  Metamaterials and meta-surfaces represent a remarkably versatile platform for
light manipulation, biological and chemical sensing, nonlinear optics, and even
spaser lasing. Many of these applications rely on the resonant nature of
metamaterials, which is the basis for extreme spectrally selective
concentration of optical energy in the near field. The simplicity of free-space
light coupling into sharply-resonant meta-surfaces with high resonance quality
factors Q>>1 is a significant practical advantage over the extremely
angle-sensitive diffractive structures or inherently inhomogeneous high-Q
photonic structures such as toroid or photonic crystal microcavities. Such
spectral selectivity is presently impossible for the overwhelming majority of
metamaterials that are made of metals and suffer from high plasmonic losses.
Here, we propose and demonstrate Fano-resonant all-semiconductor optical
meta-surfaces supporting optical resonances with quality factors Q>100 that are
almost an order of magnitude sharper than those supported by their plasmonic
counterparts. These silicon-based structures are shown to be planar chiral,
opening exciting possibilities for efficient ultra-thin circular polarizers and
narrow-band thermal emitters of circularly polarized radiation.
