Graphitization wave in diamond induced by uniformly moving laser focus

Abstract

Laser beam tightly focused inside single-crystal diamond allows conducive microstructures to be fabricated via local phase transition of diamond to graphite. An extended modified region is formed in diamond due to propagation of so called laser-induced graphitization wave, which occurs immediately after the optical breakdown and propagates towards the laser beam even though the position of the laser focus is fixed. This paper is the first to consider the behavior of the graphitization wave when the laser focus uniformly moves towards the laser, which results in the formation of conductive wires of unlimited length in diamond. It has been found that there is an initial transitional period in the wire growth, during which velocities of the laser focus and the wire front are equalized owing to the change in the distance between them. After stabilization of the graphitization front velocity, the axial fluence at the front of the growing wire also reaches a constant value. It has been found that the stable laser fluence at the wire front is practically independent of the laser pulse energy, but it grows with increasing velocity of the laser focus. Such increase finally leads to violation of the physical criterion of the continuous wire growth, since the axial fluence at the wire front becomes higher than the diamond breakdown threshold. It has been shown that the minimal fluence providing propagation of the graphitization wave in diamond can be used to predict the lateral wire dimension.