The spectroscopic analysis of large biomolecules is important in applications such as biomedical diagnostics and pathogen detection 1,2 , and spectroscopic techniques can detect such molecules at the nanogram level or lower. However, spectroscopic techniques have not been able to probe the structure of large biomolecules with similar levels of sensitivity. Here we show that superchiral electromagnetic fields 3 , generated by the optical excitation of plasmonic planar chiral metamaterials 4,5 , are highly sensitive probes of chiral supramolecular structure. The differences in the effective refractive indices of chiral samples exposed to left-and right-handed superchiral fields are found to be up to 10 6 times greater than that those observed in optical polarimetry measurements, thus allowing picogram quantities of adsorbed molecules to be characterised. The largest differences are observed for biomolecules that possess chiral planar 2 sheets, such as proteins with high β-sheet content, which suggest that this approach could form the basis for assaying technologies capable of detecting amyloid diseases and certain types of viruses.Since the building blocks of life are chiral molecular units such as amino acids and sugars, biomacromolecules formed from these units also exhibit chirality on molecular and supramolecular scales. Chirally sensitive (chiroptical) spectroscopic techniques, such as circular dichroism (CD), optical rotatory dispersion (ORD) and Raman optical activity (ROA), are therefore especially incisive probes of the threedimensional aspects of biomacromolecular structure and are widely used in biomolecular science 1,2 . Chiroptical methods typically measure small differences, or dissymmetries, in the interaction of left-and right-circularly polarised light, the chiral probe, with a chiral material 2 . However, the inherent weakness of these existing chiroptical phenomena usually restricts their application to samples of microgram level. Recently, Tang and Cohen 3 postulated that under certain circumstances superchiral electromagnetic fields could be produced that display greater chiral asymmetry than circularly polarised plane light waves. We have realised such superchiral electromagnetic fields are generated in the near fields of PCMs, and can greatly enhanced the sensitivity of a chiroptical measurement, enabling us to detect and characterise just a few picograms of a chiral material.PCMs were first fabricated, and shown to display large chiroptical effects such as optical rotation, by Schwanecke and co-workers 4 and Gonokami and co-workers 5 .The PCMs used in this study, Fig. 1 (a), are composed of left or right handed (LH / RH) Au gammadions, of length 400 nm and thickness 100 nm (plus a 5 nm Cr adhesion layer) deposited on a glass substrate and arranged in a square lattice with a periodicity of 800 nm. As a control we repeated all experiments using a metamaterial composed of achiral crosses with the same thickness and periodicity as the gammadions: these structures showed no dissymmetry in excita...

We report a new approach for creating chiral plasmonic nanomaterials. A previously unconsidered, far-field mechanism is utilized which enables chirality to be conveyed from a surrounding chiral molecular material to a plasmonic resonance of an achiral metallic nanostructure. Our observations break a currently held preconception that optical properties of plasmonic particles can most effectively be manipulated by molecular materials through near-field effects. We show that far-field electromagnetic coupling between a localized plasmon of a nonchiral nanostructure and a surrounding chiral molecular layer can induce plasmonic chirality much more effectively (by a factor of 10(3)) than previously reported near-field phenomena. We gain insight into the mechanism by comparing our experimental results to a simple electromagnetic model which incorporates a plasmonic object coupled with a chiral molecular medium. Our work offers a new direction for the creation of hybrid molecular plasmonic nanomaterials that display significant chiroptical properties in the visible spectral region.

Optical spectroscopic methods do not routinely provide information on higher order hierarchical structure (tertiary/quaternary) of biological macromolecules and assemblies. This necessitates the use of time-consuming and material intensive techniques, such as protein crystallography, NMR, and electron microscopy. Here we demonstrate a spectroscopic phenomenon, superchiral polarimetry, which can rapidly characterize ligand-induced changes in protein higher order (tertiary/quaternary) structure at the picogram level, which is undetectable using conventional CD spectroscopy. This is achieved by utilizing the enhanced sensitivity of superchiral evanescent fields to mesoscale chiral structure.

Using a modal matching theory, we demonstrate the generation of short-range, chiral electromagnetic fields via the excitation of arrays of staggered nanoslits that are chiral in two dimensions. The electromagnetic near fields, which exhibit a chiral density greater than that of circularly polarized light, can enhance the chiroptical interactions in the vicinity of the nanoslits. We discuss the features of nanostructure symmetry required to obtain the chiral fields and explicitly show how these structures can give rise to detection and characterization of materials with chiral symmetry.

effectively a disposable consumable, with properties which can be easily tuned in the production process.The TPSs we generated can be considered to be hybrid structures, referred to as a solid-inverse structure, consisting of a solid nanostructure and an identical shaped void (inverse structure) directly above it. In line with Babinet's principle, the roles of electric and magnetic fi elds are switched between solid and inverse structures. The implications of this are that symmetry equivalent electric and magnetic modes of the solid and inverse structures are spatially located directly above each other, and can consequently couple in an analogous manner to hybridization of orbitals in molecular systems. [ 23 ] We show with our chiral hybrid metafi lms, that by controlling the spatial overlap between the solid and inverse structure, using fi lm thickness, the coupling between electric and magnetic modes can be controlled enabling the chiral/optical properties to be manipulated with relative ease. This is a far more versatile approach to manipulating coupling in hybrid metamaterials than the current paradigm of altering the geometric design. [ 24 ] Our work demonstrates that fi lm thickness is an important parameter in the metamaterial design tool kit. To illustrate the potential of the tunable "disposable" TPS, we present an exemplar case where a chiral substrate, consisting of a periodic array of "shuriken" indentations which are either left (LH) or right handed (RH), is used for picogram characterization of protein structure with "plasmonic polarimetry." [ 5 ] The combination of the low cost injection-molded templates and the tunability of the fi lms they can be used to produce, make the present study a signifi cant step in the technological application of metamaterials.The shuriken TPSs were fabricated using a new approach for templating Au fi lms on nanostructured polycarbonate substrates. Injection molding enables high-throughput manufacturing of sub-micrometer resolution nanosurfaces with high levels of reproducibility and quality. [ 21,25 ] In this work, we fabricate injection-molded polycarbonate templates, Figure 1 A, that consists of chiral shuriken shaped indentations, of either left or right handedness, arranged in a square lattice. A detailed description of the injection-molding process can be found elsewhere. [ 16,19,22,26 ] The depth of the indentation is 80 nm while the distance from the end of one arm to that of the end of the arm opposite is 500 nm. The periodicity of the array is 700 nm. Due to the nature of injection molding, the edges of the structure are not perfectly sharp and the inner walls of the structures are sloped by approximately 30° (see Supporting Information).We deposited fi lms of Au with thickness ranging from 20 to 100 nm on to the nanostructured polycarbonate template to produce the TPS samples, Figure 1 B,C. The continuous nature of the Au fi lms is evidenced by an absence of charging in scanning electron microscope (SEM) images of the substrates (see Artifi cially engineere...