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

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.V. K. V. acknowledges support from the Royal Society through the University Research Fellowships. We acknowledge Royal Society grants CHG\R1\170067, PEF1\170015 and RGF\EA\180228, as well as STFC grant ST/R005842/1 and EPSRC grant EP/L015544/1. C. W. acknowledges financial support from Cancer Research UK (CRUK) Pioneer Award (C55962/A24669) and Wolfson College, Cambridge, UK. C. W. further acknowledges research support from S. Bohndiek, T. Wilkinson and G. Gordon. X. Z. and G. A. E. V. are grateful for the financial support from the FWO (G090017N) and KU Leuven internal research funds (C24/15/015)

Localized Nanoresonator Mode in Plasmonic Microcavities.

Submicron-thick hexagonal boron nitride crystals embedded in noble metals form planar Fabry-Perot half-microcavities. Depositing Au nanoparticles on top of these microcavities forms previously unidentified angle- and polarization-sensitive nanoresonator modes that are tightly laterally confined by the nanoparticle. Comparing dark-field scattering with reflection spectroscopies shows plasmonic and Fabry-Perot-like enhancements magnify subtle interference contributions, which lead to unexpected redshifts in the dark-field spectra, explained by the presence of these new modes

The origin of second harmonic generation hotspots in chiral optical metamaterials [Invited]

Novel ways to detect the handedness in chiral optical metamaterials by means of the second harmonic generation (SHG) process have recently been proposed. However, the precise origin of the SHG emission has yet to be unambiguously established. In this paper, we present computational simulations of both the electric currents and the electromagnetic fields in chiral planar metamaterials, at the fundamental frequency (FF), and discuss the implications of our results on the characteristics of experimentally measured SHG. In particular, we show that the results of our numerical simulations are in good agreement with the experimental mapping of SHG sources. Thus, the SHG in these metamaterials can be attributed to a strong local enhancement of the electromagnetic fields at the FF, which depends on the particular structure of the patterned metamaterial

Nonlinear superchiral meta-surfaces: tuning chirality and disentangling non-reciprocity at the nanoscale.

Circularly polarized light is incident on a nanostructured chiral meta-surface. In the nanostructured unit cells whose chirality matches that of light, superchiral light is forming and strong optical second harmonic generation can be observed

Low-cost multi-layer antenna sub-array for 79 GHz short-range radar applications

© The Institution of Engineering and Technology 2018. A new wideband cavity-backed aperture coupled microstrip antenna is presented for 79 GHz short-range multi-input multi-output radar applications. This design is based on a sub-array consisting of two single elements, and each element is founded on a cavity backed aperture coupled patch antenna. A microstrip to strip-line transition and a series feeding topology is used to feed the elements. The antennas are manufactured by using a new high-resolution multi-layer PCB technology. This technology is able to deliver high-resolution stacked micro-vias on very thin substrates. The performance of the designed antenna has been verified by both simulations and measurements. The bandwidth is 11.2%, and the maximum gain and efficiency are 6.05 dBi and 80% along 77–81 GHz.status: publishe

A Boundary Integral Equation Scheme for Simulating the Nonlocal Hydrodynamic Response of Metallic Antennas at Deep-Nanometer Scales

© 1963-2012 IEEE. Modeling the interaction between light and a plasmonic nanoantenna, whose critical dimension is of a few nanometers, is complex owing to the 'hydrodynamic' motion of free electrons in a metal. Such a hydrodynamic effect inevitably leads to a nonlocal material response, which enables the propagation of longitudinal electromagnetic waves in the material. In this paper, within the framework of a boundary integral equation and a method of moments algorithm, a computational scheme is developed for predicting the interaction of light with 3-D nonlocal hydrodynamic metallic nanoparticles of arbitrary shape. The numerical implementation is first demonstrated for the test example of a canonical spherical structure. The calculated results are shown to be in the excellent agreement with the theoretical results obtained with the generalized Mie theory. In addition, the capability of treating 3-D structures of general shapes is demonstrated by ellipsoids and dimers

Air traffic monitoring using optimized ADS-B CubeSat constellation

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How Ultranarrow Gap Symmetries Control Plasmonic Nanocavity Modes: From Cubes to Spheres in the Nanoparticle-on-Mirror

Plasmonic nanocavities with sub-5-nm gaps between nanoparticles support multiple resonances possessing ultra-high-field confinement and enhancements. Here we systematically compare the two fundamentally different resonant gap modes: transverse waveguide (s) and antenna modes (l), which, despite both tightly confining light within the gap, have completely different near-field and far-field radiation patterns. By varying the gap size, both experimentally and theoretically, we show how changing the nanoparticle shape from sphere to cube alters coupling of s and l modes, resulting in strongly hybridized (j) modes. Through rigorous group representation analysis we identify their composition and coupling. This systematic analysis of the Purcell factors shows that modes with optical field perpendicular to the gap are best to probe the optical properties of cavity-bound emitters, such as single molecules.We acknowledge financial support from EPSRC Grants EP/G060649/1, EP/K028510/1, and EP/L027151/1 and ERC Grant LINASS 320503. R.C. acknowledges support from the Dr. Manmohan Singh scholarship from St. John’s College. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. G.A.E.V., V.V.M., and X.Z. acknowledge the C2 project (C24/15/015) and the PDMK/14/126 project of KU Leuven, the FWO Long-Term Stay Abroad Project Grant V405115N, and the Methusalem Project funded by the Flemish government

How Ultranarrow Gap Symmetries Control Plasmonic Nanocavity Modes: From Cubes to Spheres in the Nanoparticle-on-Mirror

Plasmonic nanocavities with sub-5-nm gaps between nanoparticles support multiple resonances possessing ultra-high-field confinement and enhancements. Here we systematically compare the two fundamentally different resonant gap modes: transverse waveguide (s) and antenna modes (l), which, despite both tightly confining light within the gap, have completely different near-field and far-field radiation patterns. By varying the gap size, both experimentally and theoretically, we show how changing the nanoparticle shape from sphere to cube alters coupling of s and l modes, resulting in strongly hybridized (j) modes. Through rigorous group representation analysis we identify their composition and coupling. This systematic analysis of the Purcell factors shows that modes with optical field perpendicular to the gap are best to probe the optical properties of cavity-bound emitters, such as single molecules.We acknowledge financial support from EPSRC Grants EP/G060649/1, EP/K028510/1, and EP/L027151/1 and ERC Grant LINASS 320503. R.C. acknowledges support from the Dr. Manmohan Singh scholarship from St. John’s College. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. G.A.E.V., V.V.M., and X.Z. acknowledge the C2 project (C24/15/015) and the PDMK/14/126 project of KU Leuven, the FWO Long-Term Stay Abroad Project Grant V405115N, and the Methusalem Project funded by the Flemish government

The origin of second harmonic generation hotspots in chiral optical metamaterials

Novel ways to detect the handedness in chiral optical metamaterials by means of the second harmonic generation (SHG) process have recently been proposed. However, the precise origin of the SHG emission has yet to be unambiguously established. In this paper, we present computational simulations of both the electric currents and the electromagnetic fields in chiral planar metamaterials, at the fundamental frequency (FF), and discuss the implications of our results on the characteristics of experimentally measured SHG. In particular, we show that the results of our numerical simulations are in good agreement with the experimental mapping of SHG sources. Thus, the SHG in these metamaterials can be attributed to a strong local enhancement of the electromagnetic fields at the FF, which depends on the particular structure of the patterned metamaterial