  Using spectral interferometry technique, we measured subpicosecond
time-resolved pre-plasma scale lengths and early expansion ($<$ 12 ps) of the
plasma produced by a high intensity (2$\times$10$^{18}$ W/cm$^{2}$) pulse with
ultra-high contrast (10$^{-9}$). We measured pre-plasma scale lengths in the
range of 3-15 nm. This measurement plays a crucial role in understanding the
mechanism of laser coupling its energy to hot electrons and hence important for
laser-driven ion acceleration and fast ignition approach to fusion.
