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.

Direct measurement of pressure dependent nonlinear refractive index of CO and Xe in subcritical and supercritical states are reported. In the vicinity of the ridge (or the Widom line), corresponding to the maximum density fluctuations, the nonlinear refractive index reaches a maximum value (up to 4.8*10m/W in CO and 3.5*10m/W in Xe). Anomalous behavior of the nonlinear refractive index in the vicinity of a ridge is caused by the cluster formation. That corresponds to the results of our theoretical assumption based on the modified Langevin theory.

Using shadow photography, we observed microsecond time scale evolution of multiple cavitation bubbles, excited by tighty focused femtosecond laser pulse in water under supercritical power regime (~100 P cr ). In these extreme conditions high energy delivery into the microvolume of liquid sample leads to creation of single filament which becomes a source of cavitation region formation. When aberrations were added to the optical scheme the hot spots along the filament axis are formed. At high energies (more than 40J) filaments in these hot spots are fired and, as a result, complex pattern of cavitation bubbles is created. The bubbles can be isolated from each other or build exotic "drop-shaped" cavitation region, which evolution at the end of its "life", before the final collapse, contains the jet emission. The dynamics of the cavitation pattern was investigated from pulse energy and focusing. We found that greater numerical aperture of the focusing optics leads to greater cavitation area length. The strong interaction between bubbles was also observed. This leads to a significant change of bubble evolution, which is not yet in accordance with Rayleigh model. The phenomenon of laser-induced cavitation is well known [1], but still attracts attention to itself due to a complexity of processes, accompanying cavitation bubble evolution. The cavitation bubble rapid growth and collapse can cause shock waves, rapid jets and sonoluminescence [2]. The aggressive nature of laser-induced cavitation has found a broad range of applications, such as cell lysis [3], cell membrane poration [4] and ocular surgery [5]. Cavitation is used for mixing, pumping, switching and moving objects in microfluids. The laser-induced bubbles are used for creation of highly focused supersonic microjets. Due to their excellent controllability, high velocity and relatively low power requirements, these jets are an attractive option for needle-free drug injection [6].Tight focusing of ultrashort laser pulses with even microjoule pulse energy into the bulk of transparent dielectric leads to extreme intensities (>10 13 W/cm 2 ) in microvolume and plasma formation. The mean energy of plasma electrons reaches the value of several electronvolts. The initial plasma distribution dramatically depends on laser pulse parameters and focusing geometry [7]. Due to high teperatures the thin layer of vapor is generated, which consequently transformed into cavitation bubble. The shape of laser-induced cavitation bubble depends on the plasma density distribution and, thus, the initial spatial profile of laser intensity in the medium defines the spatio-temporal evolution of cavitation bubble. The Rayleigh model (or its more correct modifications) describes the dynamics of isolated laser-induced cavitation bubble e i

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 have investigated nonlinear laser ̶ matter interaction inside silicon under tight focusing conditions by continuously tuning driving pulse duration from femtosecond to picosecond timescales. Such tailoring of laser pulse width provides a new route for energy delivery into a microvolume avoiding two-photon absorption and plasma defocusing in the pre-focal region. As a result, we have achieved values of saturated deposited energy density and plasma electron concentration of as high as 1 kJ cm −3 and 10 19 cm −3 respectively, which is lower than the threshold of irreversible structural transformation. For further increase of energy delivery inside silicon, a two-color technique supported by extremely tight focusing can be realized, forming a roadmap to the 3D industrial micromachining of planar bulk silicon.

We report a bulk void-like micromodification of fused silica using two-color μJ-energy level tightly focused (NA = 0.5) co-propagating seeding (visible, 0.62 μm) and heating (near-IR, 1.24 μm) femtosecond laser pulses with online third harmonic diagnostics of created microplasmas as well as subsequent laser-induced void-like defects. It has been shown experimentally and theoretically that production of seeding electrons through multiphoton ionization by visible laser pulses paves the way for controllability of the energy deposition and laser-induced micromodification via carrier heating by delayed infrared laser pulses inside the material. Experimental results demonstrate wide possibilities to increase the density of energy deposited up to 6 kJ cm−3 inside the dielectric by tight focusing of two color fs-laser pulses and elliptical polarization for infrared heating fs-laser pulses. The developed theoretical approach predicts the enhancement of deposited energy density up to 9 kJ cm−3 using longer (mid-IR) wavelengths for heating laser pulses.

We present a method of ultrafast laser-induced microplasma mapping based on the third harmonic generation at tightly focused laser beams in solids. The technique gives a submicron resolution and can be applied for two-dimensional imaging of laser-induced microplasma with the electron density as low as 10−5 ncr produced by a femtosecond laser driver at the plasma formation threshold. High sensitivity and contrast are dictated by nonlinear behavior of the third harmonic yield with the laser field. The demonstrated method gives an opportunity to increase the precision of plasma volume determination in the field of material microstructuring.

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.