It is possible to pass from filamentation without breakdown, through filamentation with breakdown, to breakdown without filamentation by changing the numerical aperture (NA) and input laser intensity. The F-OB mixed mode is achievable within the F mode’s wavelength domain (visible-IR) for intensities close to the OB's threshold, by carefully selecting the NA and the input intensity level. The global focusing effect of the beam induced by a convergent lens makes the light filaments convergent. If they overlap at the geometrical focal point, their summed intensity can exceed OB's threshold.
F-OB has very special and interesting features. The dose distribution has similar characteristics to high LET radiation (a beam of ions). It is composed of a filamentary ionization zone, where the plasma density is below the critical value, and an OB zone of intense ionization, normally adjacent to the exit extremity of the filamentary zone. The length of the filamentary zone, the number of filaments, their spatial distribution, and the relative importance of the OB effect are controllable. As for the type and the spatial distribution of the primary photolytic species generated, and their dependence on the parameters of the laser beam, the sections discussing the F and OB pure modes give a clear description.
The pulse duration can be used to adjust the width of the intensity window on which the F mode and the F-OB mixed mode are possible: the difference between the intensity threshold of self-focusing (SF) and OB increases as the pulse duration diminishes. There is a pulse duration threshold for the F mode, and consequently for the F-OB mixed mode: the value for witch SF and OB have the same intensity threshold. Relative to this pulse duration threshold, for longer pulse durations OB appears at intensities lower then SF threshold, and F mode becomes impossible.
For pulse durations that make AI’s contribution to the plasma accumulation non-negligible, impurities can affect the F and OB pure modes, and therefore the F-OB mixed mode.