  Recently, the generation and detection of structured light field have drawn a
great deal of attention, due to their importance in high-capacity optical data
storage and quantum technology. In this letter, we explore the azimuthal
modulation of optical transparency in a four-level double-V type quantum system
near a plasmonic nanostructure. A Laguerre-Gaussian beam and the interaction of
the system with free-space vacuum modes have been employed to create the
phase-dependent absorption of a non-vortex probe field. First, we demonstrate
how to identify the azimuthal index associated with the conventional LG beam
via measuring the probe absorption so that the phase information of such a beam
gets encoded on the spatially-dependent absorption profile with
angularly-distributed lobes. Also, a spatially-varying optical transparency can
be formed, due to the periodic variation of the absorption spectrum. Then,
asymmetrical Laguerre-Gaussian beams are used to extend the selective spatial
transparency mechanism to asymmetric spatially-structured windows, allowing for
optical manipulation of spatial modes at arbitrary positions. Moreover, we
investigate the influence of the asymmetric parameter on the features of the
spatial inhomogeneities and show how the beams enable us to imprint the phase
information of the orbital angular momentum at a desired position.
