Precise etching of fused silica for refractive and diffractive micro-optical applications

One of the challenges of current laser material processing is the high-quality etching of transparent materials for micro-optical applications. The ablation of transparent materials with UV-, ultrashort pulse and even of VUV-lasers is characterized by a high etch rate and a high laser fluence and causes considerable surface roughness evolution. The combination of specific laser processing techniques, e.g., scanning contour mask technique and direct writing with a small laser spot, with laser-induced backside wet etching (LIBWE) allows the direct machining of dielectric materials with an almost optical quality for the fabrication of diffractive as well as refractive topographic features. The etching of multi-level elements, gratings with variable depth, micro-lenses as well as free-form surface topographies with PV-values from some 100 nm to a few micrometers, a nanometer depth accuracy and a low roughness of less than 10 nm rms is presented and demonstrates the capabilities of this approach for precision engineering.     

Comparing study of subpicosecond and nanosecond wet etching of fused silica

The effectiveness of the laser induced backside wet etching (LIBWE) of fused silica produced by subpicosecond (600 fs) and nanosecond (30 ns) KrF excimer laser pulses (248 nm) was studied. Fused silica plates were the transparent targets, and naphthalene-methyl-methacrylate (c = 0.85, 1.71 M) and pyrene-acetone (c = 0.4 M) solutions were used as liquid absorbents. We did not observe etching using 600 fs laser pulses, in contrast with the experiments at 30 ns, where etched holes were found. The threshold fluences of the LIBWE at nanosecond pulses were found to be in the range of 360-450 mJ cm⁻² depending on the liquid absorbers and their concentrations. On the basis of the earlier results the LIBWE procedure can be explain by the thermal heating of the quartz target and the high-pressure bubble formation in the liquid. According to the theories, these bubbles hit and damage the fused silica surface. The pressure on the irradiated quartz can be derived from the snapshots of the originating and expanding bubbles recorded by fast photographic setup. We found that the bubble pressure at 460 mJ cm⁻² fluence value was independent of the pulse duration (600 fs and 30 ns) using pyrene-acetone solution, while using naphthalene-methyl-methacrylate solutions this pressure was 4, 5 times higher at 30 ns pulses than it was at 600 fs pulses. According to the earlier studies, this result refers to that the pressure should be sufficiently high to remove a thin layer from the quartz surface using pyrene-acetone solution. These facts show that the thermal and chemical phenomena in addition to the mechanical effects also play important role in the LIBWE procedure.     

Material response during nanosecond laser induced breakdown inside of the exit surface of fused silica

The material response following nanosecond, UV laser induced breakdown inside of the exit surface of fused silica is investigated using multimodal time resolved microscopy. The study spans up to about 75 ns delay from the onset of material modification during the laser pulse through the observation of material ejection. A number of distinct processes were identified, including: a) the onset of optical absorption in the material arising from the buildup of an electronic excitation, b) the expansion of the hot modified region (plasma) along the surface and inside the bulk, c) the formation of radial and circumferential cracks, d) the swelling of the affected region on the surface and, e) the onset of ejection of material clusters at about 30 ns delay and its progression to a well‐defined jet by about 75 ns delay. Limited theoretical modeling is used to aid the interpretation of the data.     

Influence on the laser induced backside dry etching of thickness and material of the absorber, laser spot size and multipulse irradiation

Laser induced backside dry etching method (LIBDE) was developed - analogously to the well-known laser induced backside wet etching (LIBWE) technique - for the micromachining of transparent materials. In this procedure, the absorbing liquid applied during LIBWE was replaced with solid metal layers. Fused silica plates were used as transparent targets. These were coated with 15-120 nm thick layers of different metals (silver, aluminium and copper). The absorbing films were irradiated by a nanosecond KrF excimer laser beam through the quartz plate. The applied fluence was varied in the 150-2000 mJ/cm² range, while the irradiated area was between 0.35 and 3.6 mm². At fluences above the threshold values, it was found that the metal layers were removed from the irradiated spots and the fused silica was etched at the same time. In our experiments, we investigated the dependence of the main parameters (etch rate and threshold) of LIBDE on the absorption of the different metal layers (silver, copper, aluminium), on the size of the irradiated area, on the film thickness and on the number of processing laser pulses.     

基频和三倍频Nd:YAG激光诱导熔石英损伤特性

 采用光电探测器和数字示波器检测散射光脉冲信号,研究了基频和三倍频Nd:YAG激光诱导熔石英损伤过程,给出了泵浦光和探针光的散射光光电信号;比较了基频和三倍频激光作用下熔石英烧蚀斑显微照片,并分析了其损伤机理。结果显示:在ns脉冲激光作用下,熔石英损伤均发生在泵浦激光脉冲峰值附近,且基频光作用下损伤开始时间点比三倍频作用下早;在多脉冲或高能量激光辐照下,检测到了等离子体闪光信号,等离子体闪光发生在时间延迟21 ns附近。基于Keldysh理论计算了基频光和三倍频光作用下,熔石英光致电离速率同激光强度的关系。