From nanosecond to femtosecond science

Lasers have caused revolutionary changes in many fields of science and technology. A brief review is presented here of how lasers have provided access to measurements of short time intervals. Since 1960, six orders of magnitude, from ${10}^{-9}$ to ${10}^{-15}$ seconds, have been added to time-resolved observation of fast phenomena. New subfields of science, including femtochemistry and femtobiology, have been created. Some representative examples of transient Raman scattering and of impulsive and displacive excitations in molecules and crystals illustrate the usefulness of picosecond and femtosecond pulse techniques. In addition, the domain of power flux densities has been extended from ${10}^{12}$ to ${10}^{19}{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{t}\mathrm{s}/\mathrm{c}\mathrm{m}}^2$ by the use of short focused pulses. This has given experimental access to new phenomena, including ultrafast phase transitions in electronic structure, above-threshold ionization of atoms, and high-order harmonic generation and acceleration of relativistic electrons by light pulses.