Single-photon emitters based on individual atoms or individual atomic-like defects are highly sought-after components for future quantum technologies. A key challenge in this field is how to isolate just one such emitter; the best approaches still have an active emitter yield of only 50% so that deterministic integration of single active emitters is not yet possible. Here, we demonstrate the ability to isolate individual erbium emitters embedded in 20 nm nanocrystals of NaYF4 using plasmonic aperture optical tweezers. The optical tweezers capture the nanocrystal, whereas the plasmonic aperture enhances the emission of the Er and allows the measurement of discrete emission rate values corresponding to different numbers of erbium ions. Three separate synthesis runs show near-Poissonian distribution in the discrete levels of emission yield that correspond to the expected ion concentrations, indicating that the yield of active emitters is approximately 80%. Fortunately, the trap allows for selecting the nanocrystals with only a single emitter, and so this gives a route to isolating and integrating single emitters in a deterministic way. This demonstration is a promising step toward single-photon quantum information technologies that utilize single ions in a solid-state medium, particularly because Er emits in the low-loss fiber-optic 1550 nm telecom band.

Plasmonics has been used to enhance light− matter interaction at the extreme subwavelength scale. Intriguingly, it is possible to achieve multiple plasmonic resonances from a single nanostructure, and these can be used in combination to provide cascaded enhanced interactions. Here, we demonstrate three distinct plasmon resonances for enhanced upconversion emission from a single upconverting nanocrystal trapped in a metal nanoaperture optical tweezer. For apertures where the plasmonic resonances occur at the emission wavelengths only, a moderate enhancement of a factor of 4 is seen. However, by tuning the aperture to enhance the excitation laser as well, an additional factor of 100 enhancement in the emission is achieved. Since lanthanide-doped nanocrystals are stable emitters, this approach of using multiple subwavelength resonances can improve applications including photovoltaics, photocatalysis, and imaging. The nanocrystals can also contain only single ions, allowing for studying quantum emitter properties and applications to single-photon sources.

We demonstrate dual-wavelength (1210 and 1520 nm) excitation of upconverter nanoparticles (Er-doped nanoparticles) with plasmon-enhanced emission. Gold nanorods of 25 nm diameter with resonances at 808 nm and at 980 nm selectively enhance the upconversion emission of 2% erbium-doped NaYF4 nanoparticles at 808 nm and at 980 nm. No upconversion is seen for 1210 nm excitation alone, and 1520 nm excitation alone provides lower upconversion. The sequential 1520 and 1210 nm absorption yields the most upconversion, and the power dependence of emission supports the sequential absorption mechanism. This provides a promising avenue for harvesting from the two strongest infrared bands of the solar spectrum with selective emission tuned to either the Si or GaAs band gap.

Upconverted light from nanostructured metal surfaces can be produced by harmonic generation and multi-photon luminescence; however, these are very weak processes and require extremely high field intensities to produce a measurable signal. Here we report on bright emission, 5 orders of magnitude greater than harmonic generation, that can be seen from metal tunnel junctions that we believe is due to light-induced inelastic tunneling emission. Like inelastic tunneling light emission, which was recently reported to have 2% conversion efficiency per tunneling event, the emission wavelength recorded varies with the local electric field applied; however, here the field is from a 1560 nm femtosecond pulsed laser source. Finite-difference time-domain simulations of the experimental conditions show the local field is sufficient to generate tunneling-based inelastic light emission in the visible regime. This phenomenon is promising for producing ultrafast upconverted light emission with higher efficiency than conventional nonlinear processes.

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