Abstract
Fabricating nanocavities in which optically active single quantum emitters are precisely positioned is crucial for building nanophotonic devices. Here we show that self-assembly based on robust DNA-origami constructs can precisely position single molecules laterally within sub-5 nm gaps between plasmonic substrates that support intense optical confinement. By placing single-molecules at the center of a nanocavity, we show modification of the plasmon cavity resonance before and after bleaching the chromophore and obtain enhancements of ≥4 × 103 with high quantum yield (≥50%). By varying the lateral position of the molecule in the gap, we directly map the spatial profile of the local density of optical states with a resolution of ±1.5 nm. Our approach introduces a straightforward noninvasive way to measure and quantify confined optical modes on the nanoscale.
Highlights
Coherent coupling of light and single-molecules at room temperature is one of the fundamental goals of nanooptics that would enable widespread adoption as a building block of nanophotonic devices
For roomtemperature nano-optics, plasmonic nanocavities have gained tremendous interest due to their enhanced field confinements.[7,16−18] Integrating optically active materials into these cavities is of great importance to access the desired coherent interaction between optical field and exciton
Either the cavity must be fabricated around randomly located emitters such as quantum dots or nitrogen-vacancy centers,[14,19,20] or emitters are randomly located inside the cavity.[21]
Summary
Component delivering high coupling efficiency to the far-field, η ≥ 0.5.21,24 A two-level emitter positioned in the gap experiences high LDOS and its emission is strongly enhanced (∝1/Vc). 80 nm diameter Au nanoparticles functionalized with 5′ thiol-modified 20× poly-T strands hybridize with the DNAo. The resultant assembly yields nanoparticles on a flat metal surface with the ultranarrow gap (NPoM cavity) filled with DNA origami and a single-Cy5 molecule at the center (Figure 1d). With NPoMs providing local intensity enhancements of Igap ∼ 2500 in the gap (Figure 1c) and τb ∝ 1/Igap, the photobleaching is suppressed by more than 3 orders of magnitude.[62] The total number of photons emitted from the single dye in each NPoM before it bleaches is estimated to be 4 × 106, resulting in enhancement of total photon counts by factors >1000 times compared to each Cy5 in DNAo on glass This enhancement results from combined effects of enhanced radiative emission rates, better light collection from the NPoM antenna, and suppressed bleaching rates.[46,63] These behaviors fully corroborate our evidence for single molecule emission. It is important to note that our geometry is completely different to the quenching observed when emitters are placed close to a single Au surface
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