Enhanced Light Confinement in a Near-Field Optical Probe with a Triangular Aperture

We present a probe concept for scanning near-field optical microscopy combining the excellent background suppression of aperture probes with the superior light confinement of apertureless probes. A triangular aperture at the tip of a tetrahedral waveguide (full taper angle $\sim 90°$) shows a strong field enhancement at only one rim when illuminated with light of suitable polarization. Compared to a circular aperture of equivalent size, the resolution capability is doubled without loss of brightness. For a $\sim 60\mathrm{n}\mathrm{m}$ sized triangular aperture, we measured an optical resolution $<40\mathrm{n}\mathrm{m}$ and a transmission of $\sim {10}^{-4}$.

Figure 1

(a) Schematic of a TA probe attached to a tuning fork as a force sensor for feedback control. Probe and tuning fork are tilted such that the symmetry axis of the probe is oriented normal to the surface. (b) Enlarged side view. A tiny prism on the rear of the glass body enables the illumination of the aperture with focused laser light.

Figure 2

Characterization of a TA probe with a large aperture. (a) The SEM image shows an equilateral triangular aperture (side length $\sim 130\mathrm{n}\mathrm{m}$), whereas (b) the topography reveals a tetrahedron-shaped protrusion at the location of the aperture. (c)–(f) The simultaneously taken fluorescence images display the intensity distributions of the electric field for several different directions of the irradiated linearly polarized light (see arrows). A white triangle marks the rim of the aperture as concluded from (a) and (b).

Figure 3

Characterization of a TA probe with a small aperture. (a) SEM image: aperture size is $\sim 35\mathrm{n}\mathrm{m}$ (FWHM). (b) Topography: A small rounded tip at the location of the aperture protrudes from an otherwise flat end face. (c),(d) Optical field distributions at the aperture for two different polarization directions of the incoming light (see arrows). A white triangle marks the rim of the effective optical aperture.

Figure 4

(a) Topographic and (b) fluorescence images of aggregates of CdSe nanocrystals prepared on a structured LB film. (c) Average width of single fluorescence spots as a function of aggregate diameter. The intense blinking of CdSe QDs results in a broad distribution of optical spot widths for a given aggregate diameter. A line of regression reveals a strong correlation between optical width and aggregate size.