Probing the mechanisms of large Purcell enhancement in plasmonic nanoantennas

To move nanophotonic devices such as lasers and single-photon sources into the practical realm, a challenging list of requirements must be met, including directional emission1–5, room-temperature and broadband operation6–9, high radiative quantum efficiency1,4 and a large spontaneous emission rate7. To achieve these features simultaneously, a platform is needed for which the various decay channels of embedded emitters can be fully understood and controlled. Here, we show that all these device requirements can be satisfied by a film-coupled metal nanocube system with emitters embedded in the dielectric gap region. Fluorescence lifetime measurements on ensembles of emitters reveal spontaneous emission rate enhancements exceeding 1,000 while maintaining high quantum efficiency (>0.5) and directional emission (84% collection efficiency). Using angle-resolved fluorescence measurements, we independently determine the orientations of emission dipoles in the nanoscale gap. Incorporating this information with the threedimensional spatial distribution of dipoles into full-wave simulations predicts time-resolved emission in excellent agreement with experiments. Typical luminescent emitters have relatively long emission lifetimes (∼10 ns) and non-directional emission. Unfortunately, these intrinsic optical properties are poorly matched to the requirements of nanophotonic devices. For example, in single-photon sources, fast radiative rates are required for operation at high frequencies, and directionality is needed to achieve a high collection efficiency10. In addition, with plasmonic lasers, enhanced spontaneous emission into the cavity mode can reduce the lasing threshold9. As a result, much work has focused on modifying the photonic environment of emitters to enhance11 the spontaneous emission rate, known as the Purcell effect12. Early approaches concentrated on integrating emitters into dielectric optical microcavities and showed modest emission rate enhancements13–15. However, dielectric cavities require high quality factors for large rate enhancements, which makes these cavities mismatched with the spectrally wide emission from inhomogeneously broadened or room-temperature emitters. Plasmonic nanostructures are a natural solution to the spectral mismatch problem because of their relatively broad optical resonances and high field enhancements16–18. Despite these advantages and the capability for emission rate enhancement19, many plasmonic structures suffer from unacceptably high non-radiative decay due to intrinsic losses in the metal, or have low directionality of emission7. In plasmonic structures, the Purcell factor (defined as the fractional increase in total emission rate) has contributions from an increased radiative rate and from an increased non-radiative rate due to metal losses. It is therefore critical to specify the fraction of energy emitted as radiation, known as the radiative quantum efficiency (QE). From knowledge of the Purcell factor and the QE, the enhancement in the radiative rate can be obtained. The largest field enhancements occur in nanoscale gaps between metals, but these are challenging to fabricate reliably, especially on a large scale. Plasmonic antenna designs such as bowtie antennas rely on gaps defined laterally using electron-beam lithography20 or ion milling21, making it difficult to produce the sub-10-nm gaps for which the highest Purcell factors occur. A promising geometry that overcomes these challenges is the plasmonic patch antenna, which consists of emitters situated in a vertical gap between a metal disk and a metal plane22–24. Because this enables a planar fabrication technique, the gaps in patch antennas can be controlled with nanometre25 and sub-nanometre26 precision. To date, however, micrometre-scale plasmonic patch antennas have shown only modest emission rate enhancements (∼80) and low radiative QE23,27. In this Letter we demonstrate a nanoscale patch antenna (NPA) that has large emission rate enhancement, high radiative efficiency, directionality of emission, and deep sub-wavelength dimensions. The NPA consists of a colloidally synthesized silver nanocube (side length of ∼80 nm) situated over a metal film, separated by a well-controlled nanoscale gap (5–15 nm) embedded with emitters (Fig. 1a,b). The fundamental plasmonic mode of the film-coupled nanocube is localized in the gap (Fig. 1c) with the dominant electric field oriented in the vertical (z) direction, transverse to the gap. The resonance wavelength is determined by the size of the optical resonator, defined by the side length of the nanocube and the thickness and refractive index of the gap material25. The resonance of such NPAs can be tuned from 500 nm to 900 nm by controlling these dimensions25,28. On resonance, the maximum field enhancements in the gap can reach 200 (ref. 29), resulting in up to 30,000-fold fluorescence intensity enhancement of molecules integrated into the gap29, as well as enhanced Raman scattering24. Through full-wave simulations, the radiation pattern of the antenna at the resonance wavelength is predicted to have a single lobe oriented in the surface-normal direction (Fig. 1d). The fraction of emitted light collected by the first lens1 is calculated to be 84% using an objective lens with a numerical aperture of NA = 0.9. The scattered radiation pattern of a single NPA was measured by imaging the back of the objective lens, and showed excellent agreement with simulations (Fig. 1d). Emission at angles greater than 64° falls outside the collection cone of the NA = 0.9 objective lens, explaining the small discrepancy between measurements and simulations. Although the NPA is less directional than multi-element plasmonic antennas such as the Yagi–Uda antenna2, the main radiation lobe of the NPA is normal to the surface, an important