Photoionization Mode - Below Optical Breakdown Threshold Photoionization Mode (B/OB)

Below Optical Breakdown Threshold Photoionization Mode (B/OB)

B/OB mode is an intermediary between the optical breakdown mode (OB mode) and the filamentary mode (F mode). 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 (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.

The B/OB mode was described by A. Vogel et al. .