ABSTRACT
Gold nanocones acting as optical antennas offer an excellent geometry for focusing light near the cone tip, acting as nano-light sources with spot sizes on the order of the tip radius. However only the vertical plasmon mode oscillating in the axial direction can effectively excite the tip, whereas lateral modes oscillating along the cone base create mostly unwanted background in applications. The present work investigates the three-dimensional plasmonic mode structure of nanocones both experimentally and numerically. By tuning the nanocone aspect ratio, the modes can be spectrally tuned relative to each other, making them coincide for maximum excitation, or tuning the base mode away from the vertical mode for effective background suppression.
ABSTRACT
Coupling a single quantum emitter, such as a fluorescent molecule or a quantum dot (QD), to a plasmonic nanostructure is an important issue in nano-optics and nano-spectroscopy, relevant for a wide range of applications, including tip-enhanced near-field optical microscopy, plasmon enhanced molecular sensing and spectroscopy, and nanophotonic amplifiers or nanolasers, to mention only a few. While the field enhancement of a sharp nanoantenna increasing the excitation rate of a very closely positioned single molecule or QD has been well investigated, the detailed physical mechanisms involved in the emission of a photon from such a system are, by far, less investigated. In one of our ongoing research projects, we try to address these issues by constructing and spectroscopically analysing geometrically simple hybrid heterostructures consisting of sharp gold cones with single quantum dots attached to the very tip apex. An important goal of this work is to tune the longitudinal plasmon resonance by adjusting the cones' geometry to the emission maximum of the core-shell CdSe/ZnS QDs at nominally 650 nm. Luminescence spectra of the bare cones, pure QDs and hybrid systems were distinguished successfully. In the next steps we will further investigate, experimentally and theoretically, the optical properties of the coupled systems in more detail, such as the fluorescence spectra, blinking statistics, and the current results on the fluorescence lifetimes, and compare them with uncoupled QDs to obtain a clearer picture of the radiative and non-radiative processes.
ABSTRACT
Hybrid structures of few or single quantum dots (QDs) coupled to single optical antennas are of prime interest for nano-optical research. The photoluminescence (PL) signal from single nanoemitters, such as QDs, can be enhanced, and their emission characteristics modified, by coupling them to plasmonic nanostructures. Here, a self-aligned technique for placing nanoscale QDs with about 10 nm lateral accuracy and well-defined molecular distances to the tips of individual nanocones is reported. This way the QDs are positioned exactly in the high near-field region that can be created near the cone apex. The cones are excited in the focus of a radially polarized laser beam and the PL signal of few or single QDs on the cone tips is spectrally detected.
ABSTRACT
A fully parallel approach for the fabrication of arrays of metallic nanocones and triangular nanopyramids is presented. Different processes utilizing nanosphere lithography for the creation of etch masks are developed. Monolayers of spheres are reduced in size and directly used as masks, or mono- and double layers are employed as templates for the deposition of aluminum oxide masks. The masks are transferred into an underlying gold or silver layer by argon ion milling, which leads to nanocones or nanopyramids with very sharp tips. Near the tips the enhancement of an external electromagnetic field is particularly strong. This fact is confirmed by numerical simulations and by luminescence imaging in a confocal microscope. Such localized strong fields can amongst others be utilized for high-resolution, high-sensitivity spectroscopy and sensing of molecules near the tip. Arrays of such plasmonic nanostructures thus constitute controllable platforms for surface-enhanced Raman spectroscopy. A thin film of pentacene molecules is evaporated onto both nanocone and nanopyramid substrates, and the observed Raman enhancement is evaluated.
Subject(s)
Nanostructures/chemistry , Nanospheres/chemistry , Nanotechnology , Spectrum Analysis, RamanABSTRACT
Metallic nanocones are well-suited optical antennas for near-field microscopy and spectroscopy, exhibiting a number of different plasmonic modes. A major challenge in using nanocones for many applications is maximizing the signal at the tip while minimizing the background from the base. It is shown that nanocone plasmon resonance properties can be shifted over a wide range of wavelengths by variation of the substrate, material, size and shape, enabling potential control over specific modes and field distributions. The individual resonances are identified and studied by correlated single particle dark field scattering and scanning electron microscopy in combination with numerical simulations.
