SP-OB mixed mode

The SP-OB mixed mode is obtained within the wavelength domain of the SP mode, i.e. in the UV, for local intensities above OB-threshold. SPI, MPI and/or AI are the mechanisms responsible for the ionization. We know that SPI is very wavelength sensitive, and if no absorption band is matched we only have a pure OB mode. The dose distributions, as well as the plasma density in the SP-OB mixed mode are similar to the pure OB mode. The difference is in the nature and the relative yield of the primary photolytic species generated, due to the contribution of the SPI process, which is very selective. As for the energy spectrum of the free electrons and their dependence on different parameters of the laser beam, the section on the OB mode gives a clear description.

Converging lenses are normally used to control the local intensity at the target, as well as the size, the shape, and the position within the sample of the affected area. The pulse duration and the local intensity control the OB threshold, and therefore the passage from the pure SP mode to the SP-OB mixed mode, because they have a direct influence on the MPI and AI processes, which are responsible for the buildup of the plasma. At longer pulse durations where AI becomes dominant, the OB intensity threshold, and therefore the passage from the pure SP mode to the SP-OB mixed mode, is also affected by impurities at all wavelengths.