Photoionization Mode - Filamentary Photoionization Mode (F)

Filamentary Photoionization Mode (F)

In the F mode, filamentary or linear ionization patterns are formed. The plasma density within these filaments is below the critical value.

The self-focusing effect is responsible for the most important characteristics of the dose distribution. The diameter of these filamentary ionization traces is the same within 20% (in the order of a few micrometres). Their length, their number, and their relative position are controllable parameters. The plasma density and the yield of photolytic species are believed to be homogeneously distributed along these filaments. The local intensity reached by the laser light during propagation is also practically constant along their length. The power range of operation of the F mode is above self-focusing threshold and below optical breakdown threshold. Consequently, a necessary condition for it to exist is that the self-focusing threshold must be smaller than the optical breakdown threshold.

The F mode exhibits very important characteristics, which in combination with the other three photoionization modes makes possible the generation of a wide range of dose distributions, expanding the application range of lasers in the domain of material processing. The F mode is the only mode capable of generating linear ionization traces.

The theory needed to understand the most important features of the F mode are:

• The physics of high-(laser)field interaction with matter, to account for the plasma formation
• The theory of non-linear propagation, to account for the spatial redistribution of the laser light, intensity clamping, and the formation of filaments, as well as for frequency conversion processes.

The first concrete connection between non-linear optical effects, such as the supercontinuum generation, and photoionization was established by A. Brodeur and S.L. Chin in 1999, based on optical experimental data and modeling. In 2002 T. Brastaviceanu published the first direct measurement of the spatial distribution of photoionization induced in the self-focusing regime, in water .