Photoionization Mode - Optical Breakdown Photoionization Mode (OB)

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 1950s. 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). Once the plasma critical density is achieved, energy is very efficiently absorbed from the light pulse, and the local plasma temperature increases dramatically. An explosive Coulombian expansion follows, and forms a very powerful and damaging shockwave through the material that develops on ns timescale. In liquids one talks about cavitation bubbles. 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. This is the bases if cold laser machining using high-power sub-ps laser sources.

The optical breakdown is a very "violent" phenomenon and changes drastically the structure of the surrounding medium. 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. 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 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.