Each size and shape nanoparticle represents a different resonance frequency.  For more information about resonance these web sites may help.

http://ngsir.netfirms.com/englishhtm/Resonance.htm

http://www.pbs.org/wgbh/nova/elegant/resonance.html

In the first animation when the force applied matches the maximum spring constant resonance occurs.  Similarly, when the wavelength frequency matches the resonance energy maximum oscillation of the plasmons occur.

my aim is SPR 

your answer is for LSPR

thanks

spp wavelength < wavelength of the excitation

see this ref.:

Dragoman, M., and D. Dragoman. "Plasmonics: Applications to nanoscale terahertz and optical devices." Progress in Quantum Electronics 32.1 (2008): 1-41.