  Utilising the fact that the frequency response of a material can be
decomposed into the quasi-normal modes supported by the system, we present two
methods to directly manipulate the complex frequencies of quasi-normal modes in
the complex plane. We first consider an `eigen-permittivity' approach that
allows one to find how to shift the permittivity of the structure everywhere in
order to place a single quasi-normal mode at a desired complex frequency.
Secondly, we then use perturbation theory for quasi-normal modes to iteratively
change the structure until a given selection of quasi-normal modes occur at
desired complex frequencies.
