Purcell effect in small metallic cavities
  We have studied theoretically the Purcell factor which characterizes a change in the emission rate of an electric or magnetic dipole embedded in the center of a spherical cavity. The main attention is paid to the analysis of cavities with radii small compared to the wavelength. It is shown that the Purcell factor in small metallic cavities varies in a wide range depending on the ratio of the cavity size to the skin depth. 
M.M. Glazov, E.L. Ivchenko, A.N. Poddubny, G. Khitrova
cond-mat.mes-hall