Characterisation of Diamond-Like Carbon (DLC) laser targets by Raman spectroscopy

The search for target materials suitable for High Power Laser Experiments at ultra-low thicknesses (below ten nanometres) is ongoing. Diamond-Like Carbon is investigated as an answer for a low-Z material that can survive target chamber pump-down and laser pre-pulse. DLC was produced using Plasma-Enhanced Chemical Vapour Deposition, using with varying gas flow mixtures of argon and methane. The methane plasma deposits amorphous carbon onto the substrate and the argon plasma re-sputters the weakly bonded carbon leaving a high proportion of diamond-like bonding. Bonding natures were probed using Raman spectroscopy; analysis of the resulting spectrum showed that flow rates of 40sccm/60sccm methane to argon produced DLC films with a diamond-like (sp(3)) content of similar to 20%. Increasing the methane gas flow decreased this value to less than 5%. DLC foils were processed into laser targets by method of float off; using a sodium chloride release layer and lowering into water, this was then lifted onto an array of apertures allowing for laser irradiation of the material with no backing. DLC with 20% sp(3) content showed superior yield when compared to other materials such as metals and some plastics of the same thickness, with similar to 70% of the target positions surviving the float off procedure at < 10nm. As a result of this work DLC targets have been available for a number of experiments at the Central Laser Facility.