Modes of Photoionization
Part of Laser Applications
Part of Laser Applications
Started in 2006, last modified in Feb 9, 2011
The laser light affects materials of all types through fundamental processes such as excitation, ionization, and dissociation of atoms and molecules. These processes depend on the proprieties of the light, as well as on the proprieties of the material. Using lasers for material processing translates into understanding, and being able to control these fundamental effects. In my opinion, a better understanding can be achieved by defining distinct interaction regimes, instead of treating the whole domain as a unique block.
The concept of photoionization mode refers to very distinct interaction regimes, which are governed by a specific set of laws, and are controlled by a specific set of parameters.
Photoionization: ionization induced by light. We can have single-photon ionization, in which case the ionization is the result of the interaction between a single photon with an atom or a molecule, or multi-photon ionization, in which case more than one photon are involved.
[in water] Photolytic species: is the name for any new particle, ion, free radical, molecule, etc. that is created after the ionizing or dissociative interaction between light and matter. Products of photoionization and photodissociation, also called products of photolysis.
A photoionization mode gives rise to a photoionization pattern, having very specific characteristics in terms of spatial distribution, density, and relative yields of photolytic species induced to the host material. A particular photoionization mode is very specific in terms of the ultimate chemical and structural effects. This concept is very well defined and every photoionization mode is easily distinguishable from all others. Every photoionization mode can be induced in most dielectric materials, and mixed photoionization modes can be induced by a superposition of pure ones.
ATTENTION This section is a condensate. If you are not familiar with these concepts, I suggest you read the sections on photoionization modes (on the left menu) before reading further.
In order to understand, describe, and control the Single-Photon (SP) mode, the physics of low-intensity laser-field interaction with matter, and linear optics are needed. In the case of the Below Optical Breakdown (B/OB) mode, the physics of strong laser-field interaction with matter, and linear optics are needed to account for the most important features. Nonlinear optics can become useful for a more detailed picture. In the case of the Optical Breakdown (OB) mode, the same theoretical body is needed as for the B/OB mode, but an understanding of plasma interaction with a strong-laser-field also becomes necessary. For the Filamentary (F) mode nonlinear optics becomes important.
The wavelength of the incident laser light operates a selection between SP and F mode. B/OB and OB are possible at any wavelength.
In the visible-IR domain, for sub-picosecond pulses the F, OB, and B/OB modes are possible by adjusting the parameters of input average intensity and numerical aperture (NA) of the incident laser light. The pure F mode is obtained with low NA, and for intensities just above self-focusing (SF) threshold. The pure B/OB mode is obtained for very high NA, and for intensities below OB threshold. The OB mode is obtained for high NA and rather high intensities. F-OB mixed mode is obtained for an intermediary combination of NA and intensity. In the F-OB mixed mode, the OB zone is usually adjacent to the exit extremity of the F zone. OB and B/OB can be obtained without filamentation within this wavelength domain, with very tight focusing in the fs time regime (to avoid SF), and with not so tight focusing in the ns time regime (where OB intensity threshold is below SF intensity threshold).
In the UV domain, SP, OB and B/OB are possible. The parameter that controls their relative importance is the input intensity. The duration and the intensity of the laser pulse make the transition from B/OB to OB. If the single-photon ionization (SPI) effect is possible, OB and B/OB cannot exist alone, and are accompanied by the SP mode; therefore we have SP-OB and SP-B/OB mixed modes. Under tight focusing and for short pulse durations (at any wavelength), the average intensity is instrumental in making the transition from SP-B/OB to SP-OB.
Converging lenses are always used in practice. In the UV domain, they help inducing the SP-OB and the SP-B/OB mixed modes from the pure SP mode. In the visible-IR domain they play an important role for inducing the F-OB mode from the pure F mode, and are crucial for the B/OB mode. Converging lenses are also used to control the size, the shape, and the location within the sample of the affected area.
The F and OB modes induce very different photolytic effects. The intensity is clamped (limited to a maximum value) inside the filaments in the F mode. The plasma density stays below OB threshold (1017-1018 ) and is constant along the filaments; the heat deposited can be considered negligible. In the OB mode, the plasma density can reach values from 1020 to 1022 electrons/cm3. Being above the critical density, the plasma absorbs energy from the light pulse very efficiently. OB is also characterized by a violent Coulombian expansion inducing cavitation, and forming a very powerful and damaging shockwave. The fact that we are able to control and mix these two modes of ionization is of great interest. The F-OB mixed mode mimics the dose distribution induced by high linear energy transfer (LET) radiation, such as ion-beams, where the OB region is the equivalent of the Bragg’s peak.