INSTITUTE OF APPLIED PHYSICS - Chiral Plasmonic Nanostructures

Chiral Plasmonic Nanostructures

An object is called chiral if it is lacking any mirror symmetry and thus, chirality is a purely geometric property. A chiral plasmonic nanostructure is able to selectively absorb one circular polarization state of light, known as circular dichroism, while the orthogonal circular polarization state is allowed to pass unhindered. This effect can be used to set up thin and high-performing circular polarizers and thus replacing waveplate based circular polarizers which inherently suffer from being bulky and limited regarding ellipticity minimization. Further applications might base on the likewise ability of polarization axis rotation, known as optical activity or the formation of optical chirality in the vicinity of chiral metallic nanostructures.
To make use of the above mentioned effects the challenges for chiral plasmonic nanostructure realization are multifold. First, due to the definition of chirality, the chiral nanostructures must obey three-dimensionality. Second, in order to apply the optical effects in the visible and near-infrared spectral range, the feature sizes of the chiral plasmonic nanostructures are limited to the nanometer scale. Third, to cover large-scale surfaces, a tremendous number of chiral nanostructures have to be available.
We faced these challenges by developing an approach which we call template lithography. The idea is to project planar shapes defined by lithography onto a pre-structured template. Figure 1 shows a design study of several chiral nanostructures available by this approach. The advantage of the approach is compatibility with high-efficient cell projecting electron beam lithography (writing time: approx. 10 cm²/h) and nanoimprint replication technology.

Figure 1: Design study of chiral nanostructures realizable by means of template lithography. The shape of structure/template are: (a) propeller/cone, (b) spiral/cone, (c) split-ring/helix, (d) Z-shape/blazed grating and (e) L-shape/binary grating.(rights: IAP)

An optically very interesting design is the chiral propeller shape of figure 1(a). This design offers C4-symmetry and thus suppresses parasitic polarization conversion. The design has been realized to investigate the difference in circular dichroitic performance between two closely related designs shown in figure 2 the (a) chiral and (b) planar-chiral propeller shape. Comparing the experimentally obtained circular dichroism of both realized nanostructures visible in figure 2 (c) reveals that the chiral design of (a) is outperforming the planar-chiral design of (b).

Figure 2: SEM images of the (a) chiral and (b) planar-chiral propeller shape. (c) Experimental verification of circular dichroism and investigation of performance difference between design (a) and (b). The scale bars in (a) and (b) are 1 µm and 100 nm in the insets.(rights: IAP)

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