Enhanced fields on large metal particles: dynamic depolarization.

Influences of particle size on surface-enhancement processes are discussed in terms of a simple physical model. When the size of a silver sphere is increased, the magnitude of the enhancement exhibits a slight increase followed by a strong decrease. Simultaneously the plasmon resonance is shifted and severely broadened. To interpret these effects, a self-consistent calculation of the particle polarization is performed. Initial increase in magnitude and shift of the resonance are due to dynamic depolarization, whereas the decrease in magnitude and broadening are caused by radiation damping. The importance of higher-order multipoles is assessed by analyzing their contributions separately.