Ultrafast ablation with high-pulse-rate lasers. Part I: Theoretical considerations

Abstract

We propose a novel ultrafast pulsed laser deposition (PLD) technique, which eliminates the well-known problem of contamination of the films produced by PLD with particulates ejected from the target. The method uses low energy, picosecond duration laser pulses delivered onto a target at rates of several tens of MHz and thus differs from conventional the PLD method which utilizes high energy, nanosecond duration pulses delivered at low (≈10 Hz) repetition rates. In this article we present the theoretical background justifying the method and define the optimal conditions for efficient evaporation of a target with given thermodynamic properties. By reducing the laser pulse energy while maintaining optimum evaporation, the number of atoms evaporated by each pulse is reduced to the point where it becomes impossible for macroscopic lumps of material to be ejected with the available laser energy, thus preventing the source of particle contamination in the film. To achieve high evaporation rate, the laser pulse repetition rate is increased. We compare our theoretical predictions with literature reports of optimal evaporation in a number of experiments, while in part II of this article we describe a specific experimental study where the method is applied to the production of amorphous carbon thin films.