Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystals

Laser-active F2 centers were produced in lithium fluoride (LiF) at a concentration of 2×1018 cm−3 by irradiating focused femtosecond (fs) laser pulses from a mode-locked titanium sapphire laser (wavelength ∼800 nm, emission pulse duration ∼100 fs). This technique was used to write waveguides embedded in LiF crystals. A refractive index change estimated from a guide propagation method was approximately +1% at a wavelength of 633 nm. Refractive index-modulated volume-type gratings were also encoded inside LiF crystals by a single interfered fs laser pulse. The distributed feedback laser structure was fabricated using the gratings thus encoded, which exhibited a room-temperature F2-color center laser oscillation at 707 nm. This demonstrates a DFB color center laser operating at room temperature utilizing photon written, permanent Bragg gratings.