  Spherical silicon nanoparticles with sizes of a few hundreds of nanometers
represent a unique optical system. According to theoretical predictions based
on Mie theory they can exhibit strong magnetic resonances in the visible
spectral range. The basic mechanism of excitation of such modes inside the
nanoparticles is very similar to that of split-ring resonators, but with one
important difference that silicon nanoparticles have much smaller losses and
are able to shift the magnetic resonance wavelength down to visible
frequencies. We experimentally demonstrate for the first time that these
nanoparticles have strong magnetic dipole resonance, which can be continuously
tuned throughout the whole visible spectrum varying particle size and visually
observed by means of dark-field optical microscopy. These optical systems open
up new perspectives for fabrication of low-loss optical metamaterials and
nanophotonic devices.
