Dichroism of coupled multipolar plasmonic modes in twisted triskelion stacks

Abstract

We present a systematic investigation of the optical response to circularly polarized illumination in twisted stacked plasmonic nanostructures. The system consists in two identical, parallel gold triskelia, centrally aligned and rotated at a certain angle relative to each other. Sample fabrication was accomplished through a novel multilevel high-resolution electron beam lithography. This stack holds two plasmonic modes of multipolar character in the near-infrared range, showing a strong dependence of their excitation intensities on the handedness of the circularly polarized incident light. This translates into a large circular dichroism which can be modulated by adjusting the twist angle of the stack. Fourier-transform infrared (FTIR) spectroscopy and numerical simulations were employed to characterize the spectral features of the modes. Remarkably, in contrast to previous results in other stacked nanostructures, the system's response exhibits a behavior analogous to that of two interacting dipoles only at small angles. As the angle approaches 15°, where maximum dichroism is observed, more complex modes of the stack emerge. These modes evolve towards two in-phase multipolar excitations of the two triskelia as the angle increases up to 60°. Finally, simulations for a triangular array of such stacked elements show a sharp mode arising from the hybridization of a surface lattice resonance with the low-energy mode of the stack. This hybridized mode demonstrates the capability to be selectively switched on and off through the light polarization handedness.

Keywords: chirality; dichroism; plasmonics; triskelion; twisted stack.

Figure 1:

Schematic depiction of a triskelion displaying the dimensions of the studied nanostructure (a), a stack indicating the twist angle α, and the layers that form the structure, including the substrate and alignment markers (c). Scanning Electron Microscopy (SEM) images showing a top view of selected fabricated nanostructures (d), a cross-section image of a representative sample (e), and the spatial arrangement of one of the fabricated triangular arrays of stacked triskelia with a pitch of 1,200  nm, designed to explore the hybridization of the stack modes with a surface lattice resonance (SLR) (f). Scale bar is 400 nm.

Figure 2:

Simulated (a) and measured (b) extinction spectra for 0°, 30°, 60°, and 90° (black, red, blue, and green, respectively), under LCP (solid line) and RCP (dashed line) illumination. The baselines of the spectra are shifted arbitrarily for clarity. Simulated (c) and experimental (d) CD extracted from the data in panels (a) and (b), respectively, using Eq. (1). The shaded area in panel (d) indicates the typical size of the error bar for the CD, calculated as the root mean square deviation from 0 for the CD at 0°.

Figure 3:

Simulated charge distributions for facing triskelion interfaces for twist angles of 0° (a) and 30° (b). The simulations at different angles correspond to arbitrary values of the phase of the incoming illumination. Panel (c) presents the phase difference between the electric dipole moment orientations of the top and bottom triskelia for each twist angle. All the information displayed shares a common scale. For the 0° case, only a single polarization is presented, as the response is identical for both LCP and RCP. The geometry of the illumination, indicated by vertical gray arrows, corresponds to that of the FTIR measurements.

Figure 4:

Colormaps depicting the experimental (a, c) and simulated (b, d) extinction spectra for twist angles between 0° and 120° for both RCP (a, b) and LCP (c, d). Empty circles highlight the spectral positions of the two Lorentzian peaks fitted to both the experimental and simulated data. Dashed lines serve as guidelines to the eye.

Figure 5:

Simulated extinction spectra for an array of stacked triskelia with a pitch of 1,200 nm, embedded in a homogeneous medium of n = 1.5 with a separation of 50 nm between triskelia. The response of the array under RCP and LCP illumination is presented for different values of the twist angle. The inset shows a schematic depiction of the simulated array of twisted stacks of triskelia nanostructures.