Optical Microscopy via Spectral Modifications of a Nanoantenna

The existing optical microscopes form an image by collecting photons emitted from an object. Here we report on the experimental realization of microscopy without the need for direct optical communication with the sample. To achieve this, we have scanned a single gold nanoparticle acting as a nanoantenna in the near field of a sample and have studied the modification of its intrinsic radiative properties by monitoring its plasmon spectrum.

Figure 1

(a) Schematics of the experimental arrangement. (b) Plasmon spectra of the gold nanoparticle attached to the glass fiber tip in the absence of the sample. The polarization of the incident light is indicated by $s$ and $p$ defined in (a).

Figure 2

(a) Topography image of the sample ($40\times 40$ pixels), showing the round opening in a very thin chromium film. Inside this opening a small round protrusion is revealed (see text for details). The inset shows a cross section along cut (ii). The arrow indicates the direction of illumination. (b) The transmitted intensity integrated under the spectrum recorded at each pixel. (c) and (d) The central wavelength and the FWHM of the plasmon resonance, respectively. The circles are traced as a guide to the eye.

Figure 3

(a) Examples of the spectra recorded on the chromium and glass regions at positions $\alpha$ and $\beta$ in Fig. 2. Peak wavelengths and linewidths from cross sections (i) and (ii) in Fig. 2 are shown in (b),(c) and (g),(h), respectively. (d) An example snapshot of the simulated distribution of intensity $|H_x{|}^2$. Because of lack of space, we only show the central part of the computation box. (e) and (f) Simulated peak wavelength and linewidth of the plasmon spectra for a 2D system. The red curves show fits by the edge function $\tanh \mathbf{(}(y-y_0)/l\mathbf{)}$ where $y_0$ and $l$ are free parameters. In each case the parameter $\delta x$ represents the width from the 90% to 10% value of this function.