High index-contrast all-solid photonic crystal fibers by pressure-assisted melt infiltration of silica matrices Ning Da a , Lothar Wondraczek a, ⁎, Markus A. Schmidt b , Nicolai Granzow b , Philip St. J. Russell b a Chair of Glass and Ceramics, Department of Materials Science, University of Erlangen-Nuremberg, Erlangen 91058, Germany b Max-Planck-Institute for the Science of Light, Günther-Scharowsky-Str. 1/Bau 24, Erlangen 91058, Germany abstract article info Article history: Received 29 April 2010 Received in revised form 2 July 2010 Available online 2 August 2010 Keywords: Photonic crystal fiber; Rheology; Shear-thinning; Tellurite; Chalgogenide All-solid photonic crystal fibers (PCFs) are created by pressure-assisted filling of low-melting-point chalcogenide and tellurite glasses into silica matrix fibers with channel diameters as small as 200 nm. Overcoming to a large extent the problem of viscosity and, thus, process incompatibility of silica and non- silicate optical glasses, the technique provides a unique way of producing waveguiding devices with high core-cladding index-contrast, high optical non-linearity and a transmission range that extends into the mid infrared. In this paper, as a prerequisite for waveguide production, the rheologic properties and controlled flow of highly-viscous liquids under geometrically confined conditions are considered, and deviations from Newtonian behavior are discussed. Because the filling process requires only very small quantities of filling material that do not come into contact with the environment, and because ultra-high cooling rates can be achieved, the technique enables the use of difficult-to-handle or reactive optical glasses. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Photonic crystal fibers (PCFs) have become a versatile alternative to classical step-index optical fibers [1]. Typically, they consist of a strand of microstructured fused silica glass with an array of hollow channels running parallel to its axis, and a central core that can be either solid or hollow. These hollow channels can be filled with another material of different refractive index, and depending on the sign of the refractive index difference between fused silica and the filling material, light can be guided in a central silica core by either total reflection or as a result of a photonic bandgap of the cladding [1]. All-solid glass-glass PCFs are of interest because they make it possible to combine the optical properties of two glasses rather than a single glass plus air [2]. Much higher index contrasts, high optical nonlinearities and, for instance, the incorporation of optically active species can be realized in such devices. However, production of these structures represents a considerable technological challenge because the most interesting pairs of glasses are usually highly incompatible with respect to melting and flow behavior, thermal expansion, surface tension, wetting and interface reactions. This prevents conventional fiber drawing. Hence, a unique alternative to at least partly overcome some of these challenges is to pump a low-melting glass into a rigid PCF matrix structure [3]. Utilizing fused silica PCF templates, this allows the production of fiber waveguides with very high index contrast and suggests a large variety of potential applications such as broadband supercontinuum sources and other optically non-linear devices, fiber lasers, filters or polarizers [4]. On the other hand, besides properly designing the filling process to obtain an optically transpar- ent core structure for a technologically relevant fiber length, this approach also requires detailed knowledge of the flow, relaxation and solidification behavior of the glass that is to be pumped into the PCF. More specifically, little is known about the properties of molten glass in highly constrained μm-scale geometries and under mechanical load. Even in mildly confined conditions, inorganic melts may exhibit several peculiarities. These include positive [5] or negative [6,7] dependence of viscosity on pressure, pressure-induced structural changes [8], the evolution of anisotropic flow and structural re- arrangement [9,10], crystallization [11,12] and solubility of gases [13,14]. Compared to organic fluids (and particularly polymer melts), for reasons of experimental limitations, these phenomena are typically difficult to assess. Hence, relevant knowledge is still very limited [15], although it may have direct consequences for not only PCF fabrication but various other applications such as micromecha- nical forming processes [16] or the design of anisotropic glasses [17]. Possible choices of suitable low-melting-point glass range from classical tellurite [18] to ultra-low-melting chalcogenide [19] or a large variety of phosphate glasses (e.g. Refs. [20,21]). With respect to the thermal properties of the silica matrix, in the extreme case the pumping medium should exhibit a viscosity of less than 10 Pa s at 1150–1250 °C, depending on the silica grade. If a step-index waveguide is to be produced by filling a silica capillary, its refractive index should be higher than that of silica. For this reason, in the present study, we focus on selected tellurite and chalcogenide melts Journal of Non-Crystalline Solids 356 (2010) 1829–1836 ⁎ Corresponding author. E-mail address: lothar.wondraczek@ww.uni-erlangen.de (L. Wondraczek). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.07.002 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol