Cs. Vass

Wet etching of fused silica: a multiplex study

The machining process of transparent materials using the laser induced backside wet etching (LIBWE) procedure was studied. In the course of this, experimental investigations and numerical calculations were carried out. Fused silica plates were irradiated by an ArF excimer laser, using a naphthalene?methyl methacrylate solution as an absorbing liquid (concentration 0.85?mol?dm?3, absorption coefficient at 193?nm 52?200?cm?1). The etch rate dependence on the applied laser fluence (varied from 110 to 860?mJ?cm?2) was derived from the etch depths, measured using an atomic force microscope (AFM). The etch rate was found to be 4.7?49.5?nm/pulse, depending on the laser fluence. The surface morphology of the etched edges was also investigated by AFM. A fast photographic arrangement was used for time resolved observation of bubble development in the liquid absorbent, which is an important phenomenon of LIBWE. The internal pressure of the expanding bubbles was calculated using recorded snapshots. It was found to be 22?120?MPa 17.2?ns after the excimer pulse peak. The one-dimensional heat flow equation, including the melting of the treated fused silica layer and the vaporization of the absorbing solution, was solved using the finite difference method. The surface temperature of the fused silica was found to be a maximal 17.2?ns after the excimer pulse peak. Based on our results, we present a possible interpretation of the LIBWE procedure of fused silica.

Time-resolved study of absorbing film assisted laser induced forward transfer of Trichoderma longibrachiatum conidia

We have characterized the absorbing film assisted transfer of Trichoderma longibrachiatum conidia using a synchronized laser for illumination. The transfer laser used was a KrF excimer laser (? = 248?nm, FWHM = 30?ns) and the ejected material was illuminated parallel to the quartz plate by a nitrogen laser pumped Coumarine 153 dye laser beam (? = 453?nm, FWHM = 1?ns) electronically delayed relative to the transfer UV pulse. Our time-resolved investigations determined that the ejection velocity front of the conidia plume from the donor surface during the transfer procedure was 1150?m?s?1 at 355?mJ?cm?2 applied laser fluence. On the basis of the measured data, the acceleration of the emitted conidia at the plume front was approximately 109 ? g. The conidia survived the absorbing film assisted forward transfer and associated mechanical shear without significant damages suggesting that the technique might be applicable to other more fragile types of biological objects and applications.

Production of submicrometre fused silica gratings using laser-induced backside dry etching technique

Laser micromachining of transparent materials is a promising technique for producing micro-optical elements. Several types of both direct (e.g. ablation) and indirect (e.g. laser-induced backside wet etching: LIBWE) procedures have already been developed and presented in the last two decades. Here we present a new method (laser-induced backside dry etching (LIBDE)) in the analogy of LIBWE for the micro and nanoprocessing of transparent materials.In our experiments 1 mm thick fused silica plates were used as transparent work pieces. The plates were covered with 100 nm thick silver layers. The metal absorbing films were irradiated through the fused silica by a KrF excimer laser beam (λ = 248 nm, FWHM = 30 ns). The illuminated area was 1.05 mm2 and the fluence on the silver–quartz interface varied in the range 0–1800 mJ cm−2. We have provided evidence that LIBDE is more effective and simple than LIBWE, its etch rate being much higher at a given laser fluence. Our interference experiments proved that the LIBDE etching technique is suitable to fabricate gratings displaying submicrometre periods in transparent materials. On the basis of all these, it is suggested that this method may be useful to produce other nano and microoptical elements, too.

Fabrication and analysis of transmission gratings produced by the indirect laser etching technique

The diffraction efficiency of gratings etched into fused silica was measured and modelled. 950, 2120 and 3710 nm period gratings were fabricated with the use of two-beam interferometric laser-induced backside wet etching (TWIN-LIBWE). The spatial distribution of the modulation depth (MD), determined from AFM measurements, followed the spatial intensity distribution of the laser beam throughout the grooved areas. The diffraction efficiency of the fabricated gratings was measured to all allowed diffraction orders at several wavelengths (266, 532 and 654.5 nm). Within the frame of Gsolver code, a model was developed that takes into account the spatial distribution of MD. The computed diffraction efficiencies showed good agreement with the results of measurements.

Experimental study on droplet generation during excimer laser ablation of polyethylene glycol 1000

Droplet generation during laser ablation is important from the point of view of the quality of films deposited by pulsed laser deposition. Dependence of the emitted droplet properties and distribution on the state of matter, viscosity and fluence was investigated experimentally. Polyethylene glycol (PEG) 1000 having relatively low melting point (378C) was used in our experiments. By variation of target temperature between 23 and 808C the amount and dimensions of emitted droplets were studied. An ArF excimer laser was used for irradiation, the applied fluence was varied in the range of 0.37‐2.66 J/cm 2 . The droplets were caught onto a quartz plate placed above the target. During the transition from solid to liquid state of PEG 1000 the number and the total volume of droplets emitted during ablation were drastically decreased, reaching a minimum at 408C. Further heating of the molten target resulted in increment in the amount of emitted droplets. At 708C target temperature the maximum quantity of droplets were deposited at a fluence of 1.28 J/cm 2 . It was found that with increasing viscosity the number and total volume of droplets decrease significantly. Deposition with the smallest amount of droplets is reached, when using molten targets near to melting point with a high viscosity. # 2000 Elsevier Science B.V. All rights reserved.

A unified optical damage criterion based on the probability density distribution of detector signals

Various methods and procedures have been developed so far to test laser induced optical damage. The question naturally arises, that what are the respective sensitivities of these diverse methods. To make a suitable comparison, both the processing of the measured primary signal has to be at least similar to the various methods, and one needs to establish a proper damage criterion, which has to be universally applicable for every method. We defined damage criteria based on the probability density distribution of the obtained detector signals. This was determined by the kernel density estimation procedure. We have tested the entire evaluation procedure in four well-known detection techniques: direct observation of the sample by optical microscopy; monitoring of the change in the light scattering power of the target surface and the detection of the generated photoacoustic waves both in the bulk of the sample and in the surrounding air.