More on the size effects on the spectral figure of merit and enhancement of the local fields in the neighborhood of biconical and bipyramidal metallic nanoparticles

Optical properties of biconical and bipyramidal nanoparticles, which are equivalent to prolate and oblate spheroids, are studied. The relations for the frequency dependences for the diagonal components of the tensors of enhancement of local electric fields and spectral figure of merit tensor, as well as for the size dependences of the longitudinal and transverse spectral figure of merit, frequencies of the longitudinal and transverse surface plasmonic resonance, and effective relaxation rate are obtained. The proximity of the frequency dependence curves of the transverse and longitudinal components of the field enhancement tensors and spectral figure of merit tensor for biconic and bipyramidal particles indicates the possibility of studying the optical properties of bipyramids, most often obtained in the synthesis of nanoparticles, using the relations for bicones. The character of evolution of maxima of the longitudinal and transverse components of the field enhancement tensor under the variation of effective aspect ratio is determined. The existence of the limit to which the values of the transverse and longitudinal figure of merit asymptotically tend with increasing frequency is established. The fact of decreasing of the splitting of the surface plasmonic resonance frequency when the shape of the equivalent spheroid approaches the spherical one is proved. The qualitative coincidence of the theoretical and experimental results for the size dependence of the effective electron relaxation rate in biconic nanoparticle whose height is greater than the diameter is demonstrated.