Csaba Vass

Fabrication of 150 nm period grating in fused silica by two-beam interferometric laser induced backside wet etching method

Fused silica gratings with periods of 154 nm, 266 nm, and 550 nm have been fabricated by the method of two-beam interferometric laser induced backside wet etching (TWIN-LIBWE). The spatially filtered pulses at 266nm were splitted into two parts and interfered at an incident angle of 60(o), 30(o), and 14(o), respectively, on the backside surface of a fused silica plate contacting with the liquid absorber. The morphology of the etched gratings was characterized by atomic force microscope. According to our knowledge, the produced 154 nm period is the smallest grating constant generated by laser techniques directly in fused silica at present.

Laser-induced backside wet etching of fused silica: absorption coefficient dependence

The micromachining process of transparent materials by laser induced backside wet etching (LIBWE) was investigated. Fused silica targets were irradiated by an ArF excimer laser at 2.14 J/cm2 fluence and naphthalene solved in methyl-methacrylate with different concentrations were used as absorbing liquid. The absorption coefficient of thse solutions was measured by a plano-concave microcuvette and it found to be between 39426 and 62350 1/cm depending on the concentration of napthalene. It was demonstrated that the etch rate depends on the absorption coefficient linearly, while the roughness does not. The dependence of the etch rate can be explained as follows. The absorbed energy in the interface of the solution and the fused silica increases when increasing the absorption coefficient resulting in higher temperature liquid layer at the surface of the fused silica causing higher etch rate.

Comparison of simultaneous on-line optical and acoustic laser damage detection methods in the nanosecond pulse duration domain

We carried out single-shot laser-induced damage threshold measurements on dielectric high reflectors guided by the corresponding ISO standard. Four simultaneous on-line detection techniques were tested and compared using 532 nm, 9 ns and 266 nm, 6 ns laser pulses. Two methods, microscope aided visual inspection and detection of scattered light off the damaged surface, were based on optical signals. The other two techniques exploited the acoustic waves accompanying a damage event in ambient air and in the substrate by a microphone and a piezoelectric sensor, respectively. A unified criterion based on the statistical analysis of the detector signals was applied to assign an objective and unambiguous damage threshold value for all of our diverse detection methods. Microscope aided visual inspection showed the lowest damage thresholds for both wavelengths. However, the sensitivity of the other three techniques proved to be only slightly lower.

Grating fabrication in dielectric coatings by TWIN-LIBWE

Thin films are widely used in many applications, especially, the transparent layers are commonly used as high reflective and antireflex coatings on optical elements. Moreover, several spectroscopic applications need microstructured thin films deposited on bulk dielectric. Many procedures have been developed for etching transparent dielectrics. The conventional multistep techniques (hidrofluidic etching [1], powder blasting [2], ion etching [3]) require contact mask preparation on the target surface. The laser-based direct and indirect methods, however, can provide a good alternative for micro- and submicrometer structuring of transparent dielectrics without the complicate preparation of contact masks. The laser-induced backside wet etching (LIBWE) [4] is one of the most promising and flexible and applicable indirect technique. It was recently demonstrated that the combination of LIBWE with the two-beam interferometric method (TWIN-LIBWE) is well suited for fabrication of submicrometer period gratings onto the surface of bulk fused silica [5]. In this paper we report on the fabrication of micrometer period grating structure in SiO2, and ZrO2 thin films with the use of TWIN-LIBWE.

Influence of particulate size on the formation of polytetrafluoroethylene thin films during pulsed laser deposition

The lowest thickness of contiguous polytetrafluoroethylene (PTFE) thin films prepared by pulsed laser deposition is influenced by the size of the particulates emitted from the target. Since the particulates reach the substrate at random, by the time a contiguous layer is formed its average thickness can be several times as much as the mean size of the particulates. During our experiments the size distribution of the particulates emitted during ArF excimer laser deposition from pressed PTFE pellets was studied as the function of the applied ablating fluence in the range of 1.1 - 6.2 J/cm2. The size distribution of the particulates could be described with a first order exponential decay function. The value of the decay constant varied between 3.94 and 6.15 depending on the laser fluence. With the knowledge of the size distribution of the depositing particulates a theoretical model was used for simulating the growth of the thin film. The minimum number of the pulses required to obtain a contiguous layer and its thickness could be estimated. The thinnest layer could be obtained at the lowest investigated fluence.