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A photoionization mode gives rise to a photoionization pattern, having very specific characteristics in terms of photoionization spatial distribution and density, as well as relative yields of photolytic species. A particular photoionization mode is also very specific in terms of the ultimate chemical and structural effects induced to a given dielectric material. The concept of photoionization mode refers to very distinct interaction regimes of a high power laser pulse with a dielectric material, which are governed by a specific set of laws, and controlled by a specific set of parameters. For this reason, this concept is very well defined.
We can identify four fundamental photoionization modes based on four fundamental optical effects, that give rise to four very distinct interaction regimes: single-photon mode (SP), filamentary mode (F), optical breakdown mode (OB), and below optical breakdown threshold mode (B/OB).
Single-photon photoionization mode (SP)
The SP mode is obtained at small wavelengths (UV, X-ray), and at low intensity levels. The only photoionization process involved is single-photon ionization.
Optical breakdown photoionization mode (OB)
The OB mode is observed when a material is subjected to very powerful laser pulses. 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. Optical breakdown is related to the dielectric breakdown phenomenon which was studied and modeled successfully towards the end of the 1950's. One describes the effect as a strong local ionization of the medium, where the plasma reaches densities beyond the critical value (between 1020 and 1022 electrons/cm³). Energy is very efficiently absorbed from the light pulse, and the local plasma temperature increases dramatically. An explosive Coulombian expansion producing cavitation follows, and forms a very powerful and damaging shockwave that develops on ns timescale. If the rate of plasma formation is relatively slow, in the nanosecond time regime (for nanosecond excitation laser pulses), energy is transferred from the plasma to the lattice, and thermal damages can occur. In the femtosecond time regime (for femtosecond excitation laser pulses) the plasma expansion happens on a timescale smaller than the rate of energy transfer to the lattice, and thermal damages are reduced or eliminated.
The optical breakdown is a very "violent" phenomenon and changes drastically the local structure of the medium. To the naked eye, optical breakdown looks like a spark, and 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. First, we should mention that optical breakdown is only observed at very high intensities. For pulse durations greater than a few tens of fs avalanche ionization plays a role. The longer the pulse duration, the greater the avalanche ionization’s contribution. Multi-photon ionization processes are important in the fs time regime, and their role increases as the pulse duration decreases. The type of multi-photon ionization processes involved is also wavelength dependent.
The theory needed to understand the most important features of optical breakdown are:
- the physics of strong-(laser)field interaction with matter, to account for the plasma formation;
- the physics of strong-(laser)field interaction with plasma, to account for plasma expansion, and for thermal and mechanical effects;
- the geometrical/linear optical theory, to account at the first approximation for the spatial intensity distribution. Non-linear propagation theory is usually invoked to account for self-focusing that occurs in experiments conducted at low numerical aperture, and to account for detail features of the plasma density spatial distribution.
Below optical breakdown threshold photoionization mode (B/OB)
B/OB mode is an intermediary between OB mode and F mode. It was described only recently by A. Vogel et al.. The plasma density generated in this mode can go from 0 to the critical value i.e. optical breakdown threshold. Intensities reached inside the B/OB zone can range from multi-photon ionization threshold to the optical breakdown threshold. 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. In the UV regime, where optical breakdown intensity threshold is below self-focusing intensity threshold, tight focusing is not necessary. The shape of the ionization area is similar to that of the focal area of the beam, and can be very small in size (only a few micrometres). B/OB mode is possible only at short pulse durations, where AI's contribution to the total free electron population is very small. As the pulse duration becomes even shorter, the intensity domain where B/OB is possible becomes even wider.
The principles governing this mode of ionization are very simple. Localized plasma must be generated in predictable fashion, under the optical breakdown threshold. Optical breakdown intensity threshold is strongly correlated to the input intensity only at short pulse durations. Therefore, one important requirement, in order to systematically avoid the optical breakdown, is to operate at short pulse durations. In order for the ionization to take place, multi-photon ionization intensity threshold must be reached. The idea is to adjust the duration of the laser pulse so that multi-photon ionization, and perhaps to a lesser extent avalanche ionization, have no time to raise the plasma’s density above the critical value. In the UV, the distinction between single-photon mode (SP) and B/OB is that for the latter multi-photon ionization, single-photon ionization, and perhaps to a lesser extent avalanche ionization, are operating, whereas for the former, only single-photon ionization is operating.
B/OB relies mostly on MPI processes. Therefore, it is more selective than OB in terms of which type of atom or molecule is ionized or dissociated. The theory needed to understand the most important features of B/OB are:
- The physics of strong-(laser)field interaction with matter, to account for the plasma formation. As opposed to the OB mode, in this case the role of avalanche ionization is greatly reduced, and the effects are dominated by multi-photon ionization processes.
- The geometrical/linear optical theory, to account at the first approximation for the spatial intensity distribution. Non-linear propagation theory is usually invoked to account for self-focusing that occurs in experiments conducted at low numerical aperture, and to account for detailed features of plasma spatial distribution.
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.
Superposition of photoionization modes
It is possible to control the spatial distribution of the dose induced by laser pulses, and the relative yields of primary photolytic species, by controlling the properties of the laser beam. The dose distribution can be conveniently shaped by inducing a superposition of the four modes of photoionization. The mixed ionization modes are: SP-OB, SP-B/OB, and F-OB.