Demonstration of extrinsic chirality in self-assembled asymmetric plasmonic metasurfaces and nanohole arrays

Abstract

Chirality, the lack of mirror symmetry, can be mimicked in nanophotonics and plasmonics by breaking the symmetry in light-nanostructure interaction. Here we report on versatile use of nanosphere lithography for the fabrication of low-cost metasurfaces, which exhibit broadband handedness- and angle-dependent extinction in the near-infrared range, thus offering extrinsic chiro-optical behavior. We measure wavelength and angle dependence of the extinction for four samples. Two samples are made of polystyrene nanospheres asymmetrically covered by silver and gold in one case and silver only in the other case, with a nanohole array at the bottom. The other two samples are nanohole arrays, obtained after the nanosphere removal from the first two samples. Rich extrinsic chiral features are governed by different chiro-optical mechanisms in the three-dimensional plasmonic semi-shells and planar nanohole arrays. We also measure Stokes parameters in the same wavelength and incidence angle range and show that the transmitted fields follow the extrinsic chirality features of the extinction dissymmetry. We further study the influences of the nanostructured shapes and in-plane orientations on the intrinsic vs extrinsic chirality. The nanoholes are modelled as oval shapes in metal, showing good agreement with the experiments. We thus confirm that nanosphere lithography can provide different geometries for chiral light manipulation at the nanoscale, with the possibility to extend functionalities with optimized oval shapes and combination of constituent metals.

Keywords: Chirality; Metamaterials; Nanohole arrays; Nanomaterials; Splasmonics.

Figure 1

SEM images of the samples (a) Au–Ag–PSN, (b) Au–Ag–NHA, (c) Ag–PSN, and (d) Ag–NHA. (e) Sketch of the excitation plane. Yellow arrow indicates projection of the metal flux direction onto the sample surface plane.

Figure 2

Wavelength-incidence angle (θ) transmission maps for Au–Ag–PSN excited with (a) LCP and (b) RCP, and for Au–Ag–NHA excited with (c) LCP and (d) RCP.

Figure 3

Wavelength- incidence angle (θ) transmission maps for Ag–PSN excited with (a) LCP and (b) RCP, and for Ag–NHA excited with (c) LCP and (d) RCP.

Figure 4

Experimental wavelength- incidence angle (θ) extinction dissymmetry g_ext maps for Samples (a) Au–Ag–PSN, (b) Au–Ag–NHA, (c) Ag–PSN, and (d) Ag–NHA.

Figure 5

Normalized Stokes parameters of the transmitted field when Ag–PSN is excited with linear p-polarized light.

Figure 6

Simulated influence of the in-plane tilt angle Ψ on g_ext. Inset: the tilt definition w.r.t. incidence plane (red line, perpendicular to the surface). (a) Au–Ag–PSN behaviour of g_ext , at θ =  − 20° changes with the in-plane tilt Ψ. (b) The tilt angle of Ψ = 70° fits well the non-perfect extrinsic chirality features at θ =  − 20°.

Figure 7

(a) g_ext of oval nanohole arrays depends on the shape factor c; presented results are for Ag–NHA at θ = 20°. The best fit to the experimental data is obtained with c = 0.02 (dashed lines are the inversion of g_ext with θ =  − 20°). (b) Absorption density for strongly oval shape (c = 0.1), excited at 903 nm and θ = 20° strongly depends on the excitation handedness. (c) Adding Au on the top of Ag red-shifts the resonances: the total thickness is 55 nm, while legend presents Ag to Au thickness ratio. (d) g_ext spectral position is defined by the grating nature of the NHA; at a single angle of incidence θ = 20°, increasing the starting interparticle distance red shifts the g_ext peak.