4.6 Article

Dual Nanoresonators for Ultrasensitive Chiral Detection

Journal

ACS PHOTONICS
Volume 8, Issue 6, Pages 1754-1762

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acsphotonics.1c00311

Keywords

chiral sensing; optical chirality; circular dichroism; duality; gap antennas; hybrid metal-dielectric metasurfaces

Funding

  1. Netherlands Organisation for Scientific Research (NWO) [740.018.009]
  2. Gravitation Grant Research Center for Integrated Nanophotonics [024.002.033]
  3. European Research Council (ERC) under the European Union [948804]
  4. Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) [EXC 2089/1390776260]
  5. Bavarian Solar Energies Go Hybrid (SolTech)Programme
  6. General Secretariat for Research and Technology (GSRT)
  7. Hellenic Foundation for Research and Innovation (HFRI) [1819]
  8. European Research Council (ERC) [948804] Funding Source: European Research Council (ERC)

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This study explores the enhancement of chiral sensing using nanophotonics, achieving superchiral fields beyond the limits of single particles through hybrid metal-dielectric nanostructures. By adjusting the intensity and phase of electric and magnetic fields, and ensuring spatial overlap, optimal optical chirality can be achieved.
The discrimination of enantiomers is crucial in biochemistry. However, chiral sensing faces significant limitations due to inherently weak chiroptical signals. Nanophotonics is a promising solution to enhance sensitivity thanks to increased optical chirality maximized by strong electric and magnetic fields. Metallic and dielectric nanoparticles can separately provide electric and magnetic resonances. Here we propose their synergistic combination in hybrid metal-dielectric nanostructures to exploit their dual character for superchiral fields beyond the limits of single particles. For optimal optical chirality, in addition to maximization of the resonance strength, the resonances must spectrally coincide. Simultaneously, their electric and magnetic fields must be parallel and pi/2 out of phase and spatially overlap. We demonstrate that the interplay between the strength of the resonances and these optimal conditions constrains the attainable optical chirality in resonant systems. Starting from a simple symmetric nanodimer, we derive closed-form expressions elucidating its fundamental limits of optical chirality. Building on the trade-offs of different classes of dimers, we then suggest an asymmetric dual dimer based on realistic materials. These dual nanoresonators provide strong and decoupled electric and magnetic resonances together with optimal conditions for chiral fields. Finally, we introduce more complex dual building blocks for a metasurface with a record 300-fold enhancement of local optical chirality in nanoscale gaps, enabling circular dichroism enhancement by a factor of 20. By combining analytical insight and practical designs, our results put forward hybrid resonators to increase chiral sensitivity, particularly for small molecular quantities.

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