We were able to derive summation
rules for OAM of light in our structures
which arise due to plasmonic modes on
both sides. 5 We generated beams of light
carrying OAM up to |l| = 8 and revealed
the fundamental role of the central
aperture through specific OAM selection
rules. Moreover, OAM cut-off conditions
related to the aperture are essential in
understanding the relationship between
OAM evolutions and reciprocity. If the
selection rules are not fulfilled, the
path-reversal symmetry is broken and
our device performs as a plasmon-based OAM optical diode.
The versatility of our devices
makes them easy to integrate into 2D
plasmonic systems as optical vortex
generators for optical communication.
We believe that these concepts bear a
fundamental importance in nanophotonics and provide a basis for novel
applications in nanotechnology.
Structured light beams with phase or polarization singularities reveal
unique optical properties with numerous
applications. 1 Chiral nanostructures
draw promising routes for enhancing
singular optical signatures with new
functionalities. 2 Recently, near-field
singular optical effects have been of
interest in relation to surface plasmon
(SP) vortices which have been shown to
carry orbital angular momentum (OAM). 3
However, singular SP modes and associated spin-orbit coupling have only been
probed in the near-field. 1–4
Our research has shown that helical
nanostructures milled on both sides
of a thin suspended membrane lead to
tailoring optical OAM in the far-field.
We analyzed OAM transfer during plasmonic interaction with nanostructures
on the membrane surfaces, which led
to the generation of vortex beams with
a phase modulation of (il w) and tunable
For our experiments, we used a
suspended thin (300 nm) metallic
membrane. 5 A focused ion beam milled
shallow ( 30 nm) concentric or spiral
plasmonic structures on each side of the
membrane around a single cylindrical
aperture. The periodicity of the grooves
was chosen with respect to laser wavelength so that SP modes on the membrane are resonantly excited by normally
incident plane wave components of the
illumination beam. The SP field propagates towards the central hole and is
transmitted through it. The field is then
decoupled to the far-field by the periodic
groove on the back of the membrane, as a
quasi-plane-wave which interferes with
the direct transmission via the hole. This
interference results in fringe patterns
with the specific OAM signature.
(a) Scheme of the far-field experiment. (b) SEM image of the 5-folded spiral on a thin gold
membrane. (c) Resulting far-field transmission images with single left-handed spiral on
the front side and a right 5-fold spiral on the back side for each circular polarization state
(+/–, right/left). Angular momentum indices l are denoted in the boxes.
Yuri Gorodetski, Cyriaque Genet
and Thomas W. Ebbesen
University of Strasbourg and
CNRS, Strasbourg, France
Néel Institute, CNRS and
Joseph Fourier University,
1. J.P. Torres and L. Torner.
Twisted Photons: Applications
of Light with Orbital Angular
Momentum, First Edition,
Wiley-VCH Verlag GmbH &
Co. KGaA, (2011).
2. J.B. Pendry. Science 306,
3. Y. Gorodetski et al. Phys. Rev.
Lett. 101, 043903 (2008).
4. G. Rui et al. Opt. Lett. 36,
5. Y. Gorodetski et al. Phys. Rev.
Lett. 110, 203906 (2013).
Plasmonic Vortices Twisting the Far-Field