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 emission 1-5 , room-temperature and broadband operation 6-9 , high radiative quantum efficiency 1,4 and a large spontaneous emission rate 7 . 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 efficiency 10 . In addition, with plasmonic lasers, enhanced spontaneous emission into the cavity mode can reduce the lasing threshold 9 . As a result, much work has focused on modifying the photonic environment of emitters to enhance 11 the spontaneous emission rate, known as the Purcell effect 12 . Early approaches concentrated on integrating emitters into dielectric optical microcavities and showed modest emission rate enhancements [13][14][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 enhancements [16][17][18] . Despite these advantages and the capability for emission rate enhancement 19 , many plasmonic structures suffer from unacceptably high non-radiative decay due to intrinsic losses in the metal, or have low directionality of emission 7 . 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 ...

Intersubband transitions in n-doped multi-quantum-well semiconductor heterostructures make it possible to engineer one of the largest known nonlinear optical responses in condensed matter systems--but this nonlinear response is limited to light with electric field polarized normal to the semiconductor layers. In a different context, plasmonic metasurfaces (thin conductor-dielectric composite materials) have been proposed as a way of strongly enhancing light-matter interaction and realizing ultrathin planarized devices with exotic wave properties. Here we propose and experimentally realize metasurfaces with a record-high nonlinear response based on the coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered electronic intersubband transitions in semiconductor heterostructures. We show that it is possible to engineer almost any element of the nonlinear susceptibility tensor of these structures, and we experimentally verify this concept by realizing a 400-nm-thick metasurface with nonlinear susceptibility of greater than 5 × 10(4) picometres per volt for second harmonic generation at a wavelength of about 8 micrometres under normal incidence. This susceptibility is many orders of magnitude larger than any second-order nonlinear response in optical metasurfaces measured so far. The proposed structures can act as ultrathin highly nonlinear optical elements that enable efficient frequency mixing with relaxed phase-matching conditions, ideal for realizing broadband frequency up- and down-conversions, phase conjugation and all-optical control and tunability over a surface.

Typical emitters such as molecules, quantum dots and semiconductor quantum wells have slow spontaneous emission with lifetimes of 1–10 ns, creating a mismatch with high-speed nanoscale optoelectronic devices such as light-emitting diodes, single-photon sources and lasers. Here we experimentally demonstrate an ultrafast (<11 ps) yet efficient source of spontaneous emission, corresponding to an emission rate exceeding 90 GHz, using a hybrid structure of single plasmonic nanopatch antennas coupled to colloidal quantum dots. The antennas consist of silver nanocubes coupled to a gold film separated by a thin polymer spacer layer and colloidal core–shell quantum dots, a stable and technologically relevant emitter. We show an increase in the spontaneous emission rate of a factor of 880 and simultaneously a 2,300-fold enhancement in the total fluorescence intensity, which indicates a high radiative quantum efficiency of ∼50%. The nanopatch antenna geometry can be tuned from the visible to the near infrared, providing a promising approach for nanophotonics based on ultrafast spontaneous emission.