Suppression of spontaneous decay at optical frequencies: Test of vacuum-field anisotropy in confined space

The radiative decay of Cs atoms excited into the 5D₅/₂ level and passing between two metallic mirrors spaced by a 1. 1- gap is observed to depend upon their angular momentum. Spontaneous emission at a wavelength of 3. 49 is suppressed for the substates with maximum angular momentum normal to the mirrors, which survive without substantial decay during 13 natural lifetimes. The radiation rate is modified by application of a magnetic field which mixes sublevels having lifetimes in the gap. This experiment illustrates the anisotropy of the vacuum field in confined space.