Modification of transparent materials by temporally shaped ultrashort pulses
Final Report Abstract
In this project part important answers on the open questions addressed in the application of this project part have been found. Most importantly, the experimental setup for time-resolved phase contrast (PCM) and optical transmission microscopy (OTM) has been improved to allow time- and space-resolved observations in both microscopy modes. To the best of our knowledge, this is the first in-situ imaging of laser-induced refractive index modification in the bulk of transparent materials. We have demonstrated sub-ps temporal resolution (over an extended measurement window up to nanoseconds) with a spatial resolution which is limited only by the optical diffraction limit (submicron). Due to the improved sensitivity of the setup it was then possible to access the regime of excitation levels close to the modification threshold of the materials (a-SiO2 and BK7) – the regime which is most relevant for the technologically important fs-waveguide writing. By means of these technical improvements, we have found an answer on what time scale and for which laser pulse intensity and pulse duration refractive index modifications do appear. We have observed that the modification essential for the formation of refractive index change takes place on a time scale of tens of nanoseconds to microseconds. In detail, the following points have been demonstrated, and are worth being highlighted. At first, we note at the single pulse irradiation a drastic difference in the carrier dynamics and the materials response of BK7 and a-SiO2 under laser irradiation, emphasized by the use of phase contrast microscopy. While the response of BK7 seems to be dominated by thermal expansion resulting in the onset of a low density material exhibiting a lower index of refraction, the response of a-SiO2 looks more enigmatic. After exposure at moderate laser energies, mainly two regions are detectable. A low material density region, identified as a void, and a filament of higher refractive index than the pristine bulk. Solving the nonlinear Schrödinger equation allowed us to assign the formation of this void to a region of high energy deposition. Moreover, the void visible after irradiation in a-SiO2 coincides with a zone of strong thermal expansion. This indicates that thermo-mechanical effects play a dominant role, not only in BK7, but also in a-SiO2. Therefore, we propose that the refractive index change is mostly driven by a thermo-mechanical phenomenon. Below a certain peak temperature, the material cools down in a higher density state, and above this temperature, the material experiences a thermoplastic transformation resulting in the formation of the void. The involvement of thermo-mechanical processes has been directly proven by first the optical observation of a femtosecond laser-induced pressure wave travelling in the bulk of the transparent material at its speed of sound. Up to now, we have investigated the time scale until 10 ns which is the maximum accessible by conventional pump-probe schemes. One striking result of our work is that for a complete physical picture and for bridging the gap between short and long times, we would have to extend the dynamics investigations to longer time scales in the ns to microsecond range. This should be one task of the continuation of this project. Another import question addressed in the application is how can one guide the material-dependent interaction channels and the energy flow dynamics in laser irradiated solids by impulsive and large bandwidth excitation with THz repetition rate pulse trains. For that, we have also found a first answer by demonstrating the possibility in BK7 to inverse the material response upon application of optimized fs pulse sequences at THz repetition rate. This means that we were able to obtain an increase of refractive index (in the single shot regime) and to compensate the influence of spherical aberrations by properly temporally shaped laser pulses in the fs- to ps-time domain. In collaboration with two other international research groups, it was demonstrated for the first time that it is possible to generate low-loss fs-written waveguides in BK7 glass, where a refractive index decrease (not suitable for waveguiding) was reported in the literature before. Not all questions in the application have been answered completely. So the question still exists if an optimization procedures based on pulse envelope shaping allows for a better understanding and control of the energy flow after laser irradiation. Another question is if we can systematically compare the results for different classes of materials (e.g., glasses, crystals, polymers) and glean direct insight into the relevant energy redistribution mechanisms by comparing the specific coupling times of these materials. The systematic analysis of the material response to temporally shaped ultrashort pulses or pulse trains and the synchronization with the fast response times of the irradiated materials at high repetition rate excitation is of fundamental interest. Furthermore, the developed theoretical model should be extended for multipulse excitation to describe the dynamics of electron excitation and lattice heating to uncover the mechanisms allowing the modulation of the structures induced by laser modification in the bulk of the materials.
Publications
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Direct observation of femtosecond laser induced modifications in the bulk of fused silica by phase contrast microscopy. Journal of Laser Micro / Nanoengineering (JLMN) 1 (2006) p.155-160
A. Mermillod-Blondin, I. M. Burakov, R. Stoian, A. Rosenfeld, E. Audouard, N. Bulgakova, and I. V. Hertel
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Tailored excitation sequences for optimized laser-induced modifications in bulk transparent materials exposed to sub-ps irradiation. Photon Processing in Microelectronics and Photonics Photon Processing in Microelectronics and Photonics V 6106, T. Okada, C. B. Arnold, M. Meunier, A. S. Holmes, D. B. Geohegan, F. Träger, and J. J. Dubowski eds. (SPIE Proceedings, 2006) Vol. 6106, 610601/1-9
A. Mermillod-Blondin, R. Stoian, M. L. Boyle, A. Rosenfeld, I. M. Burakov, N. Bulgakova, E. Audouard and I. V. Hertel
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Transient response of dielectric materials exposed to ultrafast laser radiation. Appl. Phys. A 84, (2006) p. 413–42
S.W. Winkler, I.M. Burakov, R. Stoian, N.M. Bulgakova, A. Husakou, A. Mermillod-Blondin, A. Rosenfeld, D. Ashkenasi, and I.V. Hertel
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Analysis and optimization of ultrafast laser induced bulk modification in transparent materials. Freie Universität Berlin and Université Jean Monnet, Institut Superieur Des Techniques Avancees St. Etienne, France, Dissertation 2007-05-30
A. Mermillod-Blondin
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Fast electronic transport and coulomb explosion in ultrafast laser irradiated materials. In: Laser Ablation and its Applications, C. R. Phipps (Ed.), Springer Series in Optical Sciences, Vol. 129, Springer-Verlag, New York, 2007, p. 17-36 ISBN 0-387-30452-5
N. M. Bulgakova, R. Stoian, A. Rosenfeld, I. V. Hertel and E. E. B. Campbell
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Modeling of electron dynamics in laser-irradiated solids: progress achieved through a continuum approach and future prospects. Proc. SPIE 6732, 673208 (2007)
N. M. Bulgakova, A. Rosenfeld, L. Ehrentraut, R. Stoian, and I. V. Hertel
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Spatial distribution of refractive index variations induced in bulk fused silica by single ultrashort and short laser pulses. J. Appl. Phys. 101, 043506 (2007)
I. M. Burakov, N. M. Bulgakova, R. Stoian, A. Mermillod-Blondin, E. Audouard, A. Rosenfeld, A. Husakou, and I. V. Hertel
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Theoretical models and qualitative interpretations of fs material processing. Journal of Laser Micro / Nanoengineering (JLMN) 2 (2007) p. 76-86
N. M. Bulgakova, I. M. Burakov, Y. P. Meshcheryakov, R. Stoian, A. Rosenfeld, and I. V. Hertel
