Measuring the Quantum Efficiency of the Optical Emission of Single Radiating Dipoles Using a Scanning Mirror

Using scanning probe techniques, we show the controlled manipulation of the radiation from single dipoles. In one experiment we study the modification of the fluorescence lifetime of a single molecular dipole in front of a movable silver mirror. A second experiment demonstrates the changing plasmon spectrum of a gold nanoparticle in front of a dielectric mirror. Comparison of our data with theoretical models allows determination of the quantum efficiency of each radiating dipole.

Figure 1

(a) Terrylene molecules embedded in para-terphenyl (pT) were illuminated by p-polarized light that undergoes total internal reflection at the pT-air interface. A silver mirror was positioned in front of the sample with a three-dimensional piezo stage (not shown). Fluorescence light is collected and sent to an APD or CCD camera. Parameters used in the modeling are defined in this diagram. (b) The intensity distribution at the back focal plane for a single molecule (with no mirror). (c) A cross section indicated in (b). A fit to this data shows a dipole angle $\theta$ of $16°\pm 5°$.

Figure 2

Lifetime as a function of mirror position ($z$) for 2 molecules A (squares) and B (triangles). The solid curves display theoretical fits assuming ${\eta }_0=0.98$, $h=3\mathrm{nm}$, and $z_0=12\mathrm{nm}$ for molecule A, ${\eta }_0=1$, $h=22\mathrm{nm}$, and $z_0=11\mathrm{nm}$ for molecule B, and $\theta =16°$ in both molecules.

Figure 3

(a) (Left-hand axis): symbols show measured intensity of molecule $B$ in Fig. 2. The curve is the theoretical fit to the data with fitted parameters of $\varphi =53\pm 1$, $NA=1.00\pm 0.01$, and other variables as in Fig. 2. (b) (Right-hand axis): the curve $B$ of Fig. 2. (c) The pump intensity at the position of the molecule with $\varphi =53°$.

Figure 4

(a) The squares show the measured decay time and linewidth of the plasmon oscillations. The solid line shows a theoretical fit using Eq. (1). A stretch factor of 0.8 was fitted to the $z$ axis as the piezo in this system was not calibrated. (b) Circles show a plasmon spectrum recorded from the nanoparticle at the tip. The solid curve is a fit according to a modified Mie theory. (c) A schematic of the setup.