  We demonstrate that a flat-band state in a quasi-one-dimensional rhombic
lattice is robust in the presence of external drivings along the lattice axis.
The lattice was formed by periodic arrays of evanescently coupled optical
waveguides, and the external drivings were realized by modulating the paths of
the waveguides. We excited a superposition of flat-band eigenmodes at the input
and observed that this state does not diffract in the presence of static as
well as high-frequency sinusoidal drivings. This robust localization is due to
destructive interference of the analogous wavefunction and is associated with
the symmetry in the lattice geometry. We then excited the dispersive bands and
observed Bloch oscillations and coherent destruction of tunneling.
{\textcopyright} 2017 Optical Society of America.
