Abstract
Nitrogen–vacancy (NV) centers in diamond are generally recognized as highly promising as indefinitely stable highly efficient single-photon sources. We report an experimental quantification of the brightness, radiative decay rate, nonradiative decay rate and quantum efficiency of single NV centers in diamond nanocrystals. Our experiments show that the commonly observed large spread in fluorescence decay rates of NV centers in nanodiamond is inconsistent with the common explanation of large nanophotonic mode-density variations in the ultra-small high-index crystals at near-unity quantum efficiency. We report that NV centers in 25 nm nanocrystals are essentially insensitive to local density of optical states (LDOS) variations that we induce at a dielectric interface by using liquids to vary the refractive index, and propose that quantum efficiencies in such nanocrystals are widely distributed between 0 and 20%. For single NV centers in larger 100 nm nanocrystals, we show that decay rate changes can be reversibly induced by nanomechanically approaching a mirror to change the LDOS. Using this scanning mirror method, for the first time we report calibrated quantum efficiencies of NV centers, and show that different but nominally identical nanocrystals have widely distributed quantum efficiencies between 10 and 90%. Our measurements imply that nanocrystals that are to be assembled into hybrid photonic structures for cavity QED should first be individually screened to assess fluorescence properties in detail.
Highlights
Pick-and-place strategies [35,36,37] were reported in which a single nanoparticle from a diluted powder of diamond nanocrystals dispersed on a substrate is selected and pushed to a desired location by a scanning probe, such as an atomic-forcemicroscopy tip, or a manipulator in a scanning electron microscope (SEM)
We conclude that local density of optical states (LDOS) variations in nanodiamond under the hypothesis of unit quantum efficiency and a bulk decay rate of (11.6 ns)−1 do not explain the variation in measured decay rates
In order to assess the suitability of nanodiamonds as photonic LDOS probes and as building blocks for hybrid photonic systems, we have investigated the brightness, decay rate and quantum efficiency of single NV centers in nanodiamonds of 25 nm and of 100 nm median size
Summary
Since single NV centers in nanodiamonds are comparatively dim emitters that require high excitation powers, it is essential to avoid background fluorescence when performing fluorescence microscopy. As sources for NV centers, we use solutions of monocrystalline synthetic nanodiamonds (Microdiamant MSY) with a narrow size tolerance obtained from Microdiamant AG, Lengwil, Switzerland We note that these nanoparticles have been used recently by a large number of groups in experiments that rely on fluorescence [26, 27, 32, 36, 38, 39, 48,49,50]. The second type of sample (‘100 nm diamond sample’) was made from Microdiamant MSY 0–0.2, which nominally has crystal sizes ranging from 0 to 200 nm This ensemble has crystals with median diameter 108 nm, and fewer than 1% of particles above 175 nm in size, again according to the size-distribution histogram supplied by the manufacturer. A difference in reported NV center densities could be either due to a different degree of strictness in labeling a fluorescent emitter as an NV center beyond doubt (see section 2.3 for our criteria), or alternatively due to batch-to-batch variations in nanodiamond slurries
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