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      Measuring optical activity in the far-field from a racemic nanomaterial: diffraction spectroscopy from plasmonic nanogratings

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          Photograph of the experimental setup with light diffracting from a racemic nanoarray. The diffracted spectra change depending on the direction of circularly polarized illumination.


          Recent progress in nanofabrication has redrawn the boundaries of the applicability of chiroptical (chiral optical) effects. Chirality, often expressed as a twist in biomolecules, is crucial for pharmaceuticals, where it can result in extremely different chemical properties. Because chiroptical effects are typically very weak in molecules, plasmonic nanomaterials are often proposed as a promising platform to significantly enhance these effects. Unfortunately, the ideal plasmonic nanomaterial has conflicting requirements: its chirality should enhance that of the chiral molecules and yet it should have no chiroptical response on its own. Here, we propose a unique reconciliation to satisfy the requirements: a racemic plasmonic nanomaterial, consisting of equal amounts of opposite chiral unit cells. We show how diffraction spectroscopy can be used to unveil the presence of chirality in such racemic nanogratings in the far-field. Our experiments are supported by numerical simulations and yield a circular intensity difference of up to 15%. The physical origin is demonstrated by full wave simulations in combination with a Green's function – group-theory-based analysis. Contributions from Circular Dichroism in the Angular Distribution of Photoelectrons (CDAD) and pseudo/extrinsic chirality are ruled out. Our findings enable the far-field measurement and tuning of racemic nanomaterials, which is crucial for hyper-sensitive chiral molecular characterization.

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          Most cited references 44

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          Optical Constants of the Noble Metals

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            Giant optical activity in quasi-two-dimensional planar nanostructures.

            We examine the spectral dependence in the visible frequency range of the polarization rotation of two-dimensional gratings consisting of chiral gold nanostructures with subwavelength features. The gratings, which do not diffract, are shown to exhibit giant specific rotation (approximately 10(4) degrees/mm) of polarization in direct transmission at normal incidence. The rotation is the same for light incident on the front and back sides of the sample. Such reciprocity indicates three dimensionality of the structure arising from the asymmetry of light-plasmon coupling at the air-metal and substrate-metal interfaces. The structures thus enable polarization control with quasi-two-dimensional planar objects. However, in contradiction with recently suggested interpretation of experiments on larger scale but otherwise similar structures, the observed polarization phenomena violate neither reciprocity nor time-reversal symmetry.
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              Metamaterials: optical activity without chirality.

              We report that the classical phenomenon of optical activity, which is traditionally associated with chirality (helicity) of organic molecules, proteins, and inorganic structures, can be observed in artificial planar media which exhibit neither 3D nor 2D chirality. We observe the effect in the microwave and optical parts of the spectrum at oblique incidence to regular arrays of nonchiral subwavelength metamolecules in the form of strong circular dichroism and birefringence indistinguishable from those of chiral three-dimensional media.

                Author and article information

                Nanoscale Horizons
                Nanoscale Horiz.
                Royal Society of Chemistry (RSC)
                August 19 2019
                : 4
                : 5
                : 1056-1062
                [1 ]Centre for Photonics and Photonic Materials and Centre for Nanoscience and Nanotechnology, University of Bath
                [2 ]Bath
                [3 ]UK
                [4 ]Department of Electrical Engineering (ESAT-TELEMIC), KU Leuven
                [5 ]Heverlee
                [6 ]Belgium
                [7 ]Electrical Engineering Division, Department of Engineering, University of Cambridge
                [8 ]Cambridge
                [9 ]Centre for Nanoscience and Nanotechnology, University of Bath
                © 2019
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