Experimental and theoretical investigation of ultrashort pulsed laser ablation of diamond

Diamond is a wide bandgap material, which exhibits an abrupt increase of its free-electron density, when excited by an ultrashort laser pulse. The generation of free electrons transforms the insulator diamond to a conducting material with metallic optical behavior. This transformation process can be described by the multiple rate equation (MRE) model. The introduced MRE model considers strong-field excitation in the Keldysh picture as well as collisional excitation. The light attenuation results from the strong-field absorption and free-carrier absorption described in the Drude picture. Thus, the electron density and intensity distribution as functions of time, penetration depth, and laser beam radius are calculated. Furthermore, the model predicts the evolution of optical properties and estimates the ablation threshold value by the diameter and depth regression method. The calculated ablation threshold is compared to experimental results on a single crystalline chemical vapor deposited diamond by applying the diameter and depth regression method. Experimental and theoretical results are discussed with regard to the pulse duration. The discussion focuses on single pulse ablation but also addresses the multishot domain, which is essential for laser machining. At 1030 nm, the experimental single pulse ablation threshold fluence is determined to be 8.2 and 12.9 J/cm2 for pulse durations of 400 and 700 fs, respectively. This is in compliance with the simulation results.