Tip Design Offers Aperture and Apertureless Advantages

Daniel S. Burgess

A design for a metallic tip probe for scanning near-field optical microscopy promises the benefits of both aperture and apertureless techniques. The scientists at Gifu University in Japan who conceived the probe geometry suggest that it should yield the high resolution of apertureless scanning near-field optical microscopy but would not require an external light source, thus reducing background noise.

The proposed tip is a pyramid with an I-shaped aperture. Half of the pyramid is truncated, and the structure sits on a metallic slab. For an excitation wavelength of 500 nm, the pyramid would have a height of 95 nm and sides of 446 nm. In cross section, the aperture would feature a 32 × 32-nm central region bordered by two 207 × 80-nm openings. The slab would have a thickness of 64 nm and sides of 1528 nm.

When illuminated by plane-polarized light, surface plasmon polaritons would be excited on the slab and would propagate through the aperture. Upon reaching the partially truncated region of the pyramid, the surface plasmon polaritons would be focused on the sharp tip, producing strong, localized illumination on the sample.

The researchers’ models of the performance of tips truncated by 32 and 80 nm yielded full width half maxima of the resulting intensity distributions of 17 and 14 nm, respectively, in the direction parallel to the polarization of the light. Perpendicular to the polarization, the full width half maxima were 24 nm in both cases.

Applied Physics Letters, Oct. 10, 2005, 151116.