Laser plasma threshold of metals

For nanosecond laser pulses, thermal properties have a decisive influence on laser plasma thresholds of metals. The acoustic mirage effect technique allows quantitative investigations over a wide range of incident laser intensities, from subthreshold heating to dense plasma generation. Under particular consideration of copper and titanium targets, with strongly different thermal conductivities, we show that two threshold conditions must be fulfilled concurrently: the metal surface must be heated to the boiling point to provide metal vapor and the laser intensity has to be high enough to enable dielectric breakdown in the vapor. Even at very high fluences, where a dense plasma is created, thermal properties are of importance. Since part of the incident energy is consumed for evaporation, only the excess energy can heat the plasma.