Beating the diffraction limit: an optical lattice of sub-wavelength barriers*

We report on the creation of a conservative optical lattice for cold atoms with sub-wavelength features well below the diffraction limit of the light. To achieve this, we use the nonlinear optical response of three-level atoms in spatially dependent dark states. The geometric gauge potential of atoms in this spatially dependent dark state provides a conservative potential with ultra-narrow barriers, physically realizing a Kronig-Penney potential. We demonstrate optical lattices with barrier widths less than $\lambda/50$, study the band structure and dissipation mechanisms, and compare experimental findings with theory. The potential is generalizable to higher dimensions and different geometries, allowing, for example, nearly perfect box traps, narrow tunnel junctions for atomtronics applications, and dynamically generated lattices with subwavelength spacings.