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 x 10(21) cm(-3)) microplasma produced by low energy colliding IR (infrared) (1.24 mu m) and visible (0.62 mu m) 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 mu m) and IR (1.24 mu m) 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.