Abstract
Plasmonic nanopatch antennas that incorporate dielectric gaps hundreds of picometers to several nanometers thick have drawn increasing attention over the past decade because they confine electromagnetic fields to grossly sub-diffraction-limited volumes. Substantial control over the optical properties of excitons and color centers confined within these plasmonic cavities has already been demonstrated with far-field optical spectroscopies, but near-field optical spectroscopies are essential for an improved understanding of the plasmon–emitter interaction at the nanoscale. Here, we characterize the intensity and phase-resolved plasmonic response of isolated nanopatch antennas by cathodoluminescence microscopy. Furthermore, we explore the distinction between optical and electron beam spectroscopies of coupled plasmon–exciton heterostructures to identify constraints and opportunities for future nanoscale characterization and control of hybrid nanophotonic structures. While we observe substantial Purcell enhancement in time-resolved photoluminescence spectroscopies, negligible Purcell enhancement is observed in cathodoluminescence spectroscopies of hybrid nanophotonic structures. The substantial differences in measured Purcell enhancement for electron beam and laser excitation can be understood as a result of the different selection rules for these complementary experiments. These results provide a fundamentally new understanding of near-field plasmon–exciton interactions in nanopatch antennas, which is essential for myriad emerging quantum photonic devices.
Highlights
Super-resolution optical microscopies such as stimulated emission depletion (STED) microscopy, photoactivated localization microscopy (PALM), stochastic optical reconstruction microscopy (STORM), optical superlens imaging, and scanning near-field optical microscopy (SNOM) can be used to probe optical properties of matter with a spatial resolution of 10 to several tens of nanometers,32–35 though each approach comes with some challenges
The time-resolved photoluminescence (TRPL) measurements suggest that the heterostructure is well designed with measured Purcell enhancement consistent with the literature, and the samples were transferred in vacuum from the TRPL measurement to the CL measurement, so minimal environmental degradation is expected between measurements
While it seems clear that CL characterization of hybrid plasmon–exciton heterostructures may not yield the same Purcell enhancement measured in PL spectroscopies, CL microscopy is an indispensable tool for mapping localized and propagating plasmonic modes of technologically relevant nanopatch antenna structures with few nanometer spatial resolution
Summary
Scitation.org/journal/app confined to gaps of order 1 nm has enabled substantially increased flexibility in the control of the optical properties of excitons and color centers confined within the gap. Plasmonic nanogap heterostructures have been used to enhance the quantum efficiency of defect and exciton emission in the weak coupling regime, to modify optical properties in the strong coupling regime, and for applications ranging from gas sensing, water splitting, and carbon capture to the non-invasive study of nanoparticle dynamics in liquids. Electrical tuning of the resonance frequency of nanogap antennas has been demonstrated as part of the move toward more functional nanophotonic devices.. An ideal approach to super-resolution microscopy might offer true nanometer spatial resolution, access to spectrally resolved excitation and relaxation dynamics, and high-speed operation with high collection efficiency, but no such platform currently exists Techniques such as electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) microscopy are well suited to study the near-field optical properties of plasmonic heterostructures and hybrid plasmon–emitter heterostructures.. While the NMF decomposition identifies a high quality-factor corner plasmon resonance, the edge mode exhibits some additional spectral complexity and the gap plasmon mode exhibits a clear bimodal response This may be explained in part by the fact that the NMF decomposition only assumes that each component is nonnegative, but it is suggestive of mode coupling, which is not directly evident in the intensity plots. While simulations allow us to understand the behavior of model plasmonic heterostructures, CL microscopy allows us to probe experimentally achievable, imperfect heterostructures
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