From nanosecond to femtosecond science

Lasers have caused revolutionary changes in many fields of science and tech nology. A brief review is presented here of how lasers have provided access to measurements of short time intervals. Since 1960, six orders of magnitu de, from 10(-9) to 10(-15) seconds, have been added to time-resolved observ ation of fast phenomena. New subfields of science, including femtochemistry and femtobiology, have been created. Some representative examples of trans ient Raman scattering and of impulsive and displacive excitations in molecu les and crystals illustrate the usefulness of picosecond and femtosecond pu lse techniques. In addition, the domain of power flux densities has been ex tended from 10(12) to 10(19) watts/cm(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 at oms, and high-order harmonic generation and acceleration of relativistic el ectrons by light pulses. [S0034-6861(99)01002-8].