Using the Near-Field Coupling of a Sharp Tip to Tune Fluorescence-Emission Fluctuations during Quantum-Dot Blinking

We demonstrate that the cycling between internal states of quantum dots during fluorescence blinking can be used to tune the near-field coupling with a sharp tip. In particular, the fluorescence emission from states with high quantum yield is quenched due to energy transfer, while that from low-yield states is elevated due to field enhancement. Thus, as a quantum dot blinks, its emission fluctuations are progressively suppressed upon approach of a tip.

Figure 1

Experimental scheme. (a) Excitation light is incident on the sample above the critical angle, creating an evanescent field above the sample with polarization along the axis of the tip. Fluorescence data are collected as the tip oscillates vertically above an isolated QD. (b) Photoluminescence trajectory from a single QD. The different colors illustrate the threshold levels in Figs. 2a, 2b.

Figure 2

Vertical approach curves for different count-rate thresholds. Un-normalized (a, c) and normalized (b, d) fluorescence signals as a function of the tip height above a QD for gold-coated (a, b) and silicon (c, d) probes. The color-coding in (a, b) corresponds to the threshold values shown in Fig. 1b. For each tip, the various approach curves were obtained from a single measurement as the QD cycled dynamically through different quantum yield states.

Figure 3

Separation of enhancement and quenching. (a) Normalized signal at $z=0$ and corresponding fit to Eq. (2) for gold-coated (blue solid circles), silicon (green open circles), and CNT (red open squares) tips. The dashed line separates enhancement and quenching. (b) Unnormalized fluorescence signal at $z=0$ for the gold-coated tip in (a); the solid line is the best fit to Eq. (3). Panels (a) and (c) show the fitted values for $\alpha$ and $\beta$ for different tip-sample distances.