Femtosecond laser writing of depressed cladding waveguides in sapphire

A promising solution for scalable integrated optics of trapped-ion quantum processors are curved waveguides guiding visible light within sapphire bulk material. To the best of our knowledge, no curved waveguides were investigated in sapphire so far and no waveguides for visible light in undoped planar sapphire substrates were reported. Here, we demonstrate femtosecond laser writing of depressed cladding waveguides in sapphire. Laser parameters such as pulse energy, pulse duration, and repetition rate, as well as waveguide geometry parameters were optimized to guide 728 nm light. This resulted in single-mode waveguides with a propagation loss of 1.9 (3) dB/cm. The investigation of curved waveguides showed a sharp increase in total loss for curvature radii below 15 mm. Our results demonstrate the potential of femtosecond laser writing as a powerful technique for creating integrated optical waveguides in the volume of sapphire substrates. Such waveguides could be a building block for integrated optics in trapped ion quantum processors.