Search

Plasmonic nanostructures can focus light far below the diffraction limit, and the nearly thousandfold field enhancements obtained routinely enable few- and single-molecule detection. However, for processes happening on the molecular scale to be tracked with any relevant time resolution, the emission strengths need to be well beyond what current plasmonic devices provide. Here, we develop hybrid nanostructures incorporating both refractive and plasmonic optics, by creating SiO 2 nanospheres fused to plasmonic nanojunctions. Drastic improvements in Raman efficiencies are consistently achieved, with (single-wavelength) emissions reaching 10 7 counts⋅mW −1 ⋅s −1 and 5 × 10 5 counts∙mW −1 ∙s −1 ∙molecule −1 , for enhancement factors >10 11 . We demonstrate that such high efficiencies indeed enable tracking of single gold atoms and molecules with 17-µs time resolution, more than a thousandfold improvement over conventional high-performance plasmonic devices. Moreover, the obtained (integrated) megahertz count rates rival (even exceed) those of luminescent sources such as single-dye molecules and quantum dots, without bleaching or blinking.

Significance Imaging of single to a few molecules has received much recent interest. While superresolution microscopies access subdiffraction resolution, they do not work for plasmonic hot spots due to the loss of positional information that results from plasmonic coupling. Here, we show how to reconstruct the spatial locations of molecules within a plasmonic hot spot with 1-nm precision. We use a plasmonic nanoball lens to demonstrate that plasmonic nanocavities can be used simultaneously as a nanoscopic and spectroscopic tool. This work opens up possibilities for studying the behavior of a few to single molecules in plasmonic nanoresonators, while simultaneously tracking their movements and spectral features. Our plasmonic nanolens is useful for nanosensing, nanochemistry, and biofunctional imaging., Plasmonics now delivers sensors capable of detecting single molecules. The emission enhancements and nanometer-scale optical confinement achieved by these metallic nanostructures vastly increase spectroscopic sensitivity, enabling real-time tracking. However, the interaction of light with such nanostructures typically loses all information about the spatial location of molecules within a plasmonic hot spot. Here, we show that ultrathin plasmonic nanogaps support complete mode sets which strongly influence the far-field emission patterns of embedded emitters and allow the reconstruction of dipole positions with 1-nm precision. Emitters in different locations radiate spots, rings, and askew halo images, arising from interference of 2 radiating antenna modes differently coupling light out of the nanogap, highlighting the imaging potential of these plasmonic “crystal balls.” Emitters at the center are now found to live indefinitely, because they radiate so rapidly.

© 2018 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Optical metamaterials offer the tantalizing possibility of creating extraordinary optical properties through the careful design and arrangement of subwavelength structural units. Gyroid-structured optical metamaterials possess a chiral, cubic, and triply periodic bulk morphology that exhibits a redshifted effective plasma frequency. They also exhibit a strong linear dichroism, the origin of which is not yet understood. Here, the interaction of light with gold gyroid optical metamaterials is studied and a strong correlation between the surface morphology and its linear dichroism is found. The termination of the gyroid surface breaks the cubic symmetry of the bulk lattice and gives rise to the observed wavelength- and polarization-dependent reflection. The results show that light couples into both localized and propagating plasmon modes associated with anisotropic surface protrusions and the gaps between such protrusions. The localized surface modes give rise to the anisotropic optical response, creating the linear dichroism. Simulated reflection spectra are highly sensitive to minute details of these surface terminations, down to the nanometer level, and can be understood with analogy to the optical properties of a 2D anisotropic metasurface atop a 3D isotropic metamaterial. This pronounced sensitivity to the subwavelength surface morphology has significant consequences for both the design and application of optical metamaterials.

Fabricating nanocavities in which optically-active single quantum emitters are precisely positioned, is crucial for building nanophotonic devices. Here we show that self-assembly based on robust DNA-origami constructs can precisely position single molecules laterally within sub-5nm gaps between plasmonic substrates that support intense optical confinement. By placing single-molecules at the center of a nanocavity, we show modification of the plasmon cavity resonance before and after bleaching the chromophore, and obtain enhancements of $\geq4\times10^3$ with high quantum yield ($\geq50$%). By varying the lateral position of the molecule in the gap, we directly map the spatial profile of the local density of optical states with a resolution of $\pm1.5$ nm. Our approach introduces a straightforward non-invasive way to measure and quantify confined optical modes on the nanoscale.