ИСТИНА

We report theoretically predicted and experimentally verified new physical concept on laser-based micro and nanomachining inside transparent dielectrics based on oncoming interaction of tightly focused (NA = 0.5) pair of visible (SH) and infrared (IR) femtosecond laser beams with third harmonic on-line diagnosis of the electronic density in the area of laser pulses interaction. High localization of laser energy i.e. minimal collateral damage is guaranteed by two-pulse excitation regime in which SH pulse generates “seed” electrons and IR pulse involving them in avalanche heating brings to breakdown in microvolume determined by the short wavelength pulse. Since laser-induced modification of wide bandgap dielectric (for instance, fused silica) is defined by the plasma electrons distribution after the laser pulse excitation we were focused in presented research on studying ultra-fast laser microplasma ignition. We have demonstrated an effect of absorbed energy density saturation at the energy density which is associated with the saturation of electron density. Non-monotonic increasing of the third-harmonic signal and corresponding decreasing of IR laser pulse transmittance as a function of the delay between the SH and IR laser pulses tightly focused into the bulk of fused silica was revealed experimentally. Tandem action of two color pair of oncoming laser pulses with subthreshold energies in a bulk of fused silica leads to the absorbed energy density enough for residual micromodification formation which in our experimental conditions close to 4.5 kJ/cm3. In addition, we predicted that the oncoming two-color excitation of wide-bandgap dielectric in comparison with single-color one pulse interaction regime allows providing a much greater absorbed energy density and overcritical plasma which is the key point for investigating the extreme state of matter. Using the UV laser pulse for seed electrons generation and mid-IR laser pulse for ponderomotive heating is the most prospective way to increase the localization of the incoming energy giving the route to the sub-micrometer spatial resolution with high energy efficiency in forthcoming femtotechlogies and for investigating the extreme state of matter.