LPM2000 invited paper, Hertel et al.; 4088-003 – SPIE E:\eig_papers\japan\lpm2000.rtf 25. August 2000 09:48 1 Surface and bulk ultra-short pulsed laser processing of transparent materials Ingolf V. HERTEL, Razvan STOIAN, David ASHKENASI*, Arkadi ROSENFELD, and Eleanor E.B. CAMPBELL** Max Born Institute, Max-Born-Strasse 2a, D-12489 Berlin, Germany, http://www.mbi-berlin.de E-mail: hertel@mbi-berlin.de *now with LMTB, Berlin-Adlershof, Germany, **Department of Experimental Physics, University of Gothenburg and Chalmers University of Technology, S-41296, Gothenburg, Sweden Ultrashort pulsed laser ablation of dielectrics has been investigated using ex-situ morphological ex- aminations in combination with in-situ time-of-flight mass spectrometry of the ablated species. Analy- sis 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 (Al 2 O 3 ) and discuss the fundamental processes in ultrashort pulsed laser sputtering. Two dif- ferent 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. Keywords: ultra-short pulsed laser processing, dielectrics, ablation, velocity distribution, Coulomb ex- plosion, phase explosion, self-focusing. 1. Introduction Improved availability and compactness of ultra-short, sub-ps pulsed solid state lasers has stimulated a growing interest in the exploiting of the enhanced flexibility of fem- tosecond-technology for micro-machining. Disregarding price and user friendliness of present laser systems - which will improve massively in the coming years – ultra-short laser pulses offer a variety of advantages for precision micro fabrication. Presently, the first designated commercial fem- tosecond machining stations are available for an expanding market. Due to lower energetic thresholds for sub picosec- ond ablation and the controllability of individual laser pulses (e.g. by laser pulse length) the amount of energy deposited into the processed sample can be minimised and highly lo- calised. This leads to a reduction of unwanted thermal ef- fects, a minimisation of energy diffusion, and little débris (material ejection) so that very clean microstructures can be achieved with pulses optimised for the individual applica- tion. Fig. 1 illustrates as an example that the most perfect micropores can be achieved with 800 nm, 200 fs pulses in c- Al 2 O 3 (sapphire) – while other materials, e.g. CaF 2 may well require picosecond treatment for optimal results [1]. Also non-linear optical effects may be exploited: efficient multi- photon absorption allows one to modify transparent materi- als even inside the bulk and to obtain sub wavelength struc- ture sizes. Self-focusing due to the non-linear optical Kerr effect may be used to induce long, narrow 3D-modification traces into the bulk of wide band gap materials. Long, mi- crometer thin channels can be also drilled taking advantage of the high ablation rates and low heat depositin when em- ploying ultrashort laser pulses [2]. These prospects warrant a detailed understanding of the Fig. 1: Micropores in Sapphire created with ultrafast 800 nm laser pulses illustrating the influence of pulse duration (left 4.5 ps, right 200 fs – same laser fluence).