  We design a photonic sphere-array nanoantenna (NA) exhibiting both strong
directionality and wavelength selectivity. Although the geometric configuration
of the photonic NA resembles a plasmonic Yagi-Uda NA, it has different working
principles, and most importantly, reduces the inherent metallic loss from
plasmonic elements. For any selected optical wavelength, a sharp Fano-resonance
by the reflector is tunable to overlap spectrally with a wider dipole resonance
by the sphere-chain director leading to the high directionality. The work
provides design principles for directional and selective photonic NAs, which is
particularly useful for photon detection and spontaneous emission manipulation.
