Tunable circular dichroism through absorption in coupled optical modes of twisted triskelia nanostructures

Abstract

We present a system consisting of two stacked chiral plasmonic nanoelements, so-called triskelia, that exhibits a high degree of circular dichroism. The optical modes arising from the interactions between the two elements are the main responsible for the dichroic signal. Their excitation in the absorption cross section is favored when the circular polarization of the light is opposite to the helicity of the system, so that an intense near-field distribution with 3D character is excited between the two triskelia, which in turn causes the dichroic response. Therefore, the stacking, in itself, provides a simple way to tune both the value of the circular dichroism, up to 60%, and its spectral distribution in the visible and near infrared range. We show how these interaction-driven modes can be controlled by finely tuning the distance and the relative twist angle between the triskelia, yielding maximum values of the dichroism at 20° and 100° for left- and right-handed circularly polarized light, respectively. Despite the three-fold symmetry of the elements, these two situations are not completely equivalent since the interplay between the handedness of the stack and the chirality of each single element breaks the symmetry between clockwise and anticlockwise rotation angles around 0°. This reveals the occurrence of clear helicity-dependent resonances. The proposed structure can be thus finely tuned to tailor the dichroic signal for applications at will, such as highly efficient helicity-sensitive surface spectroscopies or single-photon polarization detectors, among others.

Figure 1

(a) Schematic top view of one triskelion showing the values of the main in-plane geometrical parameters. The thickness of the triskelion was set to 30 nm. (b) 3D-view showing the parallel stacking of the two triskelia in the system. The edge-to-edge distance $d$ between the two triskelia is also depicted. Note that the triskelion on top of the stack is twisted 30° anticlockwise with respect to the bottom one.

Figure 2

(a) Absorption, (b) scattering, and (c) extinction cross-sections under LCP (blue solid line) and RCP (red solid line) light for a single triskelion (see Fig. 1a). (d) Absorption, (e) scattering, and (f) extinction CS under LCP (blue solid line) and RCP (red solid line) light for a double triskelion system forming an anticlockwise 30° twist angle and at an edge-to-edge distance d = 30 nm (see Fig. 1b).

Figure 3

Absorption, scattering, and extinction CS for LCP (a–c), and RCP (d–f), illuminations, respectively, as a function of the twist angle φ for a fixed edge-to-edge distance of 30 nm. FOM defined in Eq. (1) for the CD of the absorption (g), scattering (h), and extinction (i) as a function of the twist angle.

Figure 4

Absorption, scattering, and extinction CS for LCP (a–c), and RCP (d–f), illuminations, respectively, as a function of the edge-to-edge distance for a fixed twist angle φ = 30°. FOM defined in Eq. (1) for the CD of the absorption (g), scattering (h), and extinction (i) as a function of the edge-to-edge distance.

Figure 5

Near-field distributions of the square modulus of the electric field normalized to that of the incident light under RCP and LCP illumination at a wavelength of 1100 nm for twist angles φ of 30° and 60°.

Figure 6

Near-field distributions of the square modulus of the electric field normalized to that of the incident light under RCP and LCP illumination at wavelengths of 760 nm (a, b), and 1100 nm (c, d), for twist angles φ of 30° in a plane perpendicular to the axis of the stacking and equidistant to both triskelia.