  We discuss generation of subwavelength optical barriers on the scale of tens
of nanometers, as conservative optical potentials for cold atoms. These arise
from non-adiabatic corrections to Born-Oppenheimer potentials from dressed
`dark states' in atomic $\Lambda$-configurations. We illustrate the concepts
with a double layer potential for atoms obtained from inserting an optical
subwavelength barrier into a well generated by an off-resonant optical lattice,
and discuss bound states of pairs of atoms interacting via magnetic dipolar
interactions. The subwavelength optical barriers represent an optical
`Kronig-Penney' potential. We present a detailed study of the bandstructure in
optical `Kronig-Penney' potentials, including decoherence from spontaneous
emission and atom loss to open `bright' channels.
