  A new concept for second-harmonic generation (SHG) in an optical nanocircuit
is proposed. We demonstrate both theoretically and experimentally that the
symmetry of an optical mode alone is sufficient to allow SHG even in
centro-symmetric structures made of centro-symmetric material. The concept is
realized using a plasmonic two-wire transmission-line (TWTL), which
simultaneously supports a symmetric and an anti-symmetric mode. We first
confirm the generated second-harmonics belong only to the symmetric mode of the
TWTL when fundamental excited modes are either purely symmetric or
anti-symmetric. We further switch the emission into the anti-symmetric mode
when a controlled mixture of the fundamental modes is excited simultaneously.
Our results open up a new degree of freedom into the designs of nonlinear
optical components, and should pave a new avenue towards multi-functional
nanophotonic circuitry.
