Surface-Plasmon Holographic Beam Shaping

Surface plasmon polaritons and free-space beams are often coupled through periodic gratings. Here we show that by employing holographic-based techniques for modulating the grating, one can systematically control the amplitude and phase of the free-space beam. Alternatively, arbitrarily shaped surface plasmon can be generated. By using gratings with different periods for the input and output coupling, we obtain a planar beam transformer, whose resonance angles are related through a generalized form of the Bragg law. Specifically, we demonstrate the coupling of surface plasmon polaritons into focused free-space beams, as well as into accelerating Airy beams and vortex beams.

Figure 1

Illustration of the sample layout, consisting of SPP coupling and decoupling gratings. Beam shaping is achieved by modulating the decoupling grating.

Figure 2

Airy beam generation with CGH of SPP. Cross section of the beam from the cubically modulated grating (a) and the unmodulated grating (b). Insets show the full frame images (scale maximum is normalized to 1). (c) and (d) drawings of a segment from the decoupling cubically modulated and the unmodulated gratings, respectively.

Figure 3

1D lens generated by output coupling of the SPP wave through a grating with quadratic phase. (a) Cross section of the free-space beam and (b) top view of the free-space beam. The full frame of the beam at $z=0\mathrm{cm}$, $z=10\mathrm{cm}$ (at focus), and $z=20\mathrm{cm}$ (c), (d) and (e), respectively (scale maximum is normalized to 1). (f) Drawing of a segment from the decoupling grating.

Figure 4

Simulations of the decoupling SPP. (a) The out-coupled SPP intensity. The out-coupled unwrapped phase cross section vs propagating distance for (b) quadratically modulated and (c) cubically modulated gratings $20\mu \mathrm{m}$ above the gratings, respectively, and their full phase profile (d) and (e), respectively.

Figure 5

Generation of a free-space vortex beam from an SPP. Measured beam intensity (a) without modulation of the decoupling grating and (b) with fork-shape modulation with a topological charge of $m=1$. (c), (d) Corresponding theoretical shapes. (e) Image of the decoupling grating generating a free-space vortex beam, taken with a confocal microscope (x100, NA 0.95).