We report a new regime of filamentation in water in tight focusing geometry, very similar to the socalled superfilamentation seen in air. In this regime there is no observable conical emission and multiple small-scale filaments, but instead a single continuous plasma channel is formed. To achieve this specific regime the principal requirement is the usage of tight focusing and supercritical power of laser radiation. Together they guarantee extremely high intensity in the microvolume in water (∼10 14 W cm −2 ) and clamp the energy in the ultra-thin (approximately several microns) channel with a uniform plasma density distribution in it. Each point of the 'superfilament' becomes a center of spherical shock wave generation. The overlapped shock waves transform into one cylindrical shock wave. At low energies, a single spherical shock wave is generated from the laser beam waist, and its radius tends toward saturation as energy increases. At higher energies, a long stable contrast cylindrical shock wave is generated, whose length increases logarithmically with laser pulse energy. The linear absorption decreases the incoming energy delivered to the focal spot, which dramatically complicates the filament formation, especially in the case of loose focusing. Aberrations added to the optical scheme lead to multiple dotted plasma sources for shock wave formation, spaced along the axis of pulse propagation. Increasing the laser energy launches the filaments at each of the dots, whose overlapping leads to enhancing the length of the whole filament and therefore the shock impact on the material.

We discovered that tight focusing of Cr:forsterite femtosecond laser radiation in water provides the unique opportunity of long filament generation. The filament becomes a source of numerous spherical shock waves whose radius tends to saturate with the increase of energy. These overlapping waves create a contrast cylindrical shock wave. The laser-induced shock wave parameters such as shape, amplitude and speed can be effectively controlled by varying energy and focusing geometry of the femtosecond pulse. Aberrations added to the optical scheme lead to multiple dotted plasma sources for shock wave formation, spaced along the optical axis. Increasing the laser energy launches filaments at each dot that enhance the length of the entire filament and as a result, the shock impact on the material.

We report overcritical (3.3  ×  1021 cm−3) microplasma produced by low energy colliding IR (infrared) (1.24 μm) and visible (0.62 μm) femtosecond pulses tightly focused (NA  =  0.5) into the bulk of fused silica with on-line monitoring based on third harmonic generated by the IR beam. It was established that the absorbed energy density is the key parameter that determines the micromodification formation threshold and in our experimental conditions it is close to 4.5 kJ cm−3. Non-monotonic behavior of the third harmonic signal as a function of time delay between visible (0.62 μm) and IR (1.24 μm) femtosecond pulses demonstrates the qualitative differences about the two phenomena: one is the seed electrons generation by the visible pulse via multiphoton ionization and second is the avalanche ionization by the IR pulse. We predict that the tandem two-color excitation of wide-bandgap dielectric in comparison with single-color pulse interaction regime allows providing a much higher absorbed energy density and overcritical plasma.

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