Photonics in wide-band-gap materials: The challenge of color-center waveguides in lithium fluoride

Lithium fluoride is an insulating material with outstanding optical properties, among them wide optical transparency. Its ionizing radiation sensitivity allows the formation of primary and aggregate point defects, known as color centers, some of which are laser active in the visible and near-infrared even at room temperature. Among their peculiarities, the simultaneous light emission in the green-red wavelength interval under optical excitation in the blue spectral range is suitable for the realization of solid-state photonic light sources, miniaturized optical amplifiers and lasers compatible with integrated optical circuits. Despite the material's low refractive index, broad-band light-emitting color-center active waveguides were realized at the surface of lithium fluoride crystals and thin films by direct writing with low-energy electron beams and light ions. Due to the complex behavior of radiation-induced defects at high concentrations, the balance between optical gain and losses remains a technological challenge for laser operation. This review will revise the main achievements of our group and emphasize some interesting features, along with a discussion of new experimental results, aiming to promote further developments in photonics and quantum technologies. • LiF is a radiation-sensitive transparent insulating material of low refractive index. • Optically pumped solid-state LiF color-center lasers tunable in the VIS operate at RT. • Radiation-induced color-center waveguides demonstrated in LiF crystals and thin films. • Gain in color-center LiF waveguides directly written by low-energy electron beams. • Characterization of color-center waveguides written in LiF crystals by He ions.