Fabrication of optical phase elements in bulk sapphire by UV pulses

Sapphire has many advantages over other materials commonly used for direct laser writing of optical phase elements. These include excellent thermal stability and conductivity, broad transmission spectrum, high optical damage threshold, chemical inertness, and nonlinear properties, which could lead to interesting interplay with various phase elements. The issue arises when attempting to fabricate phase elements – the aforementioned high optical damage threshold means a high intensity is required for direct laser writing. This leads to many problems, such as cracking, nanograting formation, and induction of significant mechanical stress, all of which prevent using the material's beneficial properties. We share our research into the fabrication of phase elements in sapphire, which shows that ultrashort UV laser pulses significantly reduce the induced stress and likelihood of cracking. We show fabricated high-quality photonic crystal spatial filters and diffraction gratings as examples of phase elements.