  A method for designing plasmonic particles with desired resonance spectra is
presented. The method is based on repetitive perturbations of an initial
particle shape while calculating the eigenvalues of the various quasistatic
resonances. The method is rigorously proved, assuring a solution exists for any
required spectral resonance location. Resonances spanning the visible and the
near-infrared regimes, as designed by our method, are verified using
finite-difference time-domain simulations. A novel family of particles with
collocated dipole-quadrupole resonances is designed, demonstrating the unique
power of the method. Such on-demand engineering enables strict realization of
nano-antennas and metamaterials for various applications requiring specific
spectral functions.
