Fig. 1. Experimental setup for broad-bandwidth interference SNOM (BISNOM). Light from a broad-bandwidth titanium:sapphire laser (see the spectrum in the inset in the lower left) is focused onto the sample by an MO. The polarization is controlled by an HWP. A sharply etched gold tip is brought close to the sample to scatter light from the near-field region to the far field. The scattered light is collected by the MO. The larger part of the incident light (80%) is transmitted through a BS to the reference arm of a Michelson interferometer, adjusted in power by a variable gray filter, reflected (MR), and superimposed with the light from the sample arm at the BS (see the spectral interferogram in the inset in the lower right). The light is detected either by an APD or by a monochromator followed by a fast CCD line camera.

Fig. 2. SEM images of the sample. (a) Sb2S3 nanodots are created on a flat, compact Sb2S3 film by EBL. The nanodots are regularly spaced by 2μm, and their shape varies slightly. (b) The SEM image of an individual nanodot shows a slight ellipticity and a dark shadow in the center where the material has collapsed after EBL and annealing.

Fig. 3. Quasimonochromatic SNOM measurements of an individual nanoparticle using a 40-nm bandwidth laser spectrum centered at 900 nm for excitation and the APD and lock-in amplifier for detection. (a) Optical signal demodulated at the first (blue curve), second (green curve), and fourth harmonic (red curve) of the tip modulation frequency as a function of tip–sample distance. The higher-order demodulated signals demonstrate an improved near-field contrast. The inset shows the simultaneously recorded tuning fork amplitude. (b) A topographical map and (c)–(e) maps of the optical signals S1f, S2f, and S4f, respectively. All maps show a ring-shaped intensity distribution. (f) Cross cuts through the topographical map (blue curve) and the S4f signal (black curve) along dashed lines in (b) and (e).

Fig. 4. (a) Local density of states in an Sb2S3-nanodot with a circular cross section of a 200-nm radius at a wavelength of 800 nm. (b) An x-oriented tip dipole is moved along the x axis from the left rim to the center over the surface of the elliptical nanodot. Below is shown how the dipole orientation changes relative to the surface, and the red arrows at the bottom indicate the projection on the surface. (c) The projected LDOS as probed with an x-oriented tip dipole shows minima at the left and right rims, corresponding to the small projection of the tip-dipole onto the surface at these edges. (d) Using a y-oriented tip dipole results in a 90-deg rotated image, and (e) a z-oriented dipole particularly probes the edge of the rim. (f) The asymmetric SNOM map calculated for a wavelength of 800 nm and (g) for comparison, the measured S4f map [same as Fig. 3(e)]. The main maximum as well as the asymmetric side lobes on the left and lower right and the near-zero signal regions at the top and bottom are well recreated. (h) Horizontal and vertical cuts (upper and lower graphs, respectively) of the measured SNOM map (black circles) together with the calculated SNOM signals (black curve), calculated for the nanodot curvature shown by the blue curves.

Fig. 5. Spectrally resolved approach curves measured above the flat Sb2S3 film. (a) The spectra recorded on the Sb2S3 film, demodulated at the fundamental tip modulation frequency S1f(λ) as a function of the tip–sample distance d. Due to background interference, the spectra change their shape as the tip is retracted. (b) The signal demodulated at the fourth harmonic S4f(λ) retains its shape and decreases rapidly as the tip–sample distance increases. (c) The effective tip polarizability calculated for the same tip–sample distances as used in the measurements; and (d) the calculated near-field spectra. The inset shows the reference spectrum that was measured in the experiment and used as the input spectrum for the simulations. The calculated near-field spectra are in good agreement with the measured S4f(λ) spectra.

Fig. 6. Near-field spectra recorded at different positions on an Sb2S3 nanoparticle using the monochromator and fast line camera. (a) The demodulated signals S1f(λ) to S4f(λ) recorded with the tip in close contact above the center of the nanodot; (b) on the film; and (c) above the outer area on the nanodot. (d) Comparison of the three S4f(λ) spectra shown in (a)–(c) and a calculated S4f spectrum (black curve). The inset indicates the three positions where the spectra shown in this figure are recorded.

Fig. 7. Spectrally resolved near-field maps of an Sb2S3 nanodot. (a)–(g) Near-field maps recreated from 30-nm spectral bands centered at (a) 715 nm, (b) 745 nm, (c) 775 nm, (d) 805 nm, (e) 835 nm, (f) 865 nm, and (g) 895 nm. (h) Cross cuts through the near-field maps shown in (a) (spectral range 700 to 730 nm, blue symbols) and in (f) (spectral range 850 to 880 nm, red symbols) together with the calculated SNOM maps at 715 nm (blue curve) and 865 nm (red curve).