  We investigate the magnetic response of meta-atoms that can be fabricated by
a bottom-up technique. Usually such meta-atoms consist of a dielectric core
surrounded by a large number of solid metallic nanoparticles. In contrast to
those meta-atoms considered thus far, we study here for the first time hollow
metallic nanoparticles (shells). In doing so we solve one of the most pertinent
problems of current self-assembled metamaterials, namely implementing
meta-atoms with sufficiently large resonance strength and small absorption.
Both conditions have to be met for deep sub-wavelength meta-atoms to obtain
effectively homogeneous metamaterials which may be meaningfully described by
negative material parameters. Eventually we show that by using these findings
self-assembled negative index materials come in reach.
