Direct laser writing in glasses using femtosecond lasers has been extensively studied during the last two decades. It provides a robust and efficient way to directly inscribe 3D photonic structures in the interior of bulk glasses. More specifically, the inscription of 3D embedded waveguides has been well established. However, direct laser writing of waveguides neighboring the glass surface was only demonstrated in few works. This is owed to ablation that arises when approaching the glass surface. This shortcoming was recently overcome using additional processing and/or reinforced glasses.
In silver containing glasses, direct laser writing induces the creation of silver clusters around the interaction voxel inducing a Type A refractive index change. It allows the inscription of optical waveguides using very low pulse energy, differing from standard type I waveguides which are inscribed with higher energy pulses. The low energy regime allows for the inscription of waveguides in close proximity to the surface while inhibiting surface ablation. In this work, we demonstrate direct laser writing of near-surface waveguides in a non-reinforced bulk glass sample without resorting to any additional processes, as well as a demonstration of a high-sensitivity refractive index sensor based on such near-surface waveguides. This work highlights the novelty and benefits of type A waveguides inscribed in silver containing glasses and their high potential for sensing applications compatible with laser manufacturing approaches. Finally, our results could be easily transposed to silver containing ribbon shaped glass fibers, thus paving the way towards fiber based sensing applications.
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