A GaAs/AlAs planar cavity containing a collection of InAs quantum boxes in its core region has been grown in a single step by molecular beam epitaxy, and processed by electron‐beam lithography and reactive ion etching into pillar microresonators. The optical study by photoluminescence of these localized light emitters allows a systematic and precise determination of the energies of the first confined photon modes of such microstructures, in good agreement with theoretical estimates. More generally, such probes facilitate the experimental study of the modes of complex photonic microstructures and of the spontaneous emission alteration they entail on a quasimonochromatic light emitter. ©1996 American Institute of Physics.