Revealing Hidden Topology of Complex Vector Beams via Plasmonic Interactions

Abstract

Structured light beams with space-variant polarization can be efficiently generated using voltage-tunable nematic liquid-crystal (Q-plate). By appropriately selecting the input state and the retardation of the Q-plate, an optical field acquires a spatially structured polarization distribution that is capable of encoding non-trivial topological information across the beam profile. These features can be directly read out through interaction with plasmonic nano-structures, such as circular and spiral slits. Here we show that, upon illumination, polarization-dependent excitation of surface plasmons converts the hidden topology of the polarization structure into observable intensity distributions, including plasmonic vortices and characteristic interference patterns, while the tunability of the input parameters enables a rich variety of distinct topological forms.

Figures (15)

• Figure 1: Experimental setup: (a) Scheme of the experimental arrangement showing the illumination path of the laser beam through the $Q$-plate onto the sample. (b) SEM image of the fabricated circular slit sample. (c) Leakage radiation microscopy (LRM) configuration used to record the SPs field distribution.
• Figure 2: Surface plasmon interference: (a–c) Experimentally measured SPs field for $|H\rangle$ and (d-f) for $|V\rangle$ distributions at applied voltages of 1.8$V$, 2.2$V$, and 3.8$V$, respectively.
• Figure 3: Experimental and numerical Stokes parameters: (a–c) Experimental measurements at applied voltages of 1.8$V$, 2.2$V$, and 3.8$V$, respectively, shown together with numerical results for phase retardations of $\delta \approx 1.5\pi, \pi$ and $0.5\pi$. (d–f) Measured Stokes parameters mapped onto the Poincaré sphere for three voltages and (d1-f1) calculated paths for the respective $\delta$ values. (g) $\psi$ and $\chi$ representation on the Poincaré sphere. (h) Polarization ellipse formation by $\psi$ and $\chi$.
• Figure 4: Encircling $S'_3$: Stokes parameters plotted on the $S'_2$–$S'_1$ plane through the $S'_3$ axis. Experimental results are shown for applied voltages $V_Q = 1.8V$, $2.2V$, and $3.8V$, together with numerical results corresponding to retardation $\delta \approx 1.5\pi$, $\pi$, and $0.5\pi$, respectively.
• Figure 5: Stokes-vector winding: a–c Evolution of $2\chi$ and $2\psi$ with $\theta$ for $\delta \approx 1.5\pi$, $\pi$, and $0.5\pi$, respectively. Panels a1–c1 show the corresponding experimental results for applied voltages of $1.8V$, $2.2V$, and $3.8V$. d–f Numerical plots of $2\chi(\theta)$ versus $2\psi(\theta)$ for $\delta \approx 1.5\pi$, $\pi$, and $0.5\pi$, respectively, with the corresponding experimental results shown in d1–f1 for $1.8V$, $2.2V$, and $3.8V$.