Ultrasmall volume Plasmons - yet with complete retardation effects
  Nano particle-plasmons are attributed to quasi-static oscillation with no wave propagation due to their subwavelength size. However, when located within a band-gap medium (even in air if the particle is small enough), the particle interfaces are acting as wave-mirrors, incurring small negative retardation. The latter when compensated by a respective (short) propagation within the particle substantiates a full-fledged resonator based on constructive interference. This unusual wave interference in the deep subwavelength regime (modal-volume<0.001lambda^3) significantly enhances the Q-factor, e.g. 50 compared to the quasi-static limit of 5.5. 
Eyal Feigenbaum and Meir Orenstein