Femtosecond pulsed laser ablation of thin gold film

Laser micromachining on 1000 nm-thick gold film using femtosecond laser has been studied. The laser pulses that are used for this study are 400 nm in central wavelength, 150 fs in pulse duration, and the repetition rate is 1 kHz. Plano-concave lens with a focal length of 19 mm focuses the laser beam into a spot of 3 μm (1/e² diameter). The sample was translated at a linear speed of 400 μm/s during machining. Grooves were cut on gold thin film with laser pulses of various energies. The ablation depths were measured and plotted. There are two ablation regimes. In the first regime, the cutting is very shallow and the edges are free of molten material. While in the second regime, molten material appears and the cutting edges are contaminated. The results suggest that clean and precise microstructuring can be achieved with femtosecond pulsed laser by controlling the pulse energy in the first ablation regime.     

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.     

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.     

飞秒激光在光敏玻璃上制作微凹面

利用聚焦的飞秒激光（中心波长775 nm、脉宽150 fs）照射FOTURAN光敏玻璃,经热处理及浓度为8％的氢氟酸溶液室温腐蚀50 min后,在FOTURAN玻璃表面制作了微凹面.利用原子力显微镜和扫描电子显微镜分析了微凹面的形貌,发现它具有光滑璧面和清晰边缘,直径约为几十微米.通过分别改变入射激光的单脉冲能量(970~3 250 nJ)和脉冲数目(10~3 000个),研究了它们对微凹面制作结果的影响,发现了微凹面的直径随激光入射参量的饱和效应,并解释了其原因;指出了飞秒激光在FOTURAN上制作微结构的应用.     

Femtosecond laser ablation profile near an interface: Analysis based on the correlation with superficial properties of individual materials

Femtosecond laser ablation of materials is turning to be an important tool for micromachining as well as for selective removal of biological tissues. In a great number of applications, laser ablation has to process through interfaces separating media of different properties. The investigation of the ablation behavior within materials and passing through interfaces is the main aim of this study. Especially, the analysis of the discontinuity in the ablation profile close to interfaces between distinct materials can reveal some of the phenomena involved in the formation of an ablated microcavity geometry. We have used a method that correlates the ablation cross sectional area with the local laser intensity. The effective intensity ablation properties were obtained from surface ablation data of distinct materials. The application of this method allows the prediction of the occurrence of a size discontinuity in the ablation geometry at the interface of distinct media, a fact which becomes important when planning applications in different media.     

Controlled process for polymer micromachining using designed pulse trains of a UV solid state laser

A flexible workstation equipped with a solid state laser operating at 266 nm wavelength was used to machine holes in polyethylene terephthalate, polyimide and polycarbonate. An optical pulse picker was employed to reduce the high repetition rates of the laser, while a breakthrough sensor was used to avoid over-drilling of through holes. For each material, different repetition rates and designed pulse trains were tested to improve feature quality and process efficiency. Although the three polymers had very different reactions at this wavelength they all showed an improvement in feature quality with decreasing repetition rate due to a reduction in thermal effects. Up to 10 kHz the average depth per pulse remained unchanged and afterwards a slight increase was observed but this was accompanied by large uncertainties. Bursts of pulses at 40 kHz inserted inside the low repetition rate pulse train reduced the drilling time and the amount of debris redeposited without affecting the feature quality. It was found that a number of cleaning pulses after perforation eliminates the heat affected zone around exits. Holes with entrance diameters below 20 μm and exit diameters as small as 2 μm were obtained with high repeatability.     

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.     

Femtosecond laser processing of indium-tin-oxide thin films

Micro- and nano-scale crystalline indium-tin-oxide (c-ITO) patterns fabricated from amorphous ITO (a-ITO) thin films on a glass substrate using a (low NA 0.26) femtosecond laser pulse that is not tightly focused are demonstrated. Different types of c-ITO patterns are obtained by controlling the laser pulse energies and pulse repetition rate of a femtosecond laser beam at a wavelength of 1064 nm: periodic micro c-ITO dots with diameters of ~1.4 μm, two parallel c-ITO patterns with/without periodic-like glass nanostructures at a laser scanning path and nano-scale c-ITO line patterns with a line width ~900 nm, i.e. ~1/8 of the focused beam׳s diameter (7 μm at 1/e²).     

Femtosecond laser ablation of DAST

High rate femtosecond (fs) laser ablation of the organic salt 4-N,N-dimethylamino-4-N-methyl-stilbazolium tosylate (DAST), an organic crystal with very high optical nonlinearities has been demonstrated. The threshold fluence and the ideal fluence range for damage free ablation for the wavelengths 550, 600, and 775 nm have been determined and the quality of the produced grooves has been investigated. The threshold fluences are in the order of 10–70 mJ/cm² and the ideal fluence range for damage free ablation is ranging from 30 to 300 mJ/cm², depending on the wavelength. The optimal focussing for ablation has been investigated and first results towards the structuring of a ridge waveguide are presented. We conclude that this method is most promising for waveguide patterning of DAST surfaces for integrated optics applications.     

Femtosecond laser micro-structuring of aluminium under helium

The interaction of 180 fs, 775 nm laser pulses with aluminium under a flowing stream of helium at ambient pressure have been used to study the material re-deposition, ablation rate and residual surface roughness. Threshold fluence F_th0.4 J cm⁻² and the volume ablation rate was measured to be 30<V<450 μm³ per pulse in the fluence range 1.4<F<21 J cm⁻². The presence of helium avoids gas breakdown above the substrate and leads to improved surface micro-structure by minimising surface oxidation and debris re-deposition. At 1 kHz rep. rate, with fluence F>7 J cm⁻² and >85 W cm⁻² average power density, residual thermal effects result in melt and debris formation producing poor surface micro-structure. On the contrary, surface micro-machining at low fluence F1.4 J cm⁻² with low power density, 3 W cm⁻² produces much superior surface micro-structuring with minimum melt and measured surface roughness R_a1.1±0.1 μm at a depth D50 μm. By varying the combination of fluence/scan speed during ultra-fast ablation of aluminium at 1 kHz rep. rate, results suggest that maintaining average scanned power density to <5 W cm⁻² combined with single pulse fluence <4 J cm⁻² produces near optimum micro-structuring. The debris under these conditions contains pure aluminium nanoparticles carried with the helium stream.     

The dependence of the ideality factor on the charge density of a conjugated polymer (MEH-PPV) doped with some laser dyes

Blends made of poly[2-methyl-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) conducting polymer and cresyl violet 670 (CV) and coumarin 460 (C₄₆₀) have been utilized for preparation of organic Schottky diodes. The I–V characteristics of these diodes are then determined and interpreted in terms of thermoionic emission (TIE) conduction process. Upon blending MEH-PPV with CV and C₄₆₀ dyes, the conduction is seen to completely be through TIE conduction. The turn on voltage for MEH-PPV diode is found to be 2.0 V and decreases with CV blending in contrast to the blend of MEH-PPV-C₄₆₀. The Schottky parameters are determined where the potential barrier is found to be around 0.75 V which is consistent with reported values. The charge densities of the respective blends are calculated and found to reduce by in MEH-PPV-C₄₆₀ blends while it has been enhanced in the CV samples by an order of magnitude compared that of MEH-PPV. The correlation between the ideality factor with the dielectric constant and the charge density is inspected. It is found that the ratio between the charge density of the polymeric blend and the charge density of the Al-electrode is crucial for obtaining low ideality factor.     

Femtosecond pulsed laser ablation with spatial filtering

With the rise in demand for miniaturized features with better acute edge acuity and negligible thermal damage zone, one of the key vital areas lies in the refinement of the quality of the laser beam itself. Spatial filter is routinely used in optical micromachining systems to smoothen the Gaussian profile of the machining spot in order to obtain a feature of the desired quality. However, its profile smoothening effect has never been investigated for femtosecond pulsed laser micromachining process since the extremely high peak power of femtosecond pulses will cause damage on the filtering aperture of spatial filter. During the development of an acousto-optical micromachining system using femtosecond pulses, we found that if the damage of the filtering aperture can be circumvented, spatial filter can improve the machining quality of femtosecond pulse ablation, especially when ablation is conducted at low-fluency range (just above the ablation threshold fluency). In this paper, we investigate and demonstrate both the improvement and potential that beam refinement can bring about. In our experiment, a series of test patterns were ablated with a 400 nm second-harmonic Ti:Sapphire femtosecond laser of 150 fs duration at varying pulse energy ranging from 31 to 39 nJ. The specimen used in the experiment is a platinum- (Pt) sputtered coating of 100 nm thickness on a quartz substrate. The results show a significant improvement in the constancy of the shape as well as the size of ablated feature, revealing an improved beam profile and beam energy distribution due to spatial filtering.     

飞秒激光高精细加工柴油机喷油嘴倒锥孔法

利用飞秒激光加工热效应小、高精细、对材料无选择性等特点,针对柴油发动机高压共轨系统中关键零件——喷油嘴偶件喷油孔的加工需求,设计了一种利用三光楔扫描的飞秒激光加工装置,并进行了喷油嘴倒锥孔加工工艺研究.采用微孔三座标测量机(测量准确度1.2μm)对倒锥孔尺寸准确度进行检测,通过喷油嘴高压液体流量试验台(测量准确度±0.1%)对批量制孔后的油嘴进行了流量测试,结果表明:所设计装置加工的油嘴性能可以满足欧V排放法规的应用要求,对提升汽车制造业水平及柴油车排放标准具有重要意义.     

Excimer laser micromachining of aspheric microlenses with precise surface profile control and optimal focusing capability

This paper demonstrates the unique and exceptional capability of excimer laser micromachining in fabricating aspheric microlenses with precise surface profile control. A newly developed laser scanning method is introduced for machining refractive types of microlenses, which have pre-designed surface profiles aiming at minimizing the optical focal spot sizes. The machining accuracy and machined surface roughness are examined experimentally, and very good results are obtained. Optical testing on the fabricated aspheric microlenses shows significant improvement in focusing capability and the focal spot sizes are approaching optical diffraction limits. The proposed excimer laser micromachining method is flexible, versatile, and accurate, hence can be very useful and powerful in machining 3D microstructures of complex profiles and demanding profile accuracy.     

Quasi-persistent photocurrent in the UV-irradiated MEH-PPV conjugated polymer

We report the quasi-persistent photocurrent in the MEH-PPV conjugated polymer, induced by UV-irradiation in air. It is attributed to the irradiation-induced defects, which also act to accelerate its decay.     

Quantifying the contributions of inner-filter, re-absorption and aggregation effects in the photoluminescence of high-concentration conjugated polymer solutions

The optical properties of high-concentration solutions of conjugated molecules are dominated by inner-filter and re-absorption effects. We apply a simple model to isolate the contributions from these phenomena to the non-linearity of the observed photoluminescence (PL) intensity with concentration. Poly(9,9-dioctylflourene) (PFO) solutions were studied across a range of concentrations and in a number of different solvents. The model accurately predicts the behaviour of PFO in good solvents up to concentrations above 0.1 mg/ml. In solutions of PFO in poor solvents and at high concentrations where we find a significant amount of β-phase PFO present the model is less accurate. We attribute the difference between predicted PL behaviour and that observed to non-radiative energy transfer from the bulk “glassy” phase PFO to the β-phase PFO.     

共轭高聚物双分子链中激子生成的动力学研究

通过对耦合共轭高分子双链中激子生成的动力学研究,讨论了受到光激发后,激子在分子内或分子间生成的制约因素.激子的生成除了受分子链间耦合强度的制约,还受到光激发模式的影响.结果表明,激子既可以在分子内生成,也可以在耦合分子间生成.增大分子之间的相互作用强度,可以增大在分子之间生成激子的概率.     

Femtosecond laser ablation of polypropylene for breathable film

A polypropylene (PP) film was ablated using a femtosecond laser with a center wavelength of 785 nm, a pulse width of 184 fs and a repetition rate of 1 kHz. Increments of both the pulse energy and the shot number of pulses lead to co-occurrence of photochemical and thermal effect, demonstrated by the spatial expansion of rim on the surface of PP. The shapes of the laser-ablated PP films were imaged by a scanning electron microscope (SEM) and measured by a 3D optical measurement system (NanoFocus). And, the gas and water vapor transmission rate, mechanical properties of PP film micropatterned by fs laser pulses was characterized. Our results demonstrate that a femtosecond pulsed laser is an efficient tool for breathable packaging films in modifying the flow of air and gas, where the micropatterns are specifically tailored in size, location and number of which is easily controlled by laser processing conditions.     

Femtosecond laser ablation of cadmium tungstate for scintillator arrays

Ultrafast pulsed laser ablation has been investigated as a technique to machine CdWO₄ single crystal scintillator and segment it into small blocks with the aim of fabricating a 2D high energy X-ray imaging array. Cadmium tungstate (CdWO₄) is a brittle transparent scintillator used for the detection of high energy X-rays and γ-rays. A 6 W Yb:KGW Pharos-SP pulsed laser of wavelength 1028 nm was used with a tuneable pulse duration of 10 ps to 190 fs, repetition rate of up to 600 kHz and pulse energies of up to 1 mJ was employed. The effect of varying the pulse duration, pulse energy, pulse overlap and scan pattern on the laser induced damage to the crystals was investigated. A pulse duration of ≥500 fs was found to induce substantial cracking in the material. The laser induced damage was minimised using the following operating parameters: a pulse duration of 190 fs, fluence of 15.3 J cm⁻² and employing a serpentine scan pattern with a normalised pulse overlap of 0.8. The surface of the ablated surfaces was studied using scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy. Ablation products were found to contain cadmium tungstate together with different cadmium and tungsten oxides. These laser ablation products could be removed using an ammonium hydroxide treatment.