High definition aperture probes for near-field optical microscopy fabricated by focused ion beam milling

Abstract

We have improved the optical characteristics of aluminum-coated fiber probes used in near-field scanning optical microscopy by milling with a focused ion beam. This treatment produces a flat-end face free of aluminum grains, containing a well-defined circularly-symmetric aperture with controllable diameter down to 20 nm. The polarization behavior of the tips is circularly symmetric with a polarization ratio exceeding 1:100. The improved imaging characteristics are demonstrated by measuring single molecule fluorescence. Count rates increase more than one order of magnitude over unmodified probes, and the molecule images map a spatial electric field distribution of the aperture in agreement with calculations.

Scanning near-field optical microscopy with aperture probes: Fundamentals and applications

“…These numbers are in reasonable agreement with transmission coefficients reported in the literature and measured in our laboratories. 61,63,76 It should even be possible from such considerations to estimate the effective aperture size from transmission measurements if the taper angle at the apex is known from an independent ͑e.g., SEM͒ measurement.…”

Propagation of the electromagnetic field in fully coated near-field optical probes

“…4,5 Recently a very innovative design that combines nearfield optical probes with a cantilever ͓atomic force microscopy-scanning near field optical microscopy ͑SNOM͒ hybrid probes͔ has been proposed. In this approach, the probes comprise a 12 m high amorphous SiO 2 conical nearfield tip on a silicon cantilever.…”

Probing single molecule orientations in model lipid membranes with near-field scanning optical microscopy

“…This has recently been demonstrated by van Hulst and co-workers. 7,60 These researchers demonstrated the ability to precisely define the geometry of the near-field tip aperture by micromachining the tips using focused ion beam technology. 7,60 The resulting single molecule near-field fluorescence images exhibit patterns indicative of small z/a ranges.…”

“…7,60 These researchers demonstrated the ability to precisely define the geometry of the near-field tip aperture by micromachining the tips using focused ion beam technology. 7,60 The resulting single molecule near-field fluorescence images exhibit patterns indicative of small z/a ranges. 7 At these reduced tip-sample gaps, calculations indicate that the duallobed single molecule signals are slightly less sensitive to tilt angle which means both lobes are observable at higher tilting angles than those shown here.…”