  The possibility of focusing light to an ever tighter spot has important
implications for many applications and fields of optics research, such as
nano-optics and plasmonics, laser-scanning microscopy, optical data storage and
many more. The size of lateral features of the field at the focus depends on
several parameters, including the numerical aperture of the focusing system,
but also the wavelength and polarization, phase and intensity distribution of
the input beam. Here, we study the smallest achievable focal feature sizes of
coherent superpositions of two co-propagating beams carrying opposite orbital
angular momentum. We investigate the feature sizes for this class of beams not
only in the scalar limit, but also use a fully vectorial treatment to discuss
the case of tight focusing. Both our numerical simulations and our experimental
results confirm that lateral feature sizes considerably smaller than those of a
tightly focused Gaussian light beam can be observed. These findings may pave
the way for improving the resolution of imaging systems or may find
applications in nano-optics experiments.
