BW1D.5.pdf Advanced Photonics Congress © 2012 OSA Zeosil formation by femtosecond laser irradiation J. Canning* a , M. Lancry b , K. Cook a , and B. Poumellec b a Interdisciplinary Photonic Laboratories (iPL), School of Chemistry, The University of Sydney, NSW 2006, Australia; b LPCES/ICMMO, UMR CNRS-UPS 8182, Université Paris Sud 11, Bâtiment 410, 91405 Orsay Cedex, France *john.canning@sydney.edu.au Abstract: We report the fabrication of zeosil by exploiting rapid local heating and quenching, under very high induced pressures, when silica is irradiated by femtosecond near IR laser. The release of oxygen indicates that the process is aided by the sudden conversion from a tetrahedral network prior to irradiation to rapid cooling under pressure of lower coordinated silica, an aspect which does not possible in most conventional preparations of glassy materials. Multi-photon absorption allows high localisation of these glass changes. OCIS codes: (000.0000) Femtosecond irradiation; (000.0000) gratings; (000.0000) glass, (000.0000) laser processing; zeolites; (000.0000) mesostructured silica 1. Introduction Zeolites are impressive porous, "sieve" structures involving silica doped with various level of aluminium; variations in the porous structure are also achieved by adding other trace metals. Pure silica zeolites, or “zeosil” [1], are especially interesting for their high thermal stability and hydrophobic nature [2], making them suitable for applications involving selective filtering or trapping of species. There exist many ways of preparing these materials, mostly chemical through sol-gel and gel-like reactions. This restriction to chemical processing, and the idea that they are energetically less favourable, has suggested that normal preparation via glass quenching, or by conventional glass processing, is not possible. Whilst all silica molecular sieves are metastable with respect to quartz, Petrovic et al. noted that this modest metastability posed no intrinsic thermodynamic barrier to their formation [3]. Questions regarding energetically stable large pore formation within these zeosils were evaluated in [4] where it was shown these two are close to the values of quartz, explaining why so many zeosils have been formed (~30 or so). In this work, we show that the formation of zeosils is possible by a very different route – fast rapid ionisation of silica, followed by rapid cooling under internal pressures (usually negative) created by the surrounding volume. We show that these conditions in fact already exist within normal femtosecond waveguide and grating writing, and that the process giving rise to zeosil formation essentially is what accounts for much of the nanostructure observed. 2. Femtosecond processing and silica The rapid deposition of heat through laser ionisation leads to an extreme in rapid cooling of a glass, where there is effectively (for the low repetition case) no overlap between excitation and quenching, an extremely unusual case that has no obvious analog in conventional glass processing by convective or radiative heat treatment alone. So how do we exploit this process? To answer this we need to consider conventional glass quenching and what makes silica rare. In the schematic of Figure 1 (a), an illustration of glass quenching of a typical glass former is depicted on a conventional V-T curve. The liquid state is usually less dense, and therefore lager volume, than the solid state, either amorphous or crystal, which has the optimal packing configuration. Representing the lowest free energy typically means a system, with sufficient time, drives towards the closest packing arrangement of its structure – for a random network this means the solid is denser than the liquid where local motions prevent densest packing, and that the crystal state is usually denser than the amorphous state. It is extremely difficult to see how any mesoporous structure could form under these conditions. However, silica is one of a handful of materials whose bond angles, and existing dipoles, present a serious theoretical challenge to a purely thermodynamic model of amorphous relaxation. A summary of the V-T diagram for silica can be quite confusing to interpret – the version for silica shown in Figure 1 (b) is essentially that determined by Figure 1. V-T curves for (a) typical glass forming liquids and for (b) low OH containing silica. V T liquid crystal glass T g T g T m T g T g stable metastable vitreous state (b)