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Germany: Optical Analysis New and Efficient Method of Fabrication for Optically Active Nano-Layers

Editor: MA Alexander Stark

In an article by Patrick Probst et al. published in Nature Materials, researchers led by Prof. Dr. Andreas Fery and Dr. Tobias A.F. König of the Leibniz Institute of Polymer Research Dresden describe a new design strategy for the fabrication of thin nanostructured films for active control of circularly polarized light. It is based on chiral nanoassembly by stacking two substrates with nanoparticle chains.

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Chiral, stacked nanoparticle lines efficiently filter light polarizations. They allow only left-circularly polarized light to pass (blue) and retain right-circularly polarized light (red).
Chiral, stacked nanoparticle lines efficiently filter light polarizations. They allow only left-circularly polarized light to pass (blue) and retain right-circularly polarized light (red).
(Source: Martin Mayer)

Dresden/Germany — The ripeness determination and quality control of many agricultural products, such as wine or honey, is carried out using optical analysis methods. One method for determining the degree of oechsle of wine, i.e. the sugar content, is based on the fact that sugar molecules rotate linearly polarized light in a concentration-dependent manner (optical rotation). The type of sugar determines the direction of rotation. All so-called chiral molecules, including amino acids and vitamins, exhibit this effect. Chiral molecules behave geometrically like left and right hands. They are mirror images of each other and cannot be transformed into each other by rotation and displacement. In addition to optical rotation, chiral molecules absorb left- and right-circularly polarized light differently (circular dichroism, CD), which is used, among other things, in CD spectroscopy to determine protein folding.

Spectrometers used in research and industry are usually very bulky due to the large number of rotatable components. Meta-surfaces, which allow arbitrary manipulation of the intensity, phase and polarization of light by controlled arrangement of metal nanostructures, can make an important contribution to the miniaturization of devices. In particular, actively tunable optical effects are stimulating intensive research in nanotechnology worldwide, as they can be used in the future to encode information and in nanolasers. They also open up novel applications for complex structured light and for optical computers.

Researchers from the IPF have developed a new design strategy for fabricating functional 3D arrays at the nanoscale that deliver optical effects billions of times stronger than chiral molecules without relying on complex and costly electron lithographic methods. It is based on nanochannels that can be fabricated and imprinted over large areas to arrange colloidal gold nanoparticles in parallel double lines.

Stacking two such cm²-sized substrates creates a chiral nanoarray of crossed chains with left-handed/right-handed sense of rotation that selectively interact with circularly polarized light. Metal nanostructures are known for their strong interaction with light, leading to strong selective absorption effects and concentrating strong optical fields below the diffraction limit. This occurs through light-induced excitation of electron oscillations known as localized surface plasmon resonance.

In the case of chiral plasmonic structures, a preferential absorption and scattering of left- or right-circularly polarized light (circular dichroism) results. The presented system can be repeatedly crossed at different angles and mechanically compressed to actively control all details of circular dichroism (strength, sign and spectral position). This enables an application as a tunable circular polarizer whose active layer is only a few hundred nanometers thick. Via local mechanical modulation, interesting possibilities open up for the generation of complex spectral gradients to be used as a filter array for chip-based spectrometers. Due to the dense arrangement of the chiral elements and their strong plasmonic coupling, a circular dichroism of 11° was achieved, which exceeds that of other colloidal systems by two orders of magnitude. In contrast to lithographically generated metasurfaces, the stackable design allows the use of the superchiral fields, which are particularly enhanced in the interlayer, for ultrasensitive detection of chiral molecules. For example, a proof-of-concept experiment demonstrated a 10-fold increase in sensitivity in the detection of a model protein. The research team points to a new way to generate mechanistically switchable 3D nanoassemblies. The mechanistic understanding gained can be extended to other systems and is the cornerstone for further exciting new developments that are not limited to photonic applications such as polarization elements or sensors.

References: Probst, P. T.; Mayer, M.; Gupta, V.; Steiner, A. M.; Zhou, Z.; Auernhammer, G. K.; König, T. A. F.; Fery, A., Mechano-tunable chiral metasurfaces via colloidal assembly. Nature Materials 2021, doi: 10.1038/s41563-021-00991-8.