Aluminum thin film enhanced IR nanosecond laser-induced frontside etching of transparent materials

Laser processing of glass is of significant commercial interest for microfabrication of precision optical engineering devices. In this work, a laser ablation enhancement mechanism for microstructuring of glass materials is presented. The method consists of depositing a thin film of aluminum on the front surface of the glass material to be etched. The laser beam modifies the glass material by being incident on this front-side. The influence of ablation fluence in the nanosecond regime, in combination with the deposition of the aluminum layer of various thicknesses, is investigated by determining the ablation threshold for different glass materials including soda-lime, borosilicate, fused silica and sapphire. Experiments are performed using single laser pulse per shot in an air environment. The best enhancement in terms of threshold fluence reduction is obtained for a 16 nm thick aluminum layer where a reduction of two orders of magnitude in the ablation threshold fluence is observed for all the glass samples investigated in this work.     

A preliminary study on chemical micro-machining of complex three-dimensional patterns on silicon substrates

Chemical micro-machining of complex 3-dimensional (3-D) patterns of silicon substrates was preliminarily explored by the confined etchant layer technique (CELT). Through systematic investigation, we demonstrated that cysteine as a scavenger and Br₂ as an etchant can be used to etch silicon substrates. The CELT has the potential to develop into a new means of micro-machining complex 3-D patterns on silicon substrates. However, due to the highly corrosive property of the chemicals used for the silicon etched system, great effort must be made to overcome these problems including the mold electrode with high chemical stability.Dedicated to Professor Gygy Horyi on the occasion of his 70th birthday     

超短脉冲激光辐照下金属薄膜的热行为

对双温模型的重要热学参量电子热容、电子弛豫时间、电子热导率进行量子化处理，使双温模型能适用于自由电子温度比较高的情况.利用前向差分算法，数值求解了电子-晶格双温双曲两步热传导模型，所得的结果更接近实验值.经过分析得出： 1）薄膜前表面自由电子温度达到最大值的时间约为0.27 ps,得到的损伤阈值与实验值符合较好.2）电子热容对电子温升规律影响非常大.电子热导率对自由电子温升规律也有较大的影响.3）在趋肤层内自由电子温升非常快，不同厚度自由电子温度达到最大值所需的时间延迟不明显.趋肤层以下自由电子温度升高较慢，不同厚度自由电子达到最大值所需的时间延迟明显.     

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.     

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.     

超短脉冲激光辐照金属薄膜温升效应的模拟研究

 考虑到材料的质量热容、热导率、驰豫时间等热力学参数随温度非线性变化因素的影响,利用具有人工粘性的、自适应时间步长的前向差分算法,数值求解了电子-晶格双温双曲两步热传导模型,讨论了厚度为50 nm的金膜在0.1 ps脉冲激光辐照下的温升规律。数值结果表明：薄膜前表面自由电子的温度在大约0.27 ps时达到最大值,不同厚度上自由电子达到温度平衡所需的时间大约为1.6 ps,而薄膜温度在整个厚度上达到平衡所需时间为60 ps左右。由电子温度及其温度梯度引起的热电子崩力很可能是造成材料破坏的一个主要因素。