We experimentally and theoretically analyze the radiation emitted from subw
avelength-sized apertures in nearfield optical probes. By decomposing the e
xperimentally obtained radiation patterns into vector spherical waves, we d
escribe the fields in terms of a series of multipole sources. We fit polari
zation-resolved angular intensity distributions, measured as far as 150 deg
rees from the normal, with dipole, quadrupole, and octupole radiation. We f
ind that the magnetic and the electric dipole components are dominant but t
hat the interference terms between dipoles and higher-order poles are not n
egligible. This result can be used as the basis for understanding near-fiel
d optical interactions and images. (C) 2000 Optical Society of America. OCI
S codes: 260.1960, 180.5810.