Reflectionless and invisible potentials in photonic lattices.

An arbitrarily shaped optical potential on a discrete photonic lattice, which transversely drifts at a speed greater than the maximum speed allowed by the light cone of the lattice band, becomes reflectionless. Such an intriguing result, which arises from the discrete translational symmetry of the lattice, is peculiar to discretized light and does not have any counterpart for light scattering in continuous optical media. A drifting non-Hermitian optical potential of the Kramers-Kronig type also is an invisible potential, i.e., a discrete optical beam crosses the drifting potential without being distorted, delayed, nor advanced.