Spectral Broadening in the Light Transmitted through a Rapidly Growing Plasma

It has been found that C${\mathrm{O}}_{2}$-laser intensities greatly exceeding the normal breakdown fields may be transmitted in many gases. When breakdown does occur at these high intensities, the initial plasma growth is so violent as to cause frequency shifts of \ensuremath{\sim}1 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the beam transmitted through the focus. The interpretation of the spectrum induced by this plasma nonlinearity indicates that C${\mathrm{O}}_{2}$-laser pulses of duration as short as 30 psec were produced.