Integration of Fresnel Zone Plates in the Bulk of Sapphire Crystal by Femtosecond Laser Pulses

The goal of this study was to integrate Fresnel zone plates with diameters up to 300 m in the bulk of sapphire crystal by direct laser writing technique. These zone plates are intended to be integrated directly into GaN LED substrate because of their potential to increase the light extraction efficiency of UV LED devices. For integration task a femtosecond (320 fs) high repetition rate (100 kHz) Yb:KGW laser system was used together with high precision linear positioning stages. By exposing sapphire to the focused femtosecond laser radiation, regions of modified refractive index were induced and required zone plates were patterned. The discussions about optimal fabrication parameters as well as focusing properties of fabricated zone plates are presented. It is shown that sapphire can be a suitable material for microphotonic device integration: working zone plates with diameters up to 300 m and diffraction efficiencies up to 23% were integrated below (80 m) the sapphire surface.