Photolithographic apparatus, systems, and methods that make use of sub-resolution assist devices are disclosed. In the various embodiments, an imaging mask includes an optically transmissive substrate having a sub-resolution assist device that further includes a first optical attenuation region and a spaced-apart second optical attenuation region, and an optically transmissive phase adjustment region interposed between the first optical attenuation region and the second optical attenuation region, the phase adjustment region being configured to change a phase of incident illumination radiation by altering an optical property of the substrate.