Photoionization mode

[1] Laser light affects materials of all types through fundamental processes such as excitation, ionization, and dissociation of atoms and molecules.

This new way of looking at the laser interaction with matter was first proposed by Tiberius Brastaviceanu in 2006, after his description of the "filamentary ionization mode" (Sherbrooke University, 2005).

In his Master's work he provided the empirical proof of the formation of filamentary distributions of solvated electrons in water, induced by high-power fs (femtosecond, one trillionth of a second) laser pulses in the self-focusing propagation regime, and described the theoretical context in which this phenomenon can be explained and controlled.

The SP mode is obtained at small wavelengths (UV, X-ray), or high energy per photon, and at low intensity levels.

[citation needed] It manifests a power threshold in the range of MW for the majority of dielectric materials, which depends on the duration and on the wavelength of the laser pulse.

An explosive Coulombian expansion follows, and forms a very powerful and damaging shockwave through the material that develops on ns timescale.

To the naked eye, optical breakdown looks like a spark and if the event happens in air or some other fluid, it is even possible to hear a short noise (burst) caused by the explosive plasma expansion.

There are several photoionization processes involved in optical breakdown, which depend on the wavelength, local intensity, and pulse duration, as well as on the electronic structure of the material.

Multi-photon ionization processes are important in the fs time regime, and their role increases as the pulse duration decreases.

In the visible-IR domain, B/OB mode is obtained under very tight external focusing (high numerical aperture), to avoid self-focusing, and for intensities below optical breakdown threshold.

The theory needed to understand the most important features of the F mode are: The first concrete connection between non-linear optical effects, such as the supercontinuum generation, and photoionization was established by A. Brodeur and S.L.

In 2002 T. Brastaviceanu published the first direct measurement of the spatial distribution of photoionization induced in the self-focusing regime, in water [ref 5].