Formation and development dynamics of femtosecond laser microplasma in gases

We report our experimental investigations of the formation and development dynamics of laser plasma produced in gas microvolumes (microplasma) upon multiple ionisation by tightly focused (to a spot 2–3 $\mu m$ in diameter) high-intensity (up to $\sim 10^{17}$ W cm$^{-2}$) femtosecond pulses of a Ti:sapphire laser ($\tau_p\simeq 130$ fs, $\lambda=800$ nm). Precision interferometric measurements (with a spatial resolution of $\sim 1.5\,\mu$m) were made of the spatiotemporal distribution of the refractive index and electron density in the microplasmas of the air and helium immediately during the action of the exciting femtosecond laser pulse and at the initial stage of free plasma expansion. The microplasma formation was shown to occur as a result of almost complete (up to bare nuclei) ionisation of the initial gas. For the first time the spectral continuum and the dynamics of spectral line formation in the UV and visible spectral ranges were investigated with a picosecond time resolution for the femtosecond laser-produced microplasmas of the air, N$_2$, Ar, and He at normal conditions. For the first time the generation of the second (even) laser radiation harmonic was recorded in a femtosecond subcritical-density plasma of gases.