E. I. Mareev

Laser control of filament-induced shock wave in water

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.

Controlled energy deposition and void-like modification inside transparent solids by two-color tightly focused femtosecond laser pulses

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)...

Overcritical plasma ignition and diagnostics from oncoming interaction of two color low energy tightly focused femtosecond laser pulses inside fused silica

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.

Resonant laser-plasma excitation of coherent THz phonons under extreme conditions of femtosecond plasma formation in a bulk of fluorine-containing crystals

The dynamics of coherent phonons in fluorine-containing crystals under plasma formation were studied using a nonlinear pump?probe technique based on third harmonic generation. In LiF crystal more than one phonon mode was observed. The modes are the overtones of a fundamental wave with a frequency of 0.38?THz. In CaF2 crystal phonons with frequencies of 1 and 0.1?THz were observed. In BaF2 crystal, in addition to coherent phonons with frequencies of 1?THz and 67?GHz, a significant increase of amplitude in the phonon modes with a time delay of 15?ps was detected.

Real-Time Monitoring of the Energy Deposition under the Tight Focusing of Femtosecond Laser Radiation into a Bulk Transparent Dielectric Based on Third Harmonic Signal

It has been found that the third harmonic generated under the tight focusing of a near-infrared (1.24-μm) femtosecond laser pulse indicates the energy deposition into the bulk transparent dielectric and can be used as a feedback system in the process of microstructuring. A third harmonic signal is sensitive to a change in both the size of a laser-induced microplasma and the free-electron density, which allows the detection of the microplasma with submicron longitudinal sizes. It has been shown that the method based on the third harmonic is universal and can be applied in both single-pulse and two-color microstructuring.

Femtosecond supercontinuum generation and superfilamentation in liquids and supercritical fluids

We for the first time report a generation of multioctave supercontinuum in supercritical CO2 and Xe by 0.6 mJ 1240nm femtosecond (200 fs) laser pulse. In supercritical CO2 it ranges from 350 to 1900 nm and have a plateau-like behavior in the range 1400-1900 nm, besides 50% of energy is transferred to the first Stokes component. The increase of laser energy and focusing lens numerical aperture in liquids leads to the formation of superfilament, which triggers shock waves generation, cavitation bubble formation and provides tightly divergent supercontinuum.

Supercontinuum generation under filamentation driven by intense femtosecond pulses in supercritical xenon and carbon dioxide

It is found that supercritical fluids are a unique source of multioctave supercontinuum radiation, which is generated upon filamentation of an intense femtosecond laser pulse. If the laser pulse power significantly exceeds the critical power of self-focusing, then a supercontinuum with a width of three and a half spectral octaves (from 350 to 2000 nm) is generated in supercritical xenon. The red wing of supercontinuum generated in supercritical carbon dioxide has the form of a plateau in the range from 1400 to 1900 nm, while the blue wing of the spectrum is almost completely attenuated.

Whole life-cycle of superfilament in water from femtoseconds up to microseconds

A whole life-cycle of the superfilamentation in water in tight focusing geometry was investigated. In this regime 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. They together clamp the energy in the ultra-thin (approximately several microns) channel with a uniform plasma density distribution in it. The superfilament becomes a center of cylindrical cavitation bubble area and shock wave formation. The length of the filament 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 dot, whose overlapping leads to enhance the length of the whole filament.

Cavitation and shock waves in water, stimulated by laser filament

Using shadow photography technique we have observed shock acoustic wave from optical breakdown, excited in water by tightly focused Cr:Forsterite femtosecond laser beam, and have found two different regimes of shock wave generation by varying only the energy of laser pulse. At low energies a single spherical shock wave and cavitation bubble is generated from laser beam waist, and shock wave radius tends to saturation with energy increasing. At higher energies long laser filament in water is fired, that leads to the cylindrical shock wave generation and cavitation bubble in every nonlinear focus. The diameter of cavitation bubble depends on the distance from the first nonlinear focus. From shadow pictures we also estimated maximal velocity in the shock wave front of 3200±150m/s and pressure of 240±30 MPa.

Laser Control of the Shock Wave Shape by Tightly Focused Femtosecond Laser Radiation

We observed controlled transformation of shock wave shape from spherical to cylindrical varying energy of tightly focused Cr:forsterite femtosecond laser radiation. Cylindrical hypersound shock wave (240±30 MPa) length has logarithmic dependence on laser energy.