Optical Singularities in Plasmonic Fields near Single Subwavelength Holes

Anouk de Hoogh Center for Nanophotonics, FOM Institute AMOLF, Amsterdam, Noord-Holland, The Netherlands Nir Rotenberg Center for Nanophotonics, FOM Institute AMOLF, Amsterdam, Noord-Holland, The Netherlands Kobus Kuipers Center for Nanophotonics, FOM Institute AMOLF, Amsterdam, Noord-Holland, The Netherlands

The creation of structured light fields that can carry angular momentum at the nanoscale has recently gained interest as a possible route towards on-chip applications in research areas such as biosensing or quantum optics. In the near field of nanostructures, regions with local angular momentum are often associated with optical singularities, either phase or polarization singularities in the cases of orbital- or spin-angular momentum, respectively.

We identify phase and polarization singularities through both near-field measurements and theoretical modeling of the electric near-field distributions that result from the scattering of surface plasmon polaritons (SPPs) from single subwavelength holes in optically thick gold films. We discuss the properties of the singularities, such as their topological charge, the handedness and the field amplitudes at their locations. We show that by varying the hole diameter or incident plasmon beam width we can control the position of the polarization singularities and as a result their field amplitude by over 3 orders of magnitude. Finally, we present preliminary polarization- and phase-resolved near-field measurements of the scattering of SPPs from a subwavelength hole on a gold film, in which we observe polarization singularities [1].

Unlike previous plasmonic systems whose near fields are known to contain singularities, in our case we find optical singularities in the plane of the film [2]. Additionally, because the control over the polarization singularities is achievable by tuning physically adjustable parameters of the system (the hole diameter and beam width), we can expect that our results create possibilities for its use in future on-chip applications requiring structured light fields.

a. Schematic of a SPP scattering off a subwavelength hole in an optically thick gold film. The total field is a superposition of the incident Gaussian SPP beam and the field scattered by the hole. On top the calculated amplitude map of the in-plane electric field for σ=5 μm SPP beam scattering from a hole with diameter d = 800 nm. Polarization singularities with a topological charge of 1/2 (-1/2) are marked in solid red diamonds (blue circles). b. We can control the position of the singularities by changing the hole size or beam diameter.

Fig. 1: a. Schematic of a SPP scattering off a subwavelength hole in an optically thick gold film. On top the calculated amplitude map of the in-plane electric field for m SPP beam scattering from a hole. Polarization singularities are marked in solid red diamonds and blue circles. b. We can control the position of the singularities by changing the hole size or beam diameter.

[1] A. de Hoogh, N. Rotenberg and L. Kuipers, Optical singularities in plasmonic fields near single subwavelength holes, J. Opt. 16, 114004 (2014).

[2] H.F. Schouten, T.D. Visser, G. Gbur, D. Lenstra, H. Blok, Connection between phase singularities and the radiation pattern of a slit in a metal plate, Phys. Rev. Lett. 93, 173901 (2004).

hoogh@amolf.nl









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