The appearance of the laser in the early sixties supplied physicists with highly excited and, hopefully, spectrally selective light fields. Obviously it represented a unique tool for preparation and probing of matter, in particular of the free atoms of a gas. The author, then at Heidelberg University, had wondered for some time why, in an act of light absorption, the excited atoms had escaped attention so far. Of course, the excitation per field mode of conventional light sources is weak. However, the novel laser seemed to be a light source capable of generating and addressing such an ephemeral population that is nowadays addressed as “saturation”.

© 2007 Optical Society of America

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