A microscope objective lens (shown here) focuses a red (620 nm) femtosecond laser pulse to a peak intensity of 3 X 10¹⁵ watts/cm² in a cell containing 5 atm. Ar gas, producing an ionization spark (white dot) which absorbs about 1% of the incident light. The remaining transmitted light is spectrally shifted by ultrafast phase modulation at the ionization front to a yellow-green color, which is clearly visible scattering from the exit wall of the cell and the housing of the microscope objective. Spectral analysis of this "blue-shifted" light has provided femtosecond-time-resolved measurements of ionization dynamics at high light intensity (see W. M. Wood et al., Phys. Rev. Lett. 67, 3523 (1991)). Recently a more sensitive, sophisticated measurement of such frequency shifts--"femtosecond longitudinal interferometry"--has provided femtosecond-time-resolved characterization of density oscillations (Langmuir waves) of a fully-ionized helium plasma in the wake of a very intense (3 X 10¹⁷ watts/cm²) femtosecond laser pulse (C. W. Siders et al., Phys. Rev. Lett. 76, 3570 (1996)). Such "laser wakefield oscillations" are promising media for charged particle acceleration (photograph by W. M. Wood and G. Focht).