Tip-Enhanced Fluorescence Microscopy at 10 Nanometer Resolution

We demonstrate unambiguously that the field enhancement near the apex of a laser-illuminated silicon tip decays according to a power law that is moderated by a single parameter characterizing the tip sharpness. Oscillating the probe in intermittent contact with a semiconductor nanocrystal strongly modulates the fluorescence excitation rate, providing robust optical contrast and enabling excellent background rejection. Laterally encoded demodulation yields images with $<10\mathrm{n}\mathrm{m}$ spatial resolution, consistent with independent measurements of tip sharpness.

Figure 1

Enhancement near a sharp silicon probe. Approach curves for a 5 nm diameter quantum dot (solid circles), a $5\times 20\mathrm{n}\mathrm{m}$ CdSe-ZnS nanorod (open squares), and a 20 nm diameter dye-doped latex sphere (open triangles). Additional approach curves extending to $z\sim 150\mathrm{n}\mathrm{m}$ (not shown) were flat beyond the enhancement region. The vertical scale is normalized to the count rate for a retracted probe. 10% uncertainties in the horizontal and vertical scales originate from calibration of the probe-oscillation amplitude and noise in the normalization factor, respectively. The lines connect the data points. Inset: Histogram of phase delays for the quantum dot.

Figure 2

Least-squares fit to the approach curve for (a) 20 nm diameter dye-doped sphere; (b) nanorod; and (c) spherical quantum dot. Fits to the power law of Eq. (1) (solid curves) and an exponential function (dashed curves) are shown with the data. The horizontal scales are offset by the fitted values of $a$, and unity is subtracted from the vertical scales.

Figure 3

Tip-enhanced fluorescence image of a nanorod. (a) Fluorescence demodulation signal. (b) AFM probe height (5 nm at peak). (c) Total photon count. Insets in (a) and (b) show signal profiles along the designated axis of length 250 nm. Blue curve in (d) shows the total photon count ($\times 0.2$) along the horizontal axis in (c), while red and black curves show the photon count within two ranges of tip-sample separation: $0<z<0.4\mathrm{n}\mathrm{m}$ (red) and $2.5<z<4.5\mathrm{n}\mathrm{m}$ (black). Field-of-view is $400\times 200\mathrm{n}\mathrm{m}$: 256 lines of 1024 pixels each.

Figure 4

Tip-enhanced fluorescence image of quantum dots. The degree of contrast is emphasized by false-color relief. The image contains 512 lines of 1000 pixels each. The arrows indicate the measured FWHM for two quantum dots.