David Ashkenasi

Laser-induced damage in SiO2 and CaF2 with picosecond and femtosecond laser pulses

Single- and multiple-shot damage thresholds and plasma-emission thresholds for fused silica and CaF2 are reported for 790 nm photons as a function of laser pulse width (190 fs – 4.5 ps). The results are compared with single-shot plasma-emission measurements [1] and with multiple-shot damage measurements [2]. Both the damage threshold and the plasma-emission threshold are shown to decrease with decreasing pulse width over the entire pulse-width range investigated.

Pulse-width influence on the laser-induced structuring of CaF2 (111)

We have investigated the morphology of CaF2 (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs-8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps.

Processing multilayer systems using femtosecond, picosecond, and nanosecond laser pulses at different wavelengths

We performed ablation studies on multi-layer systems at different wavelength - pulse duration combinations. The multi-layer systems of interest, 150 nm thin indium tin oxide (ITO), 200 thin polyaniline (PANI) on 1 micrometers thick photo resist, and 280 nm PPV/pedot layer-combination on 150 nm thin ITO are optically transparent and used for a variety of industrial applications. One important goal of the study was to determine the possible process window for a complete removal of only the top layer, leaving the remaining layer basically unharmed. The investigations were conducted with the following wavelength - pulse duration combinations: 800 nm and 180 fs, 800 nm and 5 ps, 266 nm and 150 fs, 266 nm and 5 ns, 532 nm and 5 ns. We generated micro dots, lines and areas to determine the damage threshold, the processing quality and the processing speed for the specified application of selective layer removal. The structures were analyzed by means of optical and atomic force microscopy. In some cases, we observed a strong pulse duration dependence in the ablation threshold, an indication for the observed difficulties using laser pulse in the ns range. Comparative studies at different wavelengths demonstrate that laser pulses in the UV are not necessarily always a first choice to achieve a precise removal of the optically transparent top layer.

Material processing of dielectrics with femtosecond lasers

Ultrashort laser pulses have considerable potential for micron and sub-micron structuring of several materials. The lower energy impact, the reduction of thermal damage, the elimination of laser-plume interaction, and the exploitation of nonlinear optical effects all contribute to a strong improvement when compared to results using pulse widths in the nanosecond range. Depending on the choice of fluence compared to the damage threshold, with ultra-short laser pulses one is able to generate different types of structures, minimizing the heat affected zone. The damage threshold drops dramatically during the first laser shots, due to defect incubation. This has important consequences for applications, such as laser machining and for the lifetime of optical components. At a fluence below surface damage threshold we were also able to generate bulk modifications of different size and location in a controllable fashion by variation of laser pulse width, energy and number of shots, utilizing the beam narrowing effects during self focusing. A study of the dependence of the structure depth on the square root of the laser power for a given pulse length provides a straightforward method for determining the non-linear index of refraction.

Machining of glass and quartz using nanosecond and picosecond laser pulses

New laser processing strategies in micro processing of glass, quartz and other optically transparent materials are being developed with increasing effort. Utilizing diode-pumped solid-state laser generating nanosecond pulsed green (532 nm) laser light in conjunction with either scanners or special trepanning systems can provide for reliable glass machining at excellent efficiency. Micro ablation can be induced either from the front or rear side of the glass sample. Ablation rates of over 100 μm per pulse can be achieved in rear side processing. In comparison, picosecond laser processing of glass and quartz (at a wavelength of 1064 or 532 nm) yield smaller feed rates at however much better surface and bore wall quality. This is of great importance for small sized features, e.g. through-hole diameters smaller 50 μm in thin glass. Critical for applications with minimum micro cracks and maximum performance is an appropriate distribution of laser pulses over the work piece along with optimum laser parameters. Laser machining tasks are long aspect micro drilling, slanted through holes, internal contour cuts, micro pockets and more complex geometries in e.g. soda-lime glass, B33, B270, D236T, AF45 and BK7 glass, quartz, and Zerodur.

Laser-induced incubation in transparent materials and possible consequences for surface and bulk microstructuring with ultrashort pulses

We performed surface and bulk processing experiments on different transparent materials with ultra short laser pulses. The investigations were performed mainly at 800 nm and at pulse widths ranging from 0.2 to 5 ps. We focused our attention on fluence and shot number dependencies to analyze possible incubation effects in the different materials and determine the damage threshold. In the multi- shot experiments we determined strong incubation effects which we attribute to laser-induced defect formation and accumulation. Inside the bulk we were able to generate dots and lines even in sub-micrometers sizes. The structures were analyzed by means of optical microscopy. Laser pulses at a pulse width above ca. 1 ps demonstrate strong self focusing which can be utilized for bulk and rear surface micro structuring. Below a certain pulse width other effects counteract self focusing and beam diffraction and fillamentation seem to dominate. Depending on focusing optics we observe strong differences in the possibility to process the bulk of transparent materials with fs laser pulses which we attribute to the effects in Kerr non- linearity. Also, the consequences of incubation effects on the structuring inside the bulk seem to depend strongly on the pulse width. We discuss the results based on possible technological relevance and the ablation mechanism involved.

Surface and bulk ultrashort-pulsed laser processing of transparent materials

Ultrashort pulsed laser ablation of dielectrics has been investigated using ex-situ morphological examinations in combination with in-situ time-of-flight mass spectrometry of the ablated species. Analysis of the energy spectrum of the ablation products provides a wealth of information on the processes occurring during femtosecond laser ablation of materials. The presentation will focus on the case of sapphire (Al2O3) and discuss the fundamental processes in ultrashort pulsed laser sputtering. Two different ablation phases have been identified, a gentle phase with low ablation rates and a strong etch phase with higher ablation rates, but with limitation in structure quality. A comparison of the energy and momentum distributions of ejected ions, neutrals and electrons allows one to distinguish between non-thermal and thermal processes that lead to the macroscopic material removal. Fast positive ions with equal momenta are resulting from Coulomb explosion of the upper layers at low fluence and low number of irradiating laser pulses (gentle etch phase). Pump-probe studies with fs laser pulses reveal the dynamics of excitation and electron mediated energy transfer to the lattice. At higher laser fluences or after longer incubation, evidence for phase explosion can be derived from both the morphology of the surface and the results of the in-situ experiments.

Ultrashort laser pulse processing of wave guides for medical applications

The availability of ultra short (ps and sub-ps) pulsed lasers has stimulated a growing interest in exploiting the enhanced flexibility of femtosecond and/or picosecond laser technology for micro-machining. The high peak powers available at relatively low single pulse energies potentially allow for a precise localization of photon energy, either on the surface or inside (transparent) materials. Three dimensional micro structuring of bulk transparent media without any sign of mechanical cracking has been demonstrated. In this study, the potential of ultra short laser processing was used to modify the cladding-core interface in normal fused silica wave guides. The idea behind this technique is to enforce a local mismatch for total reflection at the interface at minimal mechanic stress. The laser-induced modifications were studied in dependence of pulse width, focal alignment, single pulse energy and pulse overlap. Micro traces with a thickness between 3 and 8 μm were generated with a spacing of 10 μm in the sub-surface region using sub-ps and ps laser pulses at a wavelength of 800 nm. The optical leakage enforced by a micro spiral pattern is significant and can be utilized for medical applications or potentially also for telecommunications and fiber laser technology.

Laser-induced sub-surface modification of the optical properties in transparent materials: nik-engineering (TM)

A new field in laser processing is opened by the method of modifying the optical properties, i.e. the refractive index, absorption- and scattering-coefficient, at minimal mechanical stress inside the material. Focusing ultra short laser pulses inside the transparent media allows to control and modify their optical properties. This is referred to as nik-engineering (TM), relating the technique to changes of the complex refractive index, i.e. (n+ik). Three dimensional patterns of the (n + ik) modifications can be achieved in the subsurface region even on a microscopic scale. New results in nik-engineering obtained in our application laboratory are presented using different optical materials. The results in laser nik-engineering of photo-chromic glass using ultra short laser pulses at a wavelength of 800 nm is presented. A model in respect to the relevant processes leading to the observed laser-induced modifications in the optical properties of photo-chromic glass is presented. We discuss the results and the commercial potential of nik-engineering.

High aspect-ratio laser-drilling of micro-holes with a Nd:YAG master-oscillator power-amplifier (MOPA) system

A nanosecond Nd:YAG-MOPA-system with high average power at excellent beam qual-ity was used for laser drilling experiments into metals and ceramics. The output power of the oscillator (10 W@1064 nm) with a beam quality of M² = 1.3 was amplified to 95 W@1064 nm with M² = 2.3 and a pulse energy up to 500 mJ. Ablation rates in dependence of fluence, pulse-shape and ambient pressure were detected. The influence of beam quality on the geometrical shape of the hole was investigated. Additionally means for optimizing the drilling process such as burr reduction and cylindrical constancy were investigated. A maximum aspect-ratio of 1:200 was obtained.A nanosecond Nd:YAG-MOPA-system with high average power at excellent beam qual-ity was used for laser drilling experiments into metals and ceramics. The output power of the oscillator (10 W@1064 nm) with a beam quality of M² = 1.3 was amplified to 95 W@1064 nm with M² = 2.3 and a pulse energy up to 500 mJ. Ablation rates in dependence of fluence, pulse-shape and ambient pressure were detected. The influence of beam quality on the geometrical shape of the hole was investigated. Additionally means for optimizing the drilling process such as burr reduction and cylindrical constancy were investigated. A maximum aspect-ratio of 1:200 was obtained.