13 μm, and the narrowest line width was about 4 μm.     

Micromachining of circular ring microstructure by femtosecond laser pulses

In this paper, integration of interference phenomenon into femtosecond laser micromachining was reported as the femtosecond laser pulses were reshaped spatially to perform ablation. The generation of circular interference pattern was demonstrated by overlapping infrared femtosecond laser pulses. The interference pattern was subsequently focused on a copper substrate to ablate microstructures of concentric circular rings. The present technique is expected to open up new applications in the areas of rapid fabrication of micro-Fresnel lenses, hybrid microlenses and lens arrays.     

Micromachining NiTi tubes for use in medical devices by using a femtosecond laser

Recent growth in medical device technology has been substantially driven by developments in laser micromachining, which is a powerful fabrication technique in which nickel–titanium (Nitinol, NiTi) alloy materials that exhibit superelastic and shape memory properties are formed (e.g., self-expanding stents). In this study a NiTi tube curve surface process is proposed, involving a femtosecond laser process and a galvano-mirror scanner. The diameter of the NiTi tube was 5.116 mm, its thickness was 0.234 mm, and its length was 100 mm. The results indicated that during the machine process the ablation mechanism of the NiTi tubes was changed by altering the machining path. The path alteration enhanced the laser ablation rate from 12.3 to 26.7 μm/J. Thus the path alteration contributed to a wide kerf line, enabling the assisted air to efficiently remove the debris deposited at the bottom of the kerf during the laser ablation process. The results indicated that the NiTi tube curve process enhanced the laser ablation rate by two times and reduced the amount of energy accumulated within the materials by 50% or more. By altering the machining path using the scanning system, this process can decrease the production of heat affected zones (the accumulation of thermal energy) in medical device applications.     

Surface ablation of lithium tantalate by femtosecond laser

We report measurements of the laser induced breakdown threshold in lithium tantalate with different number of pulses delivered from a chirped pulse amplification Ti: sapphire system. The threshold fluences were determined from the relation between the diameter D² of the ablated area and the laser fluence F₀. The threshold of lithium tantalite under single-shot is found to be 1.84 J/cm², and the avalanche rate was determined to be 1.01 cm²/J by calculation. We found that avalanche dominates the ablation process, while photoionization serves as a free electron provider.     

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.     

Investigation of nanoparticle generation during femtosecond laser ablation of metals

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.     

Metal thin film ablation with femtosecond pulsed laser

Micromachining thin metal films coated on glass are widely used to repair semiconductor masks and to fabricate optoelectrical and MEMS devices. The interaction of lasers and materials must be understood in order to achieve efficient micromachining. This work investigates the morphology of thin metal films after machining with femtosecond laser ablation using about 1 μm diameter laser beam. The effect of the film thickness on the results is analyzed by comparing experimental images with data obtained using a two-temperature heat transfer model. The experiment was conducted using a high numerical aperture objective lens and a temporal pulse width of 220 fs on 200- and 500-nm-thick chromium films. The resulting surface morphology after machining was due to the thermal incubation effect, low thermal diffusivity of the glass substrate, and thermodynamic flow of the metal induced by volumetric evaporation. A Fraunhofer diffraction pattern was found in the 500-nm-thick film, and a ripple parallel to the direction of the laser light was observed after a few multiple laser shots. These results are useful for applications requiring micro- or nano-sized machining.     

Generation of regular femtosecond pulses in AlGaInP semiconductor laser

It is shown experimentally that stable ~ 200 fs regular pulses and optical frequency comb spectrum containing ~ 70 discrete frequencies can be achieved using semiconductor injection laser working in the conditions of superfluorescence under dc pumping current, which is lower than the threshold of laser action.     

Micromachining soda-lime glass by femtosecond laser pulses

The physical process of forming a modified region in soda-lime glass was investigated using 1 kHz intense femtosecond laser pulses from a Ti: sapphire laser at 775 nm. Through the modifications induced by the femtosecond laser radiation using selective chemical etching techniques, we fabricated reproducible and defined microstructures and further studied their morphologies and etching properties. Moreover, a possible physical mechanism for the femtosecond laser modification in soda-lime glass was proposed.     

Effect of energy above laser-induced damage thresholds in the micromachining of silicon by femtosecond pulse laser

A 400 nm second harmonic Ti : sapphire femtosecond laser was applied to structure silicon base on a direct-write process in air. A series of lines were ablated with pulses of 300-fs duration at varying power densities ranging from 50 to 100 nJ of energy on 2″ silicon (1 1 1) wafers. In this event, we investigate and report extensive laser induced thermal damage and redeposition encompassing the ablated lines at high energy levels above the damage threshold of the silicon. In addition, the effect of polarisation on the direction of micromachining is also observed and discussed. The resolution and quality of these lines were also found to hold a linear relationship to the laser energy up to its thermal threshold limit.     

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.     

Improvement of electrospun polymer fiber meshes pore size by femtosecond laser irradiation

Polymer meshes have recently attracted great attention due to their great variety of applications in fields such as tissue engineering and drug delivery. Poly(?-caprolactone) nanofibers were prepared by electrospinning giving rise to porous meshes. However, for some applications in tissue engineering where, for instance, cell migration into the inner regions of the mesh is aimed, the pore size obtained by conventional techniques is too narrow. To improve the pore size, laser irradiation with femtosecond pulses (i.e., negligible heat diffusion into the polymer material and confined excitation energy) is performed. A detailed study of the influence of the pulse energy, pulse length, and number of pulses on the topography of electrospun fiber meshes has been carried out, and the irradiated areas have been studied by scanning electron microscopy, contact angle measurements and spectroscopic techniques. The results show that using the optimal laser parameters, micropores are formed and the nature of the fibers is preserved.     

Photoetching of spherical microlenses on glasses using a femtosecond laser

We fabricated spherical microlenses on optical glasses by femtosecond laser direct writing (FLDW) in ambient air. To achieve good appearances of the microlenses, a meridian-arcs scanning method was used after a selective multilayer removal process with spiral scanning paths. A positive spherical microlens with diameter of 48 μm and height of 13.2 μm was fabricated on the surface of the glass substrate. The optical performances of the microlens were also tested. Compared to the conventional laser direct writing (LDW) technique, this work could provide an effective method for precise shape-controlled fabrication of three-dimensional (3D) microstructures with curved surfaces on difficult-to-cut materials for practical applications.     

Large negative refractive index modification induced by irradiation of femtosecond laser inside optical glasses

We report on the refractive index modification (Δn) and its cross sectional profile of the created lines inside the different types of optical glasses, containing BaO, TiO₂, or La₂O₃ as a metal oxide. The lines were fabricated by scanning a stage and focusing the femtosecond laser pulses, 800 nm wavelength, a 250 kHz repetition rate and 200 fs pulse duration, from the Ti:sapphire regenerative amplifier system. The Δn measurements were performed with the qualitative phase microscopy technique. As a result, it was found that the Δn and its sign are different depending on glass types. For example, in the glasses containing TiO₂, the Δn became smaller in the modified region and some of them showed relatively large decrease of the Δn, Δn < −0.01, with about 10 μm width. Such a glass material could be useful for the compact optics and optical devices.     

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.     

Titanium micromachining by femtosecond laser

The ultimate goal of this research was to characterize the ablation depth with respect to pulse energy, translation speed, and consecutive passes in order to obtain the parameters to have smooth microchannel surfaces. A logarithmic dependence of the channel depth on the laser pulse energy was observed with two different distinct ablation regimes. Although the same fluence values were used with two different lens sizes, the slopes of these ablation regimes were quite different. 100 mm lens has a small optical penetration length with steeper ablation slope in the first regime, whereas the 15 mm lens has the opposite. In the second part of the ablation regime, the slope was lower for 100 mm lens as compared to 15 mm lens. Furthermore, spike formation has been seen in 100 mm lens study at 0.308, 0.370, 0.431, and 0.493 J/cm² fluence values yet no spike formations have been seen in 15 mm lens study.     

Laser induced reverse transfer of gold thin film using femtosecond laser

Laser induced reverse transfer (LIRT) has been executed first time using a Mega Hertz pulse frequency femtosecond laser radiation under ambient conditions. Research has been conducted to understand the evolution of deposited structures with regard to pulse energy. Solid deposition of gold could be deposited on quartz substrate only if pulse energy falls within a certain range. For the experiment conducted in this work, it is 36-40 nJ. For energies above this range, crests appear in the middle of the deposition. There is a threshold in maximum applicable pulse energy, 82 nJ in this exepriment, beyond which further increase in pulse energy results in only traces of deposited material. Results also show that the width of deposited line increases with the increase in pulse energy and decreases with the increase in scan speed. These observations have been explained using the dynamics of ablated plume.     

High-speed and crack-free direct-writing of microchannels on glass by an IR femtosecond laser

Fabrication of high-resolution 3D structures with laser radiation on the surface of brittle materials has always been a challenging task. Even with femtosecond laser machining, micro-cracks and edge chipping occur. In order to evaluate processing modes optimal both in quality and productivity, we investigated high-speed (50 kHz) femtosecond laser processing of BK7 glass with the use of design of experiments and regression analysis. An automated inspection technique was developed to extract quality characteristics of test-objects. A regression model was obtained appropriate to fabricate microchannels with a predefined depth in the range of 1–30 µm with average accuracy of 5%. It was found that high quality machining modes are in the range of 0.91–2.27 µJ energy pulses, overlap of 53–62%, three and more number of passes. A material removal rate higher than 0.3 mm³/min was reached and microfluidic structures were formed based on data obtained.     

Energy accumulation effect and parameter optimization for fabricating of high-uniform and large-area period surface structures induced by femtosecond pulsed laser

In this paper, theoretical and experimental investigations are performed to understand the effect of laser energy accumulation on morphologies of femtosecond laser-induced periodic surface structures (LIPSSs). Based on energy accumulation effect we put forward, scanning velocity, distance of two adjacent scan lines and pulsed laser energy density are optimized to fabricate high-uniform and large-area period surface structures for the first time. Our theoretical results can offer clues to choose experiment parameters in order to obtain high-quality LIPSSs which have extensively emerging and potential applications in optoelectronics, phononics and so on.