Chiral optical response of planar and symmetric nanotrimers enabled by heteromaterial selection

Abstract

Chirality is an intriguing property of certain molecules, materials or artificial nanostructures, which allows them to interact with the spin angular momentum of the impinging light field. Due to their chiral geometry, they can distinguish between left- and right-hand circular polarization states or convert them into each other. Here we introduce an approach towards optical chirality, which is observed in individual two-dimensional and geometrically mirror-symmetric nanostructures. In this scheme, the chiral optical response is induced by the chosen heterogeneous material composition of a particle assembly and the corresponding resonance behaviour of the constituents it is built from, which breaks the symmetry of the system. As a proof of principle, we investigate such a structure composed of individual silicon and gold nanoparticles both experimentally, as well as numerically. Our proposed concept constitutes an approach for designing two-dimensional chiral media tailored at the nanoscale, allowing for high tunability of their optical response.

Figure 1. Illustration of the investigated heterogeneously composed nanodisk trimer.

(a) Sketch of a three-particle system (trimer) consisting of nanodisks of the same size and material all placed in the same plane. (b) If one of the particles at the end of the trimer is replaced by an equally sized disk made from a different material, the trimer still exhibits geometrical mirror-symmetry (grey dashed line indicates its geometric symmetry axis), but with respect to the material composition, its symmetry is broken. The trimeric nanostructure consists of three nanodisks, two made of silicon (blue) and one made of gold (yellow). (c) In the simulations, the trimer is illuminated with circularly polarized light (Gaussian beam; green arrow—right circular polarization, red arrow—left circular polarization), which impinges normally (along the negative z-direction) to the plane defined by the three disks (xy-plane). The thickness (height) and diameter of the disks were chosen to be 30 and 180 nm, respectively.

Figure 2. Numerical results for different nanodisks system.

Numerically calculated (a) transmittance and (b) reflectance spectra for an individual nanodisk trimer for left-handed circularly polarized (lhcp, dashed red line) and right-handed circularly polarized (rhcp; solid green line) light excitation. (c) Resulting circular dichroism (CD) curve. (d) Transmittance and (e) reflectance spectra for a trimer (solid blue and brown curves) and individual nanodisks (dotted and dashed curves) for linearly polarized excitation. The corresponding excitation schemes are indicated above (d). The vertical dotted cyan-coloured line indicates the spectral position of the electric dipole resonance of an individual Au nanodisk, whereas the vertical dashed magenta-coloured line marks the position of the magnetic dipole resonance of an individual Si nanodisk for the indicated excitation schemes. (f) Distributions of the instantaneous in-plane electric field (E_t=(E_x, E_y); white arrows) and the modulus of the longitudinal magnetic field component (|H_z|; colour-coded) for a wavelength of 520 nm plotted in a plane parallel to the xy-plane cutting through the centers of the disks.

Figure 3. Near-field maps for different excitation schemes.

Time-evolution of the longitudinal magnetic field component Re(H_z) plotted in a plane parallel to the xy-plane cutting through the centers of all particles for two different excitation schemes and a fixed wavelength of 520 nm. (a) Excitation with an x-polarized Gaussian light beam. (b) Excitation with a y-polarized Gaussian light beam. Each field map is normalized to its individual minima and maxima resulting in different colour-scales for different maps. The geometrical outlines of the three nanosdisks (top: Au; bottom: Si) are indicated by black circles. For right-handed (left-handed) circularly polarized excitation, the frames indicated in blue in a and green (red) in b overlap temporally.

Figure 4. Investigated nanostructure and experimental set-up.

(a) SEM micrograph of the investigated trimer assembled from one Au (white; diameter ≈171 nm) and two Si (dark grey; diameters ≈188 nm) nanospheres on a borosilicate glass (BK7) substrate. (b) Sketch of the trimer and chosen coordinate frame. It should be noted here that the investigated trimer exhibits a mirrored geometry with respect to the nanodisk trimer discussed before. (c) Simplified sketch of the experimental set-up utilized for the measurement of individual nanosphere trimers. A circularly polarized fundamental Gaussian light beam enters a microscope objective (MO) with a high numerical (NA) of 0.9. The entrance aperture of the MO is not filled, hence reducing the effective NA to ∼0.4. The focused light beam has a diameter of ∼0.8 μm (full-width at half-maximum) at a wavelength of 625 nm. The sample composed of a substrate with individual nanotrimers assembled on top is mounted on a 3D-piezo-stage, enabling precise positioning of the structure relative to the beam. A second immersion-type MO with an NA of 1.3 is located below the sample to collect the light in the forward direction, which is detected with a photo-diode. Light reflected or scattered backwards is collected by the upper MO and guided to a second photo-diode by a set of non-polarizing beamsplitters.

Figure 5. Comparison of experimental and numerical results for a heterogeneous nanosphere trimer.

(a–c) Experimental and (d–f) numerical results for an individual trimer assembled from Au and Si nanospheres on a glass substrate (see Fig. 4). Transmittance and reflectance as well as the CD spectra are shown. In the experiment, a thin layer of AZO (6 nm) was present on top of the glass substrate to prevent charging effects during fabrication.