A method of realizing nanoantenna systems at optical frequencies is suggested wherein a plasmonic nanowire is placed near a slab waveguide operating near the plasmonic resonant frequency of the nanowire. The polarizability of a nano-sized concentric cylindrical structure with the core made of an ordinary dielectrics and the shell made of a plasmonic material is calculated using Mie scattering theory. When such a core-shell nanowire is placed near a slab waveguide, the transverse magnetic (TM) surface wave guided by the waveguide can interact with the nanowire, and thus part of its energy is scattered into the open space. By reciprocity, under TM wave incidence, the induced dipole on the nanowire will launch a guided mode in the slab waveguide, thus converting part of the incident energy into the guided mode energy. A rigorous analytical treatment of the problem using the Green function of a dipole near a planar slab waveguide is developed, and the properties of such an optical antenna system are studied in detail. The finite element method is also utilized to demonstrate the idea directly.