Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna

Owing to the size mismatch between light and nanoscale objects such as single molecules, it is important to be able to control light–molecule interactions1–4. Plasmonic nanoantennas create highly enhanced local fields when pumped resonantly, leading to increased Raman scattering5, but whether fluorescence enhancement occurs depends upon a variety of factors. Although sharp metal tips6 and colloids7,8 can enhance fluorescence, the highly enhanced optical fields of lithographically fabricated bowtie nanoantennas9 provide a structure that is more controllable and amenable to integration. Using gold bowties, we observe enhancements of a single molecule’s fluorescence up to a factor of 1,340, ten times higher than reported previously7,8,10–22. Electromagnetic simulations reveal that this is a result of greatly enhanced absorption and an increased radiative emission rate, leading to enhancement of the intrinsic quantum efficiency by an estimated factor of nine, despite additional non-radiative ohmic effects. Bowtie nanoantennas thus show great potential for high-contrast selection of single nanoemitters. A single fluorescent molecule (SM) with transition dipole m acts as a nanoscale optical sensor of the local field E near a bowtie nanoantenna because its transition rate is proportional to jm Ej, and its emission can either couple to the far field via the nanoantenna or quench through ohmic losses23,24. Low-quantum-efficiency emitters have been noted to have much higher potential fluorescence brightness enhancements ( fF) than high-quantum-efficiency emitters, because their intrinsic quantum efficiency has a greater potential to be improved by the presence of the antenna25,26. Experimental measurements of fF for a SM were performed by coating electron-beam-fabricated gold bowtie nanoantennas with a relatively low fluorescence quantum efficiency of h0 2.5% but solubilized near-infrared (NIR) dye N,N 0-bis(2,6-diisopropylphenyl)-1,6,11,16-tetra-[4-(1,1,3,3-tetramethylbutyl)phenoxy]quaterrylene-3,4:13,14-bis(dicarboximide) (TPQDI) doped in a thin poly(methyl methacrylate) (PMMA) layer (Fig. 1a). In addition to its low quantum efficiency, TPQDI (Fig. 1b) was chosen for the overlap of its absorption and emission spectra with the measured bowtie plasmon resonance (Fig. 1e). A 780-nm diode laser was used to excite fluorescence from the TPQDI in a confocal microscope. Appropriate excitation and emission filters ensured that only TPQDI fluorescence reached the avalanche photodiode (APD) photon-counting silicon detector. Figure 2a shows a confocal fluorescence scan from a low TPQDI concentration in PMMA without bowtie nanoantennas. Essentially all fluorescent molecules irreversibly photobleach after a certain number of excitation cycles due to photodegradation (for example, photo-oxidation), so each spot in the image was observed until single-step digital photobleaching occurred (Fig. 2c) to ensure it corresponded to a single unenhanced TPQDI molecule. Each molecule’s dipole moment is randomly oriented with respect to the linear excitation field polarization, so each spot has a different brightness, with the brightest spots arising from molecules with dipole moments aligned along the excitation polarization. To measure the brightness of an unenhanced molecule for which the dipole moment is oriented along the excitation field, Sun,max, 201 single molecules were measured and the intensities of the brightest five averaged (Supplementary Fig. S1), yielding 2.3 detected photons per 10 ms per mW excitation power. Figure 2b shows a confocal scan from an array of 16 bowties coated with a high concentration of TPQDI in PMMA ( 1,000 molecules/diffraction-limited spot or 3 molecules/(10 nm)2). To detect a SM among the many surrounding the bowtie, the fluorescence as a function of time is shown in Fig. 2d. Discrete blinking and eventual photobleaching of 50% of the total signal can be attributed to a single molecule’s dynamics, revealing that half of the fluorescence from this particular bowtie is due to a single molecule! In other words, the digital (step-like) sudden drop near 293 s is an unambiguous signature that a single molecule photobleached, and the step size shows its contribution to the total signal, Sbowtie. Although the exact position and orientation of this molecule is 700 800 900 0