Analysis of the spectral behavior of localized plasmon resonances in the near- and far-field regimes.

The angular spectrum representation of electromagnetic fields scattered by metallic particles much smaller than the incident wavelength is used to interpret and analyze the spectral response of localized surface plasmon resonances (LSPs) both in the near-field and far-zone regimes. The previously observed spectral redshift and broadening of the LSP peak, as one moves from the far-zone to the near-field region of the scatterer, is analyzed on studying the role and contribution of the evanescent and propagating plane wave components of the emitted field. For such dipolar particles, it is found that the evanescent waves are responsible for those broadenings and shifts. Further, we prove that the shift is a universal phenomenon, and hence, it constitutes a general law, its value increasing as the imaginary part of the nanostructure permittivity grows. Our results should be of use for the prediction and interpretation of the spectral behavior in applications where the excitation of LSPs yield field enhancements like those assisting surface-enhanced Raman spectroscopy or equivalent processes.