Graphitization wave in diamond bulk induced by ultrashort laser pulses

Multi-pulse laser irradiation of diamond bulk after the optical breakdown causes extension of continuous graphitized region toward the laser beam that can be described as propagation of a "graphitization wave." Velocity of the graphitization wave in single-crystal diamond is measured experimentally as a function of local laser fluence for a few numerical apertures (NA = 0.36-0.09), pulsewidths (140 fs-5 ps), and beam orientations (along [110] or [100] diamond axes). The experimental results are used to develop the model of the crack-assisted thermal graphitization of diamond at the boundary of the laser-modified region. Velocity of the graphitization wave is determined in general case by diffusion of heat from the light-absorbing modified region. The revealed rise in the graphitization wave velocity for the [110] beam orientation can be explained by the local electric field enhancement near the crack tip, which facilitates diamond ionization and plasma-assisted energy absorption. The proposed model predicts a specific internal structure of the laser-modified region: the network of graphitic inclusions with diamond-filled gaps between them.