Cover Story: Jia Y C, Wang S X, Chen F. Femtosecond laser direct writing of flexibly configured waveguide geometries in optical crystals: fabrica-tion and application. Opto-Electron Adv 3, 190042 (2020). 

Femtosecond-laser-direct writing (FsLDW), as a true three-dimensional (3D) micromachining and microfabrication technology, allows rapid prototyping of on-demand waveguide geometries inside transparent materials via localized material modification. Due to the complexity in crystal lattice structures, direct fabrication of 3D waveguides in dielectric crystals employing FsLDW is far more challenging than that in glasses. Therefore, special designs of waveguide geometries and FsLDW fabrication skills are usually required. A review article written by the research group of Prof. Feng Chen from Shandong University summarizes the key design prototypes of 3D waveguide devices by FsLDW in optical crystals. By modifying/optimizing the fabrication strategies of the three basic geometries of FsLDW straight waveguide channels, a number of derived non-straight waveguide prototypes/designs in crystalline materials have been explored, which have been implemented a number of applications as active 3D photonic waveguide devices in electro-optic modulation, lasing, nonlinear optics, etc. Being compatible to the emerging micro-/nano-photonic solutions such as metasurfaces, plasmonic structures, and photonic crystals, these linear/nonlinear hybrid 3D waveguide devices and the 3D FsLDW technology are expected to open up new opportunities and possibilities in multifunctional photonic integrated circuits operating in both classic and quantum systems.