Nanoscale X-Ray Tomography of Fs-Laser-Written Integrated Photonics

Femtosecond direct laser writing is an established manufacturing method for photonic integrated circuits, enabling rapid prototyping of arbitrary waveguide structures in a three-dimensional geometry. It hinges on a permanent material modification caused by focusing intense laser pulses into a glass wafer (Fig. 1), where they induce refractive index changes on a scale of several microns [1], [2]. Depending on the material and exposure conditions, the induced modification can vary significantly, ranging from the relatively gentle refractive index contrast in of order 10–<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> - the preferred regime for conventional waveguides - to self-organized subwavelength nanogratings at intermediate powers, to the formation of voids and nanocracks at high powers.