Direct time-resolved plasma characterization with broadband terahertz light pulses.

We report here the results of comprehensive plasma characterization and diagnostics by analyzing time-resolved absorption spectra of short ultrabroadband (0.1-50 THz) pulses propagated through the test plasma. Spectral analysis of plasma-induced absorption of such THz pulses provides very direct, in situ, high dynamical range, potentially single-shot access to the plasma density, plasma decay time, electron temperature, and ballistic dynamics of the plasma expansion. We have demonstrated a proof-of-principle measurement of plasma created by an intense laser beam. In particular, we showed a reliable measurement of plasma densities from around 10^{16} to 10^{20}cm^{-3}. Apart from the plasma parameters, this method allowed us to reconstruct peak intensity inside the plasma spot and to observe a very early stage of plasma evolution after its excitation.