High repetition rate UV ultrafast laser inscription of buried channel waveguides in Sapphire: fabrication and fluorescence imaging via ruby R lines.

We report on the fabrication of buried cannel waveguides in Sapphire crystals by 250-kHz high repetition rate ultrafast laser inscription with 385 nm pulses. The propagation properties of the waveguides were studied as a function of the writing conditions. The micro-fluorescence analysis of the R lines generated by trace Cr³⁺ dopant in Sapphire is used to elucidate the micro-structural modifications induced in the crystal network. It is revealed that waveguide has been formed due to local dilatation of the Sapphire network generated in the surroundings of the focal volume. The refractive index increment due to the dilatation induced electronic polarizability enhancement has been estimated to be of the order of Δn ≈ 10⁻⁴.\ud \u