Dynamics of plasma formation and permanent structural transformation in ZBLAN excited by tightly focused femtosecond laser pulses

Time-resolved dynamics of plasma formation and bulk refractive-index modification in fluoride glass (ZBLAN) excited by a tightly focused femtosecond (130 fs) Ti:sapphire laser (λ_p=790 nm) was observed in situ. The femtosecond time-resolved pump–probe measurement with perpendicularly linear polarized beams was used to study the dynamics of both plasma formation and induced permanent structural transformation with refractive-index change. In the refractive-index domain, the lifetime of induced plasma formation is ～35 ps and structural transition time for forming the refractive-index change is ～80 ps. In the optical damage domain, however, the lifetime of induced plasma formation is ～40 ps and structural transition time for forming the optical damage is ～140 ps. We found that the process of refractive-index bulk modification is significantly different from that of optical cracks. From the diffraction efficiency of Kogelnik's coupled mode theory, the maximum value of refractive-index change (Δn) was estimated to be 1.3×10^?2. By the scanning of fluoride glass on the optical X–Y–Z stages, the fabrication of internal grating with refractive-index modification was demonstrated in fluoride glass using tightly focused femtosecond laser.