Defining and measuring optical frequencies: the optical clock opportunity--and more (Nobel lecture).
Four long-running currents in laser technology met and merged in 1999-2000. Two of these were the quest toward a stable repetitive sequence of ever-shorter optical pulses and, on the other hand, the quest for the most time-stable, unvarying optical frequency possible. The marriage of ultrafast- and ultrastable lasers was brokered mainly by two international teams and became exciting when a special "designer" microstructure optical fiber was shown to be nonlinear enough to produce "white light" from the femtosecond laser pulses, such that the output spectrum embraced a full optical octave. Then, for the first time, one could realize an optical frequency interval equal to the comb's lowest frequency, and count out this interval as a multiple of the repetition rate of the femtosecond pulse laser. This "gear-box" connection between the radiofrequency standard and any/all optical frequency standards came just as sensitivity-enhancing ideas were maturing. The four-way union empowered an explosion of accurate frequency measurement results in the standards field and prepared the way for refined tests of some of our cherished physical principles, such as the time-stability of some of the basic numbers in physics (e.g. the "fine-structure" constant, the speed of light, certain atomic mass ratios), and the equivalence of time-keeping by clocks based on different physics. The stable laser technology also allows time-synchronization between two independent femtosecond lasers so exact they can be made to appear as if the source were a single laser. By improving pump-probe experiments, one important application will be in bond-specific spatial scanning of biological samples. This next decade in optical physics should be a blast!